DSpace at VNU: Emission characteristics of SPAN-80 activated ZnS nanocolloids

Journal of Luminescence 132 (2012) 2135–2142 Contents lists available at SciVerse ScienceDirect Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin Emission characteristics of SPAN-80 activated ZnS nanocolloids Thu Huong Ngo a, Hong Van Bui a, Van Ben Pham a, Thi Hong Tran a, An Bang Ngac a, Nam Nhat Hoang b,n a b Faculty of Physics, HUS, Vietnam National University, Hanoi, 334 Nguyen Trai, Thanh Xuan, Ha Noi, Viet Nam Faculty of Technical Physics and Nanotechnology, UET, Vietnam National University, Hanoi, 144 Xuan Thuy, Cau Giay, Ha Noi, Viet Nam a r t i c l e i n f o abstract Article history: Received August 2011 Received in revised form March 2012 Accepted 26 March 2012 Available online April 2012 Quantum surface effects (new emission bands, blueshifts, intensity enhancement) were observed in SPAN-80 activated ZnS nanocolloids and explained in terms of time-dependent density functional theory The experimental evidences were demonstrated for both undoped and Cu, Mn-doped colloidal phases The photoluminescence spectra of these materials showed a new green band at 520 nm (ZnS:Cu) and a yellow-orange band at 576 nm (ZnS:Mn) besides a blue band at 465 nm All bands lie in the visible region and are blueshifted, show sharp emissions with narrow widths and have approximately 20-times stronger intensities in comparison with those of the bulk samples The timeresolved luminescence spectra showed that the life-times of free electrons were 0.12 ms and 1.9 ms in ZnS:Cu and ZnS:Mn correspondingly & 2012 Elsevier B.V All rights reserved Keywords: Photoluminescence ZnS TD-DFT Nanocolloid Introduction For the importance of application in nanomedicine, the optical nanocolloids continuously attract the attention of scientists worldwide The quantum effects arising from the surface modification of nanoparticles by surface active agents (surfactants) play important roles in introducing new physics and observables of colloidal nanostructures Although ZnS as the known wide band gap semiconductor (Eg E3.7 eV) (which found a variety of application in color displays, diodes, cathode ray tubes, transparent windows etc [1]) has been extensively studied, there was almost no evidence for the colloidal ZnS directly activated by SPAN-80 (alias sorbitan monooleate) Here we report the observation and the DFT-based explanation of new emission bands, their blueshifts and intensity enhancement in the undoped and Cu, Mn-doped ZnS nanocolloids synthesized by using the traditional solid-state reaction method and activated by SPAN-80 as colloidal agent (SPAN-80 nanocolloids) For the first time we introduced SPAN-80 as a novel capping agent that can be used to stimulate about 20-times stronger photoemission with remarkable narrower full-width Usually, the enhancement of ZnS photoluminescent (PL) ability might be achieved by using a capping agent [2] and by doping suitable elements, particularly Cu and Mn [3,4] Recently, the focus was paid on the development of new colloidal systems which contain light-emitting particles in strong quantum confinement regimes [5,6] The water-based optical colloids promise n Corresponding author E-mail address: namnhat@gmail.com (N.N Hoang) 0022-2313/$ - see front matter & 2012 Elsevier B.V All rights reserved http://dx.doi.org/10.1016/j.jlumin.2012.03.052 yet rich application in nanomedicine [7] A wide usability of colloidal systems (e.g ZnS:Mn) may also be found in modern inkjet pigments [8] or in light-emitting diodes (LEDs) which showed a strong photoluminescence [9] In visible region, the PL spectra of pure ZnS consist of two wide bands at around 450 and 540 nm which correspond to Zn and S vacancies particularly When doped with Cu and Mn, these self-activated characteristic bands disappeared, or diminished, and the new luminescent bands appeared at around 520 and 580 nm These two bands are the typical imprints of Cu2 ỵ and Mn2 ỵ luminescent centers in ZnS matrix [1017] In general, the photoluminescence of different ZnS colloidal systems (doped or undoped) may be quite different because of number of effects which can influence the photoemission process, e.g the surface effects arising from the interaction between nanoparticles and colloidal agents The available literature pointed to two important aspects that might occur in colloidal systems (in contrast to the bulk samples): (i) the possible blue shift of Mn2 ỵ , redshift of Zn2 ỵ , S2 luminescence and blueshift of absorption edge (which associates with the widening of band gap) [14,16,18–22]; (ii) the enhancement of PL intensity [11–13,20,23–27] In some cases, the redshift of absorption edge was observed instead, e.g in Ref [10] the ZnS:Mn nanocolloids capped with polyphosphates of sodium showed the redshift of absorption edge from 250 to 285 nm (the emission lines were observed at 424 (Zn) and 592 nm (Mn)) In Ref [11] the polyacrylic acid coated nanocolloids showed the enhanced PL but the possible shifts were not clearly resolved In many cases, only one effect was observed (e.g Ref [13]: enhanced PL in trioctylphosphine oxide capped ZnS:Mn; Ref [21]: a large blueshift and extension of band gap up to 4.6 eV in ZnS:Mn capped with sodium hexametaphosphate) but there were also cases where 2136 T.H Ngo et al / Journal of Luminescence 132 (2012) 2135–2142 both PL shift and intensity enhancement were reported (Refs [13,20] and [16]: Zn and Mn emissions shifted to 438 and 578 nm with tunable intensities in mercaptoacetic acid activated ZnS:Mn) In addition, several colloidal systems may show the shift of spectral lines due to doping content, such as the 3-mercaptopropionic acid (MPA) capped ZnS:Cu nanocolloids demonstrated the redshift of blue band from 440 to 487 nm according to the increase of Cu content [18] The ZnS:Cu,Cl/ZnS core-shell nanocrystals [24] showed both enhancement of PL intensity and redshifts (roughly 0.2 eV) of the blue bands centered around 2.5 eV (496 nm) This work also showed that the equal redshift, for both undoped and Cu-doped samples, might be caused by the increased thickness of the shell monolayer Directly or indirectly, the known experimental studies pointed out the importance of surface modification on emission characteristics This modification may be achieved by various means which include UV irradiation (blueshift of Mn2 ỵ luminescence at 2.15 eV, i.e 577 nm, with PL efficiency enhanced by 35% due to surface passivation [20]) or g-irradiation (mechanoluminescence appeared with high intensity [23]) or core/shell modification (21 times enhanced PL in ZnCdS:Mn/ZnS core–shell [26]) or activation by surfactants (color tuning of emission bands in the water-based colloids ZnS:Cu [27] and ZnS:Mn [16] activated by mercaptoacetic acid) At microscopic level, the effect of surface modification on emission characteristics is still less understood as there were only limited number of theoretical studies available [28,29] For the surfactant-activated surfaces the computational difficulty arises from large number of surfactant molecules need to be included to simulate the optical change In this paper, we report the spectral characteristics of Cu and Mn-doped ZnS nanocrystallites emulsified by using the surfactant SPAN-80 and discuss the observed spectral changes in terms of Time-Dependent (TD) Density Functional Theory (DFT) We assert that the surface activation by SPAN-80 can lead to both blueshift and enhancement of PL intensity Mechanically, the blueshift and widening of band gap may associate with growing Coulomb repulsion between filled and unfilled states and the enhancement of emission with denser valence band density of states We show that both these effects happen when the surfactant molecules SPAN-80 are attached to the surface of ZnS nanoclusters The SPAN-80 and SPAN-20 (sorbitan laurate) have been used as the surfactants to produce the triethylamine (TEA) capped ZnS nanoparticles [2] where SPAN-20 related samples showed the higher luminescence property, but both were not involved as direct capping agents The bright transparent liquid appeared at the top of the containing cuvette and was extracted The 5% SPAN-80 (weight basis) acetone solution was dropped in slowly so that the total surfactant concentration reached about 1% The colloidal thin films were prepared by spin-coating on silica substrate The liquids themselves were used for optical characterization The PL spectra were recorded at 300 K using excitation wavelengths of 325 nm from He–Cd laser and 632.8 nm from He–Ne laser on the Microspec 2300i spectrometer The time-resolved PL spectra were recorded at 300 K using N2 excitation laser (wavelength 337 nm) Results and discussion Fig shows the XRD patterns (together with the SEM images in the inset) of the bulk ZnS:Mn (a), ZnS:Cu (b) and the TEM image of undoped colloidal ZnS sample ((c), inset) These samples possess hexagonal wurtzite structure with main diffraction planes (010), (002), (011), (012), (110), (013), and (112) The diffraction patterns did not change in whole doping range (for Mn, x¼0C1.2  10 À g/g and for Cu, x ¼0C3.5  10 À g/g) The average crystallite size as calculated by using the Scherrer formula gave a value of about 10 nm Note that the reported Bohr’s radius for ZnS is about 10 nm Therefore, the samples corresponded to the strong quantum confinement regimes The average particle size determined from the SEM images was about mm which corresponds to the size of polycrystalline pieces The average particle size obtained via TEM image for colloidal system was visibly smaller $ 35 nm The emulsification process discussed in the previous section appeared to have selected only smaller particles and pasted them into the colloidal matrix Using the particle size analyzer (Horiba, LA950) we have obtained the size distribution of colloidal systems before emulsification as shown in Fig 2(a) To our experience there was a continuous distribution of size from nanometer to micrometer in every particle system, including the commercial powders of micron size So a small portion of particles with nanometer size always existed and the problem was to how to efficiently collect them It appeared from Fig 2(a) that we have obtained a liquid containing only particles of selected size, mainly below 50 nm with peaking distribution at 35 nm and narrow distribution of size from 20 to 40 nm It should be noted that, the total luminescent power of a polycrystalline system is averaged from all particles therefore the distribution of size is one of the important factors to determine the widening of Experimental Unlike in other works, where the surfactants were directly involved in the synthesis (usually by wet chemical route), here we utilized the classical solid-state reaction method to prepare the doped and undoped ZnS polycrystallites, the surfactant was involved only in emulsifying process The ZnS, ZnS:Cu, and ZnS:Mn bulk samples were first prepared in argon from the starting ZnO, CuS powder (Merck), MnS powder and MnCl2, CuCl2 beads (Sigma-Aldrich) All chemicals were of purity grade greater than 99.9% The annealing temperature and time varied from 700 1C to 1250 1C and from some hours to 36 h The structures of the final bulk materials were investigated by X-ray diffraction (XRD) method on the D8-Advance Brucker diffractometer using ˚ 2y from 101 to 701) The scanning CuKa radiation (la ¼1.5056 A, electron microscopy (SEM) study of surfaces was carried out on JEOL5410-VL microscope The bulk samples were then ground again in methanol and the resulting powder was dried at 120 1C for h The powder was then dissolved in water using ultrasound and left span at 3500 rpm in a centrifuge for 30 Fig XRD patterns and SEM images of the bulk ZnS:Mn with x¼  10 À g/g (a), ZnS:Cu with x¼  10 À g/g (b) and TEM image of undoped colloidal ZnS (c) T.H Ngo et al / Journal of Luminescence 132 (2012) 2135–2142 2137 and acetone; (4) peak V results from a typical C ¼O stretching mode occurred in both SPAN-80 and acetone; (5) peak VI shows a typical O–H resonance usually assigned to the in-plane vibration of water The features below 1500 cm À correspond mainly to C–C, C–O vibration modes etc If H(2) atoms of SPAN-80 were involved in binding, then the CH2 stretching modes should vary, but indeed we did not observe any variation in the positions of peaks III and IV from that of pure SPAN-80 If O(1) atoms of SPAN80 were involved in binding (to Zn centers) then the corresponding C¼O stretching mode of SPAN-80 (peak V) should vary, which was also not observed Instead we recognized the small downshifts of both O–H (peak I) and C–O (peak IX) vibration modes Therefore we believed that SPAN-80 was attached to ZnS nanoparticle surface via its H(1) hydrophilic end The binding of a surfactant to the particle surface has certain stabilization effect on the total energy of the particle-surfactant system In Table we give the total energy gain DE dened as DE ẳ EZnS=SPAN80ị-ẵEZnS clusterị ỵESPAN80ị Fig Size distribution for the bulk and colloidal samples (a) and FTIR spectra of pure SPAN-80 and colloidal ZnS with their PL fingerprints in the inset (b) emission band Here by introducing a suitable surfactant SPAN-80 we were able to provide a selection of narrow size segment, hence we might expect the colloidal systems to show the narrower emission widths Before going into the details of optical characteristics, we briefly summarize the structure and properties of colloidal agent The SPAN-80 (Chemical Abstract Service (CAS) registry no 133843-8) is a food additive with formula C24H44O6 and molecular weight 428.61 g/mol Its PL fingerprint is shown in the inset of Fig 2(b) The obtained PL of the solvent was relatively weak in comparison with that of ZnS nanoparticles, and could be well subtracted as the background without complication SPAN 80, commonly used water-in-oil emulsifier, has two functional ends, one hydrophilic (H(1), see the structure given in Fig 2(b)) for polar and one hydrophobic (H(2), same figure) for non-polar solvent SPAN-80 can be attached to the surface of ZnS nanoparticles by either hydrogen H(1) or H(2) (to S center) or oxygen O(1) (to Zn center) The binding of SPAN-80 can be analyzed using FTIR data given in Fig 2(b) As seen, the difference between colloidal system and pure SPAN-80 may be addressed as follows (1) peak I being assigned to O–H vibration in water and SPAN-80 (O–H(1)) shows a small down-shift which argues for the weakening of O–H bonding due to binding of H(1) to nanoparticles; (2) peak II may be assigned to the C–H stretching modes of acetone present in the solvent; (3) peaks III, IV correspond to the asymmetric and symmetric modes of CH2 stretching vibration in both SPAN-80 ð1Þ where E is the ground state energy for optimized geometry of the system given in parenthesis DE was calculated for three cases of attachment H(1), H(2) and O(1) The calculation was based on DFT approach using gradient corrected functional of Perdew–Burke– Ernzerhof [30] (i.e GGA/PBE functional) and double numeric wave function basis set with polarized and diffuse functions added (DNP 3.5 set in DMol3 [31]) It appeared from the data given that in the agreement with FTIR analysis presented above the most suitable attachment point was hydrophilic H(1), therefore we computed the electronic structure on the basis of H(1) attachment In Fig we show the PL spectra of ZnS:Cu colloids where the comparison with that of the bulk is given in the inset As seen, at the doping concentration x ¼4  10 À g/g there appeared a new green band at around 520 (or 533) nm except a blue one at 465 (or 476) nm Both are characterized by the emission– recombination of electrons located in the conduction band down to the acceptor levels (465 or 476 nm) and levels of Cu2 ỵ trapping centers (520 or 533 nm) in the band gap of ZnS (Fig 3, inset) The intensity of the 520 nm band was much greater than that of the blue band As the Cu concentration increased, the intensity of the blue band decreased while the intensity of the green band increased and reached maximum at x¼3.5  10 À g/g This suggested that the Cu2 ỵ ions might be substituted into the Zn sites However, at the higher concentration of Cu, the intensity of the green band decreased (curve (h), Fig 3, inset) This behavior was probably caused by the re-absorption by the Cu2 ỵ ions themselves From Fig it is worth to note the differences between the emission of colloidal and bulk samples At first, all emission bands of colloids were blueshifted (11–13 nm) in comparison with that of the bulks and there was above 10-times stronger emission (at the same doping concentration) from the colloidal samples The emissions from colloidal samples had also narrower band width; the FWHM (full-width at half maximum) values were about half of that for the bulk samples (i.e 50 in contrast to 100 nm) Second, in the colloidal samples with Cu concentrations above Table Total energy gain DE (eV) of emulsified ZnS for various ZnxSx clusters as obtained from the stable optimal geometries of binary ZnS/SPAN-80 system Cluster Zn12S12 Zn24S24 Zn36S36 Zn48S48 DE/H(1) DE/H(2) Hydrophobic end DE/O(1) Hydrophilic end À 1.82 À 2.38 À 2.49 À 2.50 À 1.50 À 1.57 À 0.19 À 0.16 À 0.82 À 0.28 À 1.60 À 1.50 2138 T.H Ngo et al / Journal of Luminescence 132 (2012) 2135–2142 Fig The PL spectra of colloidal ZnS:Cu as excited by 325 nm He–Cd laser at 300 K The inset shows the spectra for the bulk samples (upper) and the energy diagram of possible transitions corresponding to the blue, green, yellow and red bands (lower) unchanged The yellow-orange band was characterized by the radiation–recombination of electrons in 3d5 shell of Mn2 ỵ ions: (4G)4T1-(6S)6A1 At the Mn2 ỵ concentration over  10 À g/g, the absorption caused the transfer of energy of excited state into the thermal energy and not to the emission energy [6,7] The spectra for the colloidal samples showed even narrower FWHM in comparison with that of the Cu-doped ZnS The FWHM for most cases holds within 30–40 nm and this was valid also for the undoped sample The growth of intensity caused by colloidal agent was about 20-times larger, that is on the relative scale, twice more intensive than for the Cu-doped case For all cases, doped and undoped, we have also observed the blue-shifts (DlmaxC6–11 nm) of PL maxima for the colloids The shifts were not depending on doping but on the use of surfactant The time-resolved PL spectra for the blue band of the Cu-doped bulk sample x¼8  10 À g/g (curve d in the inset of Fig 3) and for the yellow-orange band of the Mn-doped colloid x¼4  10 À g/g (curve c in Fig 4) are shown in Fig Due to the suppression of 476 nm emission in the Cu-doped colloids, the time-resolved PL spectra obtained with colloidal samples for this band were very weak As seen in Fig when the time-delay increased from 30 to 85 ns, the intensity of the 476 nm emission band decreased about 2.6 times but its position remained unchanged The lifetime of free electrons at conduction band, as deduced from the luminescent extinguishing curve of the 476 nm band, was about 0.12 ms Similarly for the Mn-doped cases, when the time-delay increased from 80 ms to 1000 ms, the intensity of the yellow-orange band gradually decreased but its position remained constant The fluorescent lifetime of this band was determined to be 1.9 ms at 300 K This result was similar to the ones obtained by other authors for the bulk ZnS:Mn [32–34] The prolonged decay time in ZnS:Mn was ascribed to the parity (and hence spin forbiddance) of 4T1-to-6A1 transition of Mn2 ỵ ions It is worthwhile noting that there was a dependence of lifetime on nanocrystallite size For comparison we have fabricated the nanocrystallites ZnS:Mn by co-precipitation technique, which produced the nanocrystallites of size approximately nm, and we have obtained the lifetime $0.8 ms The difference of lifetime between bulk and polycrystalline material may be ascribed to the surface defects As the energy of excited states is transferred easily to the defects at the surface than to the ones inside the bulk, the lifetime of emission is correspondingly shorter in the nanocrystallites than in the bulk materials It is clear from the Fig The PL spectra of colloidal ZnS:Mn as excited by 325 nm He–Cd laser at 300 K The inset shows the spectra for the bulk samples  10 À g/g, the intensity of the 520 nm peak totally dominated over the blue band and the PL spectra showed only one peak The mechanism for such behavior might be both the size effect and the lower inter-particle re-absorption in colloidal samples due to low particle concentration in polymeric matrix The colloidal samples visibly consisted of smaller particles of narrow size distribution and this homogeneity caused the enhancement of emission ability of these samples The decisive answer requires the theoretical evaluation yet Fig shows the PL spectra of the ZnS:Mn samples with the Mn concentration varying within x¼0–1.2  10 À g/g The inset also shows the spectra of the bulk samples for comparison Besides the green and blue bands, a yellow-orange band appeared at around 576 nm As the Mn concentration increased, the intensity of the green and blue bands diminished but the intensity and width of the yellow-orange band increased gradually and reached the maximum at x¼8  10 À g/g The position of this band remained Fig Time-resolved PL spectra of 576 nm band of a colloidal sample ZnS:Mn (x ¼4  10 À g/g) as excited by 337 nm laser at 300 K The inset shows the timeresolved PL spectra of 476 nm band of a bulk ZnS:Cu (x ¼  10 À g/g), also excited by 337 nm laser at 300 K T.H Ngo et al / Journal of Luminescence 132 (2012) 2135–2142 spectra given in Fig that the Mn2 ỵ activated emission bands have only millisecond component In the PL spectra of Mn-doped ZnS (Fig 4), we might also observe that the blue band (476 and 465 nm in the bulk and colloid respectively) was weak in the bulk samples and totally disappeared in the colloids To search for a possible description of observed phenomena at microscopic level, we studied the electronic structure of colloidal ZnS using time-dependent density functional theory The DFT was usually the first choice due to high speed and accuracy at moderate computational cost At first, the cell parameters (a, c) of the hexagonal wurtzite structure of ZnS were optimized using local density approximation (LDA) functional (the GGA/PBE functional was also used for comparison) As known (i.e for the oxides), the optimized cells offered by LDA were usually smaller and the cells obtained via the gradient corrected functionals were larger than the experimental cells It also happened in many cases that the LDA results were closer to the experimental data than the ones of improved GGA versions From the data listed in Table we may also reveal that the LDA optimized cell matched better with the data given in both Ref [35] and JCPDS no 36-1450 [36] than the ones of GGA/PBE and GGA/PW91 [29] Therefore based on this cell we fine-tuned the other settings such as basis set, wave function size (cut-off), smearing value, density mixing, mixing schemes, core-electron treatment model etc to achieve the consistent approximation of band gap [10,21,22,37–39] and optical data which includes reflectivity, transmittance [38,40], absorption [10,15,18,20,21,24,38,40,41] and core-level emission [20,41] Our result of band gap 2.9 eV by GGA/RPBE functional, although still smaller than the experimental gaps, was far better approximation than the ones given in Ref [29] (2.15 eV at best) The theoretical PL bands were then determined within the frame of TD-DFT [42,43] Table compares the obtained results with the experimental data As seen in Fig 6(a), the main emission bands related to the relaxation of excited states to Zn 3d and S 3p levels in the undoped ZnS were identified at 320, 400, 445, 540 (S), 590 (Zn), 877 nm The bands should be equally assigned to both Zn and S except the one at 540 and 590 nm which should solely belong to S and Zn emissions respectively The features around 445 and 540 nm were frequently observed in ZnS photoemission but both might also broaden towards 410 and 510 nm when the impurities, such as Cu (see the curve related to Cu 4s emission), were present For the ZnS:Cu emission characteristics, the main features could be identified at 348 (S 3p), 410 (Zn, S), 510 (Cu 3d), 540 (Zn, S), 877 (Zn, S), 958 (Cu 3d), 2195 nm (Cu 3d, S) and some other in ultraviolet and infrared region The emission relating to 4s electrons of Cu might be seen at 410 nm but it should be much weaker than the 510 nm emission originating from Cu 3d electrons The Cu specific emissions might also be seen at longer wavelength, e.g at 958 and 2195 nm These bands should be absent in the pure ZnS There was a clear suppression of the 445 nm band and of an ultraviolet band at 320 nm due to doping There was also no Cu specific emission at 320 nm; instead, the weak emission might be expected at 348 nm due to the relaxation to S 3p levels Another effect caused by Cu doping was the clear blueshift of the yellow band at 590 nm (in the undoped ZnS) to the green region at 510–540 nm The partial emissions of atomic centers for the Mn-doped ZnS are shown in Fig 6(b) The Mn specific bands in visible region might be recognized at 410 (Mn), 450, 495–506, 590, 746 (Mn) and 1450 nm (Mn) In the ultraviolet region the bands were identified at 330 (Mn) and 370 nm (Mn) The 370, 410 and 746 nm bands were characteristic for the relaxation to Mn 3d levels whereas the band at 450 nm resulted from the transitions to Mn 4s levels The small emission related to Zn centers could also be seen at 360 nm but the 370 and 746 nm emissions were associated only with Mn centers The 450 nm band which were also seen in the PL of undoped and Cu-doped ZnS should be originated from Zn and S centers too Besides a strong emission usually observed at 590 nm, the features at around 500 nm might also be expected for ZnS:Mn In comparison with the PL of the undoped ZnS, there was a clear suppression of features at 400, and 877 nm and new emission bands at 360, 495 and 1450 nm were observed A redshift might be expected for 590 nm emission Table This work 3.88, 6.37 (GGA/PBE) ˚ Cell constants a, c (A) 3.77, 6.20 (LDA/num basis set) 3.79, 6.34 (LDA/plane wave) Band gap (eV) 2.6 (GGA/PBE) 2.9 (GGA/RPBE) 2.8 (LDAỵ U) PL (nm) a Other works 3.7943(2), 6.2679(5) [35]; 3.820, 6.257 [36]; 3.85, 6.29 [29] (GGA/PW91) 3.56–3.79 [37] (and references therein); 3.51, 3.84 [38]; 3.88 [22]a; 3.70 [39]; 4.36–4.77 [10]b; 4.10–4.69 [21]c GGA/RPBE (periodic structure): Zn: 421 [46]; 425 [22]a; Zn, S: 400, 445; S: 540 (ZnS) 424 [10]b; 427 [47]d Zn, S: 410, 540; Cu: 510 (ZnS:Cu) S: 438 [16]g; 440 [48]; Zn, S, Mn: 450, 500, 590; Mn: 410 (ZnS:Mn) 450 [41,46]; 440–487 [18]e; GGA/RPBE (colloidal clusters): 460 [47]d; 470 [12]h Zn, S: 425, 500 (ZnS) Cu: 520 [49]; 510 [46]; Zn, S: 425; Cu: 522 (ZnS:Cu) 525 [12]g Zn, S: 472; Mn: 575 (ZnS:Mn) Mn: 577 [20]i; 578 [16]g; 560 [45] Experimental data (bulk): 580 [13]f; 580 [14]; 476, 533 (Cu); 476, 582 (Mn) 590 [15,17]; 592 [10]b Experimental data (colloid): 465, 520 (Cu); 465, 576 (Mn) ZnS/polyvinyl alcohol (PVA) ZnS:Mn passivated by sodium hexametaphosphate (SHMP) c ZnS:Mn with core-shell structure/SHMP d ZnS:Sn/thiourea e ZnS:Cu,Cl/3-mercaptopropionic acid (MPA) f ZnS:Mn/trioctylphosphine oxide (TOPO) g ZnS:Mn/mercaptoacetic acid h ZnS:Cu capped with TOPO and SHMP i ZnS:Mn passivated by UV-irradiation b 2139 2140 T.H Ngo et al / Journal of Luminescence 132 (2012) 2135–2142 Fig Theoretical PL bands of the Cu-doped (a) and Mn-doped (b) ZnS as obtained on the basis of a periodic structure model given in the upper-left part of the figure The blueshifts of spectral lines caused by COSMO effect (c) (the inset shows the relative shifts (%) for individual emission bands) and the theoretical emissions as obtained on the basis of TD-DFT simulation using Zn9S9 model cluster activated by SPAN-80 molecules (d); the inset shows the blueshift of valence band DOS due to surfactant attachment (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article) which should broaden towards 628 nm (Zn center emission) The most apparent difference from the Cu doped case was probably the persistence of a band at 450 nm and the disappearance of the one at 540 nm The emission associated with S centers at this wavelength totally diminished and the one associated with Zn centers seemed to be blueshifted to around 500 nm To clarify the effect of surfactant binding, we first estimated the influence of solvent on emission characteristics of ZnS nanoparticles using the conductor-like screening model (COSMO) [44] Within COSMO, each solvent is represented by a dielectric constant e and its influence on solute follows from the polarization effect that solvent as continuous medium expresses on the solute The effect is similar to putting a solute within a cavity of Coulomb force field The deviation of screening effect estimated by COSMO from the exact solvation for strong polar solvents (such as water) is smaller than 1% and for the non-polar solvents (such as SPAN-80 with e E2) is less than 10% Several solvents with different dielectric constants ranging from (vacuum) to 80 (water) were tested on a small cluster Zn9S9 consisted of three hexagon layers Zn3S3 Overall, a large blueshift of emission maxima according to the increase of dielectric constant e of the solvent (Fig 6(c)) was observed The relative shifts varied also upon emission bands, i.e it reached maximum around 16% for the ultraviolet band (320 nm), 21% for the violet band (410 nm) and 32% for the green band (570 nm) For SPAN-80 (e E2 as of common oils) the relative blueshifts might be expected at 7–13% (or 20–80 nm, i.e on the energy scale roughly 0.25 eV) However, the error of the above estimation for oils was known to be as large as the estimated shifts The effect of solvent in creation of screening charges on the surface of solute nanoparticles may be more accurately evaluated at low level theory using DFT In general, we could expect that the binding of the hydrophilic end H(1) of SPAN-80 to S centers of ZnS nanoparticles would deplete electrons from H(1) centers and raise the S 3p orbital occupation level This should introduce a stronger Coulomb repulsion between occupied states (hybridization between Zn 4s and S 3p) and unoccupied states, which in turn would push the occupied states a bit lower below Fermi level Therefore, we would consequently observe a blueshift of the total DOS This scenario might be verified using the large clusters of size up to Zn36S36 (plus SPAN-80 molecules, that is 442 atoms in total) but smaller clusters were adequate to express the relative shifts The DOS-s were usually much easier to compute than the PL which required TD-DFT calculation on the geometry optimized by evaluating the excited states using single excitation (CI-Singles) From the results obtained (inset, Fig 6(d)), we T.H Ngo et al / Journal of Luminescence 132 (2012) 2135–2142 deduced that the blueshift of DOS near the energy segment of observed emissions (2.2–2.4 eV) due to the surfactant attachment was about 0.13 eV These values are half of that provided by COSMO As seen, the shift depended on energy; a smaller (negative) energy induced larger shifts Therefore, the binding of surfactant affected mostly the valence band The analysis of molecular orbitals (MO) showed that the valence MOs extended also below the surface layers and were located not only at the surface The analysis of charge population showed a small increase of negative charge (0.11e À ) upon S centers and of positive charge upon H(1) atoms of surfactant molecules The charge polarization between solute and surfactant reproduced the COSMO effect but the shifts obtained were visibly smaller It is important to mention the difference between mechanisms of charge polarization in DFT and COSMO In COSMO, there was a vacuum layer between solute and surfactant, but in DFT, there was a bonding exchange between solute and surfactant While DFT estimates the shifts directly from the modification of valence band MOs, COSMO obtains the final shifts by investigating the reaction of solute putting inside a cavity of additional Coulomb force field Another important result from the evaluation of DOS was that, the larger the clusters were, the denser and larger DOS-s appeared The smaller clusters usually showed a comb-like structure DOS due to a limited number of available states Therefore, the narrowing of PL band-widths in the spectra of colloidal samples due to the higher homogeneity in nanoparticle size could be understood Finally, Fig 6(d) shows the theoretical PL of undoped and doped ZnS nanocolloids as modeled on the basis of Zn9S9 cluster activated by SPAN-80 molecules using the TD-DFT approach The main features (in visible region) were identified at 412, 571 nm (undoped bulk); 425, 500 nm (undoped colloid); 425, 522 nm (Cu-doped colloid); and 472, 575 nm (Mn-doped colloid) Recall that the experimental data were recorded at 476 (undoped bulk), 465 (undoped colloid), 520 (colloidal ZnS:Cu), and 576 nm (colloidal ZnS:Mn) As seen, the agreement was excellent for the doped colloids: the displacements Dl between experimental and calculated data were less than nm Unfortunately, the 465 nm band was not resolved for the undoped case when Zn9S9 was used as a model cluster A clear blueshift of spectral lines due to surfactant attachment was also observed but they were all below 10 nm, therefore were not as large as predicted by COSMO The individual excitations that contributed more than 95% of total oscillation strength (intensity) of each PL band were as follows: (1) 575 nm emission: the excitations from the levels just below the Highest Occupied Molecular Orbital (HOMO), i.e HOMO-1 and HOMO-2 to the Lowest Unoccupied Molecular Orbital (LUMO); (2) 522 nm emission: HOMO-2 to LUMO þ1 and HOMO-3 to LUMO; (3) 472 nm emission: HOMO-4 and HOMO-5 to LUMO Conclusion By doping Cu2 ỵ (3d9) and Mn2 ỵ (3d5) into the bulk ZnS and subsequently activating by using the SPAN-80 as surfactant we have successfully prepared the colloidal materials which emitted light at different wavelengths, blueshifted and with stronger intensities in comparison with those of the original bulk samples The calculation using the time-dependent density functional theory showed that the green emission at 520 nm and the yellow-orange emission centered at 576 nm can be characterized by the transitions of electrons in 3d shell of Cu2 ỵ and Mn2 ỵ cations (4T16A1) particularly The blue band at 465 nm was a result of transition of delocalized (bonding) electron between Zn 2141 and S All observed emissions were about 6–13 nm blueshifted due to surfactant attachment The explanation of these shifts using the conductor-like screening model gave, however, the values larger than 20 nm The results obtained from TD-DFT calculation was half of this value and agreed in excellence with experimental data The DFT demonstrated two quantum effects when the surfactant molecule SPAN-80 was attached to the nanocluster of doped or undoped ZnS: (1) an increase in the number of allowed quantum states of the system induced denser valence DOS which in turn raised the overall oscillation strengths, thus the intensity of allowed optical transitions, (2) an increase in the valence band occupation induced an increase in Coulomb repulsion between occupied and unoccupied states which forced the blueshift of optical transitions in a scale compatible with the energy shift Acknowledgment One of the authors (NTH) would like to thank the financial support from the National Foundation for Science and Technology Development of Vietnam (NAFOSTED), project code 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emission with denser valence band density of states We show that both these effects happen when the surfactant molecules SPAN-80 are attached to the surface of ZnS. .. PL of undoped and doped ZnS nanocolloids as modeled on the basis of Zn9S9 cluster activated by SPAN-80 molecules using the TD-DFT approach The main features (in visible region) were identified at