Article

Spectroscopy of optical gain in low threshold colloidal quantum dot laser media: Dominance of single-exciton states at room temperature

If you want to read the PDF, try requesting it from the authors.

Abstract

Experimental studies of amplified spontaneous emission (ASE) and lasing from various colloidal II-VI semiconductor nanocrystals have been used as inputs to several microscopic models for underlying optical gain, usually involving permutations of quantum confined multiple excitonic states. Here we focus on particular types of CdSe/ZnCdS and CdSe/ZnS/ZnCdS colloidal quantum dot (CQD) films and elucidate on the discovery of single-exciton states at the fundamental edge as a dominant mechanism for optical gain at room temperature. Pump-probe spectroscopic techniques enable us to measure the onset of gain at ensemble-average exciton occupancy per CQD, < N > = 0.6 and 0.7 for the two types of CQD films at room temperature. Time-resolved measurements, in turn, show how optical gain persists well into the time regime associated with spontaneous emission (nanoseconds), thus providing direct evidence for how the non-radiative Auger recombination processes (~100 ps) can be thwarted. In addition to benefits of the material assets of densely packed CQD films with high luminescence efficiency (quantum yield ~90%) and nanoparticle monodispersity therein, we propose that access to the single-exciton gain regime at room temperature requires a careful spectral balance between the lowest exciton absorption resonance and its corresponding red-shifted spontaneous emission maximum ("Stokes shift").

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

Article
Organic-inorganic halide perovskites have recently developed into a potential semiconductor coherent light emitter candidate beyond their promise in solar cell applications. However, despite the ample demonstrations of perovskite lasers, experimental results on the origin of optical gain in perovskites are still elusive. Here, we analyze the excitonic gain in the green from mixed-cation halide perovskites Cs0.17[CH(NH2)2]0.83PbBr3 (Cs0.17FA0.83PbBr3) by both low temperature absorption/emission spectroscopies and ultrafast pump-probe transient absorption experiments. The perovskite thin films show a robust excitonic feature up to room temperature, with estimated exciton binding energy Eb = 43.8 meV, which can be maintained under high electronic excitations that are required for lasers. By using a high-quality (Q = 1350) vertical cavity consisting of sputtered dielectric HfO2/SiO2 distributed Bragg reflectors with perovskite optical gain medium embedded inside, we have demonstrated excitonic-gain-enabled optically pumped lasing, with improved threshold of 13.5 ± 1.4 μJ∕cm² and device longevity lifetime >35 h (10⁸ laser shots) at ambient environment under sustained pulsed optical excitations (3.493 eV, τpulse = 0.34 ns, 1 kHz). Understanding and exploiting excitonic gain from perovskite thin film materials may help to further boost the performance of perovskite-based lasers.
Article
Semiconductor lasers are the prime example of compact coherent light emitters having developed into mature technologies with ubiquitous presence in applications across much of the optical spectrum. Yet there are reasons to explore new materials, both to access those parts of the spectrum not fully accessible by these inorganic single crystal devices, while exploring alternative high throughput methods of material synthesis. In this paper, we focus on solution-processed thin film nanocrystal (NC) materials as possible candidates to expand the material repertoire of semiconductor lasers. We review work on inorganic II-VI compound colloidal quantum dots (CQDs) and nanoplatelets (NPLs) and then present recent work in our laboratory on organometallic perovskites, which have gained significant attention quite recently by their promise as high performance photovoltaic solar cells. We concentrate on fundamentals of the optical gain and means of integrating the solution grown nano/micro-crystalline thin films into high-Q cavities to identify prime challenges ahead to turn these intriguing materials into viable laser devices.
Article
Full-text available
A new strategy for structuring organic?inorganic perovskite materials for optoelectronic applications was studied. By conformally evaporating perovskite onto a nanoimprinted polymer resist, we were able to create a working perovskite distributed feedback (DFB) cavity and to tune the emission between 770 and 793 nm simply by varying the grating periodicity. DFB perovskite cavities have huge potential as inexpensive, mirror free, widely tuneable, single mode lasers that are easy to manufacture on a large scale. The DFB structure is highly versatile and can be optimized, for example, toward lower thresholds or different output energies. Broad tuneability may be achieved by utilizing the various closely related organic?inorganic perovskites and tailoring the cavity to the gain maximum of the unpatterned fi lm. With this approach, continuous tuning of the emission from 1.1 to 3.1 eV should be feasible which has relevance for telecommunication, medicine, and many other applications.
Article
Full-text available
Deformable full-colour light-emitting diodes with ultrafine pixels are essential for wearable electronics, which requires the conformal integration on curvilinear surface as well as retina-like high-definition displays. However, there are remaining challenges in terms of polychromatic configuration, electroluminescence efficiency and/or multidirectional deformability. Here we present ultra-thin, wearable colloidal quantum dot light-emitting diode arrays utilizing the intaglio transfer printing technique, which allows the alignment of red-green-blue pixels with high resolutions up to 2,460 pixels per inch. This technique is readily scalable and adaptable for low-voltage-driven pixelated white quantum dot light-emitting diodes and electronic tattoos, showing the best electroluminescence performance (14,000 cd m(-2) at 7 V) among the wearable light-emitting diodes reported up to date. The device performance is stable on flat, curved and convoluted surfaces under mechanical deformations such as bending, crumpling and wrinkling. These deformable device arrays highlight new possibilities for integrating high-definition full-colour displays in wearable electronics.
Article
Full-text available
Solution-processed inorganic and organic materials have been pursued for more than a decade as low-threshold, high-gain lasing media, motivated in large part by their tunable optoelectronic properties and ease of synthesis and processing. Although both have demonstrated stimulated emission and lasing, they have not yet approached the continuous-wave pumping regime. Two-dimensional CdSe colloidal nanosheets combine the advantage of solution synthesis with the optoelectronic properties of epitaxial two-dimensional quantum wells. Here, we show that these colloidal quantum wells possess large exciton and biexciton binding energies of 132 meV and 30 meV, respectively, giving rise to stimulated emission from biexcitons at room temperature. Under femtosecond pulsed excitation, close-packed thin films yield an ultralow stimulated emission threshold of 6 μJ cm(-2), sufficient to achieve continuous-wave pumped stimulated emission, and lasing when these layers are embedded in surface-emitting microcavities.
Article
Full-text available
We demonstrate surface emitting distributed feedback (DFB) lasers across the red, green, and blue from densely packed colloidal quantum dot (CQD) films. The solid CQD films were deposited on periodic grating patterns to enable 2nd-order DFB lasing action at mere 120, 280, and 330 μJ/cm2 of optical pumping energy densities for red, green, and blue DFB lasers, respectively. The lasers operated in single mode operation with less than 1 nm of full-width-half-maximum. We measured far-field patterns showing high degree of spatial beam coherence. Specifically, by taking advantage of single exciton optical gain regime from our engineered CQDs, we can significantly suppress the Auger recombination to reduce lasing threshold and achieve quasi-steady state, optically pumped operation.
Article
Full-text available
Low-temperature solution-processed materials that show optical gain and can be embedded into a wide range of cavity resonators are attractive for the realization of on-chip coherent light sources. Organic semiconductors and colloidal quantum dots are considered the main candidates for this application. However, stumbling blocks in organic lasing include intrinsic losses from bimolecular annihilation and the conflicting requirements of high charge carrier mobility and large stimulated emission; whereas challenges pertaining to Auger losses and charge transport in quantum dots still remain. Herein, we reveal that solution-processed organic-inorganic halide perovskites (CH3NH3PbX3 where X = Cl, Br, I), which demonstrated huge potential in photovoltaics, also have promising optical gain. Their ultra-stable amplified spontaneous emission at strikingly low thresholds stems from their large absorption coefficients, ultralow bulk defect densities and slow Auger recombination. Straightforward visible spectral tunability (390-790 nm) is demonstrated. Importantly, in view of their balanced ambipolar charge transport characteristics, these materials may show electrically driven lasing.
Article
Full-text available
Efficient, low-threshold and compact semiconductor laser sources are under investigation for many applications in high-speed communications, information processing and optical interconnects. The best edge-emitting and vertical-cavity surface-emitting lasers have thresholds on the order of 100 µA (refs 1,2), but dissipate too much power to be practical for many applications, particularly optical interconnects. Optically pumped photonic-crystal nanocavity lasers represent the state of the art in low-threshold lasers; however, to be practical, techniques to electrically pump these structures must be developed. Here, we demonstrate a quantum-dot photonic-crystal nanocavity laser in gallium arsenide pumped by a lateral p-i-n junction formed by ion implantation. Continuous-wave lasing is observed at temperatures up to 150 K. Thresholds of only 181 nA at 50 K and 287 nA at 150 K are observed--the lowest thresholds ever observed in any type of electrically pumped laser.
Article
Full-text available
Colloidal quantum dots exhibit efficient photoluminescence with widely tunable bandgaps as a result of quantum confinement effects. Such quantum dots are emerging as an appealing complement to epitaxial semiconductor laser materials, which are ubiquitous and technologically mature, but unable to cover the full visible spectrum (red, green and blue; RGB). However, the requirement for high colloidal-quantum-dot packing density, and losses due to non-radiative multiexcitonic Auger recombination, have hindered the development of lasers based on colloidal quantum dots. Here, we engineer CdSe/ZnCdS core/shell colloidal quantum dots with aromatic ligands, which form densely packed films exhibiting optical gain across the visible spectrum with less than one exciton per colloidal quantum dot on average. This single-exciton gain allows the films to reach the threshold of amplified spontaneous emission at very low optical pump energy densities of 90 µJ cm(-2), more than one order of magnitude better than previously reported values. We leverage the low-threshold gain of these nanocomposite films to produce the first colloidal-quantum-dot vertical-cavity surface-emitting lasers (CQD-VCSEL). Our results represent a significant step towards full-colour single-material lasers.
Article
Full-text available
We report a numerical study of the effect of interface alloying and band-alignment on the Auger recombination processes of core/shell nanocrystals. Numerical calculations are carried out using a two-band Kane Hamiltonian. Smooth interfaces are found to suppress Auger recombination, the strength of the suppression being very sensitive to the core size. The use of type-II structures constitutes an additional source of suppression, especially when the shell confines electrons rather than holes. We show that "magic" sizes leading to negligible Auger recombination [Cragg and Efros, Nano Letters 10 (2010) 313] should be easier to realize experimentally in nanocrystals with extended interface alloying and wide band gap.
Article
Full-text available
We demonstrate a photonic crystal nanocavity laser essentially driven by a self-assembled InAs/GaAs single quantum dot gain. The investigated nanocavities contain only 0.4 quantum dots on an average; an ultra-low density quantum dot sample (1.5 x 108 cm-2) is used so that a single quantum dot can be isolated from the surrounding quantum dots. Laser oscillation begins at a pump power of 42 nW under resonant condition, while the far-detuning conditions require ~145 nW for lasing. This spectral detuning dependence of laser threshold indicates substantial contribution of the single quantum dot to the total gain. Moreover, photon correlation measurements show a distinct transition from anti-bunching to Poissonian via bunching with the increase of the excitation power, which is also an evidence of laser oscillation with the single quantum dot.
Article
Full-text available
Suitable labels are at the core of Luminescence and fluorescence imaging and sensing. One of the most exciting, yet also controversial, advances in label technology is the emerging development of quantum dots (QDs)--inorganic nanocrystals with unique optical and chemical properties but complicated surface chemistry--as in vitro and in vivo fluorophores. Here we compare and evaluate the differences in physicochemical properties of common fluorescent labels, focusing on traditional organic dyes and QDs. Our aim is to provide a better understanding of the advantages and limitations of both classes of chromophores, to facilitate label choice and to address future challenges in the rational design and manipulation of QD labels.
Article
Full-text available
Semiconductor quantum dots (QDs) are nanometer-sized fluorescent probes suitable for advanced biological imaging. We used QDs to track individual glycine receptors (GlyRs) and analyze their lateral dynamics in the neuronal membrane of living cells for periods ranging from milliseconds to minutes. We characterized multiple diffusion domains in relation to the synaptic, perisynaptic, or extrasynaptic GlyR localization. The entry of GlyRs into the synapse by diffusion was observed and further confirmed by electron microscopy imaging of QD-tagged receptors.
Article
Full-text available
Nanocrystal quantum dots have favourable light-emitting properties. They show photoluminescence with high quantum yields, and their emission colours depend on the nanocrystal size--owing to the quantum-confinement effect--and are therefore tunable. However, nanocrystals are difficult to use in optical amplification and lasing. Because of an almost exact balance between absorption and stimulated emission in nanoparticles excited with single electron-hole pairs (excitons), optical gain can only occur in nanocrystals that contain at least two excitons. A complication associated with this multiexcitonic nature of light amplification is fast optical-gain decay induced by non-radiative Auger recombination, a process in which one exciton recombines by transferring its energy to another. Here we demonstrate a practical approach for obtaining optical gain in the single-exciton regime that eliminates the problem of Auger decay. Specifically, we develop core/shell hetero-nanocrystals engineered in such a way as to spatially separate electrons and holes between the core and the shell (type-II heterostructures). The resulting imbalance between negative and positive charges produces a strong local electric field, which induces a giant ( approximately 100 meV or greater) transient Stark shift of the absorption spectrum with respect to the luminescence line of singly excited nanocrystals. This effect breaks the exact balance between absorption and stimulated emission, and allows us to demonstrate optical amplification due to single excitons.
Article
Perovskite semiconductors are actively investigated for high performance solar cells. Their large optical absorption coefficient and facile solution-based, low-temperature synthesis of thin films make perovskites also a candidate for light-emitting devices across the visible and near-infrared. Specific to their potential as optical gain medium for lasers, early work has demonstrated amplified spontaneous emission and lasing at attractively low thresholds of photoexcitation. Here, we take an important step toward practically usable perovskite lasers where a solution-processed thin film is embedded within a two-dimensional photonic crystal resonator. We demonstrate high degree of temporally and spatially coherent lasing whereby well-defined directional emission is achieved near 788 nm wavelength at optical pumping energy density threshold of 68.5 ± 3.0 μJ/cm(2). The measured power conversion efficiency and differential quantum efficiency of the perovskite photonic crystal laser are 13.8 ± 0.8% and 35.8 ± 5.4%, respectively. Importantly, our approach enables scalability of the thin film lasers to a two-dimensional multielement pixelated array of microlasers which we demonstrate as a proof-of-concept for possible projection display applications.
Article
Synthesis of semiconductor colloidal quantum dots by low-cost, solution-based methods has produced an abundance of basic science. Can these materials be transformed to high-performance light emitters to disrupt established photonics technologies, particularly semiconductor lasers?
Article
There have been multiple demonstrations of amplified spontaneous emission (ASE) and lasing using colloidal semiconductor nanocrystals. However, it has been proven difficult to achieve low thresholds suitable for practical use of nanocrystals as gain media. Low-threshold blue ASE and lasing from nanocrystals is an even more challenging task. Here, we show that colloidal nanoplatelets (NPLs) with electronic structure of quantum wells can produce ASE in the red, yellow, green, and blue regions of the visible spectrum with low thresholds and high gains. In particular, for blue emitting NPLs, the ASE threshold is 50 µJ/cm2, lower than any reported value for nanocrystals. We then demonstrate red, yellow, green, and blue lasing using NPLs with different thicknesses. We find that the lateral size of NPLs does not show any strong effect on the Auger recombination rates and, correspondingly, on the ASE threshold or gain saturation. This observation highlights qualitative difference of multiexciton dynamics in CdSe NPLs and other quantum-confined CdSe materials, such as quantum dots and rods. Our measurements of the gain bandwidth and gain lifetime further support the prospects of colloidal NPLs as solution-processed optical gain materials.
Article
Flexible information displays are key elements in future optoelectronic devices. Quantum dot light-emitting diodes (QLEDs) with advantages in colour quality, stability and cost-effectiveness are emerging as a candidate for single-material, full colour light sources. Despite the recent advances in QLED technology, making high performance flexible QLEDs still remains a big challenge due to limited choices of proper materials and device architectures as well as poor mechanical stability. Here, we show highly efficient, large-area QLED tapes emitting in red, green and blue (RGB) colours with top-emitting design and polyimide tapes as flexible substrates. The brightness and quantum efficiency are 20,000 cd/m2 and 4.03%, respectively, the highest values reported for flexible QLEDs. Besides the excellent electroluminescence performance, these QLED films are highly flexible and mechanically robust to use as electrically driven light-emitting stickers by placing on or removing from any curved surface, facilitating versatile LED applications. Our QLED tapes present a step toward practical quantum dot based platforms for high performance flexible displays and solid-state lighting.
Article
Colloidal nanoplatelets (NPLs) have recently emerged as favorable light-emitting materials, which also show great potential as optical gain media owing to their remarkable optical properties. In this work, we systematically investigate the optical gain performance of CdSe-core and CdSe/CdS-core/crown NPLs having different CdS-crown size with one- and two-photon absorption pumping. The core/crown NPLs exhibit enhanced gain performance as compared to the core only NPLs owing to increased absorption cross-section and the efficient inter-exciton funneling, which is from the CdS-crown to the CdSe-core. One- and two-photon absorption pumped amplified spontaneous emission thresholds are found as low as 41 µJ/cm2 and 4.48 mJ/cm2, respectively. These thresholds surpass the best reported optical gain performance of the state-of-the-art colloidal nanocrystals (i.e., quantum dots, nanorods, etc.) emitting in the same spectral range as the NPLs. Moreover, gain coefficient of the NPLs is measured as high as 650 cm-1, which is 4-fold larger than the best reported gain coefficient of the colloidal quantum dots. Finally, we demonstrate a two-photon absorption pumped vertical cavity surface emitting laser of the NPLs with a lasing threshold as low as 2.49 mJ/cm2. These excellent results are attributed to the superior properties of the NPLs as optical gain media.
Article
Colloidal quantum dots (CQD) are now making their entry to full-color displays, endowed by their brightness and single-material base. By contrast, many obstacles have been encountered in their use towards lasers. We demonstrate here optically pumped distributed feedback (DFB) lasers, based on close-packed, solid films self-assembled from type-I CQDs. Notably, the single mode CQD-DFB lasers could reach such a low threshold as to be pumpable with a compact pulsed source in a quasi-continuous wave regime. Our results show the spatially and temporally coherent laser beam outputs with power of 400 μW and a quantum efficiency of 32%.
Article
Large semiconductor crystals have intrinsic electronic properties dependent upon the bulk band structure. As the crystal becomes small, a new regime is entered in which the electronic properties (excited states, ionization potential, electron affinity) should be strongly dependent upon the electron and hole in a confined space. We address the possibility of a shift in the photochemical redox potential of one carrier, as a function of crystallite size. As a semiquantitative guide, one might expect a shift on the order of h2/8em&ast;R2 due to the kinetic energy of localization in the small crystallite. We model the elementary quantum mechanics of a charged crystallite using (a) the effective mass approximation, (b) an electrostatic potential for dielectric polarization, and (c) penetration of the carrier outside the crystallite in a cases of small effective mass. Shifts of several tenths of an eV appear possible in crystallites of diameter 50 A˚. The carrier charge density reside near the crystallite surface if the effective mass is very small.
Article
By using the hole effective-mass Hamiltonian for semiconductors with the wurtzite structure, we have studied the exciton states and optical spectra in CdSe nanocrystallite quantum dots. The intrinsic asymmetry of the hexagonal lattice structure and the effect of spin-orbital coupling (SOC) on the hole states are investigated. It is found that the strong SOC limit is a good approximation for hole states. The selection rules and oscillator strengths for optical transitions between the conduction- and valence-band states are obtained. The Coulomb interaction of exciton states is also taken into account. In order to identify the exciton states, we use the approximation of eliminating the coupling of Γ6(X,Y) with Γ1(Z) states. The results are found to account for most of the important features of the experimental photoluminescence excitation spectra of Norris et al. However, if the interaction between Γ6(X,Y) and Γ1(Z) states is ignored, the optically passive Px state cannot become the ground hole state for small CdSe quantum dots of radius less than 30 Å. It is suggested that the intrinsic asymmetry of the hexagonal lattice structure and the coupling of Γ6(X,Y) with Γ1(Z) states are important for understanding the “dark exciton” effect.
Article
The mechanism of Stokes shift in semiconductor quantum dots is investigated by calculating the energy of the excitonic states. We have taken into account all possible contributions to the total electronic energy in the dot—i.e., the dielectric mismatch between the dot and surrounding medium, the effects of finite barrier height, and the electron-hole exchange interaction. The Stokes shift is calculated as a function of radius of dot and compared with experimental data on two different semiconductor based quantum dots. These results provide evidence for an exchange splitting of excitonic states, as the mechanism of Stokes shift in quantum dots.
Article
By varying the dielectric environment in new PbI4-based layer-type perovskite compounds, we have demonstrated directly the contribution by dielectric confinement to the exciton binding energy in three such ‘‘natural-quantum-well’’ semiconductors. With different dielectric environment, exciton binding energies of 320, 220, and 170 meV have been observed, dominated by the dielectric confinement. In terms of the conventional size-related electronic confinement, two of the materials represent monolayer PbI4 quantum wells while the third corresponds to a bilayer case, with a corresponding reduction in the electronic confinement. From theory, including the dielectric confinement effect, the effective mass of the exciton in a PbI4-based dielectric quantum well has been determined to be 0.09me; the corresponding quasi-two-dimensional exciton Bohr radii were 15.5, 17.0, and 20.5 Å for the three cases, respectively.
Article
We study the luminescence of surface modified CdSe nanocrystallites. There has been much speculation as to the origin of the band edge emission in these quantum confined structures. Because of their large surface to volume ratios it has been suggested that the emission originates from surface-related states. However, recent theory suggests that the band edge luminescence arises from an optically inactive fine structure state or “dark” exciton. To address this issue we modify the surface of CdSe nanocrystallites with a variety of organic and inorganic ligands. We then monitor the effect changing the surface has on the energetics of the band edge luminescence through photoluminescence and fluorescence line narrowing experiments. Our results are compared with theoretical predictions for the nonresonant and resonant luminescence. We find good agreement between experiment and theory for CdSe nanocrystallites passivated with trioctylphosphine oxide, ZnS, 4-picoline, 4-(trifluoromethyl)thiophenol, and tris(2-ethylhexyl)phosphate. The lack of dependence of our data on surface modification is consistent with a dark exciton description of the band edge luminescence. © 1997 American Institute of Physics.
Article
Colloidal quantum-dot light-emitting diodes have recently received considerable attention due to their ease of colour tunability, high brightness and narrow emission bandwidth. Although there have been rapid advances in luminance, efficiency and lifetime, device performance is still limited by the large energy barriers for hole and electron injection into the quantum-dot layer. Here, we show that by crosslinking the colloidal quantum-dot layer, the charge injection barrier in a red-light-emitting quantum-dot light-emitting diode may be considerably reduced by using a sol–gel TiO2 layer for electron transport. The device architecture is compatible with all-solution device fabrication and the resulting device shows a high luminance (12,380 cd m-2), low turn-on voltage (1.9 V) and high power efficiency (2.41 lm W-1). Incorporation of the technology into a display device with an active matrix drive backplane suggests that the approach has promise for use in high-performance, easy-to-fabricate, large-area displays and illumination sources.
Article
We describe the synthesis of ZnS-capped CdSe semiconductor nanocrystals using organometallic reagents by a two-step single-flask method. X-ray photoelectron spectroscopy, transmission electron microscopy and optical absorption are consistent with nanocrystals containing a core of nearly monodisperse CdSe of 27−30 Å diameter with a ZnS capping 6 ± 3 Å thick. The ZnS capping with a higher bandgap than CdSe passivates the core crystallite removing the surface traps. The nanocrystals exhibit strong and stable band-edge luminescence with a 50% quantum yield at room temperature.
Article
The emergence of semiconductor nanocrystals as the building blocks of nanotechnology has opened up new ways to utilize them in next generation solar cells. This paper focuses on the recent developments in the utilization of semiconductor quantum dots for light energy conversion. Three major ways to utilize semiconductor dots in solar cell include (i) metal−semiconductor or Schottky junction photovoltaic cell (ii) polymer−semiconductor hybrid solar cell, and (iii) quantum dot sensitized solar cell. Modulation of band energies through size control offers new ways to control photoresponse and photoconversion efficiency of the solar cell. Various strategies to maximize photoinduced charge separation and electron transfer processes for improving the overall efficiency of light energy conversion are discussed. Capture and transport of charge carriers within the semiconductor nanocrystal network to achieve efficient charge separation at the electrode surface remains a major challenge. Directing the future research efforts toward utilization of tailored nanostructures will be an important challenge for the development of next generation solar cells.
Article
Size-controlled spectral tunability and chemical flexibility make semiconductor nanocrystals (NCs) attractive as nanoscale building blocks for color-selectable optical-gain media. The technological potential of NCs as lasing materials is, however, significantly diminished by highly efficient nonradiative Auger recombination of multiexcitons leading to ultrafast decay of optical gain. Here we explore a novel approach to achieve NC lasing in the Auger-recombination-free regime by using type II NC heterostructures that promote spatial separation of electrons and holes. We show that such hetero-NCs can exhibit strong repulsive exciton−exciton interactions that lead to significantly reduced excited-state absorption associated with NCs containing single electron−hole pairs. This effect leads to reduced optical-gain thresholds and can potentially allow lasing in the single-exciton regime, for which Auger recombination is inactive. We use these novel hetero-NCs to demonstrate efficient amplified spontaneous emission (ASE) that is tunable across a “difficult” range of green and blue colors. The ASE in the blue range has never been previously achieved using traditional NCs with type I carrier localization.
Article
We demonstrate a new method for preparing metal sulfide nanocrystals using alkyl thiols as a sulfur precursor. Alkyl thiols have many advantages for practical synthesis because they are miscible with most organic solvents and very stable under an air atmosphere. CdS nanocrystals were made with CdO and thiols with different alkyl chains such as n-octanethiol and octadecanethiol. They exhibited uniform size, highly crystalline structure and a sharp photoluminescence spectrum. Also, CdSe/CdS core/shell nanocrystals can be prepared by single injection of a mixture consisting of alkyl thiol and Se in trioctylphosphine to a Cd precursor. A reaction scheme is proposed as alkyl thiols react with the metal precursor to form stable metal thiolate intermediates during the initial period of reaction, and the thiolate decomposes slowly to form homogeneous nuclei.
Article
High-brightness, color-tunable colloidal quantum dots are incorporated in 3D nanoporous GaN to create a nanocomposite material (CQD/NP-GaN), which is demonstrated to be an effective approach for a wavelength down-conversion nanomaterial in solid-state lighting. The white-light-emitting diode (LED) made from a blue GaN-based LED and the CQD/NP-GaN shows an increase of extraction efficiency by a factor of 2, a controllable white color, and a down-conversion quantum efficiency as high as 82%.
Article
We report simultaneous amplified spontaneous emission from two different multiexcitonic transitions -1S<sub>e</sub>–1S<sub>3/2</sub> and 1P<sub>e</sub>–1P<sub>3/2<sup>-</sup></sub> of colloidal CdSe nanocrystals (NCs) stabilized in high volume fraction in titania matrices. Two-state lasing from both multiexcitonic transitions is achieved in a surface-emitting distributed feedback CdSe NC laser. Variable stripe length measurements show that the gain from the 1P<sub>e</sub>–1P<sub>3/2</sub> transition is approximately twice that of the -1S<sub>e</sub>–1S<sub>3/2</sub> transition for 4.2 nm radius CdSe / ZnS core-shell NCs.
Article
A new type of semiconductor laser is studied, in which injected carriers in the active region are quantum mechanically confined in two or three dimensions (2D or 3D). Effects of such confinements on the lasing characteristics are analyzed. Most important, the threshold current of such laser is predicted to be far less temperature sensitive than that of conventional lasers, reflecting the reduced dimensionality of electronic state. In the case of 3D‐QW laser, the temperature dependence is virtually eliminated. An experiment on 2D quantum well lasers is performed by placing a conventional laser in a strong magnetic field (30 T) and has demonstrated the predicted increase of T 0 value from 144 to 313 °C.
Article
Spatial confinement of electronic excitations in semiconductor nanocrystals (NCs) results in a significant enhancement of nonradiative Auger recombination (AR), such that AR processes can easily dominate the decay of multiexcitons. AR is especially detrimental to lasing applications of NCs, as optical gain in these structures explicitly relies on emission from multiexciton states. In standard NCs, AR rates scale linearly with inverse NC volume. Here, we investigate multiexciton dynamics in hetero-NCs composed of CdSe cores and CdS shells of tunable thickness. We observe a dramatic decrease in the AR rates at the initial stage of shell growth, which cannot be explained by traditional volume scaling alone. Rather, fluorescence-line-narrowing studies indicate that the suppression of AR correlates with the formation of an alloy layer at the core-shell interface suggesting that this effect derives primarily from the "smoothing" of the confinement potential associated with interfacial alloying. These data highlight the importance of NC interfacial structure in the AR process and provide general guidelines for the development of new nanostructures with suppressed AR for future lasing applications.
Article
Several classes of semiconductor quantum dots (QD), including groups II-VI, III-V, IV-VI, IV, and their alloys as well as various intergroup and intragroup core-shell configurations, and nanocrystal shapes have been synthesized. One approach to enhance efficiency in QD-based PV cells compared to conventional bulk semiconductor-based PV is to create efficient multiple exciton generation from a large fraction of the photons in the solar spectrum. Three generic types of QD solar cells that could utilize MEG to enhance conversion efficiency can be defined. They include photoelectrodes composed of QD arrays that form either Schottky junctions with a metal layer, a hetero p-n junction with a second NC semiconductor layer, or the i-region of a p-i-n device, QD-sensitized nanocrystalline TiO2 films, and QDs dispersed into a multiphase mixture of electron- and hole-conducting matrices, such as C60 and hole conducting polymers.
Article
Colloidal semiconductor quantum rods have demonstrated many advantageous properties as light emitters such as high quantum yield, tunable emission wavelength, and polarized emission. This makes them an interesting optical gain material for laser applications. We report room-temperature gain lifetimes in core/shell CdSe/CdS quantum rods exceeding 300 ps, and show that the long gain lifetimes result from the significant reduction of Auger recombination in our quantum rods, even though the electrons are delocalized over the rod volume. We also fabricate devices by deposition of small droplets of quantum rod solution onto flat substrates. The evaporation dynamics of the droplets are governed by the coffee stain effect which leads to the formation of well defined micron-size stripes. These stripes consist of densely packed, laterally aligned quantum rods and provide optical feedback originating from the abrupt changes of refractive index at the stripe borders. We exploit the optical gain and the coffee stain mediated self-assembly and show that we can fabricate novel microlasers solely by deposition of droplets of quantum rod solutions on flat substrates.
Article
We explore how the size and shape of the microscopic confinement potential affects the nonradiative Auger decay rate of confined carriers. Calculations conducted in the two-band, effective mass Kane model unambiguously show that smoothing out the confinement potential could reduce the rate by more than 3 orders of magnitude relative to the rate in structures with abruptly terminating boundaries. As the confinement potential width is increased, the calculated rate decreases overall, exhibiting very deep minima at regular widths. Such minima suggest that nanocrystals of "magic sizes" can exist for which nonradiative Auger processes are strongly suppressed.
Article
Colloidal core/shell nanocrystals contain at least two semiconductor materials in an onionlike structure. The possibility to tune the basic optical properties of the core nanocrystals, for example, their fluorescence wavelength, quantum yield, and lifetime, by growing an epitaxial-type shell of another semiconductor has fueled significant progress on the chemical synthesis of these systems. In such core/shell nanocrystals, the shell provides a physical barrier between the optically active core and the surrounding medium, thus making the nanocrystals less sensitive to environmental changes, surface chemistry, and photo-oxidation. The shell further provides an efficient passivation of the surface trap states, giving rise to a strongly enhanced fluorescence quantum yield. This effect is a fundamental prerequisite for the use of nanocrystals in applications such as biological labeling and light-emitting devices, which rely on their emission properties. Focusing on recent advances, this Review discusses the fundamental properties and synthesis methods of core/shell and core/multiple shell structures of II-VI, IV-VI, and III-V semiconductors.
Article
We use photoluminescence excitation and fluorescence line narrowing spectroscopies to examine structure observed in the band-edge absorption feature of CdSe quantum dots. We study eight samples ranging from ~15 to ~50 Å in radius to probe the size dependence of this structure. We compare our results with recent theories, which predict band-edge exciton splittings in CdSe dots due to their internal crystal structure, nonspherical shape, and the exchange interaction between the electron and hole. We find reasonable agreement between our data and theory, supporting the observation of exciton fine structure.
Article
We present a theoretical analysis of the band-edge exciton structure in nanometer-size crystallites of direct semiconductors with a cubic lattice structure or a hexagonal lattice structure which can be described within the framework of a quasicubic model. The lowest energy exciton, eightfold degenerate in spherically symmetric dots, is split into five levels by the crystal shape asymmetry, the intrinsic crystal field (in hexagonal lattice structures), and the electron-hole exchange interaction. Transition oscillator strengths and the size dependence of the splittings have been calculated. Two of the five states, including the ground state, are optically passive (dark excitons). The oscillator strengths of the other three levels (bright excitons) depend strongly on crystal size, shape, and energy band parameters. The relative ordering of the energy levels is also heavily influenced by these parameters. The distance between the first optically active state and the optically forbidden ground exciton state increases with decreasing size, leading to an increase of the Stokes shift in the luminescence. Our results are in good agreement with the size dependence of Stokes shifts obtained in fluorescence line narrowing and photoluminescence experiments in CdSe nanocrystals. Mixing of the dark and bright excitons in an external magnetic field allows the direct optical recombination of the dark exciton ground state. The observed shortening of the luminescence decay time in CdSe nanoncrystals in a magnetic field is also in excellent agreement with the theory, giving further support to the validity of our model.
Article
A general scheme is established within the effective-mass approximation to calculate systematically the excitonic energy spectra in a semiconductor quantum dot including the dielectric confinement effect. This effect is found to appear most pronounced in the quantum-dot structure in comparison with the quantum-well and quantum-wire structures. A formula of the lowest exciton energy in the strong confinement regime is derived and the significance of the dielectric confinement effect is clarified. We investigate the dependence of the binding energy and the oscillator strength of the lowest-energy excitonic state on the quantum-dot radius, the electron-to-hole mass ratio, and the dielectric-constant ratio between the quantum dot and the surrounding medium. The subband mixing effect due to the electron-hole Coulomb interaction gives a finite oscillator strength to excitonic transitions which are forbidden in the absence of the Coulomb interaction. This effect is shown unambiguously in the calculated excitonic energy spectra. Furthermore, the electron-hole exchange interaction in a quantum dot is discussed. The short-range part of the exchange energy is shown to increase in proportion to the inverse of the volume of the quantum dot as the quantum-dot size is reduced. On the other hand, the long-range part of the exchange energy is found to be sensitively dependent on the shape of the quantum dot. In particular, it vanishes for the optically allowed excitonic states in a spherical quantum dot.
Article
We use external magnetic fields to identify the band edge emitting state in CdSe quantum dots. The field dependence of emission decays and LO phonon spectra show the importance of exciton spin dynamics in the recombination mechanism. To interpret our results we calculate the band edge exciton structure, including the effects of the electron-hole exchange interaction and a nonspherical shape. The exchange term, negligible in the bulk, is strongly enhanced by quantum confinement and allows the observation of an optically passive ``dark'' excitonic state.
Article
We have resolved single-exponential relaxation dynamics of the 2-, 3-, and 4-electron-hole pair states in nearly monodisperse cadmium selenide quantum dots with radii ranging from 1 to 4 nanometers. Comparison of the discrete relaxation constants measured for different multiple-pair states indicates that the carrier decay rate is cubic in carrier concentration, which is characteristic of an Auger process. We observe that in the quantum-confined regime, the Auger constant is strongly size-dependent and decreases with decreasing the quantum dot size as the radius cubed.
Article
The optical performance of quantum dot lasers with different dots-in-a-well (DWELL) structures is studied as a function of the well number and the indium composition in the InGaAs quantum well (QW) surrounding the dots. While keeping the InAs quantum dot density nearly constant, the internal quantum efficiency η<sub>i</sub>, modal gain, and characteristic temperature of 1-DWELL and 3-DWELL lasers with QW indium compositions from 10 to 20% are analyzed. Comparisons between the DWELL lasers and a conventional In<sub>0.15</sub>Ga<sub>0.85</sub>As strained QW laser are also made. A threshold current density as low as 16 A/cm<sup>2</sup> is achieved in a 1-DWELL laser, whereas the QW device has a threshold 7.5 times larger. It is found that η<sub>i </sub> and the modal gain of the DWELL structure are significantly influenced by the quantum-well depth and the number of DWELL layers. The characteristic temperature T<sub>0</sub> and the maximum modal gain of the ground-state of the DWELL structure are found to improve with increasing indium in the QW It is inferred from the results that the QW around the dots is necessary to improve the DWELL laser's η<sub>i </sub> for the dot densities studied