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nanoparticles ( nps ) with sizes from a few nanometers to tens of nanometers are artificial materials which exhibit extraordinary optical and electrical properties due to its ultra - small sizes and in particular to quantum confined nature of their energy levels .    the extraordinary properties of size - tunable nps has given rise to their wide spread applications in nanophotonics , plasmonics , biomedicine , lasing , photovoltaic etc @xcite .
semiconductor nps or quantum dots ( qds ) are found a number of applications in the biomedical imaging due to the size- dependent flexibility of its luminescent properties .
the nps with the sizes 2 - 10 nm , which is about of the size of small biomolecules , have the great potential for various applications in the modern biomedicine . in difference from organic biological labels ,
the luminescent qds have several advantages .
at first they have a wide absorption band and can be used with a variety of laser sources having different output wavelengths . on the other side ,
the luminescence band of qds can be tuned from uv to ir by changing their sizes and this property allows combining several types of qds at the one excitation wavelength .
it is also important to notice that qds are more photostable under the long time of laser irradiation .
the existence of the mentioned properties of qds could dramatically improve the use of fluorescent markers in the biological imaging .    during the last decade
various methods have been investigated for the synthesis of nps . recently , a simple and flexible method of the pulsed laser ablation in liquid media ( plal ) has been demonstrated for the laser synthesis of metal nanoparticles @xcite .
this method also has been applied for synthesis of colloidal solutions of semiconductor qds 7,8,9 .
laser ablation in liquids media is especially good - looking for biomedical applications , since nps could be formed directly in specially prepared bio - conjugated media and such nps colloidal solution can be exactly tuned for the specific biological target
.    the formation of semiconductor nanoparticles via plal has been investigated in different experimental conditions , and photoluminescence properties of nanoparticles produced by the plal have been found to be strongly dependent as on the physical and chemical properties of liquid environment , as well as on the parameters of the laser irradiation .
laser ablation syntheses of nanoparticles could be explained in terms of the dynamic formation mechanism of the particle growth @xcite . according this mechanism , a material of target evaporated in the laser plume very quickly aggregate into small embryonic particles ,
therefore such laser pulse parameters as pulse energy and duration could be highly critical in this process . in the paper
@xcite the influence of a laser beam transverse electromagnetic mode structure on the luminescence properties of laser synthesized gaas nanoparticles has been studied . in the present work we studied luminescence properties of the si 
ni hybrid colloidal nanoparticles system fabricated in pure water by the laser ablation method using a nanosecond nd : yag laser .
the pulsed passive q - switched nd : yag laser with wavelength of 1064 nm , pulse duration of 8ns and repetition rate of 10hz was used to synthesize nanoparticles in pure water .
the laser beam was focused on the surface of a bulk target ( the plate of bulk si or ni ) allocated in the glass cuvette with distilled water ( see figure ( 1 ) ) .
exposition of the laser irradiation was two hours . to produce hybrid metal - semiconductor system ,
at first we irradiated a ni bulk target , then in the same cuvette with water solution of ni nanoparticles was located the si bulk target to produce si nanoparticles .
hybrid nanoparticles system was produced also in similar scheme when ni and si nanoparticles were produced separately in different cuvettes and after that mixed in one solution .
note that luminescence spectra of the obtained hybrid nanoparticles were the same in these two experimental approaches .
note also that a luminescence of separately prepared si nanoparticles was not observed .
[ fig1 ]    for luminescence measurements the water solution of nanoparticles was relocated into quartz cuvette with the transparent walls in the uv spectral range .
the luminescence of the colloidal qds was excited by the irradiation at 400 nm wavelength of the continuous wave diode laser with the power of 10mw .
the luminescence spectra were analyzed by spectrometer constructed on the base of monochromator lomo mdr- 23-pmt .
electrical signal amplifier was used to enhance output of the pmt .
luminescence spectrum of colloidal si - ni qds is presented in the figure ( 2 ) .
[ fig2 ]    the considerable blue shift of the photoluminescence has been registered with the location of the maximum at the wavelength of 455 nm which is connected with the size - effect and significant difference from the well known luminescence of a bulk si material located in the near infrared region .
the peak on the 400 nm is the excitation laser wavelength .
the bandwidth of the luminescence spectra on the halfheight was about 40 nm that demonstrates the narrow size distribution of laser generated qds , which is achieved without application of any size selection schemes .
[ fig3 ]    possible contributions to the luminescence from a quartz material of the cuvette and water has been eliminated by a comparison with the photoluminescence from the same cuvette filled with the clean water or with the solution of coumarin 120 organic dye ( see figure ( 3 ) ) .
the long - term stability of the luminescence properties of the produced qds was observed during the one year .
here we give some interpretation of the results of our experiment by estimations based on the known properties of silicon nanoparticles and quantum confinement effect , as well as the effect of the charged nickel nanoparticles on luminescence spectra of the colloidal si quantum dots .
as is known @xcite , metalic nanoparticles generated in diverse media by the method of a laser ablation carry a positive or negative electrical charge . in our case ,
the charged ni nanoparticles within the si 
ni hybrid system induce a static electrical field which causes stark shift of the electron - hole energy levels in the silicon qd . in the following ,
we consider a point charge at a certain distance from the surface of the silicon qd ( see figure ( 4 ) ) .
the assessment of the si nanoparticles luminescence spectrum shifting under the influence of a static electric field of an external charge allows us to explain experimental results by the stark effect in the inhomogeneous ( coulomb ) field .
the external point charge @xmath0 on the distance @xmath1  from the center of the silicon quantum dot creates a screened electrostatic field at the point @xmath2 inside the qd , potential of which can be given in the following form @xcite :    @xmath3    where @xmath4 is the legendre polynomial ,
@xmath5 -screening dielectric constant of si qds , @xmath6 -angle between the vectors @xmath7 and @xmath8 . the shift of the energy levels of an exciton caused by an external electrostatic field can be evaluated using the wavefunction @xmath9 of the electron - hole system :    @xmath10 \left\vert \psi_{0}\left ( \mathbf{r}_{e},\mathbf{r}_{h}\right ) \right\vert ^{2}. \label{2}\ ] ]    taking into account the form of the potential of the external field ( [ 1 ] ) , the energy shift ( [ 2 ] ) can be represented in the form of the sum    @xmath11    where    @xmath12@xmath13    the first term in the sum ( [ 3 ] ) : @xmath14 .
we will estimate only the term corresponding to @xmath15 taking into account the smallness of the other terms .
thus ,    @xmath16    which can be written in the following form :    @xmath17    where @xmath18 is the size of an exciton , @xmath19 are the mean values of the charge distribution density of the hole and the electron , respectively . to calculate the energy spectral shifting in accordance with the conditions of the above described experiment ,
let us estimate @xmath20 .    as is shown in @xcite , the emitted photon energy depends on silicone nanocrystallite size . with the decreasing of the size of nanoparticles the quantum confinement affects on the band gap of the exciton thereby increasing emission energy .
during the experiment with the solely silicon nanoparticles the luminescence had not been observed in the visible spectrum at the irradiation by the wavelength of 400 nm .
this fact evidences that bandgap energy of the si qds is more than 3 ev , which is agreed with the results of the work @xcite .
according to the work @xcite , such energies corresponds to silicon nanoparticles with diameter less than 1.4 nm . to determine the other parameters we will take for screening dielectric constant @xmath21 , which corresponds to the diameter @xmath22 nm of si quantum dots at the passivation with the hydrogen atoms , according to the paper @xcite .
as we consider si nanoparticles generated in a liquid medium ( pure water in our case ) , dangling bonds of the silicon atoms on the dot surface should be saturated mostly by the hydrogen atoms .
note that in the process of the laser ablation hydrogen molecules are produced due to water molecules dissociation in the surroundings of the laser plume . as is shown in the work @xcite ,
in the case of the hydrogen passivated si nanoparticles the size of the exciton is approximately equal to the diameter of the qd .
[ fig4 ]    since we consider the nickel nanoparticles in close proximity from the silicone qds , then @xmath23 , and coulomb potential is strong enough to pull the hole away from the electron . in this case
, the quantity @xmath24 can reach as large as @xmath25 @xcite .
then , we have @xmath26 ev .
this shift of bandgap energy corresponds to a red shift of si quantum dot luminescence spectrum by about @xmath27 nm ( it is clear that depending on the distance of the point charge from the surface of silicon qd this shift will be changed ) .
this shift can explain the observation of blue luminescense in the case of hybrid si - ni nanoparticles system in opposite to the case of the solely si nanoparticles in the colloidal solution .
summarizing , by the luminescense picture of the mixed metallic - semiconductor nanoparticles in the colloidal solution one can define the sizes of quantum dots .
the si colloidal qds in liquids can be used in a number of optical applications , in particular , as luminescent markers in the biomedical imaging in vivo and in vitro .
the knowledge of peculiarities of their luminescent properties is very important for such kinds of biomedical applications .
more complex assemblies of nanocrystals can be easily constructed by the presented technique of laser synthesis of colloidal qds including complexes of nanoparticles of different materials .
these complex artificial structures will exhibit new physical and chemical properties that require further investigation and could be applied in the optical nanotechnologies .
the results of this study may have far - reaching consequence for nanoparticles applications .
we would like to thank prof . h. k. avetissian and dr .
g. f. mkrtchian for valuable discussions .
this work was supported by state committee of science of ra .
99 alivisatos a p 1996 _ j. phys .
chem . _ * 100 * 13226 13239 .
the luminescence properties of the colloidal hybrid si  
 ni nanoparticles system fabricated in the pure water by pulsed laser ablation is considered . 
 the red - shifted photoluminescence of this system because of the stark effect in the coulomb field of the charged ni nanoparticles has been registered in the blue range of the spectrum .    
 _ keywords _ : laser ablation , colloidal quantum dots , si - ni nanoparticles , photoluminescense , stark shift