We describe a procedure for dispersion bulk Si into ultrasmall nanoparticles, much smaller than what is available today, with high throughput and excellent size definition and control. The quantum dots are ∼ 1 nm in diameter and contain 29 Si atoms[1-2]. We show most unexpected and totally surprising phenomenon. Unlike bulk Si, an optically inert indirect gap material, the particles are extremely active optically in the blue, exceeding the activity of fluorescein, such that single particles are readily detected and imaged, using two-photon near- infrared femto second excitation. This comes nearly a decade after Canham first surprised the scientific community by reporting red luminescence from anodized Si. In addition to being ultrabright, the particles have other useful properties that are unlike those of bulk including being a source of stimulated emission and collimated beam emission[4-5], and harmonic generation. Single electron charging and the confinment energy spacing, both are much larger than the thermal energy. They can be formed into colloids, crystals, films, and collimated beams for applications in the electronics, optoelectronics and biomedical industries. The synthesis utilizes straightforward, low-cost, environmentally benign present-day commercial technologies and material: abundant Si, common chemicals and electricity. The procedure is readily amenable to large-volume production, easy recovery, and controlled delivery.
Using density functional with exchange correlation, configuration interaction and Monte Carlo theory, we constructed a structural propotype for 29 atoms (magic number for the Td symmetry and spherical shape), giving a size, an energy gap and an absorption spectrum that agree with experiment. In the prototype, twenty four atoms form a network of reconstructed Si-Si species on the surface, with five atoms constituting a tetrahedral core.
The results suggest that the surface reconstructed network is the source of the new properties. In these particles, the elasticity drops, atoms become amenable to large displacement, and are subjected to unballanced atomic forces, conditions under which novel molecular structure or configuration, distinct from bulk, and which otherwise does not exist may form. This structure may constitute a transition between the bulk and atomistic phases.