Аннотация
The presented investigations aim to pinpoint the crossover of lead sulfide (PbS) quantum dots (QDs) with a shrinking diameter to a molecular state. We have measured the transmittance of strongly confined PbS QDs of 2.7, 3.1, 4.7 and 4.8 nm size in the temperature range of 10–300 K. Fitting these results with the classical Fan theory revealed that the average phonon energy coincides with the reduction of the QD diameters, pointing to a limitation of coherent atom movements in strongly confined matter. The phonons vanish at QDs limited to 90 atoms corresponding to a size of 1.7 nm, which we define as the critical size for the solid-to-molecule crossover. The result is confirmed by a calculation based on the uncertainty principle.