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.

We report a two-step synthesis of highly luminescent CdS/ZnSe core/shell nanocrystals (emission quantum yields up to 50%) that can produce efficient spatial separation of electrons and holes between the core and the shell (type-II localization regime). Our synthesis involves fabrication of cubic-singony CdS core particles that are subsequently overcoated with a layer of ZnSe in the presence of surfactant-ligands in a noncoordinating solvent. Studies of different growth regime of the ZnSe shell indicate that one approach to obtaining high emission efficiencies is through alloying the CdS/ZnSe interface with CdSe, which leads to the formation of an intermediate ZnCdSe layer with a graded composition. We perform theoretical modeling of these core/shell nanocrystals using effective mass approximation and applying first-order perturbation theory for treating both direct electron-hole coupling and the core/shell interface-polarization effects. Using this model we determine the range of geometrical parameters of the core/shell structures that result in a type-II localization regime. We further applied this model to evaluate the degree of electron-hole spatial separation (quantified in terms of the electron-hole overlap integral) based on measured emission wavelengths. We also discuss the potential applicability of these nanocrystals in lasing technologies and specifically the possibility of single-exciton optical gain in type-II nanostructures.

We study theoretically two electron-hole pair states (biexcitons) in core/shell hetero-nanocrystals with type II alignment of energy states, which promotes spatial separation of electrons and holes. To describe Coulomb interactions in these structures, we apply first-order perturbation theory, in which we use an explicit form of the Coulomb-coupling operator that takes into account interface-polarization effects. This formalism is used to analyze the exciton-exciton interaction energy as a function of the core and shell sizes and their dielectric properties. Our analysis shows that the combined contributions from quantum and dielectric confinement can result in strong exciton-exciton repulsion with giant interaction energies on the order of 100 meV. Potential applications of strongly interacting biexciton states include such areas as lasing, nonlinear optics, and quantum information.

Single-walled carbon nanotubes (SWNTs) are π-conjugated, quasi-one-dimensional structures consisting of rolled-up graphene sheets that, depending on their chirality, behave as semiconductors or metals 1 ; owing to their unique properties, they enable groundbreaking applications in mechanics, nanoelectronics and photonics 2,3 . In semiconducting SWNTs, medium-sized excitons (3-5 nm) with large binding energy and oscillator strength are the fundamental excitations 4-8 ; exciton wavefunction localization and one-dimensionality give rise to a strong electron-phonon coupling 9-11 , the study of which is crucial for the understanding of their electronic and optical properties. Here we report on the use of resonant sub-10-fs visible pulses 12 to generate and detect, in the time domain, coherent phonons in SWNT ensembles. We observe vibrational wavepackets for the radial breathing mode (RBM) and the G mode, and in particular their anharmonic coupling, resulting in a frequency modulation of the G mode by the RBM. Quantumchemical modelling 13 shows that this effect is due to a corrugation of the SWNT surface on photoexcitation, leading to a coupling between longitudinal and radial vibrations.Electron-phonon coupling in SWNTs is usually studied using Raman spectroscopy; this technique is useful for investigating ground-state vibrations 14 , whereas photoexcited-state vibrational dynamics remain largely unknown because, in the frequency domain, phonon replicas are hardly detectable in the presence of substantial inhomogeneous broadening. Time-domain observation of phonon dynamics has much lower sensitivity with respect to conventional Raman, but it enables direct measurement of excitedstate dynamics, vibrational dephasing and mode coupling in a distinct way 15,16 . Coherent phonon detection allows resolution in time of wavepacket dynamics that is otherwise averaged-out in standard Raman scattering.To detect coherent phonons in SWNTs, we use a standard pump-probe configuration, in which the observed quantity is the modulation depth in the differential transmission 17 ( T /T); details of the experimental setup are provided in the Methods section. Figure 1a shows T /T dynamics of SWNTs grown by the high-pressure carbon monoxide procedure dispersed in polymethylmethacrylate films following excitation with a sub-10-fs visible pulse (1.8-2.4 eV bandwidth), probed at an energy of 2.1 eV. The signal exhibits an initial photobleaching, which quickly turns into photoinduced absorption (PA). The fast photobleaching decay is ascribed to relaxation of the higher-lying exciton (second in an increasing energy scale) to the lower one, taking place with a 40-fs time constant 18 . The PA signal is generated by this lower exciton 4,5 and decays on the ps timescale, in agreement with previous results [19][20][21] . As shown in Fig. 1a, there is a clear oscillation in the T /T amplitude. The Fourier transform (FT) of the oscillatory component (Fig. 2a) shows a strong peak at 252 cm −1 (132-fs period). This frequency can be recognized as the RBM...

We study inverted core/shell nanocrystals (NCs) in which a core of a wide gap semiconductor (ZnSe) is overcoated with a shell of a semiconductor of a narrower gap (CdSe). By monitoring radiative recombination lifetimes for a series of these NCs with a fixed core radius and progressively increasing shell thickness, we observe a continuous transition from Type-I (both electron and hole wave functions are distributed over the entire NC) to Type-II (electron and hole are spatially separated between the shell and the core) and back to Type-I (both electron and hole primarily reside in the shell) localization regimes. These observations are in good agreement with the calculated dependence of the electron−hole overlap integral on shell thickness.

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.

Non-blinking excitonic emission from near-infrared and type-II nanocrystal quantum dots (NQDs) is reported for the first time. To realize this unusual degree of stability at the single-dot level, novel InP/CdS core/shell NQDs were synthesized for a range of shell thicknesses (~1–11 monolayers of CdS). Ensemble spectroscopy measurements (photoluminescence peak position and radiative lifetimes) and electronic structure calculations established the transition from type-I to type-II band alignment in these heterostructured NQDs. More significantly, single-NQD studies revealed clear evidence for blinking suppression that was not strongly shell-thickness dependent, while photobleaching and biexciton lifetimes trended explicitly with extent of shelling. Specifically, very long biexciton lifetimes—up to >7 ns—were obtained for the thickest-shell structures, indicating dramatic suppression of non-radiative Auger recombination. This new system demonstrates that electronic structure and shell thickness can be employed together to effect control over key single-dot and ensemble NQD photophysical properties.

Optical gain in ultrasmall semiconductor nanocrystals requires that some of the nanoparticles in the ensemble be excited with multiple electron−hole pairs (multiexcitons). A significant complication arising from this multiexciton nature of optical amplification is the ultrafast gain decay induced by nonradiative Auger recombination. Here, we develop a simple model for analyzing optical gain in the nanocrystals in the presence of exciton−exciton (X−X) interactions. This analysis indicates that if the X−X interaction is repulsive and its energy is large compared to the ensemble line width of the emitting transition, optical gain can occur in the single-exciton regime without involvement of multiexcitons. We further analyze theoretically and experimentally X−X interactions in type-II heteronanocrystals of CdS (core)/ZnSe (shell) and ZnTe (core)/CdSe (shell) and show that they can produce giant repulsion energies of more than 100 meV resulting from a significant local charge density generated as a result of spatial separation between electrons and holes. We observe that the dynamical and spectral properties of optical gain in type-II nanocrystals are distinctly different from those of multiexciton gain in traditional type-I nanocrystals and are consistent with those expected for the single-exciton regime. An important implication of these results is the possibility of a significant increase in the optical-gain lifetime, which could simplify applications of chemically synthesized nanocrystals in practical lasing technologies and perhaps allow for lasing using electrical injection.