Plasma Synthesis and Surface Passivation of Silicon Quantum Dots with Photoluminescence Quantum Yields higher than 60%
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0934-I01-04
Plasma Synthesis and Surface Passivation of Silicon Quantum Dots with Photoluminescence Quantum Yields higher than 60% Lorenzo Mangolini1, David Jurbergs2, Elena Rogojina2, and Uwe Kortshagen1 1 Mechanical Engineering, University of Minnesota, 111 Church St. SE, Minneapolis, MN, 55455 2 Innovalight Inc., Santa Clara, 95054 ABSTRACT Silicon nanocrystals with diameters of less than 5 nm show efficient room temperature photoluminescence (PL). Previous reports of PL quantum yields for ensembles of silicon quantum dots have usually been in the few percent range, and generally less than 30%. Here we report the plasma synthesis of silicon quantum dots and their subsequent wet-chemical surface passivation with organic ligands while strictly excluding oxygen. Photoluminescence quantum yields as high as 62% have been achieved at peak wavelengths of about 789 nm. INTRODUCTION Bulk silicon is the material of choice for microelectronics fabrication; however, its optical properties are poor due to its indirect band gap. In the bulk, the slow electron-hole radiative recombination has to compete with nonradiative recombination at defect sites and through threebody Auger processes, leading to the low optical emission [1]. The observation of intense room temperature photoluminescence (PL) from silicon nanocrystals hence created significant excitement [2, 3]. It is generally believed that the improved optical properties of silicon nanocrystals smaller than 5 nm are due to a combination of effects: The enhanced recombination rate of electrons and holes due to the increased overlap of the electron and hole wavefunction confined in the dot [4], and the reduction of the rate of nonradiative defect mediated and three-body Auger recombination events [1]. In spite of the enhancement of silicon’s optical properties at the nanoscale, these properties have so far not rivaled those of direct band gap semiconductors such as the II-VI and III-V compounds. The excellent optical activity, for instance, of CdSe and CdS quantum dots with proper surface passivation manifest itself in PL quantum yields of ~80-90% [5-9], and radiative lifetimes of nanoseconds or shorter. In comparison, the quantum yields of ensembles of silicon quantum dots have generally been reported to be on the order of a few percent [10-12]. However, over the past few years there has been increasing evidence that a further improvement of the optical properties can be achieved through a careful passivation of the Si quantum dot surface. Limited reports discussed high quantum yields of silicon quantum dots passivated by a native silicon oxide [13]; however, ensemble quantum yields as high as 23% [14] and 30% [15] were achieved by grafting covalently bonded organic molecules onto the silicon quantum dot surface. Careful single quantum dot PL studies [16] produced further hints that this may not yet present the limit of the optical activity of Si. The authors of this study [16] reported quantum yields as high as 88% for selected individual quantum dots. However, the overall
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