Directed Atom-by-Atom Assembly of Dopants in Silicon.

The ability to controllably position single atoms inside materials is key for the ultimate fabrication of devices with functionalities governed by atomic-scale properties. Single bismuth dopant atoms in silicon provide an ideal case study in view of proposals for single-dopant quantum bits. However, bismuth is the least soluble pnictogen in silicon, meaning that the dopant atoms tend to migrate out of position during sample growth. Here, we demonstrate epitaxial growth of thin silicon films doped with bismuth. We use atomic-resolution aberration-corrected imaging to view the as-grown dopant distribution and then to controllably position single dopants inside the film. Atomic-scale quantum-mechanical calculations corroborate the experimental findings. These results indicate that the scanning transmission electron microscope is of particular interest for assembling functional materials atom-by-atom because it offers both real-time monitoring and atom manipulation. We envision electron-beam manipulation of atoms inside materials as an achievable route to controllable assembly of structures of individual dopants.

Transfer-matrix formalism for the calculation of optical response in multilayer systems: from coherent to incoherent interference.

We present a novel way to account for partially coherent interference in multilayer systems via the transfer-matrix method. The novel feature is that there is no need to use modified Fresnel coefficients or the square of their amplitudes to work in the incoherent limit. The transition from coherent to incoherent interference is achieved by introducing a random phase of increasing intensity in the propagating media. This random phase can simulate the effect of defects or impurities. This method provides a general way of dealing with optical multilayer systems, in which coherent and incoherent interference are treated on equal footing.

Tuning the electronic coupling and magnetic moment of a metal nanoparticle dimer in the nonlinear dielectric-response regime.

We show that the electronic coupling and magnetic moment of a silver nanoparticle dimer can be readily tuned by applying an electric field in the nonlinear dielectric-response regime. For a given interparticle separation, the electronic coupling becomes tunable as soon as the system crosses over from the linear to nonlinear regime. Remarkably, this transition takes place for modest strengths of the electric field. Further increase of the field strength may close the HOMO-LUMO gap of the dimer due to Stark shifts, accompanied by the emergence of a net magnetic moment from two nonmagnetic building blocks. These findings, obtained within density functional theory, exhibit the delicate coupling between the electronic and magnetic degrees of freedom and point to new approaches to gain multifunctionality of nanoparticle aggregates.

Electronic coupling and optimal gap size between two metal nanoparticles.

We study the electronic coupling between two silver nanoparticles using ab initio density functional theory for real atoms. We show that the electronic coupling depends on both the gap size of the dimer system and the relative orientation of the particles. As the two particles are separated from touching contact, the dimer undergoes a bond-breaking step, which also establishes the striking existence of an optimal gap size defined by a maximal static polarizability of the dimer. For some dimers, the electronic coupling before the bond breaking can be strong enough to give rise to a net magnetic moment of the dimer, even though the isolated particles are nonmagnetic. These findings may be instrumental in understanding and controlling the physical and chemical properties of closely packed nanoparticle aggregates.