![]() ![]() Most of the previous studies have concentrated on type I core/shell NCs, in which overcoating with a shell of a wide gap semiconductor is used to improve photoluminescence (PL) quantum efficiencies of the NCs (e.g., refs 9-15). The type I regime corresponds to the situation in which both an electron and a hole are predominantly located in the same part of the heterostructure (in the core or in the shell), while in the type II regime, electrons and holes are spatially separated and occupy different parts of the hetero-NC. Core/shell NC heterostructures can show either type I or type II carrier localization, depending on the energy offsets between the core and the shell materials, effective masses, and the relation between the core diameter and the shell thickness. Furthermore, we show that the use of appropriately tailored hetero-NCs can, in principle, allow lasing in the single-exciton regime, which completely eliminates the problem of Auger recombination. E-mail: Īuger recombination and simultaneously reduce excited-state absorption arising from singly excited NCs, which makes it possible to demonstrate efficient ASE in the green to blue range of the optical spectrum. This approach allows us to suppress * To whom correspondence may be addressed. Here, we utilize heterostructuring of individual NCs as a means of controlling x-x interactions and enhancing the NC optical-gain performance. For example, although CdSe NCs exhibit strong optical-gain performance in the red to yellow spectral ranges, they do not show efficient ASE in the range of green to blue colors. One such problem is associated with ultrafast optical gain decay induced by nonradiative Auger recombination.6-8 In CdSe NCs, optical gain occurs if the average number of electron-hole (e-h) pairs (excitons) per nanoparticle, N, is greater than 1.1,2 This requirement indicates that optical gain in these NCs relies on multiexciton states (biexciton, triexcitons, etc.), and therefore, its dynamical and spectral properties are strongly affected by exciton-exciton (x-x) interactions.1-3,5,7-8 In particular, these interactions open a highly efficient nonradiatiVe decay channel (picosecond time scale) associated with an Auger-type process, in which the e-h recombination energy is transferred to a third particle (an electron or a hole).7 Because of rapid shortening of Auger lifetime, with decreasing NC radius, R, it becomes progressively more difficult to achieve the optical gain and ASE regimes for shorter wavelengths that require the use of NCs of small sizes. Despite the success of these first experiments, indicating the feasibility of NC lasing, there are still several fundamental problems that hinder applications of NC materials in laser technologies. Tunable optical gain and amplified spontaneous emission (ASE) have recently been demonstrated in the visible and near-IR spectral ranges using CdSe1-4 and PbSe5 NCs, respectively. The ASE in the blue range has never been previously achieved using traditional NCs with type I carrier localization.Ĭhemically synthesized semiconductor nanocrystals (NCs) are considered to be promising building blocks for novel colorselectable optical-gain media1,2 because of NC-size-controlled emission colors and potentially low, temperature-insensitive gain thresholds. We use these novel heteroNCs to demonstrate efficient amplified spontaneous emission (ASE) that is tunable across a “difficult” range of green and blue colors. ![]() 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 show that such hetero-NCs can exhibit strong repulsive excitonexciton interactions that lead to significantly reduced excited-state absorption associated with NCs containing single electron-hole pairs. 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. 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. Size-controlled spectral tunability and chemical flexibility make semiconductor nanocrystals (NCs) attractive as nanoscale building blocks for color-selectable optical-gain media. Klimov* Los Alamos National Laboratory, Los Alamos, New Mexico 87545 ReceiVed: ApIn Final Form: June 3, 2004 Ivanov, Jagjit Nanda, Andrei Piryatinski, Marc Achermann, Laurent P. Light Amplification Using Inverted Core/Shell Nanocrystals: Towards Lasing in the Single-Exciton Regime Sergei A. ![]()
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