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a-SiOx multilayer nanocomposites
ARTICLE IN PRESS Physica E 41 (2009) 436–440
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Band-edge absorption spectroscopy of CdSe=a-SiOx multilayer nanocomposites P.E. Shepelyavyj, I.Z. Indutnyj, V.P. Bryksa 1, Vas.P. Kunets 2, V.P. Kunets Lashkarev Institute of Semiconductor Physics, NAS of Ukraine, 41 Prospect Nauki, 03028 Kiev, Ukraine
a r t i c l e in f o
a b s t r a c t
Article history: Received 27 May 2008 Received in revised form 6 September 2008 Accepted 8 September 2008 Available online 2 October 2008
CdSe=a-SiOx multilayer nanocomposites containing CdSe nanocrystals between a-SiOx thin layers have been produced by sequential thermal vacuum deposition of CdSe and SiOx on quartz substrates. Strong confinement effect has been observed in absorption spectra. Average size of nanocrystals was estimated from the absorption spectra discriminating between confinement, oxidation and artificial effects. Areal density of nanocrystals in CdSe sublayers was estimated to be 1012 cm2 . Oxidation of nanocrystals was observed as a decreasing of optical absorption in the region above the optical band-gap energy. The binding energies of Cd–O and Se–O bonds have been calculated as a function of the inter-atomic distances for the Cd17 Se17 cluster bounded to SiO4 fragment in the framework of self-consistent Hartree–Fock scheme by means of the semi-empirical PM3 method in approximation of NDDO (neglect of diatomic differential overlapping). The greater energy has been found for Cd–O bonds (123:5 kcal=mol) that evidences the preferred oxidation of Cd facets of CdSe nanocrystals. Crown Copyright & 2008 Published by Elsevier B.V. All rights reserved.
PACS: 78.20.Ci 78.55.Et 78.67.n 78.67.Bf 78.67.Pt Keywords: Multilayer nanocomposites Nanocrystals Band-edge absorption Oxidation
1. Introduction Semiconductor nanocrystals have attracted great interest over the past few decades because of their potential applications in the fast developing fields of material science, nanotechnology, opto- and nanoelectronics, chemical sensing, biology, and medicine. Size-dependant properties of nanocrystals come from the confinement effects and the high surface to volume ratio. In the first experimental [1] and theoretical [2] papers CdS(Se) and CuCl(Br) nanocrystals (quantum dots) embedded into borosilicate glasses have been investigated. Afterwards, a lot of structures were synthesized based on II–VI and IV–VI semiconductors in different matrices such as a-SiOx , polymers, organic solvents, and water [1,3–7]. Because of the unique optical properties, nanocrystals are attractive for applications in visible and near infra-red regions where matrices are transparent [8–10]. For the mono-dispersive nanocrystals in the so-called ‘‘strong confinement’’ regime (¯r oaB , here r¯ is the average radius of Corresponding author. Tel.: +1 916 335 0011.
E-mail address: [email protected] (V.P. Kunets). Present address: Department of Physics, Humboldt-Universita¨t zu Berlin, Newtonstrasse 15, 12489 Berlin, Germany. 2 Present address: Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA. 1
nanocrystals, and aB is the Bohr exciton radius in a bulk) one or more narrow absorption bands are usually observed in the region of the fundamental band edge. The energy spectrum of confined excitons can be determined from the absorption spectra with a high accuracy by the method proposed in Refs. [11,12]. The absorption edge is decomposed for separate bands, taking into account the size dispersion, and r¯ is used as a fitting parameter. This method was successfully applied to CdSx Se1x doped glasses. The narrow absorption bands have been observed for CdSe quantum dots grown by the wet chemical route in the coordinating solvents [4,13] where the size dispersion is only 5%. The long-wavelength band corresponding to 1Se ! 1Sh optical transitions determines the confinement optical band-gap shift (DEg ) with regard to the bulk gap. In the ‘‘weak confinement’’ regime, (¯r XaB Þ the absorption edge is smooth, this is typical for bulk. Therefore, special analytical procedure was proposed in Ref. [11] to determine r¯ and DEg from such a smooth edge. For the CdSe quantum dots in a borosilicate glass the absorption edge became smooth as early as at r¯ 41:5aB [5]. The wide size dispersion also smoothes spectra and complicates analysis. Absorption spectra of the multilayer CdSe=a-SiOx nanocomposites are usually smooth even at nanocrystal radii less than aB (strong confinement) [3]. Such behavior is not clearly understood yet. Moreover, the typical absorption coefficients of these nanocomposites in the band-to-band spectral region are much smaller than in the
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ARTICLE IN PRESS P.E. Shepelyavyj et al. / Physica E 41 (2009) 436–440
bulk. While the absorption reaches 105 cm1 in individual nanocrystals even being increased in small ones due to oscillator strength increasing [14], it comes only to 10–100 cm1 in nanocomposites for nanocrystal concentration of about 1015 cm3 . For nanocomposites it becomes crucial because the thicknesses and total numbers of CdSe and SiOx sublayers can be varied in a wide range. This ‘‘dilution’’ effect significantly complicates the analysis of the band-edge absorption as well as its comparison with the bulk spectra. Besides, the optical interference effects should be also excluded. Therefore, the attempt has been made in Ref. [15] to restore the optical absorption using the photocurrent technique. Unfortunately, this technique is not applicable at low nanocrystal concentration. In this paper, we report on the absorption spectroscopy of CdSe nanocrystals grown in CdSe=a-SiOx (x 1:5) multilayer nanocomposites. We have determined the optical band-gap of nanocrystals and their average sizes from the analysis of the fundamental band-edge absorption discriminating between confinement, oxidation and artificial peculiarities of the spectra. The oxidation of nanocrystals is revealed as the changes in the spectra at high absorption level in comparison with the reference film. Theoretical modeling proves the preferred oxidation of the cadmium facets.
2. Experimental
437
a CdSe/SiOx SiOx
CdSe
b Etrap
0.15 eV
c-band 0.55 eV EF 1.75 eV 2.9 eV
Uh = 1.3 eV V-band
CdSe=a-SiOx multilayer nanocomposites have been prepared in the shape of rectangle platelets (0:5 1:5 cm2 ) by alternating thermal evaporation of CdSe and SiOx (x 1:5) in vacuum from two independent Ta sources on quartz substrates at room temperature. The nanocomposite have been formed on a central part of the substrate while both its components (CdSe and SiOx ) have been simultaneously deposited on the left and the right side of the sample (Fig. 1a) forming the reference films of CdSe and a-SiOx . A special screen system was developed for this purpose. The deposition conditions were: a vacuum around 2 103 Pa, a deposition rate of 0:01nm s1 for CdSe and 0:1 nm s1 for SiOx. CdSe has been deposited using the step-by-step method [16] while a-SiOx has been deposited by continuous evaporation. During the deposition the thicknesses of CdSe and SiOx sublayers (dCdSe and dSiOx ) and the deposition rates have been controlled by the preliminarily calibrated quartz monitors. After deposition the thicknesses of the structures and the reference films have been measured by the optical interferometer MII-4. The thickness of a-SiOx sublayers was always the same (dSiOx ¼ 11:0 nmÞ and sufficiently thick to prevent direct contact between nanocrystals in neighboring layers. For as-deposited structures this method gives the quasi-isolated CdSe nanocrystals with the near-spherical shape that is confirmed by TEM [16]. The quality of a-SiOx matrix has been controlled through the absorption of the reference film. Two groups of nanocomposites with the different nanocrystal radii were prepared. In the first group the nominal thickness of CdSe sublayers (dCdSe ) was 3.03 nm while the number N of CdSe=a-SiOx pairs was just 40 (Table 1). In the second group, dCdSe ¼ 1:61 and N ¼ 75. Thus, the total thickness of CdSe in both nanocomposites was the same, coinciding with the thickness of ref the reference film, dCdSe ¼ 121 nm. It was specially done to get the same optical densities (D ¼ lgðI0 =IÞ) in both group of nanocomposites. Besides, the thicknesses and numbers of absorptive layers of ref CdSe were chosen in the way to get KdCdSe 1, where K105 cm1 for direct optical transitions. In this case the measurement errors are minimal. This approach was specially used to exclude the role of the transparent a-SiOx matrix comparing the absorption spectra of nanocomposites and reference films of CdSe. Indeed, the total nanocomposite thickness is increased from 560 to 940 nm when the number of CdSe=a-SiOx pairs is risen from 40 to 75 (see
Fig. 1. (a) Scheme of the multilayer CdSe=a-SiOx nanocomposite. Both control layers of CdSe and a-SiOx are shown on the left and the right side, respectively. (b) Real space band-diagram of multilayer CdSe=a-SiOx nanocomposites [24]. Direct and indirect in real space optical transitions as well as electron trapping to the traps in a-SiOx matrix are shown.
Table 1), but the SiOx matrix does not add to the absorption in the region of the transparency. In this way, we could exclude the aforementioned ‘‘dilution’’ effect. Transmission (T), reflections (R and R0 ) from the sides of nanocomposite and quartz substrate, respectively, have been measured using KSVU (LOMO) spectrophotometer, equipped with a special optical attachment. Spectral resolution was better than 0.7 nm. All optical constants were calculated using the method developed earlier in Ref. [18]. The refractive indexes of a-SiOx films and quartz substrate are close. Therefore, the reflection from a-SiOx /quartz interface was only 3% and the interference effects could be neglected. The absorption coefficients have been calculated using the formula: 1 1R K ¼ ln 1 a þ aR0 þ aT , (1) d T where a ¼ ðn3 1Þ2 =ð4 n3 Þ and n3 is the refractive index of the quartz substrate (n3 ¼ 1:44).
3. Confinement effects The absorption spectrum of the reference film of CdSe looks like a typical one for the bulk crystal (Fig. 2, curve 1). In the longwavelength region (1.65–1:78 eVÞ the absorption coefficient K is risen from 1 104 to 4 104 cm1 exponentially. Such exponential tail obeys the Urbach rule [19] in bulk crystals, polycrystalline films and even CdSe quantum dots grown in a glass matrix [20] where the typical temperature dependence of the absorption edge is observed. The temperature dependence of the absorption edge of CdSe=a-SiOx nanocomposites should be investigated elsewhere. The small coefficients at the photon energies less than 1.65 eV ref
could not be measured because here KdCdSe o0:1. In the region
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Table 1 Parameters of CdSe=a-SiOx nanocomposites and reference films of CdSe and a-SiOx Parameter
Ref. film of CdSe I group
CdSe=a-SiOx (40 twin layers) II group
CdSe=a-SiOx (75 twin layers)
Ref. film of a-SiOx
D* (nm)
121 5 (I and II group)
560 5
940 5
440 5 (I group) 820 5 (II group)
1:747 0:005 0
3.03 11.0 1:934 0:005 0:19 0:01 2:1 0:1 6.0
1.61 11.0 1:996 0:005 0:25 0:01 1:8 0:1 5.1
o0:7 [17]
0.55
0.34
dCdSe (nm) dSiO (nm) Optical band-gap, Eg (eV) Band-gap shift, DEg (eV) Average radius, r¯ (nm) Areal density, N s , 1012 Fill factor, Z
2:9ðx 1:5Þ
*Measured by interferometry method.
3.0x1010
105
Eg1 = 1.747 eV Eg2 = 1.934 eV Eg3 = 1.996 eV
1
2.5x1010
x 4.63 1
104
7
(Khν)2, cm-2 eV
absorption, cm-1
2
x 7.77 6
5
2.0x1010 3 1.5x1010
1.0x1010
5.0x109
2 103 1.4
3 1.6
1.8
4 2.0 2.2 2.4 photon energy, eV
2.6
2.8
3.0
0.0 1.6
1.8
2.0 energy, eV
2.2
2.4
Fig. 2. Band-edge absorption of the reference film of CdSe (1) and of CdSe=a-SiOx nanocomposites, dCdSe ¼ 3:03 nm (2) and dCdSe ¼ 1:61 nm (3), respectively. Curves 5 and 6 are the normalized spectra 2 and 3 (see text). Curve 4 is the spectra of the reference film of a-SiOx . Curve 7 represents the spectra 5 after subtraction of the confinement shift.
Fig. 3. Semi-quadratic plotting of the absorption edge for the reference film of CdSe (1) and the normalized spectra of CdSe=a-SiOx at dCdSe ¼ 3:03 (2) dCdSe ¼ 1:61 nm (3), respectively.
between 1.78 and 2.15 eV the absorption obeys 12 power law (Fig. 3, curve 1) which is typical to the direct optical transitions at G-point of the Brilluene zone. The shoulder above 2:20 eV (where
which adds to the nanocomposite thickness but not to its absorption. The more is the number of a-SiOx sublayers in the structure, the more pronounced is the ‘‘dilution’’ effect (compare curves 2 and 3). At the same time, a-SiOx matrix gives rise to absorption in the short-wavelength region (2.2–2:7 eV). The blue shift of the absorption edge is caused by the confinement of the photo-excited electron–hole pairs in CdSe nanocrystals. Since the nominal thicknesses of CdSe sublayers (3.03 and 1.61 nm) in both nanocomposites are far smaller than the Bohr exciton radius in the bulk (aB ¼ 5:6 nm) the strong confinement regime takes place increasing the optical band-gap of nanocrystals. This shift is increased with decreasing of the thickness of CdSe sublayers (curves 2 and 3). To determine carefully the optical band-gap increasing the ‘‘dilution’’ effect should be excluded. As the total thickness of nanocomposite is used as a denominator in formula (Eq. (1)) the following normalization procedure has been applied. We have multiplied the absorption spectra 2 and 3 by the following 560 coefficients 121 ¼ 4:63 and 940 121 ¼ 7:77, respectively, which were obtained by division of the total nanocomposite thicknesses
K105 cm1 ) originates from the spin–orbit split of valence band. To determine the optical band-gap we approximated the edge by the standard dependence: K¼
A
_o
ð_o Eg Þ1=2 ,
(2)
where A is a constant. Corresponding semi-quadratic plotting (Fig. 3, curve 1) gives Eg ¼ 1:747 eV which correlates with the band-gap of bulk crystals 1:732 eV [21] and polycrystalline films 1.65–1:75 eV [22]. The absorption spectra of CdSe=a-SiOx nanocomposites show some differences (Fig. 2, curves 2 and 3). (a) Their absorption coefficients are much smaller all over the spectral region. (b) The noticeable blue shift of the edge is observed. (c) The absorption is risen in the region 2.2–2:7 eV. It is obvious that the smaller absorption coefficients come from the presence of a-SiOx matrix
ARTICLE IN PRESS P.E. Shepelyavyj et al. / Physica E 41 (2009) 436–440
(560 and 940 nm) by the thickness of the reference film of CdSe (121 nm). In this way, we have obtained two normalized spectra (5) and (6) which exclusively demonstrates the real absorption in CdSe nanocrystal subsystem (Fig. 2). Both of them clearly show a noticeable blue shift of the absorption edge. As a preliminary estimation we have determined the shift at the absorption level of about 4 104 cm1 . It was found to be 0.23 and 0.28 eV for the structure with dCdSe ¼ 3:03 nm and dCdSe ¼ 1:61 nm, respectively, that is evidence for the increasing of the optical band-gap with decreasing of the nanocrystals sizes. To determine precisely the optical band-gap increasing (DEg ) we have applied the standard semi-quadratic plotting as it was proposed earlier in Ref. [11] for CdS0:13 Se0:87 quantum dots. It is important to emphasize that the normalized spectra have been used for this purpose. The corresponding plots give us the optical band-gaps 1.934 and 1.996 eV for dCdSe ¼ 3:03 nm and dCdSe ¼ 1:61 nm, respectively. The optical band-gap shifts were found to be DEg ¼ 0:19 and 0.25 eV (Table 1). They have been used for determination of the average sizes of nanocrystals and other relative parameters. The energy band diagram of CdSe=a-SiOx nanocomposites as well the kinetics of electron transitions are not studied thoroughly yet. The relative positions of the energy bands on both sides of a heterointerface are supposed to be the same as in CdSe=a-SiOx multilayers. In turn, the band diagram of multilayers with CdSe sublayer thicknesses between 2.5 and 10:0 nm and the ratio of a-SiOx to CdSe thicknesses of about 1.5 was built in Refs. [23,24] based on the photoconductivity and optical data. Unlike that of type-I CdSe quantum dots embedded into the borosilicate glasses which usually contain about 72% of SiO2 , the type-II band diagram is formed here with the potential well for holes (U h ¼ 1:3 eVÞ and the conduction band offset of about 0.15 eV (see Fig. 1b). Thus, only holes can be confined inside nanocrystals while electrons tunnel into a-SiOx . In Fig. 1b only one energy level for the holes localized in nanocrystal is shown in the quantum well of the valence band. The type-II band diagram is inherent to Ge/Si quantum dots [25] where the confined states for holes and electrons emerge on different sides of a heterointerface and, therefore, they are spatially separated. The electron transitions between these states are spatially indirect forming so-called spatially indirect excitons. The formation of such exciton is shown in Fig. 1b by inclined arrow. The localization of a hole in nanocrystal modifies the potentials in the neighboring space giving rise to the potential well for electrons around nanocrystal and leads to the bound states in this well. The difference from Ge/Si quantum dots is that the photoexcited electron is trapped most probably on the electron traps in a-SiOx . Therefore, we suppose that the size-dependent shift of the absorption edge in Fig. 2 is caused by the hole confinement in CdSe nanocrystals. The confinement energy of the holes is described by the formula developed by Efros and Efros [2]: E01 ¼ s
_2 j201 2mh r¯ 2
,
(3)
where mh is the hole effective mass, j01 ¼ 3:14 is the first root of the Bessel function, l ¼ 0 and n ¼ 1 are the orbital and main quantum numbers, respectively. In formula (3) s describes the asymmetry of size dispersion which was shown to be almost symmetrical here [3], therefore s ¼ 1. The marked asymmetry to the large sizes was observed after annealing. At the same time, formula (3) is valid only for the deep localized states where the tunneling effects for holes are neglected. Taking into account the obtained earlier band-gap shifts DEg ¼ 0:19 and 0:25 eV we have estimated the average radii of nanocrystals. They were found to be 2.1 and 1.8 nm, respectively. Notice, that in both cases the diameters of nanocrystals are larger than the nominal thicknesses
439
of the deposited CdSe sublayers (3.03 and 1.61 nm) proving the formation of CdSe nanocrystals instead of the super-thin films on the rough surface of a-SiOx . Possible mechanism of such process has been discussed elsewhere [16]. Knowing the average radii of nanocrystals we have estimated their areal densities in CdSe sublayers: 6:0 1012 and 5:1 1012 cm2 in both nanocomposites, respectively. Here, the decreasing of the nominal thickness of CdSe sublayer of about 1.3 times due to oxidation effects has been taking into account (see next section). It is obvious that the areal densities are proportional to the nominal thicknesses of the deposited CdSe. The effective volume fill factor Z of CdSe sublayers was then estimated as the ratio of the total volume of nanocrystals to the volume of the hypothetical film with the thickness to be equal to 2¯r . Thus, we have found Z ¼ 0:55 at dCdSe ¼ 3:03 nm and Z ¼ 0:34 at dCdSe ¼ 1:61 nm. Notice, Z is also decreased with decreasing of dCdSe (see Table 1). In general, there are several reasons which make impossible the observation of the typical oscillating absorption edge for nanocrystals or quantum dots. They are (a) the large size dispersion of nanocrystals, (b) the non-spherical shape of nanocrystals, and (c) the interaction between nanocrystals. Unlike that of CdSe-doped glasses, where the distance between quantum dots is far more exceeding their sizes, the isolated and almost spherical nanocrystals are grown in CdSe=a-SiOx at the thickness ratio dSiO =dCdSe 20. At the thickness ratio dSiO =dCdSe 1 the a-SiOx sublayers behave as the high quality amorphous films so that the amorphous/nanocrystalline multiple quantum well are formed [3,16]. In our case, the thickness ratios were 3.63 and 6.83, so that the arrangements of quasi-isolated nanocrystals have been formed. We suppose that the size dispersion contributes to the widening of the absorption bands, but the main factor which leads to the smooth absorption edge is the type-II energy bands structure and the random fluctuations of the conduction band off-sets at the interfaces with the amorphous atomic-network.
4. Oxidation of CdSe nanocrystals The normalized spectra 5 and 6 in Fig. 2 show the real absorption in nanocrystal subsystems. Besides of the welldisplayed confinement shift of the edge, they also demonstrate the lower absorption in the region between 2.05 and 2:30 eV where the absorption coefficients became smaller more than 1.3 times as compared to the reference film. In absolute units this decrease is of about 1:5 104 cm1 which is remarkable value. This important peculiarity is clearly demonstrated by curve 7 which was obtained from curve 5 by subtraction the confinement shift. This decreasing of K evidences the losses of CdSe in asprepared nanocomposites as compared to the reference film. We suppose the losses are caused by the oxidation of nanocrystals. CdO component has been also detected by Raman and PL for CdS nanocrystals in SiO2 matrix [26] and the thermally oxide CdSe films [27]. However, to our knowledge it was not detected by the absorption spectroscopy yet. According to the model proposed in Ref. [27] CdO comes from the substitution of Se sites at the nanocrystal surface by the oxygen atoms. It gives rise to the new Raman band shifted from the 1 LO mode of the bulk by the ratio ðM Se =M O Þ1=2 ¼ 2:20, where MSe and M O are the atomic masses of Se and O, respectively. CdO is also evidenced by the sizedependent behavior of the band-gap emission [26]. In Ref. [28] cadmium oxide has been detected by X-ray diffraction. It is well known that the curvature and stresses at the rough surface of a-SiOx strongly affect the CdSe embryo formation and the reaction rates [25]. At the very beginning the embryos of CdSe are formed at the places with the greatest curvature and the
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bonds at the CdSe=a-SiOx interface are Cd–O bonds. Their formation or break-off changes essentially the charges of the facets and the electric field inside nanocrystal. Thus, we suppose the most effective formation of Cd–O bonds take place at Cd facets of nanocrystal. Se–O bonds are also possible but they are less expected. It was also supposed in Ref. [23] that the halkogen atoms could be dispersed elsewhere in SiOx matrix due to their relatively high mobility after the replacing by oxygen. Besides, we do not repel the possibility of oxidation of CdSe molecules at the deposition process even before the embryo appearing in each following CdSe sublayer.
200
binding energy, kkal/mol
100
0 Se-O
Cd-O -100
1.0
1.5 2.5 2.0 inter-atomic distance, A
3.0
Fig. 4. Bonding energies for Cd–O and Se–O bonds as a function of the inter-atomic distance. On the inset: the hexagonal Cd17 Se17 cluster (a ¼ 4:2 A and c ¼ 7:01 A) and SiO4 fragment.
lattice stresses. At this stage CdO component appears from the side of the underlying layer of a-SiOx . Interactions between neighboring Cd and O atoms generate Cd–O bonds at the heterointerface balancing the incomplete bonding structure of the nanocrystal surface and replacing the most outward Se sites by oxygen. The stress effects seem forced more deep penetrations of oxygen atoms into nanocrystals. On the next stage nanocrystals are grown. Finally, CdO is also formed when the a-SiOx layer cover the nanocrystal. The growth mechanism of CdSe nanocrystals on a rough a-SiOx surface should be investigated elsewhere. Here, we present only calculations of the chemical bond energies for Cd–O and Se–O which can potentially arise at the CdSe=SiOx heterointerface. These calculations evidence the preferable formation of CdO bonds. In general, Cd–O, Se–O, Cd–Si, and Se–Si chemical bonds can be generated at the CdSe=a-SiOx interface at the growth process. However, among the binary compounds having these bonds only cadmium silicide is not observed naturally. In our model we consider Cd17 Se17 hexagonal cluster which is associated with SiO4 fragment (inset in Fig. 4). The anion facet of the cluster is charged negatively and the cation facet is charged positively. We simulate a charge condition at the anion and the cation facets calculating the energies of the Cd–O and Se–O chemical bonds. Ab initio calculations were made within the framework of the selfconsistent Hartree–Fock scheme by means of the semi-empirical PM3 method in the approximation of NDDO (neglect of diatomic differential overlapping). The result calculations have shown that the greatest energy corresponds to Cd–O (123.5 kcal/mol) bonds at the equilibrium inter-atomic distance of about 1.9 A˚ (Fig. 4). Somewhat smaller energy is obtained for Se–O bonds (109.9 kcal/mol) with the equilibrium distance of about 1.6 A˚ between the atoms. Moreover, the charges induced at the anion and cation facets are changed when the oxygen atom belonging to SiO4 fragment approach the cluster: the positive charge is increased at the cation facet and the negative charge is decreased at the anion facet. These results show that the most expected
5. Conclusion The band-edge absorption of CdSe=a-SiOx nanocomposites fabricated by the sequential vapor deposition of CdSe and a-SiOx on quartz substrates clearly demonstrates the increasing of the optical band-gap of CdSe nanocrystals due to the quantum confinement effect. Besides, the decreasing of the absorption in the region of the direct band-to-band optical transitions as compared to the reference spectra testifies on the nanocrystal oxidation. Ab initio calculations of the energies of the most probable chemical bonds at the CdSe=a-SiOx heterointerface show the preferable formation of Cd–O bonds. The average radius of nanocrystals was estimated from the increasing of the optical band-gap. It is appeared to be larger than the nominal thickness of CdSe sublayers that evidences the growth of nanocrystals.
Acknowledgment This work was supported by the Board of Directors of the Lashkarev Institute of Semiconductor Physics of the National Academy of Science of Ukraine. References [1] A.I. Ekimov, A.A. Onushchenko, J. Experiment. Theoret. Phys. Lett. JETP Lett. 34 (1982) 346. [2] Al.L. Efros, A.L. Efros, Semiconductors 16 (1982) 772. [3] D. Nesheva, J. Optoelectron, Adv. Mater. 3 (2001) 882. [4] C.B. Murray, D.J. Norris, A.L. Bawendi, J. Amer. Chem. Soc. 115 (1993) 8706. [5] A.L. Alivisatos, Science 271 (1996) 933. [6] Z.A. Peng, X. Peng, J. Amer. Chem. Soc. 123 (2001) 183. [7] F. Henneberger, J. Puls, Ch. Spiegelberg, A. Schulzgen, H. Rossman, V. Jungnickel, A.I. Ekimov, Semicond. Sci. Technol. 6 (1991) A41. [8] T. Torimoto, S.-Y. Murakami, M. Sakuraoka, K. Iwasaki, K.-I. Okazaki, T. Shibayama, B. Ohtani, J. Phys. Chem. B 110 (2006) 13314. [9] C. Woelfle, R. Claus, Nanotechnology 18 (2007) 025402. [10] E.H. Surgent, Adv. Mater. 17 (2005) 515. [11] N.R. Kulish, V.P. Kunets, M.P. Lisitsa, Phys. Solid State 39 (1997) 1667. [12] V.P. Kunets, Semicond. Phys. Quantum Electron. Optoelectron. 2 (1999) 23. [13] X. Peng, J. Wickham, A.P. Alivisatos, J. Amer. Chem. Soc. 120 (1998) 5343. [14] T. Vossmeyer, L. Katsikas, M. Gienig, I.G. Popovic, K. Diesner, A. Chemseddine, A. Eychmiiller, H. Weller, J. Phys. Chem. 98 (1994) 7665. [15] D. Nesheva, Z. Levi, Z. Aneva, V. Nikolova, H. Hofmeister, J. Phys. Condens. Matter 12 (2000) 751. [16] D. Nesheva, H. Hofmeister, Solid State Comm. 114 (2000) 511. [17] M.J. Lee, R.M. Langford, S.W. Wright, C.P. Judge, R.J. Chater, T.J. Tate, J. Electron. Mater. 29 (2000) 418. [18] I.Z. Indutnyi, A.I. Stetsun, in: Proceedings of the SPIE, vol. 2113, 1993, p. 55. [19] F. Urbach, Phys. Rev. 92 (1953) 1324. [20] V.P. Kunets, N.R. Kulish, Vas.P. Kunets, M.P. Lisitsa, Semicond. Phys. Quantum Electron. Optoelectron. 5 (2002) 9. [21] M. Cardona, J. Phys. Chem. Solids 24 (1963) 1543. [22] C. Baban, G.I. Rusu, P. Prepelita, J. Optoelectron. Adv. Mater. 7 (2005) 817. [23] D. Nesheva, D. Arsova, R. Ionov, J. Mater. Sci. 28 (1993) 2183. [24] D. Nesheva, C. Raptis, Z. Levi, Phys. Rev. 58 (1998) 7913. [25] A.V. Dvurechenskii, A.I. Yakimov, Semiconductors 35 (2001) 1095. [26] H. Wang, Y. Zu, P.P. Ong, J. Appl. Phys. 90 (2001) 964. [27] D.P. Masson, D.J. Lockwood, M.J. Graham, J. Appl. Phys. 82 (1997) 1632. [28] H. Wang, Y. Zhu, P.P. Ong, J. Cryst. Growth 220 (2000) 554.
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Physica E journal homepage: www.elsevier.com/locate/physe
Band-edge absorption spectroscopy of CdSe=a-SiOx multilayer nanocomposites P.E. Shepelyavyj, I.Z. Indutnyj, V.P. Bryksa 1, Vas.P. Kunets 2, V.P. Kunets Lashkarev Institute of Semiconductor Physics, NAS of Ukraine, 41 Prospect Nauki, 03028 Kiev, Ukraine
a r t i c l e in f o
a b s t r a c t
Article history: Received 27 May 2008 Received in revised form 6 September 2008 Accepted 8 September 2008 Available online 2 October 2008
CdSe=a-SiOx multilayer nanocomposites containing CdSe nanocrystals between a-SiOx thin layers have been produced by sequential thermal vacuum deposition of CdSe and SiOx on quartz substrates. Strong confinement effect has been observed in absorption spectra. Average size of nanocrystals was estimated from the absorption spectra discriminating between confinement, oxidation and artificial effects. Areal density of nanocrystals in CdSe sublayers was estimated to be 1012 cm2 . Oxidation of nanocrystals was observed as a decreasing of optical absorption in the region above the optical band-gap energy. The binding energies of Cd–O and Se–O bonds have been calculated as a function of the inter-atomic distances for the Cd17 Se17 cluster bounded to SiO4 fragment in the framework of self-consistent Hartree–Fock scheme by means of the semi-empirical PM3 method in approximation of NDDO (neglect of diatomic differential overlapping). The greater energy has been found for Cd–O bonds (123:5 kcal=mol) that evidences the preferred oxidation of Cd facets of CdSe nanocrystals. Crown Copyright & 2008 Published by Elsevier B.V. All rights reserved.
PACS: 78.20.Ci 78.55.Et 78.67.n 78.67.Bf 78.67.Pt Keywords: Multilayer nanocomposites Nanocrystals Band-edge absorption Oxidation
1. Introduction Semiconductor nanocrystals have attracted great interest over the past few decades because of their potential applications in the fast developing fields of material science, nanotechnology, opto- and nanoelectronics, chemical sensing, biology, and medicine. Size-dependant properties of nanocrystals come from the confinement effects and the high surface to volume ratio. In the first experimental [1] and theoretical [2] papers CdS(Se) and CuCl(Br) nanocrystals (quantum dots) embedded into borosilicate glasses have been investigated. Afterwards, a lot of structures were synthesized based on II–VI and IV–VI semiconductors in different matrices such as a-SiOx , polymers, organic solvents, and water [1,3–7]. Because of the unique optical properties, nanocrystals are attractive for applications in visible and near infra-red regions where matrices are transparent [8–10]. For the mono-dispersive nanocrystals in the so-called ‘‘strong confinement’’ regime (¯r oaB , here r¯ is the average radius of Corresponding author. Tel.: +1 916 335 0011.
E-mail address: [email protected] (V.P. Kunets). Present address: Department of Physics, Humboldt-Universita¨t zu Berlin, Newtonstrasse 15, 12489 Berlin, Germany. 2 Present address: Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA. 1
nanocrystals, and aB is the Bohr exciton radius in a bulk) one or more narrow absorption bands are usually observed in the region of the fundamental band edge. The energy spectrum of confined excitons can be determined from the absorption spectra with a high accuracy by the method proposed in Refs. [11,12]. The absorption edge is decomposed for separate bands, taking into account the size dispersion, and r¯ is used as a fitting parameter. This method was successfully applied to CdSx Se1x doped glasses. The narrow absorption bands have been observed for CdSe quantum dots grown by the wet chemical route in the coordinating solvents [4,13] where the size dispersion is only 5%. The long-wavelength band corresponding to 1Se ! 1Sh optical transitions determines the confinement optical band-gap shift (DEg ) with regard to the bulk gap. In the ‘‘weak confinement’’ regime, (¯r XaB Þ the absorption edge is smooth, this is typical for bulk. Therefore, special analytical procedure was proposed in Ref. [11] to determine r¯ and DEg from such a smooth edge. For the CdSe quantum dots in a borosilicate glass the absorption edge became smooth as early as at r¯ 41:5aB [5]. The wide size dispersion also smoothes spectra and complicates analysis. Absorption spectra of the multilayer CdSe=a-SiOx nanocomposites are usually smooth even at nanocrystal radii less than aB (strong confinement) [3]. Such behavior is not clearly understood yet. Moreover, the typical absorption coefficients of these nanocomposites in the band-to-band spectral region are much smaller than in the
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bulk. While the absorption reaches 105 cm1 in individual nanocrystals even being increased in small ones due to oscillator strength increasing [14], it comes only to 10–100 cm1 in nanocomposites for nanocrystal concentration of about 1015 cm3 . For nanocomposites it becomes crucial because the thicknesses and total numbers of CdSe and SiOx sublayers can be varied in a wide range. This ‘‘dilution’’ effect significantly complicates the analysis of the band-edge absorption as well as its comparison with the bulk spectra. Besides, the optical interference effects should be also excluded. Therefore, the attempt has been made in Ref. [15] to restore the optical absorption using the photocurrent technique. Unfortunately, this technique is not applicable at low nanocrystal concentration. In this paper, we report on the absorption spectroscopy of CdSe nanocrystals grown in CdSe=a-SiOx (x 1:5) multilayer nanocomposites. We have determined the optical band-gap of nanocrystals and their average sizes from the analysis of the fundamental band-edge absorption discriminating between confinement, oxidation and artificial peculiarities of the spectra. The oxidation of nanocrystals is revealed as the changes in the spectra at high absorption level in comparison with the reference film. Theoretical modeling proves the preferred oxidation of the cadmium facets.
2. Experimental
437
a CdSe/SiOx SiOx
CdSe
b Etrap
0.15 eV
c-band 0.55 eV EF 1.75 eV 2.9 eV
Uh = 1.3 eV V-band
CdSe=a-SiOx multilayer nanocomposites have been prepared in the shape of rectangle platelets (0:5 1:5 cm2 ) by alternating thermal evaporation of CdSe and SiOx (x 1:5) in vacuum from two independent Ta sources on quartz substrates at room temperature. The nanocomposite have been formed on a central part of the substrate while both its components (CdSe and SiOx ) have been simultaneously deposited on the left and the right side of the sample (Fig. 1a) forming the reference films of CdSe and a-SiOx . A special screen system was developed for this purpose. The deposition conditions were: a vacuum around 2 103 Pa, a deposition rate of 0:01nm s1 for CdSe and 0:1 nm s1 for SiOx. CdSe has been deposited using the step-by-step method [16] while a-SiOx has been deposited by continuous evaporation. During the deposition the thicknesses of CdSe and SiOx sublayers (dCdSe and dSiOx ) and the deposition rates have been controlled by the preliminarily calibrated quartz monitors. After deposition the thicknesses of the structures and the reference films have been measured by the optical interferometer MII-4. The thickness of a-SiOx sublayers was always the same (dSiOx ¼ 11:0 nmÞ and sufficiently thick to prevent direct contact between nanocrystals in neighboring layers. For as-deposited structures this method gives the quasi-isolated CdSe nanocrystals with the near-spherical shape that is confirmed by TEM [16]. The quality of a-SiOx matrix has been controlled through the absorption of the reference film. Two groups of nanocomposites with the different nanocrystal radii were prepared. In the first group the nominal thickness of CdSe sublayers (dCdSe ) was 3.03 nm while the number N of CdSe=a-SiOx pairs was just 40 (Table 1). In the second group, dCdSe ¼ 1:61 and N ¼ 75. Thus, the total thickness of CdSe in both nanocomposites was the same, coinciding with the thickness of ref the reference film, dCdSe ¼ 121 nm. It was specially done to get the same optical densities (D ¼ lgðI0 =IÞ) in both group of nanocomposites. Besides, the thicknesses and numbers of absorptive layers of ref CdSe were chosen in the way to get KdCdSe 1, where K105 cm1 for direct optical transitions. In this case the measurement errors are minimal. This approach was specially used to exclude the role of the transparent a-SiOx matrix comparing the absorption spectra of nanocomposites and reference films of CdSe. Indeed, the total nanocomposite thickness is increased from 560 to 940 nm when the number of CdSe=a-SiOx pairs is risen from 40 to 75 (see
Fig. 1. (a) Scheme of the multilayer CdSe=a-SiOx nanocomposite. Both control layers of CdSe and a-SiOx are shown on the left and the right side, respectively. (b) Real space band-diagram of multilayer CdSe=a-SiOx nanocomposites [24]. Direct and indirect in real space optical transitions as well as electron trapping to the traps in a-SiOx matrix are shown.
Table 1), but the SiOx matrix does not add to the absorption in the region of the transparency. In this way, we could exclude the aforementioned ‘‘dilution’’ effect. Transmission (T), reflections (R and R0 ) from the sides of nanocomposite and quartz substrate, respectively, have been measured using KSVU (LOMO) spectrophotometer, equipped with a special optical attachment. Spectral resolution was better than 0.7 nm. All optical constants were calculated using the method developed earlier in Ref. [18]. The refractive indexes of a-SiOx films and quartz substrate are close. Therefore, the reflection from a-SiOx /quartz interface was only 3% and the interference effects could be neglected. The absorption coefficients have been calculated using the formula: 1 1R K ¼ ln 1 a þ aR0 þ aT , (1) d T where a ¼ ðn3 1Þ2 =ð4 n3 Þ and n3 is the refractive index of the quartz substrate (n3 ¼ 1:44).
3. Confinement effects The absorption spectrum of the reference film of CdSe looks like a typical one for the bulk crystal (Fig. 2, curve 1). In the longwavelength region (1.65–1:78 eVÞ the absorption coefficient K is risen from 1 104 to 4 104 cm1 exponentially. Such exponential tail obeys the Urbach rule [19] in bulk crystals, polycrystalline films and even CdSe quantum dots grown in a glass matrix [20] where the typical temperature dependence of the absorption edge is observed. The temperature dependence of the absorption edge of CdSe=a-SiOx nanocomposites should be investigated elsewhere. The small coefficients at the photon energies less than 1.65 eV ref
could not be measured because here KdCdSe o0:1. In the region
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Table 1 Parameters of CdSe=a-SiOx nanocomposites and reference films of CdSe and a-SiOx Parameter
Ref. film of CdSe I group
CdSe=a-SiOx (40 twin layers) II group
CdSe=a-SiOx (75 twin layers)
Ref. film of a-SiOx
D* (nm)
121 5 (I and II group)
560 5
940 5
440 5 (I group) 820 5 (II group)
1:747 0:005 0
3.03 11.0 1:934 0:005 0:19 0:01 2:1 0:1 6.0
1.61 11.0 1:996 0:005 0:25 0:01 1:8 0:1 5.1
o0:7 [17]
0.55
0.34
dCdSe (nm) dSiO (nm) Optical band-gap, Eg (eV) Band-gap shift, DEg (eV) Average radius, r¯ (nm) Areal density, N s , 1012 Fill factor, Z
2:9ðx 1:5Þ
*Measured by interferometry method.
3.0x1010
105
Eg1 = 1.747 eV Eg2 = 1.934 eV Eg3 = 1.996 eV
1
2.5x1010
x 4.63 1
104
7
(Khν)2, cm-2 eV
absorption, cm-1
2
x 7.77 6
5
2.0x1010 3 1.5x1010
1.0x1010
5.0x109
2 103 1.4
3 1.6
1.8
4 2.0 2.2 2.4 photon energy, eV
2.6
2.8
3.0
0.0 1.6
1.8
2.0 energy, eV
2.2
2.4
Fig. 2. Band-edge absorption of the reference film of CdSe (1) and of CdSe=a-SiOx nanocomposites, dCdSe ¼ 3:03 nm (2) and dCdSe ¼ 1:61 nm (3), respectively. Curves 5 and 6 are the normalized spectra 2 and 3 (see text). Curve 4 is the spectra of the reference film of a-SiOx . Curve 7 represents the spectra 5 after subtraction of the confinement shift.
Fig. 3. Semi-quadratic plotting of the absorption edge for the reference film of CdSe (1) and the normalized spectra of CdSe=a-SiOx at dCdSe ¼ 3:03 (2) dCdSe ¼ 1:61 nm (3), respectively.
between 1.78 and 2.15 eV the absorption obeys 12 power law (Fig. 3, curve 1) which is typical to the direct optical transitions at G-point of the Brilluene zone. The shoulder above 2:20 eV (where
which adds to the nanocomposite thickness but not to its absorption. The more is the number of a-SiOx sublayers in the structure, the more pronounced is the ‘‘dilution’’ effect (compare curves 2 and 3). At the same time, a-SiOx matrix gives rise to absorption in the short-wavelength region (2.2–2:7 eV). The blue shift of the absorption edge is caused by the confinement of the photo-excited electron–hole pairs in CdSe nanocrystals. Since the nominal thicknesses of CdSe sublayers (3.03 and 1.61 nm) in both nanocomposites are far smaller than the Bohr exciton radius in the bulk (aB ¼ 5:6 nm) the strong confinement regime takes place increasing the optical band-gap of nanocrystals. This shift is increased with decreasing of the thickness of CdSe sublayers (curves 2 and 3). To determine carefully the optical band-gap increasing the ‘‘dilution’’ effect should be excluded. As the total thickness of nanocomposite is used as a denominator in formula (Eq. (1)) the following normalization procedure has been applied. We have multiplied the absorption spectra 2 and 3 by the following 560 coefficients 121 ¼ 4:63 and 940 121 ¼ 7:77, respectively, which were obtained by division of the total nanocomposite thicknesses
K105 cm1 ) originates from the spin–orbit split of valence band. To determine the optical band-gap we approximated the edge by the standard dependence: K¼
A
_o
ð_o Eg Þ1=2 ,
(2)
where A is a constant. Corresponding semi-quadratic plotting (Fig. 3, curve 1) gives Eg ¼ 1:747 eV which correlates with the band-gap of bulk crystals 1:732 eV [21] and polycrystalline films 1.65–1:75 eV [22]. The absorption spectra of CdSe=a-SiOx nanocomposites show some differences (Fig. 2, curves 2 and 3). (a) Their absorption coefficients are much smaller all over the spectral region. (b) The noticeable blue shift of the edge is observed. (c) The absorption is risen in the region 2.2–2:7 eV. It is obvious that the smaller absorption coefficients come from the presence of a-SiOx matrix
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(560 and 940 nm) by the thickness of the reference film of CdSe (121 nm). In this way, we have obtained two normalized spectra (5) and (6) which exclusively demonstrates the real absorption in CdSe nanocrystal subsystem (Fig. 2). Both of them clearly show a noticeable blue shift of the absorption edge. As a preliminary estimation we have determined the shift at the absorption level of about 4 104 cm1 . It was found to be 0.23 and 0.28 eV for the structure with dCdSe ¼ 3:03 nm and dCdSe ¼ 1:61 nm, respectively, that is evidence for the increasing of the optical band-gap with decreasing of the nanocrystals sizes. To determine precisely the optical band-gap increasing (DEg ) we have applied the standard semi-quadratic plotting as it was proposed earlier in Ref. [11] for CdS0:13 Se0:87 quantum dots. It is important to emphasize that the normalized spectra have been used for this purpose. The corresponding plots give us the optical band-gaps 1.934 and 1.996 eV for dCdSe ¼ 3:03 nm and dCdSe ¼ 1:61 nm, respectively. The optical band-gap shifts were found to be DEg ¼ 0:19 and 0.25 eV (Table 1). They have been used for determination of the average sizes of nanocrystals and other relative parameters. The energy band diagram of CdSe=a-SiOx nanocomposites as well the kinetics of electron transitions are not studied thoroughly yet. The relative positions of the energy bands on both sides of a heterointerface are supposed to be the same as in CdSe=a-SiOx multilayers. In turn, the band diagram of multilayers with CdSe sublayer thicknesses between 2.5 and 10:0 nm and the ratio of a-SiOx to CdSe thicknesses of about 1.5 was built in Refs. [23,24] based on the photoconductivity and optical data. Unlike that of type-I CdSe quantum dots embedded into the borosilicate glasses which usually contain about 72% of SiO2 , the type-II band diagram is formed here with the potential well for holes (U h ¼ 1:3 eVÞ and the conduction band offset of about 0.15 eV (see Fig. 1b). Thus, only holes can be confined inside nanocrystals while electrons tunnel into a-SiOx . In Fig. 1b only one energy level for the holes localized in nanocrystal is shown in the quantum well of the valence band. The type-II band diagram is inherent to Ge/Si quantum dots [25] where the confined states for holes and electrons emerge on different sides of a heterointerface and, therefore, they are spatially separated. The electron transitions between these states are spatially indirect forming so-called spatially indirect excitons. The formation of such exciton is shown in Fig. 1b by inclined arrow. The localization of a hole in nanocrystal modifies the potentials in the neighboring space giving rise to the potential well for electrons around nanocrystal and leads to the bound states in this well. The difference from Ge/Si quantum dots is that the photoexcited electron is trapped most probably on the electron traps in a-SiOx . Therefore, we suppose that the size-dependent shift of the absorption edge in Fig. 2 is caused by the hole confinement in CdSe nanocrystals. The confinement energy of the holes is described by the formula developed by Efros and Efros [2]: E01 ¼ s
_2 j201 2mh r¯ 2
,
(3)
where mh is the hole effective mass, j01 ¼ 3:14 is the first root of the Bessel function, l ¼ 0 and n ¼ 1 are the orbital and main quantum numbers, respectively. In formula (3) s describes the asymmetry of size dispersion which was shown to be almost symmetrical here [3], therefore s ¼ 1. The marked asymmetry to the large sizes was observed after annealing. At the same time, formula (3) is valid only for the deep localized states where the tunneling effects for holes are neglected. Taking into account the obtained earlier band-gap shifts DEg ¼ 0:19 and 0:25 eV we have estimated the average radii of nanocrystals. They were found to be 2.1 and 1.8 nm, respectively. Notice, that in both cases the diameters of nanocrystals are larger than the nominal thicknesses
439
of the deposited CdSe sublayers (3.03 and 1.61 nm) proving the formation of CdSe nanocrystals instead of the super-thin films on the rough surface of a-SiOx . Possible mechanism of such process has been discussed elsewhere [16]. Knowing the average radii of nanocrystals we have estimated their areal densities in CdSe sublayers: 6:0 1012 and 5:1 1012 cm2 in both nanocomposites, respectively. Here, the decreasing of the nominal thickness of CdSe sublayer of about 1.3 times due to oxidation effects has been taking into account (see next section). It is obvious that the areal densities are proportional to the nominal thicknesses of the deposited CdSe. The effective volume fill factor Z of CdSe sublayers was then estimated as the ratio of the total volume of nanocrystals to the volume of the hypothetical film with the thickness to be equal to 2¯r . Thus, we have found Z ¼ 0:55 at dCdSe ¼ 3:03 nm and Z ¼ 0:34 at dCdSe ¼ 1:61 nm. Notice, Z is also decreased with decreasing of dCdSe (see Table 1). In general, there are several reasons which make impossible the observation of the typical oscillating absorption edge for nanocrystals or quantum dots. They are (a) the large size dispersion of nanocrystals, (b) the non-spherical shape of nanocrystals, and (c) the interaction between nanocrystals. Unlike that of CdSe-doped glasses, where the distance between quantum dots is far more exceeding their sizes, the isolated and almost spherical nanocrystals are grown in CdSe=a-SiOx at the thickness ratio dSiO =dCdSe 20. At the thickness ratio dSiO =dCdSe 1 the a-SiOx sublayers behave as the high quality amorphous films so that the amorphous/nanocrystalline multiple quantum well are formed [3,16]. In our case, the thickness ratios were 3.63 and 6.83, so that the arrangements of quasi-isolated nanocrystals have been formed. We suppose that the size dispersion contributes to the widening of the absorption bands, but the main factor which leads to the smooth absorption edge is the type-II energy bands structure and the random fluctuations of the conduction band off-sets at the interfaces with the amorphous atomic-network.
4. Oxidation of CdSe nanocrystals The normalized spectra 5 and 6 in Fig. 2 show the real absorption in nanocrystal subsystems. Besides of the welldisplayed confinement shift of the edge, they also demonstrate the lower absorption in the region between 2.05 and 2:30 eV where the absorption coefficients became smaller more than 1.3 times as compared to the reference film. In absolute units this decrease is of about 1:5 104 cm1 which is remarkable value. This important peculiarity is clearly demonstrated by curve 7 which was obtained from curve 5 by subtraction the confinement shift. This decreasing of K evidences the losses of CdSe in asprepared nanocomposites as compared to the reference film. We suppose the losses are caused by the oxidation of nanocrystals. CdO component has been also detected by Raman and PL for CdS nanocrystals in SiO2 matrix [26] and the thermally oxide CdSe films [27]. However, to our knowledge it was not detected by the absorption spectroscopy yet. According to the model proposed in Ref. [27] CdO comes from the substitution of Se sites at the nanocrystal surface by the oxygen atoms. It gives rise to the new Raman band shifted from the 1 LO mode of the bulk by the ratio ðM Se =M O Þ1=2 ¼ 2:20, where MSe and M O are the atomic masses of Se and O, respectively. CdO is also evidenced by the sizedependent behavior of the band-gap emission [26]. In Ref. [28] cadmium oxide has been detected by X-ray diffraction. It is well known that the curvature and stresses at the rough surface of a-SiOx strongly affect the CdSe embryo formation and the reaction rates [25]. At the very beginning the embryos of CdSe are formed at the places with the greatest curvature and the
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bonds at the CdSe=a-SiOx interface are Cd–O bonds. Their formation or break-off changes essentially the charges of the facets and the electric field inside nanocrystal. Thus, we suppose the most effective formation of Cd–O bonds take place at Cd facets of nanocrystal. Se–O bonds are also possible but they are less expected. It was also supposed in Ref. [23] that the halkogen atoms could be dispersed elsewhere in SiOx matrix due to their relatively high mobility after the replacing by oxygen. Besides, we do not repel the possibility of oxidation of CdSe molecules at the deposition process even before the embryo appearing in each following CdSe sublayer.
200
binding energy, kkal/mol
100
0 Se-O
Cd-O -100
1.0
1.5 2.5 2.0 inter-atomic distance, A
3.0
Fig. 4. Bonding energies for Cd–O and Se–O bonds as a function of the inter-atomic distance. On the inset: the hexagonal Cd17 Se17 cluster (a ¼ 4:2 A and c ¼ 7:01 A) and SiO4 fragment.
lattice stresses. At this stage CdO component appears from the side of the underlying layer of a-SiOx . Interactions between neighboring Cd and O atoms generate Cd–O bonds at the heterointerface balancing the incomplete bonding structure of the nanocrystal surface and replacing the most outward Se sites by oxygen. The stress effects seem forced more deep penetrations of oxygen atoms into nanocrystals. On the next stage nanocrystals are grown. Finally, CdO is also formed when the a-SiOx layer cover the nanocrystal. The growth mechanism of CdSe nanocrystals on a rough a-SiOx surface should be investigated elsewhere. Here, we present only calculations of the chemical bond energies for Cd–O and Se–O which can potentially arise at the CdSe=SiOx heterointerface. These calculations evidence the preferable formation of CdO bonds. In general, Cd–O, Se–O, Cd–Si, and Se–Si chemical bonds can be generated at the CdSe=a-SiOx interface at the growth process. However, among the binary compounds having these bonds only cadmium silicide is not observed naturally. In our model we consider Cd17 Se17 hexagonal cluster which is associated with SiO4 fragment (inset in Fig. 4). The anion facet of the cluster is charged negatively and the cation facet is charged positively. We simulate a charge condition at the anion and the cation facets calculating the energies of the Cd–O and Se–O chemical bonds. Ab initio calculations were made within the framework of the selfconsistent Hartree–Fock scheme by means of the semi-empirical PM3 method in the approximation of NDDO (neglect of diatomic differential overlapping). The result calculations have shown that the greatest energy corresponds to Cd–O (123.5 kcal/mol) bonds at the equilibrium inter-atomic distance of about 1.9 A˚ (Fig. 4). Somewhat smaller energy is obtained for Se–O bonds (109.9 kcal/mol) with the equilibrium distance of about 1.6 A˚ between the atoms. Moreover, the charges induced at the anion and cation facets are changed when the oxygen atom belonging to SiO4 fragment approach the cluster: the positive charge is increased at the cation facet and the negative charge is decreased at the anion facet. These results show that the most expected
5. Conclusion The band-edge absorption of CdSe=a-SiOx nanocomposites fabricated by the sequential vapor deposition of CdSe and a-SiOx on quartz substrates clearly demonstrates the increasing of the optical band-gap of CdSe nanocrystals due to the quantum confinement effect. Besides, the decreasing of the absorption in the region of the direct band-to-band optical transitions as compared to the reference spectra testifies on the nanocrystal oxidation. Ab initio calculations of the energies of the most probable chemical bonds at the CdSe=a-SiOx heterointerface show the preferable formation of Cd–O bonds. The average radius of nanocrystals was estimated from the increasing of the optical band-gap. It is appeared to be larger than the nominal thickness of CdSe sublayers that evidences the growth of nanocrystals.
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