Physicists from the Paul-Drude-Institut für Festkörperelektronik (PDI; Berlin, Germany), NTT Basic Research Laboratories (Atsugi, Japan), and the U.S. Naval Research Laboratory (NRL; Washington, DC) have used a scanning tunneling microscope to create quantum dots (QDs) with identical, deterministic sizes.1 The perfect reproducibility of these dots opens the door to QD architectures completely free of uncontrolled variations, an important goal for technologies from nanophotonics to quantum information processing as well as for fundamental studies.
Quantum dots can be thought of as artificial atoms because, like real atoms, they confine their electrons to quantized states with discrete energies. But the analogy breaks down quickly, because while real atoms are identical, QDs usually comprise hundreds or thousands of atoms -- with unavoidable variations in their size and shape and, consequently, in their properties and behavior. External electrostatic gates can be used to reduce these variations; however, making numbers of QDs that are truly identical (down to the numbers and positions of their individual atoms) would eliminate them.
The team assembled the dots atom-by-atom, using a scanning tunneling microscope (STM), and relied on an atomically precise surface template to define a lattice of allowed atom positions. The template was the surface of an indium arsenide (InAs) crystal, which has a regular pattern of indium vacancies and a low concentration of native indium "adatoms" adsorbed above the vacancy sites. The adatoms can be moved with the STM tip by vertical atom manipulation. The team assembled quantum dots consisting of linear chains of 6 to 25 indium atoms.
Stefan Fölsch, a physicist at the PDI who led the team, explained that "the ionized indium adatoms form a quantum dot by creating an electrostatic well that confines electrons normally associated with a surface state of the InAs crystal. The quantized states can then be probed and mapped by scanning tunneling spectroscopy measurements of the differential conductance." These spectra show a series of resonances labeled by the principal quantum number n. Spatial maps reveal the wave functions of these quantized states, which have n lobes and n – 1 nodes along the chain, exactly as expected for a quantum-mechanical electron in a box.
Because every QD is essentially identical, QD "molecules" consisting of several coupled chains will reflect the same invariance. Steve Erwin, a physicist at NRL and the team's theorist, pointed out that "this greatly simplifies the task of creating, protecting, and controlling degenerate states in quantum dot molecules, which is an important prerequisite for many technologies."
Complex QD geometries possible
By combining the invariance of QD molecules with the intrinsic symmetry of the InAs vacancy lattice, the team created degenerate states that are surprisingly resistant to environmental perturbations by defects. In the example shown in the figure, a QD molecule with perfect threefold rotational symmetry was first created and its two-fold degenerate state demonstrated experimentally. By intentionally breaking the symmetry, the team found that the degeneracy was progressively removed, completing the demonstration.
The reproducibility and high fidelity offered by these QDs makes them excellent candidates for studying fundamental physics that is typically obscured by stochastic variations in size, shape, or position of the chains. Looking forward, the team also anticipates that the elimination of uncontrolled variations in QD architectures will offer many benefits to a broad range of future QD technologies in which fidelity is important.
New types of metamaterials can be imagined that consist of arrays of these QDs, each spaced apart from the others with atomic precision. In addition, these are QDs with among the fewest numbers of individual atoms known; their novel properties could be useful in nanophotonics, especially in photonic/nanoplasmonic circuits that are many times smaller than conventional photonic circuits.
REFERENCE:
1. Stefan Fölsch et al., Nature Nanotechnology (2014); doi: 10.1038/nnano.2014.129
