Doping--the intentional introduction of impurities into a material--is fundamental to controlling the properties of bulk semiconductors. This has stimulated similar efforts to dope semiconductor nanocrystals. Despite some successes, many of these efforts have failed, for reasons that remain unclear. For example, Mn can be incorporated into nanocrystals of CdS and ZnSe (refs 7-9), but not into CdSe (ref. 12)--despite comparable bulk solubilities of near 50 per cent. These difficulties, which have hindered development of new nanocrystalline materials, are often attributed to 'self-purification', an allegedly intrinsic mechanism whereby impurities are expelled. Here we show instead that the underlying mechanism that controls doping is the initial adsorption of impurities on the nanocrystal surface during growth. We find that adsorption--and therefore doping efficiency--is determined by three main factors: surface morphology, nanocrystal shape, and surfactants in the growth solution. Calculated Mn adsorption energies and equilibrium shapes for several nanocrystals lead to specific doping predictions. These are confirmed by measuring how the Mn concentration in ZnSe varies with nanocrystal size and shape. Finally, we use our predictions to incorporate Mn into previously undopable CdSe nanocrystals. This success establishes that earlier difficulties with doping are not intrinsic, and suggests that a variety of doped nanocrystals--for applications from solar cells to spintronics--can be anticipated.

We demonstrate high-quality, highly fluorescent, ZnSe colloidal nanocrystals (or quantum dots) that are doped with paramagnetic Mn 2+ impurities. We present luminescence, magnetic circular dichroism (MCD), and electron paramagnetic resonance (EPR) measurements to confirm that the Mn impurities are embedded inside the nanocrystal. Optical measurements show that by exciting the nanocrystal, efficient emission from Mn is obtained, with a quantum yield of 22% at 295 K and 75% below 50 K (relative to Stilbene 420). MCD spectra reveal an experimental Zeeman splitting in the first excited state that is large (28 meV at 2.5 T), depends on doping concentration, and saturates at modest fields. In the low field limit, the magnitude of the effective g factor is 430 times larger than in undoped nanocrystals. EPR experiments exhibit a six-line spectrum with a hyperfine splitting of 60.4 × 10 -4 cm -1 , consistent with Mn substituted at Zn sites in the cubic ZnSe lattice.Nanometer-scale semiconductor crystallites, also referred to as nanocrystals or quantum dots, have been extensively studied to explore their unique properties and potential applications. 1 Interesting behavior arises in these materials due to the confinement of optically excited electron-hole pairs by the crystallite boundary. However, while the basic explanation of this phenomenon, known as the quantum size effect, was provided early in the investigation of these materials, 2-4 a detailed understanding required the advent of high-quality colloidal nanocrystals, which were uniform in size, shape, crystallinity, and surface passivation. Once such materials became available, 5 tremendous progress was made in a variety of physical studies. Consequently, many of the properties of semiconductor nanocrystals are now understood in detail. 1 In addition, high-quality crystallites have led to more complicated nanocrystal-based structures, such as quantum-dot solids, 6 light-emitting devices, 7 and even photonic crystals. 8 These successes have encouraged researchers to go beyond pure nanocrystals and investigate particles that are intention-*

Fine and hyperfine splittings arising from electron, hole, and nuclear spin interactions in the magnetooptical spectra of individual localized excitons are studied. We explain the magnetic field dependence of the energy splitting through competition between Zeeman, exchange, and hyperfine interactions. An unexpectedly small hyperfine contribution to the splitting close to zero applied field is described well by the interplay between fluctuations of the hyperfine field experienced by the nuclear spin and nuclear dipole/dipole interactions. DOI: 10.1103/PhysRevLett.86.5176 PACS numbers: 78.67.Hc, 71.70.Ej The spin of an electron in a 10 nm GaAs quantum dot (QD) interacts with ϳ10 5 nuclear spins. This hyperfine interaction, though relatively weak, may ultimately limit spin coherence of localized electrons in QDs or shallow impurities -a concern that strongly influences developing visions of information technologies based on spin [1][2][3]. Nevertheless, there may be ways around even this intrinsic scattering process; for example, by optically polarizing all nuclear spin and thereby dramatically reducing phase space [1]. Furthermore, one could imagine using the nuclear spin for information storage or to control the electronic spin [4]. However, it is necessary to develop a more precise understanding of spin interactions in nanostructures in the presence of external magnetic and optical fields before such creative ideas can be explored.In this Letter, we present and analyze spectroscopic signatures of spin via fine and hyperfine structure splittings in the magneto-optical spectra of individual GaAs QDs under polarized and nearly resonant laser excitation. We find it necessary to consider the interaction of the electron spin with an external magnetic field (Zeeman interaction), exchange Coulomb interactions between the electron and hole, and hyperfine interactions between the electronic spin and the spins of the nuclei. Because of the hyperfine interaction, it is necessary to consider also the nuclear spin system. We are led then to consider, for the nuclei, the Zeeman interaction, dipole-dipole interactions between neighboring nuclear spins [5], and the hyperfine interaction. We quantify in experiment and theory how interactions manifest themselves in the spectral fine structure of a single exciton, discovering and explaining a remarkably complex dependence on magnetic field arising from competition between these various spin interactions.We have studied QDs formed by monolayer-high interface islands in a 4.2 nm GaAs quantum well with 25 nm Al 0.3 Ga 0.7 As barriers. The quantum wells were grown using molecular beam epitaxy with two-minute growth interrupts at the interfaces to allow large interface islands to develop. Individual QDs were excited and detected through 1 2 micron diameter apertures in an aluminum shadow mask patterned on the sample surface. A split-coil superconducting magnet was used in backscattering Faraday geometry. Previous studies have demonstrated that optical pumping could lead to large nu...

Kennedy, T. A.; Butler, J. E.; Linares, R. C.; and Doering, P.J., "Long coherence times at 300 K for nitrogen-vacancy center spins in diamond grown by chemical vapor deposition" (2003). All Faculty Publications. 469.

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where 7 = (3127) So" dz r2(e' + I)-' -0.86. The additional factor kBT reflects the linear density of states at the nodes (3). Thus, the QP current in the clean d-wave superco~lductor varies as T2 in the limit T + 0, which is consistent with Fig. 4B. Equation 2 provides a good fit to the measured K~~ with a relaxation time l/T = 0.38 os.These co~nparisons show that it is reasonable to id en ti^ the total field-induced change AK(H,, T) with the value of K, in zero field. However. the inference that K = 0 in the plateau region presents a challenge to our u~ldersta~lding of the Q P state. The simplest way to have K, = 0 is to assume that the QLP density nQ, vanishes (above Hk). This could arise from a gap A, that opens gradually with field, leading to an exponential decay, namely 7xQp

We examine the impact of growth kinetics on the incorporation of Mn dopants into ZnSe nanocrystals. We synthesize such particles, also known as colloidal quantum dots, and use optical spectroscopy to extract information about the average number of Mn impurities per nanocrystal as the reaction proceeds. We find that this number increases with particle growth until the Zn and/or Se precursors are depleted in the reaction solution. If the reaction is continued further, then ripening of the colloid begins and the average number of Mn per nanocrystal decreases, even as the particles slowly increase in size. We show that this effect, which is detrimental for enhanced doping, can be avoided if the reactant concentration is maintained by addition of more reactants. We consider several explanations and conclude that intraparticle ripening, in which material is redistributed on the same nanocrystal due to evolution of the particle shape, is the most consistent with experimental observations.

We find that detuning an optical pulse train from electronic transitions in quantum dots controls the direction of nuclear spin flips. The optical pulse train generates electron spins that precess about an applied magnetic field, with a spin component parallel to the field only for detuned pulses. This component leads to asymmetry in the nuclear spin flips, providing a way to stabilize and control the nuclear spin polarization. This effect is observed using two-color, time-resolved Faraday rotation and ellipticity.