Giant Uv-Induced Spectral Shift of Lithium Borate Nanoparticles
Embedded Into the Polymer Matrix
I.V.Kityk
Elecrical Engineeirng Department, Czestochowa Univeristy Technology, Armii Krajowej 17,
Czestohcowa, Poland
ABSTRACT:
We have found that UV polarized treatment of the Li2B4O7 nanocrystlas (TBL NC)
embedded into the olygoether acrylate photopolymer samples with the pulsed 7 ns laser
polarized nitrogen laser pulses at 337 nm causes substantial red spectral shift of the energy
gap from 7.3 eV up to 6.3 eV. The dependence of the spectral shift versus the fundamental
power density as well as versus the temperature and UV pump power density was investigated
The spectral shift
is disappeared during the 15 min. after the interruption of optical
treatment. The effect exists only in the nanocrystlalites with sizes below 200 nm.
Key words:optical materials, nanoparticles
1. INTRODUCTION
The crystalline inorganic nanocrystallites (NC) embedded into the polymer matrices present
an interest for the different optoelectronic applications [1-3]. They possess different properties
which originate from interfaces between the polymer matrices and incorporated polymers
[4,5]. The wide gap crystals and the corresponding NC like borates of an especial interest due
to the promising optoectronic properties. Particular interest present the wide energy gap
lithium tetraborate (LTB) Li2B4O7 crystals [6]. To be appropriate the polymer matrix should
have the close values of the refractive indices to avoid any light scattering. So we have chosen
the oligoetheracryalte photopolymer matrix [7]. The titled photopolymer matrices possess
besides the good optical fitness very good photothermal optical properties and are stable with
respect to the external laser light.
It is well known that the semiconducting NC with the sizes below 70 nm show
substantial changes of the band energy dispersion [8] and corresponding effective energy
gaps. This is caused by the reconstructed surface states, which are very sensitive to the
surrounding polymer matrices due to enhanced values of the ground state dipole moments.
So one can expect that UV-induced light below the energy gap may cause additional photooccupation of the localized trapping levels and as a consequence it may be a possibility to
perform the UV-spectral shit of the effective energy gap in such kinds of NC.
In the present work we explore influence of the UV-pulsed power density, temperature
and NC sizes of the polymer nanocomposites and temperature on the possible shift of the
effective energy gap.
2. SAMPLE PREPARATION
The investigated crystals were grown by a modified Bridgman method as described in the
Ref. 9. The sizes of the crystals were equal to about 2 cm in the lengths and the diameter
about 0.7 cm. The monitoring of their quality was done by X-ray diffraction method and
optical polarized method. The NC were milled using acoustical field and were incorporated
into the photopolymer olygoether acrylate matrices as described in the Ref. 10. We have
prepared the samples with the content of the NP equal to about 0.5 %, 1.0 %, 2 %, 3 %, $ %,
% % , in weighting units. The control of their TEM structure was done by JEOL-100. From
the Fig. 1 one can see that the NC are incorporated into the polymer matrices.
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Fig. 1. TEM picture of the titled nancompsites.
The treatment are performed by the 337 nm, 7 ns UV pulse laser with the variations of the laser
power densities up to 1 GW/cm2. The U-7000 Automated Vacuum UV System was used as
vacuum spectrophotometer with resolution about 7 nm. The laser beam was focused by MgF2
windows on the samples with the diameters about 1 mm. The probing beam form the UV source
also was incident on the samples which were diafragmed. The UV-induced laser pulses
possessed the frequency repetition about 10 Hz and the beam of the pumping laser and of the
probing spectrophotometer was overlapped.
RESULTS and DISCUSSION
In the Fig. 2 are presented the UV-absorption edges for the titled nanocompsites under
influence of the pulsed UV-laser irradiation of different powers. One can clearly see that the
absorption edge is shifted from 7.3 eV up to 6.31 eV at 1.1 GW/cm2 laser irradiation.
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Fig. 2. UV-induced spectral shift of energy gap under influence of the UV-induced laser light.
The absorption is shown after the saturation during the 3-4 min. All the measurements were done
at T=77 K. This fact may indicate on a principal role for electron-phonon interactions
participating in the absorption edge. The lowering of temperature stimulates the substantial
changes in trapping of the photoexcited electrons [11]. As a consequence the optically induced
electron-phonon anharmonicities forming the internal effective electric field may play substantial
role in the effect observed.
1200
RT
T=180 K
T=77 K
-1
Absoprtion (cm )
1000
800
600
400
200
0
5,0
5,5
6,0
6,5
7,0
7,5
8,0
E (eV)
Fig. 3.The absorption edge measured at different temperatures.
It is principal that the observed spectral shift is very sensitive to the temperature and exists only
at low temperatures (see Fig. 3)
Another principal factor here is a content dependence of the energy gap versus the content of the
NC (Fig. 4). One can see that the maximal energy gap exists at 3 % of TBL nanocrystlalites in
weighting units. Such dependence may indicate on a principal role played by the NC interfaces
which possess a fixed effective nanointerfaces which contribute to the effects observed. As a
consequence such effects may be very crucial for the observed UV-induced energy gap shift. It is
necessary to underline that the effect exists only during the phototreatment and disappears after
several minutes after interruption of the UV-laser treatment. Moreover the effect is absolutely
absent for the crystallites with sizes about 200 nm. So it is not observed for the bulk crystals.
This phenomenon may be of interest for the UV-optical trigger because it allows to
operate by the UV-absorption transparency, which is principal for the UV-optoelectronic.
B
6,9
6,8
Eg (eV)
6,7
6,6
6,5
6,4
6,3
0
1
2
3
4
5
NC content(%, in weight.units)
Fig. 4.Dependence of effective energy gap at diferent NP content.
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CONCLUSIONS
UV polarized treatment of the Li2B4O7 NC (TBL NC) embedded into the olygoether acrylte
photopolymer films with the pulsed 7 ns laser polarized nitrogen laser pulses at 337 nm causes
substantial red spectral shift of the energy gap from 7.3 eV up to 6.31 eV at 1.1 GW/cm2 UV
laser irradiation at T=77 K. With the increasing temperature and the lower UV-induced laser
power densities the effect is decreased. The optically induced electron-phonon anharmonicities
forming the internal effective electric field may play substantial role in the effect observed. exists
at 3 % of TBL nanocrystlalites in weighting units. Such dependence may indicate on a principal
role played by the NC interfaces which possess a fixed effective nanointerfaces which contribute
to the effects observed. As a consequence such effects may be very crucial for the observed
UV-induced energy gap shift. It is necessary to underline that the effect exists only during the
phototreatment and disappears after several minutes after interruption of the UV-laser treatment.
Moreover the effect is absolutely absent for the crystallites with sizes about 200 nm. So it is not
observed for the bulk crystals. The principal mechanisms may also include some effect related to
the photoinduced anharmonic phonons [12, 13]. Besides principal role may play different kinds
of defects [14].
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