The size dependence of optical properties in colloidal ZnxCd1-xS:Mn quantum dots in gelatin

Journal Pre-proof The size dependence of optical properties in colloidal ZnxCd1-xS:Mn quantum dots in gelatin D.V. Volykhin, V.G. Klyuev PII:

S1386-9477(19)30748-9

DOI:

https://doi.org/10.1016/j.physe.2019.113709

Reference:

PHYSE 113709

To appear in:

Physica E: Low-dimensional Systems and Nanostructures

Received Date: 18 May 2019 Revised Date:

28 August 2019

Accepted Date: 6 September 2019

Please cite this article as: D.V. Volykhin, V.G. Klyuev, The size dependence of optical properties in colloidal ZnxCd1-xS:Mn quantum dots in gelatin, Physica E: Low-dimensional Systems and Nanostructures (2019), doi: https://doi.org/10.1016/j.physe.2019.113709. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

1 The size dependence of optical properties in colloidal ZnxCd1-xS:Mn quantum dots in gelatin

D. V. Volykhin*, V.G. Klyuev Department of Optics and Spectroscopy, Voronezh State University, Voronezh 394018, Russia *corresponding author, e-mail: [email protected]

Abstract Size-selected colloidal ZnxCd1-xS quantum dots were successfully prepared by aqueous synthesis in gelatin. Quantum dots with an average size of a few nanometers were obtained by changing the synthesis conditions such as the amount of precursors and temperature. Variation of the composition and size of particles leads to a change in the optical, in particular, luminescent characteristics of quantum dots. Estimation of particle sizes were performed by several methods based on X-Ray diffraction and optical data. The mechanisms of trap state photoluminescence in ZnxCd1-xS quantum dots are proposed. Models of increasing / decreasing the photoluminescence intensity depending on the composition and particle size are presented. Keywords: quantum dots, luminescence, zinc sulfide, cadmium sulfide, quantum size effect, aqueous synthesis 1 Introduction Investigation of physical properties of nanosized semiconductor particles is a very interesting and important task nowadays. A large number of applications in many fields of modern science and technology makes these nanosized objects one of the most perspective materials for investigation. Today, much attention is riveted to quantum dots (QDs) [1]. Interesting optical characteristics of QDs such as emission color and its quantum yield tuning due to quantum size, concentration and surface coating effects [1-9], relatively high emission quantum yield [10,11], nonlinear optical properties [12-14], photostability [15] etc. made them good candidates for application in modern devices. Since the 80s of the 20th century, active research has been carried out on these nanoscale objects. Methods for synthesis nanoparticles in solid matrices [2], as well as in colloidal solutions [3] have been developed. A large number of theoretical and experimental papers on the quantum size effect have been published, for example refs [1-8]. Today, there is an intensive study of optical properties of semiconductor nanosized objects (QDs, core/shell structures, nanorods, nanobelts etc.). Not a small role is played by the choice of material for such objects.

2 There is a large selection of suitable materials: CdS [8,9], ZnS [16,17], CdSe [15,18,19], PbS [20, 21], TiO2 [22], Ag2S [23] etc. Moreover, mixed semiconductor materials like Ag-In-S systems [24], CdSxSe1-x [25], ZnSxSe1-x [26], ZnxCd1-xSe [27] etc. can provide new unique physical properties. Today, one of the most promising materials for optical applications is mixed zinc-cadmium sulfide ZnCdS. Due to the similarity of crystal lattices of ZnS and CdS (syngony, lattice constant) it becomes possible to obtain mixed ZnCdS solid solution. Along with zinc and cadmium selenides and tellurides, mixed zinc and cadmium sulfides have a wide range of luminescence: from almost the infrared region to medium ultraviolet radiation. Variation of particle sizes and composition of ZnxCd1-xS allows one to achieve tuning of both the exciton luminescence and intrinsic defects emission. This fact has long been known both for bulk crystals [28,29] and for nano-sized crystallites [30-32]. Moreover, doping QDs with different impurities (Mn, Ni, Cu, Eu etc.) often leads to a significant change of their optical properties [33-40]. QDs size variation creates additional opportunities to control the physical properties of these nanoscale objects. Choice of dielectric matrix for nanocrystals growth is very important for its potential application. Gelatin is a good material for this, because of its low cost, low toxicity and improved photostability [41-45]. Such advantages make QDs in gelatin well applicable in medicine and biology (bio-imaging). Moreover, QDs in gelatin matrix can be used in LED devices [46]. At the same time, the presence of electron and hole traps makes it possible to use investigated QDs as donors or acceptors of electronic excitation for external sources, such as organic dyes (hybrid association) [13,14]. A large number of works on the study of QDs shows that the quantum size effect in optical properties manifests itself differently for particles synthesized in different matrixes or stabilizers. Most researchers only consider the size effect in sulfides, selenides tellurides of zinc or cadmium. Thus, there is an extremely small number of works that consider the quantum confinement effect in mixed zinc and cadmium sulfides. Cizeron and Pileni performed the last known detailed study of such effects out back in the 90s [47]. They considered the method of mixed QDs synthesis in reverse micelles. In their works, optical characteristics of ZnxCd1-xS QDs were considered in sufficient detail. However, quite controversial models of luminescence appearance have been proposed; the weak size dependence of ZnS QDs emission is not explained. At the same time, more data on ZnxCd1-xS QDs synthesized by various methods is needed to build common size dependencies. Moreover, the size dependences of Mn-doped ZnxCd1-xS QDs are currently not widely considered.

3 Thus, the main objective of this work is to establish the size dependences of ZnxCd1-xS:Mn QDs optical characteristics in a gelatin matrix. Thus, our work is devoted to the study of size dependencies of ZnxCd1-xS:Mn QDs optical properties.

2 Materials and methods 2.1 QDs preparation We have used photographic inert gelatin as the polymer matrix to synthesize different sizes of pure and Mn-doped ZnxCd1-xS QDs. Detailed description of ZnxCd1-xS QDs preparation by low-temperature aqueous synthesis in gelatin showed in refs. [9,32] QDs sizes were varied by changing synthesis conditions such as the amount of precursors and temperature [48-51] QDs size variation is reached by changing synthesis conditions, such as temperature (40° C, 60° C and 85° C) and the amount of precursors (0.0013 M – 0.0154 M). Manganese source (MnCl2•4H2O) in aqueous solution was added in the amount necessary to establish 10 mol. % impurity concentration in QDs solution. Such percentage choice is due to the highest manganese emission intensity at this concentration [49]. Thus, three series of QDs samples were synthesized with different putative particle sizes: small (S), medium (M) and big (B). Gelatin matrix was partially removed by injection of the pancreas bacteria in QDs solution at 40° C for XRD measurements [49-51].

2.2 Methods of investigation and equipment Crystal structure of ZnxCd1-xS QDs was investigated by XRD measurements. XRD patterns from QDs were obtained with DRON-3 diffractometer (Burevestnik, USSR/Russia) operated at 50 kV and 60 mA using MoKα radiation wavelength λ = 0.71075 Å. Investigations of QDs optical properties were performed by UV-Vis absorption and PL measurements. UV-Vis absorption spectra of investigated QDs were collected using a USB2000 spectrophotometer with USB-DT combined tungsten-deuterium halogen light source (Ocean Optics, USA). MDR-23 (LOMO, Russia) diffraction monochromator and R928P photomultiplier with a C4900-51 power supply (Hamamatsu, Japan) was used for PL measurements. PL excitation sources were mercury lamp (LOMO, Russia) with release of the mercury emission band of λex1 = 313 nm by UFS-2 and IGS-3 light filters for ZnS QDs and HPL-H77GV1BT-V1 diode module (HPL, Taiwan) with λex2 = 380 nm for other kinds of QDs.

3 Results and discussion

4 3.1 Structural characteristics of ZnxCd1-xS:Mn QDs. Morphological properties of ZnxCd1-xS QDs prepared with different synthesis conditions are characterized by XRD data. XRD patterns of investigated QDs are shown in fig. 1. Broadened diffraction reflexes are observed for all samples, which indicate the formation of nanocrystals. At the same time, a reflexes width decrease is observed with an increase in the synthesis temperature and the amount of precursors of ZnxCd1-xS QDs. Hence, we can conclude that QDs sizes are changed by varying the synthesis conditions.

Fig. 1. XRD patterns of ZnxCd1-xS QDs (bottom-up “S”, “M” and “B” sample series, respectively) and bulk crystals (vertical solid lines). As is shown in fig. 1 all ZnxCd1-xS QDs have a set of three broad and well distinguishable diffraction peaks at different angles. XRD reflexes broadening makes it difficult to unambiguously identify the crystal structure. However, it has been suggested in refs. [51-53] that CdS particles smaller than 5 nm are formed with a cubic lattice structure. Thus, we assume that positions of XRD peaks corresponds to (111), (200), (220) and (311) lattice planes of cubic crystal lattice (zinc blende) with
43 space group in all cases [32,49]. Positions of bulk ZnS and CdS XRD peaks (solid vertical lines in fig. 1) are calculated with the use of Crystallography Open Database (COD) [54], data from ref. [55] and VESTA software [56]. It should be noted that for small Zn0.5Cd0.5S and CdS QDs samples there is one large peak instead of two (220) and (311) peaks. Broad peaks in some QDs samples that are up to 2θ = 10 o probably belong to some gelatin residues (triple helices) [57,58]. Gradual shift of XRD peaks with increase Zn and decrease Cd content indicate formation of mixed ZnCdS crystal lattice [32]. For Mn-doped and undoped QDs samples, there are no visible changes in XRD patterns [49].

5 It is possible to roughly estimate QDs sizes using well-known Scherrer formula, which relates XRD peaks width to the particle size [59]: =





(1)

where D – particle diameter, λ = 0.71075 Å – X-ray emission wavelength (Mo), B – diffraction peak width at the half maximum (in radians), θ – Bragg’s angle, K = 0.94 – particle shape factor (for spherical crystallites). XRD peak with Miller indices (111) was used for the calculations with Scherrer formula. Particle sizes estimated by Scherrer formula are given in Table 1.

3.2 Optical absorption properties of pure and Mn-doped ZnxCd1-xS QDs. Optical absorption spectra of ZnxCd1-xS QDs with different particle sizes and composition are shown in fig. 2. There is a characteristic feature (shoulder) in optical absorption spectra of ZnxCd1-xS QDs in all cases. This feature is attributed to the transition between the hole level 1S3/2 and electron level 1Se of QDs [60]. All investigated QDs are characterized by the presence of broad absorption band. This is likely due to some particle size distribution in QDs ensemble.

6

Fig. 2. UV-Vis absorption (dashed lines) and normalized PL spectra (solid lines) of ZnxCd1-xS QDs with different sizes (“S”, “M” and “B’ series) and composition: (a) x = 1, (b) 0.7, (c) 0.5, (d) 0.3, (e) 0.

Position of characteristic shoulder in absorption spectra of ZnxCd1-xS QDs changes when composition and particle sizes are varied. This is clearly due to quantum size and concentration effects. The shift of the absorption spectra was conveniently estimated by evaluating the effective band gap (energy of 1S3/2→1Se transition) of ZnxCd1-xS QDs. An effective band gap of QDs (Egeff) was estimated from the position of optical absorption spectra second derivative minimum [49-51] Values of Egeff for investigated QDs are higher than the band gap for of ZnxCd1-xS bulk crystals in all cases (fig. 2). This is a well-known phenomenon called quantum

7 size effect. Absorption band of ZnxCd1-xS QDs shifts to longer wavelengths with increase of synthesis temperature and the amount of precursors in all cases (fig. 2). This behavior indicates increase of average particle size in ensemble. This fact, as well as XRD data indicate the effectiveness of our approach to the synthesis to produce ZnxCd1-xS QDs with different particle sizes. Effective band gap dependencies on size and composition of ZnxCd1-xS QDs are presented in fig. 3a.

Fig. 3. Effective band gap (a) and photon energy in PL intensity maximum (b) vs. composition dependencies of ZnxCd1-xS QDs with fitted curves. There is an obvious nonlinear dependence of effective band gap values of ZnxCd1-xS QDs vs. their composition. Such nonlinear dependence was observed for bulk [28,29] and nanosized ZnxCd1-xS [32,61]. Effective band gap vs. x parameter dependence of ZnxCd1-xS QDs can be performed as the following equation [62, 63]:    = 1 −  +  − 1 − 

(2)

where ECdS and EZnS – effective band gap of CdS and ZnS QDs respectively,  = 2 +

  − 4/! – bowing parameter, E1/2 – effective band gap for Zn0.5Cd0.5S QDs. The calculations give the following bowing parameters: 1.06, 0.98 and 1.34 eV for small, middle and big synthesis series respectively. Curves plotted according to the equation (2) are shown in fig. 3. The evaluation of particle size of pure CdS and ZnS QDs by the optical absorption spectra was carried out by a well-known Kayanuma formula (modified Brus formula) [64]: 



&'





−  "#$% = ()' *+∗ + +∗ / − ,

.

,1( ' 2)

∗ − 0.248)6 ,

(3)

8 where Egeff – effective band gap of QD, Egbulk – single (bulk) crystal band gap, R – QD radius, me* and mh* - electron and hole effective masses respectively, h – Planck’s constant, e – electron ∗ charge, ε – material permittivity, )6 =

!7 '  8 :

:

2' &' 9 ∗ < ∗ = ;, ;.

- Rydberg’s effective energy.

Recently, D. L. Ferreira and coworkers in ref. [65] proposed a more accurate formula for estimating particle sizes, the so-called DLF equation. The formula is presented below: 



−  "#$% =

&'

(+,∗ )

! + ' ?@

&'

∗ )' (+.

where finite confining parameter @E = F height K =

LM;,NHO; BLMPOQR !

∗ ?@& ! + ∆B& C, @ , @& , D − 0.248)6

(7 ' G+H∗ ) ' &



J '

 B'

(4)

B

and ?@E  = *1 + I + I I B/ , barrier H

H

H

, third term of eq. (3) ∆B& C, @ , @& , D is related to screened

Coulomb interaction between the electron and the hole, more details see in ref. [65].

We used the following numerical data to particle sizes estimation using equations (3) and (4): for ZnS QDs Egbulk = 3.66 eV (cubic structure) [66], me* = 0.34m0, mh* = 1.76m0 [67], ε = 5.7 [68]; for CdS QDs Egbulk = 2.36 eV (cubic structure) [69], me* = 0.205m0, mh* = 1.6m0 [60], ε = 5.5 [70] (where m0 is free electron mass), Egmedium = 6.9 eV for QDs obtained by aqueous synthesis [65, 71]. The approximate values of mixed ZnxCd1-xS QDs electron or hole effective masses are calculated using the following expression [72]: ∗ ∗ ∗  =  + 1 − 

(5)

Thus, estimated values of Zn0.5Cd0.5S effective masses are me* = 0.243m0 and mh* = 1.68m0, bulk bang gap is 2.83 eV [28]. Values of Egeff and particle sizes of ZnxCd1-xS QDs are presented in figs. 2, 3 and Table 1. Note that the calculations by formula (3) is sufficiently rough. This is because the effective mass approximation gives an overestimated result for small particles (strong confinement regime) [4,5,32]. Particle sizes values estimated by the DLF equation (4) are on average almost 20% less than the results calculated by the Kayanuma formula (3). According to the conclusions of the authors of ref. [65], the result obtained by the formula (4) is approximate to the real, in contrast to formula (3). Indeed, QDs particle sizes estimated using formula (4) are closer to results obtained from TEM images [32]. Particle sizes, determined by the Scherrer formula, turned out to be the lowest of all presented estimations, due to the fact that lattice defects could seriously broaden the XRD peak.

9 The absorption data also contains information on the particle size distribution in the ensemble. According to Pesika and coworkers in ref. [73] particle size distribution n(R) derived from absorption data can be represented as: V

SC ∝ −

 U

G )

(6)

where A – absorption, K = XC W – spherical quantum dot volume. This model assumes W

that all the particles have a spherical shape, and the absorption coefficient is independent of particle size. We used DLF formula (4) to estimate the particle size distribution with (6) expression. Size distribution of ZnS QDs from our previous work [49] estimated by Eq. (4) and Eq. (5) with comparison it with transmission electron microscopy (TEM) data is shown in fig. 4.

Fig. 4. Size distribution of ZnS QDs from ref. [49] determined with absorbance (filled curve) and TEM data (bars).

The most probable particle size (diameter) of ZnS QDs (fig. 4) determined by Pesika method is 2.4 nm and by TEM data is 2.2 nm. At the same time, there is a correspondence of distributions in the region of large particles (particles with a size of more than 2.4 nm). However, in the size distribution estimated from the absorption spectra, particles with a size of less than ~ 2.2 nm are not observed. Thus, in our case, Pesika method does not provide reliable data on the size distribution of small particles in an ensemble. This is due to broad excitonic peak and its overlap with other peaks in absorbance spectrum [73]. However, it is suitable for estimation of most probable radius and size distribution with the exception of the smallest particles in ensemble. So, estimated by Pesika method size distributions of QDs in present work are shown in fig. 5.

10

Table 1. Particle sizes of ZnxCd1-xS QDs estimated by different methods. Egeff (sec. derivative), eV ZnS S 4.44±0.02 M 4.32±0.02 B 4.23±0.02 Zn0.5Cd0.5S S 3.55±0.02 M 3.40±0.02 B 3.22±0.02 CdS S 3.19±0.02 M 2.97±0.02 B 2.88±0.02 Sample

QDs size, nm DLF formula XRD (Scherrer Kayanuma formula [64] [65] formula) [59] 1.7±0.3 2.8 2.2 1.9±0.3 3.1 2.5 2.2±0.3 3.3 2.8 1.7±0.3 3.1 2.6 1.9±0.3 3.5 2.9 2.2±0.3 4.3 3.7 1.6±0.3 3.2 2.6 1.8±0.3 3.7 3.1 2.1±0.3 4.0 3.4

Pesika method [73] 2.4 2.6 2.8 2.4 3.2 3.6

Fig. 5. Particle radius distributions of ZnS and CdS QDs derived from absorption spectra for small, middle and big synthesis series. The most probable particle sizes in ensemble are shown in Table 1. These sizes differ from those obtained directly by the DLF formula (second derivative). This is probably due to a slightly different evaluation method, despite using formula (4) in both cases. Thus, Pesika technique can be used to determine the particle size distribution as an alternative to TEM data. However, there is a failure of this method for the smallest particles in an ensemble.

3.3 Steady-state photoluminescence of pure and Mn-doped ZnxCd1-xS QDs. All pure ZnxCd1-xS QDs have one broad emission band in PL spectra located in almost all optical spectral range (from 440 nm to 650 nm) (fig. 2). A large width and a Stokes shift (see table 2) of this bands indicate trap state nature of QDs PL. The width of such luminescence bands are determined by broad emission centers distribution, some particle size distribution in

11 ensemble and complex origin of PL. Presence of crystal lattice defects (interstitial atoms, vacancies etc.) in investigated QDs leads to trap state PL occurrence and absence of band edge (excitonic) emission, which is characterized by a narrow emission band and a small Stokes shift. Such PL behavior is a result of fast capture of charge carriers by above mentioned lattice defects [74]. Thus, ZnxCd1-xS QDs in gelatin are characterized by the predominance of trap state emission and absence of excitonic luminescence. The redshift of QDs trap state PL with Cd content increase (x parameter decrease) indicate formation of mixed ZnxCd1-xS (fig. 3 b). For visual clarity, the dependences PL maximum vs. composition in the fig. 3b are fitted using the ]

exponential growth function: Y = YZ + [\ ^ (these curves have no physical meaning). The formation of a mixed ZnS-CdS crystal lattice leads to a change in effective band gap, as well as trap states locations. Moreover, a change in the mutual concentration of Zn and Cd can lead to the appearance, increase or decrease of certain crystal lattice defects number, which are centers of radiative recombination.

Table 2. Variation of ZnxCd1-xS QDs PL bands the Stokes shift with particle size and composition. Zn concentration in QDs, mol. % 0 30 50 70 100

Stockes shift of PL band, eV S 0.96 1.00 1.21 1.16 1.63

Pure QDs M 0.90 1.04 1.21 1.31 1.53

B 0.92 1.12 1.08 1.27 1.47

S 1.28 1.51 1.65 2.41

Mn-doped QDs M B 1.17 1.04 1.33 1.14 1.58 1.42 2.31 2.19

As is known, CdS QDs PL occurs predominantly by the donor-acceptor mechanism [9,52,75,76]. It was alleged that the PL centers are a shallow electron trap and a deep hole trap. Authors in refs. [9,77] suggest model of CdS QDs PL where emission occurs due to the recombination on interstitial cadmium atom (donor) and sulfur vacancy (acceptor). Previously in ref. [32] we proposed a model of ZnxCd1-xS QDs PL in gelatin where emission occurs on interstitial cadmium atom (donor) and cadmium vacancy (acceptor). Such donor-acceptor pairs are formed due to the replacement of the sulfur atom in crystal lattice by oxygen during synthesis. Thus, several groups of researchers proved that the trap state PL in CdS QDs occurs precisely by a donor-acceptor mechanism. Previously blue PL of nanosized and bulk ZnS (fig. 2) was related to recombination of electron from sulfur vacancy and hole from «valence band» (1S3/2 level), surface states or zinc

12 vacancy [78-83]. Moreover, PL of ZnS QDs may occur due to recombination between zinc vacancy and interstitial zinc atom. Such pair of defects can occur due to oxygen implantation in ZnS crystal lattice during synthesis [32]. Thus, there are several competing channels of PL occurrence in ZnS. Despite the fact that the energy of the exciton transition in ZnS QDs varies in the range from 4.23 eV (big) to 4.44 eV (small), the energy of the luminescence band maximum lies in the range from 2.76 eV to 2.83 eV. A weak PL band size dependence of ZnS QDs previously observed for nanoparticles synthesized in reverse micelles [61]. However, explanation for this behavior not provided. Such size dependence of ZnS QDs trap state PL is probably due to its dominant emission mechanism. In this case, the radiative transition may occur by the recombination of a localized electron with a free hole. Such transition can occur due to recombination of electron from sulfur vacancy and hole from «valence band» (Lambe-Klick model) [79-81,83]. A simple estimation within the framework of the effective masses approximation indicates that an increase/decrease in the value of the effective band gap for ZnS QDs with decreasing/increasing diameters is mainly due to the size effect for electron states in the “conduction band”. Indeed, a simple calculation using effective mass approximation [64] shows that with particle size variation, the shift of «conduction band» energy levels is more than 6 times greater than the shift of «valence band» levels. Indeed, heavy hole effective mass in ZnS (1.76m0) is about six times larger than the effective mass of an electron (0.28m0). It is likely that PL center is a deep trap, so particle size change has little effect on the energy position of this level [75]. Consequently, the position of PL band maximum associated with this emission center varies slightly with size. Schematic illustration of PL mechanism described above is shown in fig. 6.

13 Fig. 6. Schematic illustration of possible PL mechanism in ZnS QDs with different diameters (synthesis series: «S», «M», «B»).

Particle size increase leads to the red shift of ZnxCd1-xS QDs PL in all cases (fig. 3). It’s a clearly due to a well-known quantum size effect [75]. Moreover, there is a change in PL spectra shape after QDs size increase. There are several reasons for such PL spectra shape changes. Probably, PL spectra of ZnxCd1-xS QDs consist of few emission bands with different origin. Thus, it may be due to a variety of radiative recombination sources with different emission centers distribution [51]. Another reason may be a possible variation of particle size distribution for QDs samples with different average sizes in ensemble. A rise in the magnitude of PL band Stokes shift with zinc concentration increase was observed in almost all cases (table 2). Such behavior may indicate different influence of concentration effect on «valence and conduction» states and defect states. In the case of the quantum size effect investigations, the situation is ambiguous. In some cases, the Stokes shift remains the same, in others, changes in its magnitude are observed. This can be explained by the possible different distribution of luminescence centers in the QDs, as well as by the different influence of the size effect on these centers. The behavior of the Stokes shift magnitude of Mn band in Mn-doped samples turned out to be predictable: an increase with zinc concentration increase and a decrease with particle size increase. This is due to the intracenter character of Mn luminescence. Indeed, the position of the manganese band remains almost unchanged: 2.01 – 2.08 eV. Relative PL intensity of pure ZnS QDs grows with particle size increase (fig. 7 a). In the case of pure CdS QDs samples, on the contrary, the opposite is observed: the relative PL intensity decreases with increasing particle size. Such PL intensity growth of ZnS QDs with particle size increase can be explained by the following. If emission intensity of ZnS QDs is determined by a number of luminescent centers (point defects) that are located near particle surface, then with surface area increase, the number of such centers also increases, which is manifested in PL intensity growth. Effect of PL intensity growth with particle size decrease in nanosized CdS was observed earlier in refs. [51,75]. To explain this fact, the assumption of the donor – acceptor mechanism of PL in CdS QDs is used. The decline in PL intensity with particle size increase can be explained by the decrease of recombination probability of charge carriers trapped on centers located at ever greater distances from each other [84]. PL intensity of Zn0.5Cd0.5S «S», «M» and «B» QDs samples is higher than that of CdS QDs samples (fig. 7 b). This is probably due to the mixing luminescent centers of ZnS and CdS QDs, which leads to the appearance of new possible radiative recombination pathways (for

14 example, new donor-acceptor pairs). It is likely that a decrease of non-radiative transitions also occurs. It is possible that when zinc is introduced into CdS QD crystal lattice, some non-radiative recombination channels turn out to be blocked. At the same time, a decrease in the PL intensity with an increase in particle size is observed, which is similar to the situation with CdS QDs. The luminescence decay of Zn0.5Cd0.5S QDs in gelatin for longer wavelengths is becoming slower [49], which is a confirmation of the donor-acceptor mechanism [9]. Thus, to explain the increase of luminescence intensity with decreasing particle size, it is possible to use the assumption previously applied for CdS QDs: decrease the distance between PL centers with particle size decrease.

Fig. 7. PL intensity histograms for pure and Mn-doped ZnS (a) and CdS, Zn0.5Cd0.5S QDs (b) with different sizes (synthesis series).

Mn-doped QDs, except CdS, are characterized by the presence of a new wide emission band. This band is attributed to a well-known internal Mn transition 4T1 (4G) → 6A1 (6S) in d shell due energy transfer from QD [33,49,85-87]. The appearance of such a band in PL spectra indicates the successful introduction of manganese ions into ZnxCd1-xS crystal lattice. In the case of CdS QDs, Mn related band is clearly not observed, probably due to its total overlap with trap state PL band. For other QDs Mn band is predominant (except ZnS:Mn QDs “small” sample), that indicates energy transfer from QDs to manganese centers. Such energy transfer can occur directly from an exciton or indirectly from QDs trap states [49,88]. Trap state PL quenching in Mn-doped samples with increasing particle size indicates a possible increase of excitation transfer efficiency to manganese centers. Manganese PL band position maximum in ZnxCd1-xS QDs varies in range from 596 nm (2.08 eV) to 617 nm (2.00 eV). Such a fluctuation is probably caused by the different degree of bands overlap of the trap state PL and manganese emission. Maximum PL intensity of Mn-doped mixed ZnxCd1-xS QDs in almost all cases is larger in

15 comparison with pure samples (fig. 7 a, b). As the particle size increases, trap state PL quenching is observed in comparison with impurity emission (fig. 2). These facts indicate effective excitation transfer from QDs excitons or trap states to Mn centers. Thus, doping ZnxCd1-xS QDs with manganese makes it possible to achieve an intense orange emission band (~600 nm) in the PL spectra. At the same time, varying the concentration of the components and the synthesis temperature makes it possible to control the optical characteristics of Mn-doped ZnxCd1-xS QDs.

4 Conclusion Optical investigations of colloidal pure and Mn-doped ZnxCd1-xS QDs in photographic gelatin are presented in this work. Possibility of optical properties (absorption, PL band position, shape and intensity) tuning by changing the size and composition of mixed ZnxCd1-xS QDs are showed. The variation of these characteristics was proved using XRD and optical absorption investigations. All QDs have characteristic absorption bands with an exciton peak in the range 2.88 - 4.44 eV. The presence of wide emission bands associated with the recombination of charge carriers on lattice defects is characteristic for all pure QDs. The size and concentration dependences of QDs optical characteristics are consistent with the data obtained in other studies for QDs (particle size in the range 1.5 – 4.0 nm) synthesized by other methods. The possibility of a relatively accurate particle size estimation with use of optical data is shown. The possible mechanisms of trap state PL in ZnxCd1-xS QDs are shown. Such can be the donor – acceptor mechanism, as well as the recombination of a hole from the «valence band» with an electron localized at the trap level (Lambe – Klick model). The assumption of a donor – acceptor mechanism was based on QDs PL decay slowdown in the long wavelength region. Lambe-Klick mechanism is proposed based on a weak size dependence of ZnS QDs PL. The observed behavior of QDs PL intensity was associated with the proposed emission mechanism. An increase of ZnS QDs PL intensity with crystal sizes increase may be due to new luminescence centers appearance. In the case of cadmium-containing compounds, a decrease in the PL intensity for larger QDs is associated with an increase in the distance between donors and acceptors. It is shown that the incorporation of Mn impurity into ZnxCd1-xS QDs leads to the appearance of a new PL band. This well-known manganese emission, in our case, has a sufficiently high intensity, probably due to the effective excitation transfer from ZnxCd1-xS QDs (excitons or defects states) to the corresponding Mn emission centers. Changes in the position of Mn band and its intensity are associated with varying degrees of overlap with the intrinsic PL band, the difference in the crystalline field, the number of manganese centers, and with a change in the

16 efficiency of excitation transfer to Mn centers. All these conditions, obviously, can be different for different sizes and compositions of ZnxCd1-xS:Mn QDs.

5 Acknowledgements We are grateful to Ilicheva N. A. for assistance in paper preparation. Measurements on DRON-3 diffractometer performed in the Center of Collective Use of Voronezh State University.

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Highlights: 1. Optical properties of ZnCdS:Mn quantum dots depend on the synthesis conditions 2. Size estimation of nanocrystals using optical data is possible by several methods 3. Luminescence in ZnCdS:Mn quantum dots can occur through several mechanisms