ALKALI HALIDE NANO AND MICRO FILMS PRODUCED BY PULSED LASER
DEPOSITION : AN ELECTRON MICROSCOPY STUDY
Dwight Acosta
1
, Elsi Mejía
2
, Citlali Sanchez
2
1
Instituto de Física, UNAM, Ciudad de México
2
Centro de Ciencias Aplicadas y Desarrollo Tecnológico, Ciudad de México.
The alkali halide (AH) compounds, are materials with cubic symmetry and
constituted by an assembly of positive sodium ions and negative chloride ions. These
compounds are insulators with wide band gap and by its simple crystalline composition are
often used as model structure to validate and try new theories and experiments. Thus, recently
these materials have been researched as promising host for self-assembly of luminescent
nanostructures. The AH are synthesized with different growth processes in bulk and thin film
configuration, and present electrical anisotropy according to crystallography faces thus for
the NaCl, (100) surface is electrically neutral and the (111) surface is electrically charged.
Arranging and control of atomic structures and molecular on insulating thin film are of great
interest in device miniaturization.
In the insulating materials, and in particular in NaCl, the
epitaxial growth of thin films and its self-assembly process is highly important for
nanotechnological applications due to its high-quality in the growth of layer by layer .
Monocrystalline NaCl and KCl materials, from International Crystal Laboratories,
were used respectively as targets during Pulsed Laser Deposition process, which in turn,
was carried out in an UHV chamber at pressure 10 mTorr using an excimer laser at 248 nm,
with an output energy 50 mJ at 10 Hz. The PLD system used operation was controlled with a
computer. The substrates were located in front o the target and several distances target-
substrate were tried : 45, 55, 60, 70, 80 mm. The substrate temperature was increased from
room temperature up to 300, 400 and 500 ºC respectively and the number pulses was rised
sequentially from 4800 to 24000 shots.
From sequential SEM and STEM micrographs analysis the simultaneous ocurrence of
three growing process can be stablished:1)The atomic and molecular clusters formation, 2)
the micro and nano grains coalescence process 3) the self ensamble of the nanocrystal
cluster as a function of substrate temperature, deposition chamber pressure and laser pulses
number. Electron micrographs suggest that nucleation and coalescence rates are different for
nano and micro AH crystallites, during film synthesis and the big crystallites do not growth at
the expenses of small ones while the flow of alkali halide plumel from target is impinging
continuously on substrate
Alkali halides (NaCl and KCl) have been succesfully deposite on carbon and resin
substrates using the pulsed laser deposition technique for different deposition times and
different substrates temperature. TEM,SEM and STEM micrographs reveal details of
coalescense and growth processes at different stages :a) nanocrystallites and b)
microcrystallites. The last ones reveal the growth in a self assemble way following
crystallites faceting. It has been found that even for short deposition times the coalescence
and growth occurs simultaneously at nano and micro size levels and the resulting alkali halide
films are produced after a continuos coalescence and assembly processes.
The results suggest that beside the clasics film atomistic growth models other
possibilities based on nano cristal assembly, must be considered in order to explain the Alkali
Halides grains growth and thin films formation respectively during the PLD deposition
process.
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