Chemical Quantum Dots in Bell Laboratories

ConspectusChemical quantum dots are small semiconductor crystallites (1.5 to 5 nm in diameter). Too small to behave as bulk semiconductors, they have band gaps and luminescence colors that vary with size in a controllable and predictable manner. This is the quantum size effect. Quantum dots are essentially new classes of large molecules. This Account describes our efforts to synthesize, characterize, and understand them, starting in 1983 at the Bell Telephone Laboratories in Murray Hill, NJ. The culture and management style of Bell Laboratories was critical to the success of this effort. There was widespread collaboration among scientists of differing backgrounds and interests. Research was carried out entirely with internal AT&T corporate funding. It is doubtful this research could have been successfully carried out in an academic setting at that time.We began with simple aqueous colloidal precipitations of II-VI salts such as CdS and ZnSe. To achieve better control and monodispersity, precipitations were done in the small water pools of inverse micelle solutions, using both inorganic and organometallic reagents. We found that with slow addition of reagents, existing particles would grow larger without nucleation of new particles. The particles were capped with phenyl radicals, causing them to become hydrophobic and enabling a powder of capped quantum dots to be recovered. CdSe/ZnS core/shell particles were made by sequential addition of ZnS reagents to an inverse micelle CdSe colloid. ZnS surface passivation greatly improved core CdSe luminescence by passivation of surface states. Quantum dot powders could be dissolved and refluxed at high temperature in Lewis base solvents such as tributyl phosphine oxide. High temperature annealing removed defects and improved structure and luminescence quantum yield, making high quality dots.We then focused on silicon because of its overwhelming importance in the computer and communications industry. To make dots of covalent, strongly bound silicon, a different synthesis was devised. Gaseous disilane in flowing He was cracked at 900 °C to make an aerosol of silicon crystallites of wide size distribution. This was bubbled through ethylene glycol to make a robust colloid. The smallest particles were separated by a combination of HPLC and size selective precipitation methods. These small oxide capped silicon dots emitted in the red range, about 0.9 eV above the bulk silicon band gap in the near IR. Despite this strong quantum size effect, and a large increase of band gap luminescence compared with bulk silicon, optical spectra showed that the fundamental transition remained dipole forbidden.