2P031
Exciton dynamics of silicon quantum dots –Femtosecond near-IR
transient absorption spectroscopic study–(Kwansei Gakuin Univ.*, Univ. of Hyogo**)○Dong Chen*, Li Wang*, Seiichi Sato**, Hioshi Yao**, Naoto Tamai*
Introduction
Silicon is an important material for application in electronic and photovoltaic industry.
Recently, silicon nanoparticles attract a great deal of attention due to their size-tunable,
non-toxic and electronic properties. Especially, the fascination caused by the process called
multiple exciton generation (MEG) that multiple pairs of excitons could be generated just by asingle high-energy photon is getting more focused by the researchers. MEG in siliconquantum dots (Si QDs) has a strong potential for application in photovoltaic devices. As aresult, multiple excitons decay dominantly via Auger recombination that is a process in which
an exciton transfers its recombination energy to another particle (an exciton, an electron or ahole) and excites it to a higher energy state. Auger recombination rate is enhanced by therelaxation of the translational momentum conservation in nanocrystals (NCs) that must to besatisfied in bulk materials [1]. The enhancement of Auger recombination rate in NCs
diminishes the potential of MEG in photovoltaics as well as for light sources [2] as it reduces
the lifetime of multiple excitons localized in the same NC. So it is very important to reduce or
suppress Auger recombination rate in NC materials. For that purpose, microscopic
understanding of multibody Auger recombination in quantum confined system is important
both from fundamental viewpoints and for applications. In the present study, we prepared
several kinds of silicon nanoparticles by a ball milling method, and exciton dynamics andAuger recombination of silicon nanoparticles were examined by femtosecond near-IR
transient absorption spectroscopy.generated
Experiments
We prepared three Si NCs synthesized via a ball milling method, and dissolved in theethanol (80%) + water (20%), propanol, and propanol (80%) + water (20%), respectively. Eachsamples were tagged as Si A, Si B and Si C. Transmission electron microscopy (TEM) wasused to analyze the size of Si NCs. The samples were excited at second harmonic (400 nm) ofan amplified Ti: sapphire laser (800 nm, 60 fs and 500 Hz) for femtosecond near-IR transientabsorption spectroscopy. All the experiments were conducted at room temperature.
Results and Discussion
To estimate the bandgap energy (Eg) of Si NCs, absorption spectra of each samples weretransferred into the graph of (hνα)1/2 against photon energy as similar to the method
mentioned Meier’s Group [3], where α is an absorption coefficient. The results wereillustrated in Fig. 1. Eg of Si A, Si B, and Si C were estimated to be 1.85, 1.76 and 1.70 eV,

respectively. A typical TEM image of Si A is shown in the inset of Fig. 1, where the average
diameter is estimated to be 2.6 nm. This value is little smaller than the size estimated fromEg (3.1nm) [4]. The excitation energy at 400 nm (3.1 eV) is below the MEG threshold (<2 Eg),
so the multiple excitons in one Si NC could appear only by absorption of multiple photons.
The transient absorption spectra of Si NCs are very broad and positive signal in near-IRregion (900 ~ 1400 nm), suggesting the intraband transition in the conduction band. Fig.2
illustrates the transient absorption dynamics of Si C sample at various excitation intensities
observed at 1000 nm. The dynamics were globally analyzed by two or three exponential decayfunctions. The fast decay component of 0.5 ~ 0.8 ps was detected as a main component
irrespective of a kind of Si NCs (A ~ C) and excitation intensity. The fast component isprobably due to the carrier trapping by the defects. Slow component of a few tens ps was alsoobserved. However, the amplitude of a fast component increased with increasing theexcitation intensity. This result suggests that multiple excitons are easily trapped to thedefect sites in Si NCs.
References
1. Semiconductor and Metal Nanocrystals, edited by V. I. Klimov (Marcel Dekker, 2004)
2. V. I. Klimov, J. Phys. Chem. B 2006, 110, 16827-16845

3. C. Meier; A. Gondorf; S. Luttjohann; A. Lorke, J. Appl. Phys. 2007, 101, 1031121-10311218
4. Silicon Nanocrystals, edited by L. Pavesi and R. Turan (WILEY-VCH, 2010)10 nm

Fig. 1 (hνα)1/2 of Si A, Si B and Si C areplotted as a function of photon energy.
TEM image of Si A is shown in the inset.

Fig. 2 The transient absorption dynamics ofSi C at different excitation intensities
observed at 1000 nm.