Paper NS-MoM5
Transforming Luminiscent Silicon Nanocrystals Into a Direct-Bandgap Semiconductor via Surface-Capping-Induced Strain
Monday, October 29, 2012, 9:40 am, Room 12
| Session: |
Nanoparticles and Quantum Structures |
| Presenter: |
P. Hapala, Institute of Physics of ASCR, Czech Republic |
| Authors: |
P. Hapala, Institute of Physics of ASCR, Czech Republic
P. Jelinek, Institute of Physics of ASCR, Czech Republic
K. Kusova, Institute of Physics of ASCR, Czech Republic
I. Pelant, Institute of Physics of ASCR, Czech Republic
|
| Correspondent: |
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We report combined experimental and theoretical work pointing out the possibility to convert Silicon Nanoparticles (SiNcs) to a luminescing direct-band gap material via the concerted action of the quantum confinement and tensile force induced by proper surface passivation.
The transformation of silicon, originally a very poor light emitter due to indirect band gap, into a light-emitting medium is key challenge from the application point of view. One promising way to achieve this ultimate goal is through dramatic shrinkage in the size of the crystal down to nanoscale. The observation of an efficient room-temperature luminescence [1] from SiNcs initiated the debate about the nature of their band structure. The most common silicon-oxide-capped SiNcs maintain the indirect band gap showing long radiative lifetimes (>100 μs) [2].
Beside the size of the nanocrystal, a proper surface also plays an important role in the light emission process. Recently we have shown that SiNcs sized 2.5-3 nm with methyl-based surface passivating layer [3] exhibit luminescence properties (short radiative lifetime ~10 ns and enhanced quatum yield ~20%) analogical to direct-band gap semiconductor. This property is further supported by single-crystal luminescence experiment giving emission pattern very similar to that observed in direct-band gap CdSe nanoparticles [4].
To understand the impact of surface passivation on the electronic structure of SiNcs we performed large-scale total energy DFT calculations including up to 1500 atoms and different functional surface groups (-H,-CH3,-OH). Our calculations show that the presence of methyl group leads to significant elongation of the Si-Si distance in core region. Further we restore band structure of SiNc mapping real space molecular orbitals into the momentum space [6].
The resotred band structure clearly shows, that combination of tensile stress and the quantum confinement strongly modifies dispersion of the conduction band along the Γ-X direction, with significant lowering of the Γ15 even as lifting the conduction minimum band Δ1.
References
[1] L. Canham, App. Phys. Lett. 57, 1046 (1990).
[2] D. Kovalev et al, Phys. Rev. Lett. 81, 2803 (1998).
[3] K. Kůsová et al ACS Nano, 4(8), 4495 (2010).
[4] X. Wang et al Nature 459, 686 (2009).
[5] K. K Kůsová et al (submitted).
[6] P. Hapala and P. Jelinek (ArXiv:cond-mat/1204.0421).