Photoluminescence study of CdSe nanorods embedded in a PVA matrix

Journal of Luminescence 135 (2013) 327–334

Contents lists available at SciVerse ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Photoluminescence study of CdSe nanorods embedded in a PVA matrix Mamta Sharma, S.K. Tripathi n Centre of Advanced Study in Physics, Department of Physics, Panjab University, Chandigarh 160014, India

a r t i c l e i n f o

abstract

Article history: Received 30 June 2012 Received in revised form 1 September 2012 Accepted 13 September 2012 Available online 4 October 2012

Nanometer-sized semiconductor CdSe nanorods have been successfully grown within polyvinyl alcohol (PVA) matrix by in situ technique. PVA:n-CdSe nanorods are characterized by X-ray diffraction, transmission electron microscopy, UV–vis spectrophotometer and photoluminescence spectroscopy. The photoluminescence spectra of PVA:n-CdSe nanorods are studied at different excitation wavelengths. PVA:n-CdSe nanorods have demonstrated to exhibit strong and well-defined green photoluminescence emission. The long-term stability of the PL properties of PVA:n-CdSe nanorods is also investigated in view of possible applications of polymer nanocomposites. The linear optical constants such as the extinction coefficient (k), real (e1) and imaginary (e2) dielectric constant, optical conductivity (sopt) are calculated for PVA:n-CdSe nanorods. The optical properties i.e. good photostability and larger stokes shift suggesting to apply PVA:n-CdSe nanorods in bioimaging applications. & 2012 Elsevier B.V. All rights reserved.

Keywords: A. Nanostructures B. Chemical synthesis C. X-ray diffraction D. Optical properties

1. Introduction Semiconductor nanoparticles show unique size dependent optical properties due to the quantum confinement effect and are of great interest for applications in optoelectronics, photovoltaics, lasing, and bio-labeling [1,2]. The optical properties of these nanomaterials are strongly affected because of the quantum confinement of electrical carriers within nanoparticles and the large local electric field enhancement, and significantly enhanced surface effects. So, by controlling the dimensions and the chemistry of their surfaces, optical properties of the nanomaterials can be significantly tailored. Cadmium selenide (CdSe) nanoparticles are considered to be one of the model systems for investigating the unique optical and electronic properties of quantum confined semiconductors [3,4]. CdSe have attracted considerable attention arising from their unique optical properties, including extended optical absorption in the ultra-violet region, bright photoluminescence (PL), narrow emission band, size tunable PL and photostability [5–8], and their potential promising PL materials for optical electronic device applications [9–10]. CdSe is a wide bandgap semiconductor (Eg  1.74 eV) materials with corresponding bulk absorption edge around 710 nm. CdSe of appropriate size can have an absorption edge and emission peak anywhere in the visible spectrum due to the quantum confinement effect. The emission properties of semiconductor nanomaterials have different sources like band edge luminescence, defect-related

n

Corresponding author: Tel.: þ 91 172 253446; fax: þ 91 172 2783336. E-mail addresses: [email protected], [email protected] (S.K. Tripathi).

0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.09.016

luminescence and luminescence due to doping. The nanomaterials obtained from organometallic precursors and high-temperature synthesis, defect-free nanoparticles are often achieved to realize band edge luminescence with high efficiency. On the other hand, the room temperature or low-temperature synthesis routes give rise to photoluminescence arising from defect levels in the band gap. These defects are usually related to the surface states. In spite of the use of surfactants, these invariably exist in the nanoparticles. It is possible to produce the dopant levels in the band gap and the photoluminescence occur through these levels. Under certain circumstances, photoluminescence due to surfactants or matrix material can be strong enough to dominate the overall luminescence from the particles. Colloidal nanocrystals or nanoparticles tend to aggregate in solution due to their large surface energy. To stabilize nanomaterials, various stabilizers (surfactants, polymers or coupling agents) have been employed to modify the surface functionalities for obtaining stable nanocrystals [11–13]. In the present study, polyvinyl alcohol (PVA) is chosen as stabilizing materials in the composite fabrication due to its polar and hydrophilic properties, good thermo-stability, chemical resistance, easy processability and transparency. In this paper, PVA:n-CdSe nanorods are fabricated by a dropcasting method. Transmission electron microscopy (TEM) shows uniform size and highly crystallized quantum dots. The optical absorption and fluorescent emission properties of the polymer nanocomposites were investigated by UV–vis spectrophotometer and fluorescence spectrophotometer, respectively. The authors have studied the structural characterization [14] of PVA:n-CdSe films at room temperature in their previous report. In present work, we are reporting the optical properties of PVA:n-CdSe at 60 1C.

328

M. Sharma, S.K. Tripathi / Journal of Luminescence 135 (2013) 327–334

2. Material and methods The chemical bath technique is used to prepare PVA:n-CdSe nanorods via a single-step reaction. Polyvinyl alcohol (PVA) was chosen as the polymer matrix for its aqueous solubility. The high viscosity of the polymer solution would be helpful in controlling the growth of selenide nanocrystals. Furthermore, from the application point of view, the polymer matrix would protect the selenide particles against photooxidation. PVA:n-CdSe nanorods are prepared by in situ generation of CdSe nanorods in PVA matrix. 2.1. Preparation of selenium source Sodium selenosulfate (Na2SeSO3) has been chosen as the selenium source. Sodium selenosulphate aqueous solution (0.50 M) is prepared by adding 1.0 M of sodium sulfite in 50 ml of distilled water, by adding 0.05 mol of selenium powder. The solution has been stirred for 7 h at 80 1C. The solution is kept overnight. Upon filtration, sodium selenosulfate solution is sealed. The beaker containing sodium selenosulfate solution is covered with an Al foil and stored in an oven at warmer conditions i.e. 60 1C to prevent decomposition for its unstability at room temperature. 2.2. Synthesis of PVA:n-CdSe nanorods PVA solution is obtained by adding 6.0 g PVA to 100 ml deionized water and stirring at 60 1C until a viscous transparent solution is achieved. 0.1 M of metal sources i.e. cadmium acetate has been dissolved in 20 ml of deionized water. Ammonia (2.0 M) is used to turn metal ions into complex ions and to reduce the free metal ion concentration. In 50 ml flask, 20 ml PVA solution has been taken. Ammonia solution is then slowly added drop wise until a clear PVA solution is obtained. The final pH value of PVA solution is adjusted with dilute acetic acid to about 10. Sixteen milliliter cadmium salt solution is added to PVA solution with constant stirring and then 2 ml of selenosulphate solution is introduced in order to achieve the desired Cd:Se ratio. The Cd:Se ratio is changed during the addition of Cd and Se solution to PVA matrix to check the effect of selenium ion concentration on structural and optical properties of PVA:n-CdSe. The mixture is

stirred for 6–7 h at 60 1C temperature to obtain a solution. The solution is casted on a glass substrate. Upon solvent evaporation, PVA:n-CdSe film has been obtained. The film is washed with deionized water to remove other soluble salts. The reaction mechanism is shown in Fig. 1.

2.3. Characterizations XRD study has been done using a Phillips PW-1710 X-ray diffractometer using CuKa radiation in the 2y range from 101 to 601. Transmission electron microscopy (TEM) has been done using Hitachi H7500 electron microscope, operating at 110 kV. The sample is deposited on a carbon coated copper grid. The normal incidence absorption and transmission spectra of the sample have been measured by a UV/VIS/NIR computer controlled spectrophotometer Perkin Elmer LAMBDA 750 in the transmission range of 400–700 nm at room temperature (300 K). Photo-luminescence (PL) spectrum of the sample is recorded in the visible region on a computer-controlled luminescence spectrophotometer LS-55 (Perkin Elmer Instruments) with accuracy of 71.0 nm. The Xe discharge lamp is used as an excitation source. The photoluminescence Quantum Yield (QY) of the PVA:nCdSe is calculated by using the gradient method [15] using Rhodamine 6G as a reference fluorescent dye. Briefly, ethanol solutions at different concentrations of Rhodamine 6G are prepared, and their absorption and fluorescence spectra are recorded (using 10 mm optical path fluorescence cuvette). The concentration range of these solutions is such that their optical densities at their excitation wavelength are between 0.01 and 0.1, to avoid self-absorption effects in the photoluminescence spectra [16]. The optical densities and the integrated fluorescence intensities of the various samples are plotted. The series of points is then interpolated with a straight line of slope mDye and which in principle should have intercept equal to zero. The same approach is adopted for each PVA:n-CdSe nanorods, that is, for each of them different solutions of NCs (in water) at various concentrations are prepared, and their absorption and integrated PL are plotted and fitted with a straight line, yielding therefore for each type of nanocrystal an interpolation line of slope mNC and intercept close to zero. The PL QY from each nanocrystal sample is then calculated

+ Cd(OCOCH 3)2 (0.1M)

PVA

+

[(0.5M) sodium selenosulfate by Reaction of Se and Sodium Sulphite]

Solution is stirrer for 5-6 hours at RT CdSe/PVA Nanorods

Fig. 1. Reaction mechanism of PVA:n-CdSe nanorods.

M. Sharma, S.K. Tripathi / Journal of Luminescence 135 (2013) 327–334

using the following equation !2 mNC ZNC QYNC ¼ QYDye mDye ZDye

ð1Þ

where QYDye ¼0.95 is the QY of Rhodamine 6 G, Zethanol ¼1.36 and Zsolvent ¼1.33 are the refractive indices of the solvents in which the dye and the nanocrystals sample are dissolved, respectively [17]. The dispersion of nanostructures in a polymer matrix is an important issue of any nanocomposite. The nanostructures have a tendency to agglomerate and inadequate dispersion results in nonuniform films. It has a significant impact on the properties of nanocomposites. The dispersion of nanorods has been characterized via optical microscopy at multiple length scales.

3. Results and discussion 3.1. Structural characterization Fig. 2 shows the XRD spectrum of PVA:n-CdSe nanorods. The XRD pattern of PVA is amorphous in nature [18]. The degree of crystallization of PVA:n-CdSe nanorods is found to be increased after the addition of CdSe nanorods to PVA. This is due to decrease in the number of hydrogen bonds, which are formed between its layers and are responsible for the crystallization of polymer. The XRD spectrum of PVA:n-CdSe shows the highest intensity reflection peak at 2y ¼29.931 along with two small intensity peaks at 42.261 and 48.821. The peak positioned at these 2y values corresponds to (2 0 0), (2 2 0) and (3 1 1) cubic phase of CdSe [19,20]. CdSe can exist in both single cubic (zinc blende) and hexagonal (wurtzite) structures. We have also found that PVA:n-CdSe shows the mixed structure i.e. both cubic and hexagonal. The peaks positioned at 2y value of 23.431 correspond to (1 0 1) plane of hexagonal phase of bulk CdSe [21]. The XRD pattern of CdSe also shows the presence of the some small intensity peaks at about angle 31.51 and 37.81 corresponding to the (1 0 3), (1 0 2) planes of hexagonal CdSe. Presence of sharp or prominent peaks in the XRD pattern confirms that PVA:n-CdSe nanocomposite film is crystalline in nature. The average crystallite size can be obtained by using the Debye–Scherrer Formula D¼

kl b cos y

ð2Þ

where D is the average crystallite size, b is full width at half maxima and k¼0.9 is Scherrer constant. The value of the average crystallite

Fig. 2. XRD spectra of PVA:n-CdSe nanorods.

329

size of PVA:n-CdSe nanocomposite as calculated by Eq. (2) is 4.1 nm. The calculated value of d corresponding to 2y ¼23.431 ˚ The comparison of calculated d value with the value is 3.79 A. standard d value clearly indicates the existence of hexagonal phase [22]. The d value for different peak positions 2y ¼ 29.931, 42.261 ˚ 2.13 A, ˚ 1.84 A, ˚ respectively. The comparison and 48.821 are 2.98 A, of these calculated d values with the standard d value clearly indicates the existence of cubic phase [20]. Fig. 3 depicts a TEM micrograph of PVA:n-CdSe. TEM image shows three arm nanorods or tripods structure. The lengths of the arms are not very uniform, while their widths have a relatively narrow size distribution (about 4–5 nm). There is no clamping of PVA and CdSe. Cadmium selenide nanorods are homogenously dispersed in PVA. However, the particle sizes obtained from TEM images are larger than that calculated from the XRD results using the Scherrer’s equation. The difference between the results of XRD and TEM may be due to aggregation of the smaller particles of the selenides. The data calculated from the XRD results reflected the size of a ‘‘single’’ crystal, while the TEM photograph shows the aggregates of the particles, which are formed because of the high surface energy of the nanometer-sized crystals. In literature, there are reports regarding the synthesis of PVA:nCdSe nanorods prepared by chemical bath method (CBD). Ma et al. [23] have synthesized PVA-capped CdSe nanorods at room temperature and the diameter of the particles was confined within 8 nm. Yang et al. [24] have reported the CdSe nanorods in PVA matrix at high temperature (160–180 1C). The diameter of these CdSe nanorods is 15–100 nm and length is 10–30 mm. Seoudi et al. [25] have reported PVA capped CdSe nanorods with diameter 25 nm. In this study, we have obtained PVA:n-CdSe nanorods having smaller dimensions (i.e. average diameter 1.9–5.1 nm). 3.2. Optical characterization 3.2.1. Absorption spectra Fig. 4 shows UV–visible absorption spectra of PVA:n-CdSe nanorods. The wavelength corresponding to absorption edge can be estimated directly from the graph using the intersection of the sharply decreasing region of the UV–vis spectra with the baseline known as ‘‘graphical method’’ [26]. On the other hand, due to the inherent variation on acquiring the best intersection point from the absorbance spectra, the assessment of the ‘‘optical band gap’’ has been accepted as a more accurate method for obtaining the wavelength value.

Fig. 3. TEM micrograph of PVA:n-CdSe at room temperature.

330

M. Sharma, S.K. Tripathi / Journal of Luminescence 135 (2013) 327–334

12000

(αhν) (eV/m)

2

0.40 0.35

10000 8000

2

Absorbance

can be written as " # 2 h 1 1 1:8e2 Eg ¼ Ebulk  þ þ g n 2 mn mh 4pe0 er R 8R e

14000

0.30

6000 4000 2000

0.25

0

1.8

2.1

2.4

2.7

3.0

3.3

3.6

hν (eV)

0.20 350

400

450

500 550 600 Wavelength (nm)

650

700

750

Fig. 4. Absorption spectra of PVA:n-CdSe nanorods (Inset show the plot of (ahn)2 vs. hn for PVA:n-CdSe nanorods). (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)

The fundamental absorption, which corresponds to the transition from valence band to conduction band, can be used to determine the band gap of the material. The relation between a and the incident photon energy (hn) can be written as [27]  n A hnEg a¼ hn

ð3Þ

where A is a constant, Eg is the optical band gap of the material and the exponent n depends on the type of transition. The n may have values 1/2, 2, 3/2 and 3 corresponding to the allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions, respectively. Fig. 4 inset shows the plot of (ahn)2 vs. hn for PVA:n-CdSe nanorods. The value of Eg is calculated by extrapolating the straight line portion of (ahn)2 vs. hn graph to hn axis taking n ¼0.5. The value of optical band gap for PVA:n-CdSe is 2.69 eV. The wavelength corresponding to absorption edge is 460 nm calculated from a simple relation l (nm) ¼(1238/Eg(eV)). The observed values of Eg are higher than the value of bulk optical band gap of CdSe nanorods due to quantum confinement. The factor which has a great effect on the size of CdSe nanorods is the PVA matrix effect. This blue shift could be attributed to the size reduction effect of PVA matrix to CdSe nanorods. For semiconductor nanomaterials, some theoretical models have been developed to deal with the band gap widening by quantum confinements [28]. From effective mass approximation formula, one can know about the band gap widening of semiconductor nanomaterials. When applying the EMA formula for nanorods, the band gap of nanorods depends on the rod diameter and not on its length as also reported by Awad et al. [29] and other researchers [30,31]. Several factors, such as the stress effect, the quantum confinement effect can affect the optical band gap. Under the EMA, the band gap can be calculated by the following equation [32,33] Eg ¼

Ebulk þEconf g

þEcoul

ð5Þ

where Ebulk ¼ 1:74 eV is the bulk band gap of CdSe [34], R¼d/2, is g the Planck constant, mne , mnh are the effective mass of the semiconductor conduction band electron and the valence band hole (For CdSe: mnh ¼ 0:13m0 , mne ¼ 0:45m0 ), respectively, Eg is the band gap of material obtained from UV–vis spectra and R the radius of the nanorods. The value of d is found to be 3.68 nm from UV–vis data. The coulomb term of electron–hole interaction is small compared to electron–hole confinement kinetic energy, which supported the blue shift result. The determination of optical constants of PVA:n-CdSe is done on chemically deposited films and the transmittance spectrum of PVA:n-CdSe film is shown in Fig. 5. The Swanepoel model [35] is used for calculation of refractive index of CdSe film. The model behind Swanepoel’s assumes that the sample is a film of nonuniform thickness deposited on a transparent substrate having a refractive index (s). The film has a complex refractive index nn ¼n-ik, where n is the refractive index and k is the extinction coefficient. According to this method, which is based on the approach of Manifacier et al. [36], the refractive index in the region where a E0 is calculated by the following equation qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffi n¼

H2 s2

Hþ

ð6Þ

where H¼ 

4s2

 2 T

s2 þ 1

s2 þ1 2

ð7Þ

where s is the refractive index of glass, and T is the transmittance of a sample. The extinction coefficient (k) is a measure of the fraction of light lost due to scattering and absorption per unit distance of the penetration medium. The extinction coefficient (k) can be written as k¼

al

ð8Þ

4p

where a is the absorption coefficient, and l is the wavelength. The spectral distributions of n and k vs. wavelength l as calculated using Swanepoels method for the PVA:n-CdSe nanorods are shown in Fig. 6(a) and (b). The decrease in the value of n with l is due to the significant normal dispersion behavior of the

0.70 0.65

Transmittance

0.45

0.60 0.55 0.50

ð4Þ

where Eg is the band gap of CdSe, Eg is the band gap of bulk CdSe, Econf is the electron–hole pair confinement kinetic energy and Ecoul is the coulomb interaction energy between the hole and the electron. Ecoul was estimated in first order perturbation theory using single band EMA wave  functions  and is given by 2 Ecoul ¼  (1.8e2/4pe0erR) and Econf ¼ h =8R2 ½1=mne þ 1=mnh , corresponding to the lowest energy transition. Now the above equation

0.45 0.40 300

350

400

450

500 550 λ (nm)

600

650

Fig. 5. Transmittance spectra of PVA:n-CdSe film.

700

750

M. Sharma, S.K. Tripathi / Journal of Luminescence 135 (2013) 327–334

331

3.4 0.090

n

3.2

0.085

2.8

0.080

k

3.0

2.6

0.075

2.4 2.2 300

400

500 600 λ (nm)

0.070 300

700

12

500 600 λ (nm)

700

0.65 0.60

10

0.55

8

ε2

ε1

400

0.50 0.45

6 4 1.5

0.40 0.35 2.0

2.5 3.0 λ (nm)

3.5

4.0

1.5

2.0

2.5 3.0 λ (nm)

3.5

4.0

Fig. 6. Variation of (a) linear refractive index (b) extinction coefficient (k) (c) real part of dielectric constant (e1) and (d) imaginary part of dielectric constant e2 with wavelength for PVA:n-CdSe nanorods.

films. The value of k first decreases with wavelength up to about 380 nm and then increases with wavelength. The variation of k values in investigated frequency range shows that some interactions take place between photons and electrons. The fact that the material is nanocrystalline extra absorption of light is shown to be taken place at grain boundaries. The similar behavior is reported on CdSe and other semiconductor films [37]. The real (e1) and imaginary (e2) parts of the dielectric constant for PVA:n-CdSe nanorods have also been calculated using the relations

eðoÞ ¼ e1 þie2

ð9Þ

where

e1 ¼ n2 k2 and e2 ¼ 2nk

ð10Þ

The values of the complex dielectric constant (real and imaginary parts) (e1 and e2) have been plotted in Fig. 6(c) and (d). The variation of the dielectric constant with photon energy indicates that some interactions between photons and electrons are produced in this energy range. The dissipation factor (tand) can be calculated using relation [38] tand ¼

e2 e1

ð11Þ

Fig. 7(a) shows the plot of the dissipation factor (tan d) as a function of frequency. It is clear from the figure that dissipation factor increases with increase in frequency. The similar behavior is reported on CdSe and other semiconductor films [37]. Optical response is most conveniently studied in terms of optical conductivity. The absorption coefficient a can be used to calculate the optical conductivity (sopt) as follows [39]

anc ð12Þ 4p where a is the absorption coefficient and c is the velocity of light. Fig. 7(b) shows the variation of optical conductivity (sopt) as a function of photon energy hn. The increase of optical conductivity sopt ¼

at high photon energies is due to the high absorbance and also

Fig. 7. Variation of (a) dielectric loss with frequency and (b) optical conductivity with wavelength for PVA:n-CdSe film.

may be due to the electron excited by the photon energy. This behavior agrees well with the reported CdSe film. However, the value of optical conductivity is found to be less as compared to value reported by Wahab et al. [37] which may be due to the presence of PVA matrix effect.

3.2.2. Photoluminescence spectra Fig. 8 shows the PL spectra of PVA:n-CdSe nanorods excited at different excitation wavelength range from 350 to 400 nm. The graph clearly shows the photoemission peaks at about 540 nm. This emission peak is blue shifted as compared to bulk CdSe (709 nm). This feature indicated the quantum-confined effect of

332

M. Sharma, S.K. Tripathi / Journal of Luminescence 135 (2013) 327–334

1.0 350 nm 360 nm PL intensity (a.u.)

0.8

370 nm 380 nm 390 nm

0.6

400 nm

0.4

0.2

0.0 480

500

520

540

560

580

600

λ (nm) Fig. 8. Photoluminescence spectra of PVA:n-CdSe nanorods at different excitation wavelength. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)

Fig. 9. Variation of PL intensity with wavelength. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)

the PVA:n-CdSe nanorods. The band gap of PVA:n-CdSe nanorods obtained from PL peak is 2.29 eV. It is clear from Fig. 8 that the emission band at 540 nm does not change with the increase in the excitation wavelength. The full width at half maximum (FWHM) of the band edge is maintained at 29.18 nm for full excitation wavelength scan. As the excitation wavelength is more than 400 nm, no noticeable emission was observed. Fig. 9 shows the PL intensity as a function of the excitation wavelength. There is an increase in PL intensity with excitation wavelength. There are two types of photo-emission an excitonic and a trapped emission. The trapped emission is board and it may be due to the formation of deep or shallow traps. An excitonic emission is normally sharp [40,41]. In our case, the observed peak at 540 nm is an excitonic emission peak with 30 meV binding energy. As the energy of the light emitted is not equal to the band gap. It is less by an amount equal to the exciton energy (Ex). The value of Ex depends on the material and it is given by the following relation: hv ¼ Eg Ex

ð13Þ

where hv is the emission energy calculated from PL spectra and Eg is the band gap of material and Ex is the excitonic energy. The Brus relation as stated in Eq. (5) is used for theoretically

determination of band gap of materials by considering the value of R from TEM measurements. The band gap measured experimentally from PL spectra (hv) and theoretically calculated band gap (Eg) from Brus relation Eq. (5) is 2.29 eV and 2.26 eV, respectively. By substituting the value of hv and Eg in Eq. (13), the value of Ex is obtained as 30 meV. Shabaev et al. [42] and Chen et al. [43] have also reported that the optical properties of CdSe nanorods are controlled by excitons with binding energy. This excitonic emission peak is stokes shifted with respect to the absorption edge wavelength. The appearances of such peak are also reported by Ramrakhiani et al. [44] for chemically synthesized CdSe doped in PVA. PVA:n-CdSe nanorods show absorption and emission peak at 460 nm and 540 nm, respectively. PVA:n-CdSe nanorods show larger stokes shift  80 nm. The larger value of stokes shift for semiconductor nanoparticles dispersed in polymeric media has been reported [45,46]. The PL spectrum is red shifted as compared to the absorption spectrum, which is indicative of the large stokes shift of the nanorods. The red emission is assigned to the presence of defect containing materials. The long tails of absorption spectra also show the presence of surface defects on the PVA:n-CdSe nanorods. Fig. 10(a) and (b) shows the 3D emission–excitation spectrum of PVA:n-CdSe nanorods together with the contour plot of 3-D spectrum in the Iðlex , lem Þ for emission peak 540 nm at excitation wavelength 350–400 nm. The colors in the Fig. 9(b) correspond to the PL intensity range varying from red (minimum) to green (maximum). The emission band peaking at 540 nm is excited at lex 4 350 nm. This band blue shifts relative to the bulk CdSe (709 nm). The change of Eex from 350 to 400 nm causes variations of the PL amplitude, whereas the spectral features are poorly influenced; the emission spectrum is characterized by a broad band, peaking at Epeak ¼2.29 eV. By substituting the value of QYDye ¼0.95, Zethanol ¼1.36 and Zsolvent ¼ 1.33 and the slopes of graph between absorbance and integrated fluorescence for Rhodamine 6G and CdSe in Eq. (1), the value of QY for PVA:n-CdSe nanorods is found to be 29%.

3.2.3. Confocal fluorescence microscopy Optical microscopy images of PVA:n-CdSe nanorods are shown in Fig. 11. It shows bright field micrographs of the resulting luminescent PVA:n-CdSe nanorods in films form with phases on the order of 100 mm and the corresponding fluorescence micrographs obtained by excitation with light. The PVA:n-CdSe nanorods have demonstrated to exhibit strong and well-defined green photoluminescence indicating that the films have endowed with luminescence features as shown in Fig. 11(a). Both up and down sides of the resulting luminescent PVA:nCdSe nanorods definitely showed the same luminescence behavior, which means that the deposition of CdSe nanorods in the PVA is uniform throughout the thickness, because the PVA is fully rinsed with the dispersion in the fabrication process. The luminescent PVA:n-CdSe nanorods are stable enough to show obvious green photo-luminescence after storage for several months. Under UV light, PVA:n-CdSe nanorods in solution form also show strong green emission as shown in Fig. 11(b). The strong green emission of PVA:n-CdSe nanorods is also confirmed from strong emission peak (540 nm) of PVA:n-CdSe nanorods shown in Fig. 8. These nanocomposites blink during the scanning process. This image gives only information about the color emission of PVA:n-CdSe nanorods at the micrometer level not about the morphology of the sample. As we are seeing at micrometer scale, there may be bunches of nanorods that impart color. The uniformity of the color implies an excellent dispersion of PVA:n-CdSe nanorods suggesting that the film casting process

M. Sharma, S.K. Tripathi / Journal of Luminescence 135 (2013) 327–334

0.7

PL Intensity (a.u.)

0.6

333

fresh sample 1 month

0.5 0.4 0.3 0.2 0.1 0.0 480

500

520 540 560 Wavelength (nm)

580

600

Fig. 12. PL spectra of PVA:n-CdSe nanorods stored under ambient conditions for freshly prepared sample and 1 month.

Fig. 10. (a) 3D plot of emission–excitation spectra as function of PL intensity and (b) Contour plot of emission–excitation spectra. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)

The long-term stability of the PL properties of PVA:n-CdSe nanorods is also investigated as part of this study, in view of possible applications of polymer nanocomposites. The as-prepared PVA:n-CdSe nanorods stored under ambient conditions did not exhibit any quenching of luminescence even after more than 1 month, and the apparent increase in the PL intensity of trap emission could possibly be attributed to photooxidation as shown in Fig. 12. These PL properties suggest the possibility of such polymer nanocomposites to use as fluorophores. In the present study, PVA:n-CdSe shows three arm nanorods or tripods structure. PVA:n-CdSe nanorods also show good photostability and large stokes shift. On the basis of these properties, it would be helpful to apply these materials in biological labeling, bio-imaging or biosensing applications and might one day serve as ideal candidates for in vitro and in vivo applications. These luminescent PVA:n-CdSe nanorods are stable enough to show obvious green photo-luminescence and enhanced intensity after storage in air for several months. To meet with complete knowledge of optical properties of II–VI semiconductor, we also studied the optical constant parameters of PVA:n-CdSe nanorods that would helpful in using these materials in optical communication and optical devices [50].

4. Conclusion

Fig. 11. Fluorescence optical microscopy of (a) PVA:n-CdSe nanorods in film form and (b) CdSe/PVA nanorods in solution under UV light. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)

did not cause significant aggregation of NCs due to the good compatibility between PVA-stabilized CdSe nanorods and the polymeric PVA matrices. Due to these properties, PVA:n-CdSe nanorods are expected to be widely used for biological labeling, bio-imaging, or biosensing applications. Such nanocomposites might one day serve as ideal candidates for in vitro and in vivo applications [47–49].

CdSe nanorods dispersed in the polymer matrix are prepared by in situ technique at room temperature. XRD spectra show the mixed structure i.e. hexagonal and cubic. TEM spectra confirmed the formation of nanorods. The blue shift in peak occurs at absorption and emission spectra in comparison to bulk are explained on the basis of quantum confinement effect. The 3D contour plot of emission–excitation wavelength spectra is studied as a function of PL intensity. A broad photoluminescence band with a large stokes shift is observed for PVA:n-CdSe nanorods. PVA:n-CdSe nanorods show green emission properties and good photostability.

Acknowledgments This work is financially supported by DST (Major Research Project), New Delhi. Mamta Sharma is thankful to UGC, New Delhi for providing fellowship.

334

M. Sharma, S.K. Tripathi / Journal of Luminescence 135 (2013) 327–334

References ˜ oz-Sanjose´, [1] S. Gime´nez, T. Lana-Villarreal, R. Go´mez, S. Agouram, V. Mun I. Mora-Sero´, J. Appl. Phys. 108 (2010) 064310. [2] J. Jie, W. Zhang, I. Bello, C. Lee, S. Lee, Nano Today 5 (2010) 313. [3] J.E. Brandenburg, X. Jin, M. Kruszynska, J. Ohland, J. Kolny-Olesiak, I. Riedel, H. Borchert, J. Parisi, J. Appl. Phys. 110 (2011) 064509. [4] S. Hachiya, Y. Onishi, Q. Shen, T. Toyoda, J. Appl. Phys. 110 (2011) 054319. [5] Y. Li, F. Qian, J. Xiang, C.M. Lieber, Mater. Today 9 (2006) 18. [6] K. Kumari, U. Kumar, S.N. Sharma, S. Chand, R. Kakkar, V.D. Vankar, V. Kumar, J. Phys. D: Appl. Phys. 41 (2008) 235409. [7] K. Prabakar, S. Minkyu, S. Inyoung, K. Heeje, J. Phys. D: Appl. Phys. 43 (2010) 012002. [8] V. Krishnakumar, G. Shanmugam, R. Nagalakshmi, J. Phys. D: Appl. Phys 45 (2012) 165102. [9] G.S. He, L.S. Tan, Q. Zheng, P.N. Prasad, Chem. Rev. 108 (2008) 1245. [10] M. Sharma, A.B. Sharma, N. Mishra, R.K. Pandey, Mater. Res. Bull. 46 (2011) 453. [11] Y. Da-Qin, F. Wei, W. Hong-Cai, L. Xiao-Zeng, Q. Jun-Feng, Chin. Phys. B 19 (2010) 017304. [12] A.S. Khomane, Mater. Res. Bull. 46 (2011) 1600. [13] S.H. Mohamed, H.M. Ali, J. Appl. Phys. 109 (2011) 013108. [14] M. Sharma, A. Narang, G. Kaur S.K. Tripathi, AIP Conf. Proc. 1313 (2010) 189. [15] J.N. Demas, G.A. Crosby, J. Phys. Chem. 75 (1971) 991. [16] S. Dhami, A.J. De Mello, G. Rumbles, S.M. Bishop, D. Phillips, A. Beeby, Photochem. Photobiol. 61 (1995) 341. [17] D.R. Lide, CRC Handbook of Chemistry and Physics, 81st ed., CRC Press, Boca Raton, FL, 2001. [18] M. Sharma, S.K. Tripathi, J. Phys. Chem. Solids 73 (2012) 1075. [19] G. Bing, F. Ya-Xian, C. Jing, W. Hui-Tian, H. Jun, J. Wei, J. Appl. Phys. 102 (2007) 083101. [20] R.B. Kale, C.D. Lokhande, Appl. Surf. Sci. 223 (2004) 345. [21] K. Kandasamy, H.B. Singh, S.K. Kulshreshtha, J. Chem. Sci. 121 (2009) 293. [22] JCPDS data file no 8-459. [23] X.D. Ma, X.F. Qian, J. Yin, H. Xi, Z. Zhu, J. Colloid Interf. Sci. 252 (2002) 77. [24] Q. Yang, K. Tang, C. Wang, Y. Qian, S. Zhang, J. Phys. Chem B 106 (2002) 9227.

[25] R. Seoudi, S.A. Mongy, A.A. Shabaka, Physica B 403 (2008) 1781. [26] H.S. Mansur, A.A.P. Mansur, Mater. Chem. Phys. 125 (2011) 709. [27] U. Woggon, Optical Properties of Semiconductor Quantum Dots, SpringerVerlag, Berlin Heidalberg New York, 1997. [28] D.H. Feng, Z.Z. Xu, T.Q. Jia, X.X. Li, S.Q. Gong, Phys. Rev. B 68 (2003) 035334. [29] H. Awad, T. Abdallah, M.B. Mohammed, K. Easawi, S. Negm H. Talaat, J. Phys. Conf. Ser. 214 (2010) 012130. [30] V. Prasad, C. D’Souza, D. Yadav, A.J. Shaikh, N. Vigneshwaran, Spectrochim. Acta A 65 (2006) 173. [31] L. Li, J. Hu, W. Yang, A.P. Alivisatos, Nano Lett. 1 (2001) 349. [32] L. Brus, J. Phys. Chem. 90 (1986) 2555. [33] L. Al., A.L. Etros, Efros. Sov. Phys. Semicond. 16 (1982) 772. [34] C. Kumar, Semiconductor Nanomaterials, Wiley-Vch, 2010, p. 73. [35] R. Swanepoel, J. Phys. E: Sci. Instrum. 17 (1984) 896. [36] J.C. Manifacier, J. Gasiot, J.P. Fillard, J. Phys. E: Sci. Instrum. 9 (1976) 1002. [37] L.A. Wahab, H.A. Zayed, A.A. Abd El-Galil, Thin Solid Film 520 (2012) 5195. [38] F. Yakuphanoglu, A. Cukurovali, I. Yilmaz, Physica B 351 (2004) 53. [39] J.I. Pankove, Optical Processes in Semiconductors, Dover Publications Inc, New York, 1975, p. 91. [40] Y. Lin, J. Zhang, E.H. Sargent, E. Kumacheva, Appl. Phys. Lett. 81 (2002) 3134. [41] M.G. Bawendi, P.J. Carrol, W.L. Wilson, L.E. Brus, J. Chem. Phys. 96 (1992) 946. [42] A. Shabaev Al L Efros, Nano Lett. 4 (2004)1821. [43] X. Chen, A.Y. Nazzala, M. Xiao, Z. Adam Peng, X. Peng, J. Lumin. 97 (2002) 205. [44] M. Ramrakhiani, V. Nogriya, J. Lumin. (2011), http://dx.doi.org/10.1016/ j.jlumin.2011.09.046. [45] D.V. Talapin, J.H. Nelson, E.V. Shevchenko, S. Aloni, B. Sadtler, A.P. Alivisatos, Nano Lett. 7 (2007) 2951. [46] W. Cai, Z. Li, J. Sui, Nanotechnology 19 (2008) 465606. [47] S. Deka, A. Quarta, M.G. Lupo, A. Falqui, S. Boninelli, C. Giannini, G. Morello, M.D. Giorgi, G. Lanzani, C. Spinella, R. Cingolani, T. Pellegrino, L. Manna, J. Am. Chem. Soc. 131 (2009) 2948. [48] H. Mattoussi, G. Palui, H.B. Na, Adv. Drug. Deliver. Rev. 64 (2011) 138. [49] A.D. Saran, M.M. Sadawana, R. Srivastava, J.R. Bellare, Colloid Surf. A 384 (2011) 393. [50] P.R. Watekar, H. Yang, S. Ju, W.T. Han, Opt. Express 17 (2009) 3157.