Laser-Induced Photoluminescence of Nanoparticles

      By laser-vaporizing and reassembling solids in background gases, novel materials can be synthesized in nanometer-sized dimensions where quantum size effects become important.   Recently, we formed 1 to 10 nm diameter nanoparticles of Si by laser ablation into 1-10 Torr Ar or He gas, and observed the particles via laser-induced photoluminescence.  Unlike laser-induced fluorescence from atoms or small molecules, we believe we have observed photoluminescence from small, gas-suspended nanoparticles.  This luminescence was collected, and appeared very similar to the photoluminescence attributed to quantum-confined nanocrystals of silicon formed in other ways, or that of porous silicon.

     Laser ablation into a background gas creates a bright plasma which can be detected (with sensitive ICCD-imaging) to perhaps a few milliseconds.  To induce the photoluminescence, a second laser pulse (25 ns long, UV, 308 nm) sheet was introduced after a variable time delay.  The particles luminesced for 1.8 microseconds after this excitation, and this light was imaged by ICCD-photography to provide a spatial map of the nanoparticle cloud.  Alternatively, we collected the directly-scattered 308-nm light from the
nanoparticles (Rayleigh scattering, intensity proportional to nanoparticle diameter to the sixth power) to track non-luminescing particles as well.  We used these images to understand when and where nanoparticles grow, what causes the photoluminescence, how it can be maximized, and where the nanoparticles go after they are formed.    These first time-resolved PL measurements from nanoparticles suspended in the gas phase were used to maximize the luminescence of the nanoparticles in the gas phase, prior to their deposition as photoluminescent thin films.

    The background gas mass has a major effect on the plume dynamics.   Ablation of Si (m=28) into 1 Torr of (heavier, m=40) Ar results in a uniform, stationary plume of nanoparticles while Si ablation into lighter He (m=4) results in a turbulent ring of particles which propagates forward at 10 m/s.[1]  The graphic shows a side-view of photoluminescent nanoparticles propagating up to 10 cm away from a silicon target at time delays of 4 to 10 ms after the initial ablation event.
 
     Using these diagnostics, individual nanoparticles which were unambiguously formed in the gas phase were collected on transmission electron microscope grids for particle size and composition analysis using Z-contrast imaging and electron energy loss spectroscopy.   The effects of gas flow on nanoparticle formation, photoluminescence, and collection are described in [1].

     Time-resolved photoluminescence (PL) spectra are reported for gas-suspended 1-10 nm diameter SiOx particles formed by laser ablation of Si into 1-10 Torr He and Ar.   Three spectral bands (1.8, 2.5 and 3.2 eV) similar to PL from oxidized porous silicon were measured, but with a pronounced vibronic structure.   Maximized violet (3.2 eV) PL from the gas-suspended nanoparticles was correlated with an ex situ SiOx (x=1.4) overall particle stoichiometry.  Cryogenically-collected gas-suspended nanoparticles produced weblike-aggregate films exhibiting very weak PL.  Standard anneals restored strong PL bands without vibronic structure, but otherwise in agreement with the PL measured from the gas-suspended nanoparticles.[2]

 This research was sponsored by the Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corp., for the U.S. Department of Energy, under contract DE-AC05-96OR22464.
 

References

1.  "Time-Resolved Imaging  of Gas Phase Nanoparticle Synthesis by Laser Ablation" D.B. Geohegan, A.A. Puretzky, G. Duscher, and S.J. Pennycook, Appl.Phys. Lett. 72, 2987 (1998) .  Download PDF file (195k)

2. "Photoluminescence from Gas-Suspended SiOx Nanoparticles Synthesized by Laser Ablation" D.B. Geohegan, A.A. Puretzky, G. Duscher, and S.J. Pennycook, Appl.Phys. Lett.73, 438 (1998) .   Download PDF file (429k)