O.A.Yeshchenko a,1, I.M.Dmitruk a, S.V.Koryakov a and Yu.A.Barnakov b
a
Physics Department, National Taras Shevchenko Kyiv University,
6 Akademik Glushkov prosp., 03127 Kyiv, Ukraine
b
Advanced Materials Research Insitite, University of New Orleans, New Orleans, LA 70148
We report that for the first time the nanoparticles of II-V semiconductor (ZnP2) wereprepared and studied. ZnP2 nanoparticles were prepared by incorporation into zeolite Na-Xmatrix. Absorption, diffuse reflection (DR) and photoluminescence (PL) spectra of theZnP2 nanoclusters incorporated into the supercages of zeolite Na-X were measured at thetemperature 77 K. Five bands B1-B5 are observed in both the DR and PL spectrademonstrating the blue shift from the line of free exciton in bulk crystal. We attribute theB1-B5 bands to some stable nanoclusters with size less than the size of zeolite Na-Xsupercage. We observed Stokes shift of the PL bands from the respective absorption bands.The nonmonotonic character of its dependence on the cluster size can be explained as theresult of competition of the Frank-Condon shift and the shift due to electronic relaxation.
1. Introduction
Many different methods have been used for preparation of the semiconductor nanoparticles.These are, e.g. preparation of nanoparticles in solutions [1], glasses [2] or polymers [3]. However, itis not easy to control the size distribution of small clusters with countable number of atoms andmolecules in these methods. Matrix method based on the incorporation of materials into the 3Dregular system of voids and channels of zeolites crystals could be one of the possible solution [4,5].Zeolites are crystalline alumosilicates with cavities, size of which can vary in the range from one toseveral tents nanometers. It depends on the type of alumosilicate framework, ratio Si/Al, origin ofion-exchanged cations, which are stabilized negative charge of framework and etc. Zeolite NaX,which have been used in the present work has Si/Al ratio equal 1, Fd3m symmetry and two types ofcages: one is sodalite cage – truncated octahedron with diameter 8 A and supercage, which formed bythe connection of sodalites in diamond law with the diameter of about 13 Å [6]. All cages areinterconnected by shared small windows and arranged regularly. Thus, the cages can be used for thepreparing of small semiconductor clusters of zinc diphosphide (ZnP2).
Many works have been reported on the nanoparticles of II-VI [1,7,8], III-V [9,10], IV-VI[11], I-VI [12], II-VII, IV-VII, V-VII [13] and IV [14] types of semiconductors. But to the best of our
knowledge the nanoparticles of II-V type have not been studied yet. The present paper is the firststudy of ZnP2 clusters. Wet chemistry methods are not applicable to scale up a production ofultrasmall ZnP2 clusters due to their high reactivity at air. It is hard to expect its stability in glass meltas well. Thus, incorporation into zeolite cages seems to us one of the most suitable methods ofpreparation of ZnP2 clusters.
Quantum confinement of charge carriers in small particles leads to new effects in opticalproperties of the particles. Those are the blue shift of exciton spectral lines originating from theincrease of the kinetic energy of charge carriers and the increase of the oscillator strength per unitvolume [15,16]. These effects are quite remarkable when the size of the particle is comparable withor smaller than Bohr radius of exciton in bulk crystal. Since the exciton Bohr radius in bulkmonoclinic ZnP2 (β-ZnP2) is quite small (15 Å) [17], the problem to reach the quantum confinementfor this material is quite difficult. Therefore, zeolite is the good candidate to solve this problem, as itscages are rather small and can contain only small clusters.
Bulk β-ZnP2 crystal is the direct-gap semiconductor. As this crystal is strongly anisotropic, itsoptical spectra are characterised by three exciton series. The lowest energy exciton peak is observedat 1.55913 eV [18]. As the bulk crystal has rather small energy gap (1.6026 eV), the blue shiftedexciton lines of ZnP2 nanoparticles are expected to be in the visible spectral region.
2. Technological and experimental procedures
For the preparation of ZnP2 nanoclusters we used crystals of β-ZnP2 and synthetic zeolite ofNa-X type. The size of zeolite single crystals was about 50 µm. Zinc diphosphide was of 99.999%purity. The framework of zeolite Na-X consists of sodalite cages and supercages with the insidedianeters of 8 and 13 Å, respectively. ZnP2 molecule seems to us to be too large to be incorporatedinto small sodalite cage, because of the existence of many Na cations. Therefore, it is naturally toassume that only the supercages can be the hosts for ZnP2 nanoparticles. Zeolite and ZnP2 crystalwere dehydrated in quartz ampoule in vacuum about 2·10-5 mm Hg for one hour at 400°C. After thatampoule was sealed. We used 100-mm length ampoule for space separation of ZnP2 source andzeolite in it. ZnP2 was incorporated into the zeolite matrix through the vapour phase at 840°C insource region and 835°C in zeolite region for 100 hours. The cooling of ampoule we carried outgradually with inverted temperature gradient.
In our experiments we used the samples with 5 wt% loading level of ZnP2 into zeolite. Since
Samples in quartz ampoule were dipped into liquid nitrogen during the experiment. A tungsten-halogen incandescent lamp was used as a light source for the absorption and diffuse reflection
1
Corresponding author. E-mail address: yes@univ.kiev.ua (O.A. Yeshchenko).
measurements. An Ar+ laser with wavelength 4880 Å was used for the excitation of theluminescence.
3. Results and discussion
Absorption, diffuse reflection (DR) and photoluminescence (PL) spectra of the ZnP2 clustersincorporated into the 13 Å supercages of zeolite Na-X were measured at the temperature 77 K. Theabsorption and DR spectra of the studied nanoparticles are presented in fig.1. Conventionalabsorption spectrum was obtained from the transmission one measured from the layer of zeolite withZnP2 with the thickness of 0.3 mm. The absorption spectrum has no features. The absorptioncoefficient increases monotonically with increase of the photon energy in the range from 1.66 eV to2.60 eV approximately. The absorption spectrum can be obtained from the diffuse reflectionspectrum converting it with Kubelka-Munk function K(hω)=[1−R(hω)]2R(hω), where R(hω) is
2
the diffuse reflectance normalised by unity at the region of no absorption. Obtained spectrum is moreinteresting than conventional absorption one as it demonstrates clear band structure. Five bandssigned as B1-B5 is observed in the spectrum (fig. 1). The spectral positions of these bands arepresented in table 1. All these bands demonstrate the blue shift (table 1) from the line of free exciton(1.55913 eV) in the bulk β-ZnP2 crystal. The observed blue shift allows us to attribute these bands tothe absorption of ZnP2 nanoclusters incorporated into supercages of zeolite. It is often observed fornanoclusters that clusters with certain number of atoms are characterised by high binding energy(more stable clusters) and are more abundant in the sample. This effect is well known, e.g. for thenanoparticles of II-VI semiconductors [19,20]. We estimate that the largest ZnP2 cluster in zeoliteNa-X supercage with the diameter of 13 Å is (ZnP2)7. Under the assumption of the full loading of thesupercages by nanoparticles for 5 wt% loading level of ZnP2 into zeolite one can estimate the theaverage number of the ZnP2 molecules per a supercage to be 5.7. Thus, B1-B5 bands can be attributedto these stable clusters containing up to 7 of ZnP2 molecules. Probably they are stoichiometric sincewe do not have any evidence of ZnP2 decomposition at the temperature of 840°C used for itssublimation. Results of theoretical search of the stable clusters in this size range will be printed in thenext paper. The photoluminescence spectrum (fig. 2) shows the same structure as the DR spectrum,i.e. PL spectrum consists of the same five B1-B5 bands. Their spectral positions are presented in table1. The PL bands are characterised by blue shift from the exciton line in bulk crystal as well (table 1).
The observed blue shift of the absorption and luminescence bands is the result of the quantumconfinement of electrons and holes in ZnP2 clusters. As the exciton Bohr radius a in bulk crystal is15 Å that is larger than radius of zeolite supercage R = 6.5 Å, the strong confinement regime takesplace in the clusters. The experimental value of the blue shift obtained from the spectral positions ofB5 band in DR spectrum is 0.282 eV. In Ref. [21] the finite confining potential was used to calculate
blue shift. However, for experimental blue shift of 0.282 eV the theory gives the value of clusterdiameter of about 30 Å. That is considerably larger than the diameter of zeolite supercage (13 Å). So,the value of blue shift calculated by the effective mass approximation are substantially different fromthe experimental one. It means probably that the effective mass approximation fails for consideredsmall clusters. One more explanation of the obtained small value of the blue shift is the shift ofelectron and hole energy levels to the low energy due to the tunnelling between the neibouringsupercages.
As it can be seen from fig.1 the intensities of the bands increase at the increase of therespective photon energy in DR spectrum. Meanwhile, the opposite situation takes place for PLspectrum (fig.2). Here the intensities of bands increase at the decrease of the energy. This effect canbe explained in such way. Lower intensities of the PL bands corresponding to the smaller clusterscan be explained by reabsorption of their emission by larger clusters. The reabsorption is due tooverlap of the corresponding bands, absorption into excited states and phonon-assisted transitions.
One can see from the table 1 that the luminescence bands have the Stokes shift from theabsorption ones. The value of this shift is from 0.078 to 0.135 eV. Such shift is well known both inthe molecular spectroscopy and in the spectroscopy of nanoparticles. It is known that this kind ofStokes shift (so-called Frank-Condon shift) is due to vibrational relaxation of the excited molecule ornanoparticle to the ground state. The dependence of the red shift in the spectra of ZnP2 clusters inzeolite on the size of cluster is nonmonotonic. For smallest clusters the increase of the Stokes shift atthe decrease of the nanoparticle size is observed. Such dependence can be explained by the theorydeveloped, e.g. in Ref. [22] where the first-principle calculations of excited-state relaxations innanoparticles and the dependence of respective Stokes shift on particle size were performed. As it isshown in Ref. [22], for smallest clusters the observed Stokes shift is the Frank-Condon shift which isthe result of the structure relaxation of the cluster in excited and ground states. Meanwhile, for largerclusters the opposite dependence is observed, namely the Stokes shift increases with the increase ofthe cluster size. Perhaps, such behaviour is the result of the relaxation of initially excited electron-hole pair to its self-consistent equilibrium configuration with hole in the centre of cluster.
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Table 1. Spectral characteristics of ZnP2 nanoclusters in zeolite Na-X.
BandSpectral position (eV)Absorption
PL2.2682.1151.9951.8541.706
Blue shift ofabsorption bands (eV)
0.8080.6410.5090.3960.282
Stokes shift (eV)
0.0990.0850.0780.1010.1353
← Cluster sizeB1B2B3B4B5
2.3672.2002.0681.9551.842
Fig. 1. The conventional absorption (A) and obtained by Kubelka-Munk method absorption spectra(DR) of ZnP2 nanoclusters in zeolite Na-X at the temperature 77 K.
Fig. 2. The photoluminescence spectrum of ZnP2 nanoclusters in zeolite Na-X at the temperature 77K.
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