Site-Selective Ion Beam Synthesis and Optical Properties of Individual CdSe Nanocrystal Quantum Dots in a SiO₂ Matrix

Cadmium selenide nanocrystal quantum dots (NC-QDs) are site-selectively synthesized by sequential ion beam implantation of selenium and cadmium ions in a SiO₂ matrix through submicrometer apertures followed by a rapid thermal annealing step. The structural and optical properties of the NC-QDs are controlled by the ion fluence during implantation and the diameter of the implantation aperture. For low fluences and small apertures the emission of these optically active emitters is blue-shifted compared to that of the bulk material by >100 meV due to quantum confinement. The emission exhibits spectral diffusion and blinking on a second time scales as established also for solution-synthesized NC-QDs

Alloy Fluctuations Act as Quantum Dot-like Emitters in GaAs-AlGaAs Core–Shell Nanowires

GaAs-Al_<i>x</i>Ga_1–<i>x</i>As (AlGaAs) core–shell nanowires show great promise for nanoscale electronic and optoelectronic devices, but the application of these nonplanar heterostructures in devices requires improved understanding and control of nanoscale alloy composition and interfaces. Multiple researchers have observed sharp emission lines of unknown origin below the AlGaAs band edge in photoluminescence (PL) spectra of core–shell nanowires; point defects, alloy composition fluctuations, and self-assembled quantum dots have been put forward as candidate structures. Here we employ laser-assisted atom probe tomography to reveal structural and compositional features that give rise to the sharp PL emission spectra. Nanoscale ellipsoidal Ga-enriched clusters resulting from random composition fluctuations are identified in the AlGaAs shell, and their compositions, size distributions, and interface characteristics are analyzed. Simulations of exciton transition energies in ellipsoidal quantum dots are used to relate the Ga nanocluster distribution with the distribution of sharp PL emission lines. We conclude that the Ga rich clusters can act as discrete emitters provided that the major diameter is ≥4 nm. Smaller clusters are under-represented in the PL spectrum, and spectral lines of larger clusters are broadened, due to quantum tunneling between clusters

Dynamic Acoustic Control of Individual Optically Active Quantum Dot-like Emission Centers in Heterostructure Nanowires

We probe and control the optical properties of emission centers forming in radial heterostructure GaAs-Al_0.3Ga_0.7As nanowires and show that these emitters, located in Al_0.3Ga_0.7As layers, can exhibit quantum-dot like characteristics. We employ a radio frequency surface acoustic wave to dynamically control their emission energy, and occupancy state on a nanosecond time scale. In the spectral oscillations, we identify unambiguous signatures arising from both the mechanical and electrical component of the surface acoustic wave. In addition, different emission lines of a single emission center exhibit pronounced anticorrelated intensity oscillations during the acoustic cycle. These arise from a dynamically triggered carrier extraction out of the emission center to a continuum in the radial heterostructure. Using finite element modeling and Wentzel–Kramers–Brillouin theory we identify quantum tunneling as the underlying mechanism. These simulation results quantitatively reproduce the observed switching and show that in our systems these emission centers are spatially separated from the continuum by >10.5 nm