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Growth of tin oxide thin films composed of nanoparticles on hydrophilic and hydrophobic glass substrates by spray pyrolysis technique
Accepted Manuscript Title: Growth of tin oxide thin films composed of nanoparticles on hydrophilic and hydrophobic glass substrates by spray pyrolysis technique Author: Abdul Rasheed P M. Satheesh M. Carmen Mart´ınez-Tom´as Vicente Mu˜noz-Sanjos´e Sreekumar Rajappan Achary M. Junaid Bushiri PII: DOI: Reference:
S0169-4332(15)02206-0 http://dx.doi.org/doi:10.1016/j.apsusc.2015.09.112 APSUSC 31326
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Received date: Revised date: Accepted date:
30-4-2015 9-9-2015 13-9-2015
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Growth of tin oxide thin films composed of nanoparticles on hydrophilic and hydrophobic glass substrates by spray pyrolysis technique Abdul Rasheed P1, M. Satheesh1, M Carmen Martínez-Tomás2, Vicente Muñoz-Sanjosé2, Sreekumar Rajappan Achary1and M. Junaid Bushiri1,* Nano Functional Materials Lab, Department of Physics, Cochin University of Science and Technology,
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Kochi-682022, Kerala, India.
Departamento de Física Aplicada y Electromagnetismo, Universitat de Valencia, c/Dr Moliner 50,
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2
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Burjassot, Valencia-46100, Spain.
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Highlight
We have grown SnO2 thin films composed of nanoparticles with average particle size ~ 3-4 nm by spray pyrolysis technique on hydrophilic and hydrophobic substrates.
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Films grown over hydrophobic substrates are having comparatively larger lattice volume.
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Larger particles with low density distribution are observed in SnO2 thin films grown over hydrophobic substrates compared to the films grown over hydrophilic substrates.
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Substrate dependent photoluminescence emission is observed from the prepared samples.
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SnO2 thin films composed of nanoparticles grown on hydrophobic substrates may find useful in optoelectronic applications and also as gas sensors.
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Abstract
In this paper, we have demonstrated the growth of tin oxide (SnO2) thin films composed of nanoparticles on hydrophobic (siliconized) glass and hydrophilic (non-siliconized) glass substrates by using the spray pyrolysis technique. X-ray diffraction (XRD) analysis confirmed the formation of SnO2 thin films with tetragonal rutile-phase structure. Average particle size of nanoparticles was determined to be in the range of 3-4 nm measured from the front view images obtained by a field emission gun scanning electron microscope (FESEM), while the size of nanoparticle clusters, when present, were in the range of 11-20 nm. Surface morphology of SnO2 films grown over
*
Corresponding author,Tel.: + 91 9495348631. E-mail address: [email protected]
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hydrophobic substrates revealed larger isolated particles which are less crowded compared to the highly crowded and agglomerated smaller particles in films on hydrophilic substrates. Blue shift in the band gap is observed in samples in which the average particle size is slightly larger than the exciton Bohr radius (2.7 nm). Photoluminescence (PL) analysis of samples grown over hydrophobic substrates exhibited an intense defect level emission and a weak near band edge emission. The enhanced visible emission from these SnO2 thin films is attributed to lattice defects formed during the film growth due to the mismatch between the film and the
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hydrophobic substrate surface. Key words: SnO2 thin films, nanoparticles, spray pyrolysis, hydrophobic substrate, photoluminescence.
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Introduction
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Modern technology demands isolated nanoparticles and nanolayers of semiconductors due to their attractive optical and electrical properties attributed to spatial confinement of
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electrons in these systems [1-3]. These isolated nanoparticles can have very interesting applications in optoelectronics, telecommunication, optical sensors, low-threshold lasers and also in photocatalysis [4-7]. In particular, Tin oxide (SnO2) is an important wide band gap (3.60 eV)
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semiconductor material which is quite stable at atmospheric conditions. SnO2 has low electrical resistivity and high optical transmittance in thin film form and offers potential applications in
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fabricating solar cell electrodes, gas sensors, flat panel displays and LEDs [8-13]. SnO2 films can be prepared by various vacuum assisted methods such as RF magnetron sputtering, thermal
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evaporation and pulsed laser deposition [14-16]. Chemical based techniques such as chemical
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vapour deposition, MOCVD, chemical spray pyrolysis are also used for the growth of these films [17-19]. A lot of works report on the growth and physical properties of SnO2 thin films composed of microstructures. Nevertheless, the synthesis of semiconductor structures in nanoregime is very important in order to take advantage of their peculiar optical properties related, amongst others, to quantum confinement effect. Previous studies on growth of SnO2 ultrathin films composed of nanoparticles in the size range 1-6 nm using pulsed laser deposition [13], 310 nm by reactive RF magnetron sputtering [20] and 12 nm by using pulsed laser deposition [21] can be found in the literature. Interestingly, spray pyrolysis is a low cost method for the deposition of thin layers composed of low dimensional structures at ambient conditions in large area without vacuum ambience [22, 23]. Successful growth of ZnO nanoparticles on substrates like glass, c-sapphire, quartz etc. was reported by spray pyrolysis technique [24]. The nature of substrates influences the growth process of the material that contributes to the structure, morphology as well as the optical and electrical properties of deposited thin films [25]. Strain induced self-assembly of islands in heteroepitaxial systems is an important method for the growth of nanoscale particles [26]. In most cases heteroepitaxy involves lattice mismatch Page 2 of 19
between deposit and substrate which will produce strain in the layer and offers the opportunity to create metastable structures with novel physical and chemical properties [27]. Silicate glass slides are generally used for the deposition of semiconductor oxides by spray pyrolysis method. These substrates are normally hydrophilic in nature and can be modified into hydrophobic by siliconization process. Hydrophobic surfaces, thus obtained have usually high surface roughness
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compared to hydrophilic ones and hence the possibility of growing strained layers on them can be expected. Consequently more crystalline defects are expected to be formed during the film
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growth for relieving the strain [27]. Such induced defects in semiconductor oxide thin films can be useful for the fabrication of light emitting diodes as well as in gas sensor applications [28,
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29]. It is worth to note that hydrophobic substrates obtained by siliconization can prevent a rapid spreading of the precursor solution due to a reduced diffusivity and can provide more sites for the nucleation process [30]. The growth of the deposited film will depend on the relation
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between the surface energy and the interfacial energy [31]. In this work, we report on the growth of SnO2 thin films composed of nanoparticles on hydrophobic glass substrates as well as on non-
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siliconized glass substrates by using spray pyrolysis technique, providing particular emphasis on
Experimental
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the photoluminescence (PL) emission properties.
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Tin oxide thin films were prepared by chemical spray pyrolysis technique using stannic chloride (SnCl4.5H2O) dissolved in distilled water as precursor solution and compressed air as
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spraying gas. Hydrophilic and hydrophobic glass slides (made by siliconization) were used as substrates. Hydrophilic glass slides were cleaned by a standard cleaning procedure by using nitric acid, acetone and distilled water. Hydrophobic substrates were prepared by soaking well cleaned glass slides in a standard siliconizing solution (Sigmacote) provided by Sigma-Aldrich. Substrate-to-nozzle distance was optimized and kept at 28 cm throughout the spraying process and substrates were preheated for 2 hours. SnO2 thin films were deposited by spraying the precursor solution onto preheated substrates kept at a temperature of 350 ºC. Spraying of precursor solution was interrupted after each 2 minutes for duration of 1 minute in order to avoid a decrease in the substrate temperature. As prepared samples were maintained at 350 ºC for 30 minutes and then allowed to cool to room temperature. SnO2 thin film samples were deposited using two different precursor molarities (0.2 and 0.4 M) both on hydrophilic as on hydrophobic glass substrates. Structural characterization of samples was performed by using a X-ray diffractometer (Rigaku-Dmax C) using the Cu-Kα line (λ=1.5414 Å) with 2θ values between 10 and 80º. The Page 3 of 19
surface of samples was examined by using a field emission gun scanning electron microscope (FESEM), Hitachi make (S-4800) at an accelerated voltage of 20kV. A spectrophotometer (JASCO V570 UV-VIS-NIR) working in the range of 200-800 nm at room temperature was used for optical measurements. Room temperature photoluminescence characterization of samples was done by using a high resolution Horiba Jobin Yvon LabRam system with a He-Cd laser (325
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nm) as excitation source.
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Results and Discussion
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XRD patterns of samples (Fig.1) show that the diffraction peak positions are in agreement with the standard JCPDS data (Card No. 041-1445) of tetragonal rutile-phase SnO2. It can be seen that the diffraction peaks intensity increases with respect to the molarity of the
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precursor solution used for spraying on both type of substrates which indicates a better crystallinity as the molarity increases. Although the peak corresponding to the (110) orientation
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exhibited higher intensity, Harris texture analysis [32] shows that the (200) is the preferential orientation (Table 1). The degree of preferred orientation sigma in all the samples is much lower than that corresponding to a perfectly oriented sample, indicating that the films are poorly
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oriented. Although the diffraction peaks of films grown with low solubility solutions have low
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intensity, the values of sigma indicate that the hydrophobic glass substrates produce more oriented films. The presence of a higher roughness of the surface in the hydrophobic glass
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substrates can explain this behaviour, as they could have better nucleation conditions, lowering the surface interfacial energy, giving rise to more density of nucleation points and better oriented films, as it will be explained later. The broad diffraction peaks indicate that the formed tin oxide particles have very small crystallite sizes.
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Fig.1. XRD pattern of spray pyrolytically grown SnO2 thin films composed of nanoparticles synthesized by using 0.2 M precursor solution over (a) hydrophilic glass (b)
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hydrophobic glass and by using 0.4 M precursor solution over (c) hydrophilic glass (d)
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hydrophobic glass substrates.
Table 1: Texture coefficients and degree of preferred orientation from XRD pattern
Sample Perfectly oriented 0.2 M hydrophilic glass
(110) (101) (200) (211) (310) sigma 2.000 1.185 0.752 1.344 0.693 1.026 0.248
0.2 M hydrophobic glass 1.023 0.487 1.392 0.769 1.329 0.340
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0.4 M hydrophilic glass
1.178 0.651 1.369 0.756 1.045 0.265
0.4 M hydrophobic glass 1.170 0.260 1.857 0.385 1.329 0.600
A small, but clear shift in diffraction peaks towards lower angles for films grown over
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hydrophobic glass substrates with respect to films grown over hydrophilic glass substrates can be observed. This can be attributed to the increase in unit cell volume induced by epitaxial strain
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introduced in the material due to interfacial mismatch between hydrophobic substrate and grown film. The lattice parameter a of the tetragonal cell was obtained (with an uncertainty of about 1-
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3%) for each sample by plotting, from at least three peaks of the (h k 0) family, the diffraction order (h2 + k2) versus 2sinθ /λ being θ the Bragg angle and λ the wavelength of the X-ray beam. The large values in hydrophobic substrates as compared to those in hydrophilic ones indicate the
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presence of a tensile strain in these samples. Parameter c of the tetragonal cell was calculated from the position of (211) peaks and the value of the parameter a of the tetragonal cell
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previously determined. Table 2 shows the values of lattice parameters as well as the unit cell volume. Films grown over hydrophobic substrates are having comparatively larger lattice
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volume which can be attributed to the presence of tensile strain in these samples [33]. In many cases epitaxial strain due to lattice mismatch in heterostructures is accommodated through the
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formation and ordering of oxygen vacancies and act as a strain relief mechanism [33, 34]. The probability of increase in defects in samples on hydrophobic substrates due to epitaxial strain is
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evidenced from the photoluminescence spectra which will be discussed later.
Table 2: Lattice parameters and unit cell volume from XRD pattern
Sample 0.2 M hydrophilic glass
Lattice parameter (Å) a 4.75
c 3.10
Volume (Å3) a×a×c 69.94
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0.2 M hydrophobic glass
4.82
3.12
72.49
0.4 M hydrophilic glass
4.74
3.17
71.22
0.4 M hydrophobic glass
4.82
3.12
72.49
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The average crystallite size values calculated from the XRD data using Scherrer’s
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formula [35] are given in Table 3. As SnO2 films grown over hydrophobic glass substrates have higher crystallite sizes, the assumption that the hydrophobic substrates may have more sites for
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the nucleation process and the formation of grains is reinforced. This can be explained on the fact that a hydrophobic surface obtained by siliconization generally has a rough surface and
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consequently hydrophobicity will increase with roughness. The increase in roughness contributes to the enhancement of surface area which geometrically boosts hydrophobicity (Wenzel model) [36]. Further, air may be trapped between hydrophobic solid-liquid interface which will increase
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the hydrophobic nature as explained by the Cassie model [36]. At any instance, there will be more nucleation sites remaining active on such a rough surface [37]. So adatoms acquiring
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energy from the hot substrate will migrate into nearby nucleation sites, get nucleated and then coalesced to form larger and better oriented grains.
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FESEM images (Fig. 2) show that SnO2 thin films are composed of nanoparticles in the
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case of samples deposited over hydrophilic glass substrates (precursor molarity 0.2 and 0.4 M) and over hydrophobic glass substrate with a precursor molarity 0.2 M (Fig. 2(a), 2(b) and 2(c)). Whereas, SnO2 thin film composed of clusters of nanoparticles were formed over hydrophobic glass substrate with a precursor molarity 0.4 M (Fig. 2(d)). The surface of the films is found to be fairly uniform and homogenous. The size of nanoparticles were determined from the FESEM images and showed as histograms in Fig. 2. The size of nanoparticles is found to be in the range 2-7 nm while that of nanoparticle clusters are in the range 11-20 nm. The average crystallite size of the sample deposited over hydrophobic glass substrate with a precursor molarity of 0.4 M obtained from XRD data is 24 nm which is close to the cluster size values determined from FESEM image (Fig. 2(d)). The higher value of crystallite size obtained from XRD data suggests that nanoparticles constituting a cluster are coherently oriented (i.e., under the same Bragg diffraction conditions) [13]. SnO2 thin films grown over hydrophilic glass substrates show a high density distribution of smaller particles while films grown over hydrophobic surface have larger particles and their Page 7 of 19
density is comparatively lesser than that of films on hydrophilic glass substrates. The comparably lesser population and increase in size of nanoparticles on hydrophobic glass surface is attributed to the coalescence process driven by the so-called Ostwald ripening phenomenon. It is a thermodynamically driven spontaneous process in which larger particles grow at the expense of smaller particles by the diffusion of adatoms in order to decrease the total energy of the
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system [38]. As a result, average particle size increases while density of particles decreases. The rate of adatom diffusion from smaller particles to larger particles decreases as particle size
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increases and finally ceases once all small particles are consumed [39]. Due to the difference in surface wettability, a hydrophobic surface prefers drop-wise condensation while a hydrophilic
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surface prefers film-wise condensation [40]. The heat transfer rate yielded by drop-wise condensation is one order of magnitude higher than that given by the film-wise condensation because in the latter case the film spans the entire surface and acts as a thermal insulator
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reducing the heat transfer rate [41]. As Ostwald ripening is a thermodynamically driven process the feasibility for its occurrence is much higher on the hydrophobic samples due to an increased
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heat transfer rate through the substrate. Nanoparticle clusters are observed only on the hydrophobic glass substrate sample (precursor molarity 0.4 M). This phenomenon is attributed to the increase in the rate of growth of nanoparticles due to the increase of precursor molarity
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which reduces the rate of Ostwald ripening [42]. In addition, mobility of the adatoms decreases
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with increasing size which in turn decreases the probability of coalescence by collision. So, larger particles get hydrophobically accumulated around the nucleation centers to form clusters
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of nanoparticles with gaps between the clusters. Similar hydrophobic organization of monolayer protected Au clusters, on functionalised gold substrates, has been reported by Aslam et al [43]. Such hydrophobic organization will be important in designing complicated semiconducting nanoclusters possessing interesting optical properties on transparent substrates [43].
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ip t cr us an M d te Ac ce p Fig. 2. FESEM images of spray pyrolytically grown SnO2 thin films composed of nanoparticles synthesized by using 0.2 M precursor solution over (a) hydrophilic glass (b) hydrophobic glass and by using 0.4 M precursor solution over (c) hydrophilic glass (d) hydrophobic glass (nanoparticle clusters can be observed in this sample as marked in the image) substrates.
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Fig. 3. Reflectance spectra of spray pyrolytically grown SnO2 thin films composed of nanoparticles synthesized by using 0.2 M precursor solution over (a) hydrophilic glass (b)
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hydrophobic glass substrates.
d
hydrophobic glass and by using 0.4 M precursor solution over (c) hydrophilic glass (d)
Fig. 4. Tauc plot of spray pyrolytically grown SnO2 thin films composed of nanoparticles synthesized by using 0.2 M precursor solution over (a) hydrophilic glass (b) hydrophobic glass and by using 0.4 M precursor solution over (c) hydrophilic glass (d) hydrophobic glass substrates. Page 10 of 19
There is a small shift in optical reflectance edges which leads to the corresponding variation in the optical band gap (Fig. 3). Optical band gaps can be calculated from Tauc plot as shown in Fig. 4 using Kubelka-Munk relation [44] and the obtained values are given in Table 3. SnO2 nanoparticles are expected to exhibit quantum confinement effects when their particle
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radius is of the order of the corresponding exciton Bohr radius, 2.7 nm [45]. So, there will be strong confinement if the particle size is lesser than the Bohr radius and a weak confinement
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effect will be observed if the particle size is slightly larger than the exciton Bohr radius [46]. Samples deposited on both hydrophilic and hydrophobic glass substrates with 0.2 M precursor
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solution show weak confinement since their particle sizes are slightly larger than 2.7 nm and as a result the band gaps get blue shifted as the particle size decreases. Similar result is observed from films grown with 0.4 M solution on hydrophilic substrates used in the present investigation. No
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confinement effect is observed from films grown with 0.4 M solution on hydrophobic substrate since their size, 4.2 nm, is higher than 2.7 nm. The higher band gap observed in these films is
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ascribed to Burstein Moss shift [47]. Oxygen vacancies induced during the film growth increase the electrons near the conduction band edge producing donor levels which will increase the band
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gap [47].
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composed of nanoparticles.
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Table 3: Crystallite size, particle size and band gap of spray pyrolytically grown SnO2 thin films
Sample
Average crystallite
Average particle
size (nm) from
size (nm) from
XRD
FESEM
Band gap (eV)
0.2 M hydrophilic glass
4.2
3.2
3.72
0.2 M hydrophobic glass
7.4
3.3
3.62
0.4 M hydrophilic glass
7
3.9
3.47
0.4 M hydrophobic glass
24
4.2
3.91
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Fig. 5. Photoluminescence spectra of spray pyrolytically grown SnO2 thin films
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composed of nanoparticles synthesized by using 0.2 M precursor solution over (a) hydrophilic glass (b) hydrophobic glass and by using 0.4 M precursor solution over (c)
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hydrophilic glass (d) hydrophobic glass substrates.
Fig. 5 shows the room temperature photoluminescence spectra of as prepared SnO2
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samples measured with an excitation wavelength of 325 nm. Typically SnO2 has a near band
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edge emission (NBE) in the UV region which could be due to excitons and a defect level emission (DLE) in the visible region which can be ascribed to intrinsic defects such as oxygen
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vacancies, tin interstitials and dangling bonds [23,46]. Gaidi et al have reported room temperature photoluminescence of SnO2 nanoparticles, but only after annealing the samples at 700 °C [13]. In our study we have obtained room temperature luminescence without any post deposition annealing treatment. The PL spectra of as prepared samples have mainly two peaks, one broad peak in the UV region centred at 380 nm (3.26 eV) from samples grown over hydrophilic glass substrates and a second broad peak in the visible range 495-594 nm (2.5-2.1 eV) from all samples. The intensity of peaks in the visible region is found to be increased with increasing molarity of the precursor solution irrespective of the nature of substrates used for the deposition of thin films. The broad NBE emission from 0.2 M hydrophilic sample centred at 380 nm is attributed to defects close to the conduction band since the positions of emission are lower than the corresponding band gap of the sample [13]. Similar emission with lesser intensity is observed for 0.4 M hydrophilic glass samples which may be due to the electronic transition from the conduction band to the valence band and also from defects close to the conduction band. All the samples have broad defect level emissions in the range 495-594 nm with varying intensity. Page 12 of 19
This could be attributed to electronic transitions due to various defect levels present in the band gap such as oxygen vacancies. Oxygen vacancies are considered to be the most prominent defects in semiconductor oxides acting as luminescent centres by creating defect levels inside the band gap and trapping electrons from the valence band [48]. The oxygen vacancies do not contribute to the formation of occupied donor levels which are just below the conduction band,
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but the occupied levels associated to the vacancy appear at around 1 eV above the top valence band (i.e., 2.6 eV below the conduction band) [49]. So electrons coming back from the
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conduction band after excitation may get trapped in the shallow defects close to the band edge. They either recombine with holes in the valence band or continue to relax to deeper defect states present in three different charge states in the oxides:
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before recombining, contributing to a broad visible PL emission [48]. Oxygen defects can be ,
and
states [50]. As
is
assumed to be a very shallow donor, most oxygen vacancies will be in their paramagnetic
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states under flat band conditions [50]. It has been reported that visible broad emissions are due to the recombination of electrons in singly occupied oxygen vacancies (
) with photo excited
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holes in the valence band [48]. In addition, the interactions between oxygen vacancies and interfacial tin vacancies cause the formation of a significant number of trapped states, which
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region [51].
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form a series of metastable energy levels within the band gap resulting in emission in the visible
One can observe that intensity of NBE emission is suppressed and DLE emission is
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enhanced from films grown over hydrophobic glass substrates. Hydrophobic glass substrates, having roughened surface will increase possibility of lattice mismatch between the substrate and the film which will induce the accumulation of strain during the film growth. This strain will promote the formation of lattice defects within the crystal. Such defects could be both radiative and nonradiative and will reduce the excitonic recombination resulting in poor UV emission efficiency. Radiative defects such as oxygen vacancies formed during the film growth are responsible for visible PL emission. Weak visible PL emission from 0.2 M hydrophobic glass sample indicates that defects formed during the film growth are mainly nonradiative. Nonradiative defects are formed in a material where lattice distortion is high [47]. This is true for this sample as evident from the XRD pattern which shows its poor crystalline nature. Increased crystallite size in 0.4 M hydrophobic glass sample decreases the concentration of nonradiative recombination centres enhancing visible emission [23]. Another factor regarding films on hydrophobic glasses is the presence of isolated nanoparticles, which will have large surface area and may have large number of defects such as oxygen vacancies resulting in the enhancement of Page 13 of 19
trap emission in the visible region. A red shift in the visible PL peak position is observed from samples on hydrophobic glass substrates, which can be attributed to the combined effect of tensile strain and larger crystallite size observed in these samples [52]. The present PL observation suggests that SnO2 thin films grown on hydrophobic substrates can be used for the quenching of UV emission and enhancement of
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photoluminescence in the visible region. Such substrate dependent tuning of photoluminescence properties of SnO2 is important for the fabrication of white LEDs and other optoelectronic
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devices. Isolated nanoparticles in SnO2 thin films grown over hydrophobic substrates have large exposed surface area and high surface energy due to more number of surface atoms which
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accelerates adsorption of oxygen in ambient atmosphere and are good for the fabrication of gas sensors similar to that of ZnO nanorods reported in the literature [29]. Interestingly, the presence states enhances the adsorption of oxygen without lowering the expansion
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of large amount of
level of depletion layer which may also contribute to gas sensing efficiency [29].
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Conclusions
We report the self-assembled growth of spray pyrolytically grown SnO2 thin films composed of nanoparticles on hydrophilic and hydrophobic glass substrates by using the spray
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pyrolysis technique. SnO2 thin films deposited on hydrophobic substrates are more oriented
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compared to hydrophilic substrate samples and the growth of larger grains is attributed to the availability of more nucleation sites. SnO2 thin films grown on hydrophobic substrates have
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larger particles with low density distribution compared to films grown on hydrophilic substrates owing to Ostwald ripening phenomenon. Band gap of samples having particle sizes slightly larger (3.2, 3.3 and 3.9) than the exciton Bohr radius are blue shifted due to quantum confinement effect. The increase in band gap observed from 0.4 M hydrophobic glass sample as compared to other samples is attributed to Burstein Moss shift. Defect level emission is enhanced in the PL spectra from films on hydrophobic substrates with the suppression of near band edge emission. The enhanced visible emission from SnO2 thin films grown on hydrophobic surfaces is attributed to unintentionally induced lattice defects in the films during their growth on these surfaces. Thus SnO2 thin films composed of nanoparticles grown on hydrophobic substrates may find useful in optoelectronic applications and also as gas sensors owing to the occurrence of isolated nanoparticles and large amount of
states.
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Acknowledgments The authors are thankful to KSCSTE, Government of Kerala for the financial support of this work (No: 023/SRSPS/2007/CSTE). The authors acknowledge Prof. M. R. N. Murthy, I.I.Sc. Bangalore for providing facilities for the preparation of hydrophobic glass substrates and
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Prof. M. K. Jayaraj, Dept. of Physics, CUSAT, for providing PL measurements under DST, India nano mission initiative program. We also thank the SCSIE–University Valencia for providing the
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FESEM facilities related with (MINECO) projects TEC2011-28076-C02-01/02 and TEC201460173-C2-1-R, Generalitat Valenciana projects PrometeoII/20115/004 and ISIC/2012/008.
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J. Phys. Chem. C, 114 (2010) 18390.
[29] L. Liao, H. B. Lu, J. C. Li, H. He, D. F. Wang, D. J. Fu, C. Liu, W. F. Zhang, J. Phys. Chem. C 111 (2007) 1900.
[30] M. Borgström, J. Johansson, L. Landin, and W. Seifert, Appl. Surf. Sci. 165 (2000) 241.
[31] S. Sinha, C. -H. Wang, A. K. M. Maidul Islam, Y. -W. Yang, and M. Mukherjee, AIP Conf. Proc. 1512 (2013) 746.
[32] G. B. Harris, Philos. Mag. 43 (1952) 113. [33] U. Aschauer, R. Pfenninger, S.M. Selbach, T. Grande, N.A. Spaldin, Phys. Rev. B, 88 (2013) 054111. [34] J. Gazquez, S. Bose, M. Sharma, M. A. Torija, S. J. Pennycook, C. Leighton, and M. Varela, APL Mat. 1 (2013) 012105. [35] B. D. Cullity, Elements of X-ray Diffraction, Addison-Wesley Publishing Co., USA, 1956. Page 16 of 19
[36] A. Lafuma, D. Quere, Nat. Mater. 2 (2003) 457. [37] B. S. Sikarwar, S. Khandekar and K. Muralidhar, ECI 8th International Conference on Boiling and Condensation Heat Transfer, 2012. [38] D. Z. Hu, Stress evolution during growth of InAs on GaAs measured by an in-situ cantilever
ip t
beam setup, Dissertation, Humboldt-Universität zu Berlin, 2007. [39] B. D. Min, Y. Kim and E. K. Kim, J. Korean Phys. Soc.33 (1998) S291.
cr
[40] C. Fang, J. E. Steinbrenner, F. M. Wang, K. E. Goodson, J. Micromech. Microeng. 20 (2010) 045018.
us
[41] K. E. Ruleman, Heat transfer via dropwise condensation on hydrophobic
microstructured surfaces, Thesis (S. B. ), Massachusetts Institute of Technology,
an
Department of Mechanical Engineering, 2009.
[42] H. Zhang, L. Wang, H. Xiong, L. Hu, B. Yang, W. Li, Adv. Mater. 15 (2003) 1712.
M
[43] M. Aslam, I. S. Mulla, K. Vijayamohanan, Langmuir 17 (2001) 7487. [44] P. Kubelka, F. Munk, Z. Tech. Phys. (Leipzig) 12 (1931) 593.
te
Phys. Lett. 84 (2004) 1745.
d
[45] E. J. H. Lee, C. Ribeiro, T. R. Giraldi, E. Longo, E. R. Leite and J. A. Varela, Appl.
[46] V. Kumar, V. Kumar, S. Som, J. H. Neethling, M. Lee, O. M. Ntwaeaborwa, H. C.
Ac ce p
Swart, Nanotechnology 25(2014) 135701. [47] P. Chetri, A. Choudhury, Physica E 47 (2013) 257. [48] A. Singhal, B. Sanyal, A. K. Tyagi, RSC Adv. 1 (2011) 903. [49] F. Trani, M. Causa, D. Ninno, G. Cantele, and V. Barone, Phys. Rev. B 77 (2008) 245410.
[50] K. Vanheusden, W. L. Warren, C. H. Seager, D. R. Tallant, J. A. Voigt, B. E. Gnade, J. Appl. Phys. 79 (1996) 7983. [51] Y. Wang, M. Guo, M. Zhang, X. Wang, Scr. Mater. 61 (2009) 234. [52] J. Ni, X. Zhao, X. Zheng, J. Zhao, B. Liu, Acta Mater. 57 (2009) 278.
Page 17 of 19
Table 1: Texture coefficients and degree of preferred orientation from XRD pattern
Sample
(110) (101) (200) (211) (310) sigma 2.000
Perfectly oriented
1.185 0.752 1.344 0.693 1.026 0.248
ip t
0.2 M hydrophilic glass
0.2 M hydrophobic glass 1.023 0.487 1.392 0.769 1.329 0.340 1.178 0.651 1.369 0.756 1.045 0.265
cr
0.4 M hydrophilic glass
us
0.4 M hydrophobic glass 1.170 0.260 1.857 0.385 1.329 0.600
an
Table 2: Lattice parameters and unit cell volume from XRD pattern
Ac ce p
0.4 M hydrophilic glass 0.4 M hydrophobic glass
c
Volume (Å3) a×a×c
3.10
69.94
4.82
3.12
72.49
4.74
3.17
71.22
4.82
3.12
72.49
d
0.2 M hydrophobic glass
4.75
te
0.2 M hydrophilic glass
Lattice parameter (Å) a
M
Sample
Page 18 of 19
Table 3: Crystallite size, particle size and band gap of spray pyrolytically grown SnO2 thin films composed of nanoparticles. Average crystallite
Average particle
size (nm) from
size (nm) from
XRD
FESEM 3.2
0.2 M hydrophobic glass
7.4
3.3
0.4 M hydrophilic glass
7
3.9
0.4 M hydrophobic glass
24
3.72
ip t
4.2
(eV)
us
0.2 M hydrophilic glass
Band gap
3.47
3.91
an
4.2
3.62
cr
Sample
Ac ce p
te
d
M
Page 19 of 19
S0169-4332(15)02206-0 http://dx.doi.org/doi:10.1016/j.apsusc.2015.09.112 APSUSC 31326
To appear in:
APSUSC
Received date: Revised date: Accepted date:
30-4-2015 9-9-2015 13-9-2015
Please cite this article as:
Growth of tin oxide thin films composed of nanoparticles on hydrophilic and hydrophobic glass substrates by spray pyrolysis technique Abdul Rasheed P1, M. Satheesh1, M Carmen Martínez-Tomás2, Vicente Muñoz-Sanjosé2, Sreekumar Rajappan Achary1and M. Junaid Bushiri1,* Nano Functional Materials Lab, Department of Physics, Cochin University of Science and Technology,
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1
Kochi-682022, Kerala, India.
Departamento de Física Aplicada y Electromagnetismo, Universitat de Valencia, c/Dr Moliner 50,
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2
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Burjassot, Valencia-46100, Spain.
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Highlight
We have grown SnO2 thin films composed of nanoparticles with average particle size ~ 3-4 nm by spray pyrolysis technique on hydrophilic and hydrophobic substrates.
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Films grown over hydrophobic substrates are having comparatively larger lattice volume.
•
Larger particles with low density distribution are observed in SnO2 thin films grown over hydrophobic substrates compared to the films grown over hydrophilic substrates.
•
Substrate dependent photoluminescence emission is observed from the prepared samples.
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SnO2 thin films composed of nanoparticles grown on hydrophobic substrates may find useful in optoelectronic applications and also as gas sensors.
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•
Abstract
In this paper, we have demonstrated the growth of tin oxide (SnO2) thin films composed of nanoparticles on hydrophobic (siliconized) glass and hydrophilic (non-siliconized) glass substrates by using the spray pyrolysis technique. X-ray diffraction (XRD) analysis confirmed the formation of SnO2 thin films with tetragonal rutile-phase structure. Average particle size of nanoparticles was determined to be in the range of 3-4 nm measured from the front view images obtained by a field emission gun scanning electron microscope (FESEM), while the size of nanoparticle clusters, when present, were in the range of 11-20 nm. Surface morphology of SnO2 films grown over
*
Corresponding author,Tel.: + 91 9495348631. E-mail address: [email protected]
Page 1 of 19
hydrophobic substrates revealed larger isolated particles which are less crowded compared to the highly crowded and agglomerated smaller particles in films on hydrophilic substrates. Blue shift in the band gap is observed in samples in which the average particle size is slightly larger than the exciton Bohr radius (2.7 nm). Photoluminescence (PL) analysis of samples grown over hydrophobic substrates exhibited an intense defect level emission and a weak near band edge emission. The enhanced visible emission from these SnO2 thin films is attributed to lattice defects formed during the film growth due to the mismatch between the film and the
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hydrophobic substrate surface. Key words: SnO2 thin films, nanoparticles, spray pyrolysis, hydrophobic substrate, photoluminescence.
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Introduction
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Modern technology demands isolated nanoparticles and nanolayers of semiconductors due to their attractive optical and electrical properties attributed to spatial confinement of
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electrons in these systems [1-3]. These isolated nanoparticles can have very interesting applications in optoelectronics, telecommunication, optical sensors, low-threshold lasers and also in photocatalysis [4-7]. In particular, Tin oxide (SnO2) is an important wide band gap (3.60 eV)
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semiconductor material which is quite stable at atmospheric conditions. SnO2 has low electrical resistivity and high optical transmittance in thin film form and offers potential applications in
d
fabricating solar cell electrodes, gas sensors, flat panel displays and LEDs [8-13]. SnO2 films can be prepared by various vacuum assisted methods such as RF magnetron sputtering, thermal
te
evaporation and pulsed laser deposition [14-16]. Chemical based techniques such as chemical
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vapour deposition, MOCVD, chemical spray pyrolysis are also used for the growth of these films [17-19]. A lot of works report on the growth and physical properties of SnO2 thin films composed of microstructures. Nevertheless, the synthesis of semiconductor structures in nanoregime is very important in order to take advantage of their peculiar optical properties related, amongst others, to quantum confinement effect. Previous studies on growth of SnO2 ultrathin films composed of nanoparticles in the size range 1-6 nm using pulsed laser deposition [13], 310 nm by reactive RF magnetron sputtering [20] and 12 nm by using pulsed laser deposition [21] can be found in the literature. Interestingly, spray pyrolysis is a low cost method for the deposition of thin layers composed of low dimensional structures at ambient conditions in large area without vacuum ambience [22, 23]. Successful growth of ZnO nanoparticles on substrates like glass, c-sapphire, quartz etc. was reported by spray pyrolysis technique [24]. The nature of substrates influences the growth process of the material that contributes to the structure, morphology as well as the optical and electrical properties of deposited thin films [25]. Strain induced self-assembly of islands in heteroepitaxial systems is an important method for the growth of nanoscale particles [26]. In most cases heteroepitaxy involves lattice mismatch Page 2 of 19
between deposit and substrate which will produce strain in the layer and offers the opportunity to create metastable structures with novel physical and chemical properties [27]. Silicate glass slides are generally used for the deposition of semiconductor oxides by spray pyrolysis method. These substrates are normally hydrophilic in nature and can be modified into hydrophobic by siliconization process. Hydrophobic surfaces, thus obtained have usually high surface roughness
ip t
compared to hydrophilic ones and hence the possibility of growing strained layers on them can be expected. Consequently more crystalline defects are expected to be formed during the film
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growth for relieving the strain [27]. Such induced defects in semiconductor oxide thin films can be useful for the fabrication of light emitting diodes as well as in gas sensor applications [28,
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29]. It is worth to note that hydrophobic substrates obtained by siliconization can prevent a rapid spreading of the precursor solution due to a reduced diffusivity and can provide more sites for the nucleation process [30]. The growth of the deposited film will depend on the relation
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between the surface energy and the interfacial energy [31]. In this work, we report on the growth of SnO2 thin films composed of nanoparticles on hydrophobic glass substrates as well as on non-
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siliconized glass substrates by using spray pyrolysis technique, providing particular emphasis on
Experimental
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the photoluminescence (PL) emission properties.
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Tin oxide thin films were prepared by chemical spray pyrolysis technique using stannic chloride (SnCl4.5H2O) dissolved in distilled water as precursor solution and compressed air as
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spraying gas. Hydrophilic and hydrophobic glass slides (made by siliconization) were used as substrates. Hydrophilic glass slides were cleaned by a standard cleaning procedure by using nitric acid, acetone and distilled water. Hydrophobic substrates were prepared by soaking well cleaned glass slides in a standard siliconizing solution (Sigmacote) provided by Sigma-Aldrich. Substrate-to-nozzle distance was optimized and kept at 28 cm throughout the spraying process and substrates were preheated for 2 hours. SnO2 thin films were deposited by spraying the precursor solution onto preheated substrates kept at a temperature of 350 ºC. Spraying of precursor solution was interrupted after each 2 minutes for duration of 1 minute in order to avoid a decrease in the substrate temperature. As prepared samples were maintained at 350 ºC for 30 minutes and then allowed to cool to room temperature. SnO2 thin film samples were deposited using two different precursor molarities (0.2 and 0.4 M) both on hydrophilic as on hydrophobic glass substrates. Structural characterization of samples was performed by using a X-ray diffractometer (Rigaku-Dmax C) using the Cu-Kα line (λ=1.5414 Å) with 2θ values between 10 and 80º. The Page 3 of 19
surface of samples was examined by using a field emission gun scanning electron microscope (FESEM), Hitachi make (S-4800) at an accelerated voltage of 20kV. A spectrophotometer (JASCO V570 UV-VIS-NIR) working in the range of 200-800 nm at room temperature was used for optical measurements. Room temperature photoluminescence characterization of samples was done by using a high resolution Horiba Jobin Yvon LabRam system with a He-Cd laser (325
ip t
nm) as excitation source.
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Results and Discussion
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XRD patterns of samples (Fig.1) show that the diffraction peak positions are in agreement with the standard JCPDS data (Card No. 041-1445) of tetragonal rutile-phase SnO2. It can be seen that the diffraction peaks intensity increases with respect to the molarity of the
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precursor solution used for spraying on both type of substrates which indicates a better crystallinity as the molarity increases. Although the peak corresponding to the (110) orientation
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exhibited higher intensity, Harris texture analysis [32] shows that the (200) is the preferential orientation (Table 1). The degree of preferred orientation sigma in all the samples is much lower than that corresponding to a perfectly oriented sample, indicating that the films are poorly
d
oriented. Although the diffraction peaks of films grown with low solubility solutions have low
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intensity, the values of sigma indicate that the hydrophobic glass substrates produce more oriented films. The presence of a higher roughness of the surface in the hydrophobic glass
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substrates can explain this behaviour, as they could have better nucleation conditions, lowering the surface interfacial energy, giving rise to more density of nucleation points and better oriented films, as it will be explained later. The broad diffraction peaks indicate that the formed tin oxide particles have very small crystallite sizes.
Page 4 of 19
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Fig.1. XRD pattern of spray pyrolytically grown SnO2 thin films composed of nanoparticles synthesized by using 0.2 M precursor solution over (a) hydrophilic glass (b)
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hydrophobic glass and by using 0.4 M precursor solution over (c) hydrophilic glass (d)
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hydrophobic glass substrates.
Table 1: Texture coefficients and degree of preferred orientation from XRD pattern
Sample Perfectly oriented 0.2 M hydrophilic glass
(110) (101) (200) (211) (310) sigma 2.000 1.185 0.752 1.344 0.693 1.026 0.248
0.2 M hydrophobic glass 1.023 0.487 1.392 0.769 1.329 0.340
Page 5 of 19
0.4 M hydrophilic glass
1.178 0.651 1.369 0.756 1.045 0.265
0.4 M hydrophobic glass 1.170 0.260 1.857 0.385 1.329 0.600
A small, but clear shift in diffraction peaks towards lower angles for films grown over
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hydrophobic glass substrates with respect to films grown over hydrophilic glass substrates can be observed. This can be attributed to the increase in unit cell volume induced by epitaxial strain
cr
introduced in the material due to interfacial mismatch between hydrophobic substrate and grown film. The lattice parameter a of the tetragonal cell was obtained (with an uncertainty of about 1-
us
3%) for each sample by plotting, from at least three peaks of the (h k 0) family, the diffraction order (h2 + k2) versus 2sinθ /λ being θ the Bragg angle and λ the wavelength of the X-ray beam. The large values in hydrophobic substrates as compared to those in hydrophilic ones indicate the
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presence of a tensile strain in these samples. Parameter c of the tetragonal cell was calculated from the position of (211) peaks and the value of the parameter a of the tetragonal cell
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previously determined. Table 2 shows the values of lattice parameters as well as the unit cell volume. Films grown over hydrophobic substrates are having comparatively larger lattice
d
volume which can be attributed to the presence of tensile strain in these samples [33]. In many cases epitaxial strain due to lattice mismatch in heterostructures is accommodated through the
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formation and ordering of oxygen vacancies and act as a strain relief mechanism [33, 34]. The probability of increase in defects in samples on hydrophobic substrates due to epitaxial strain is
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evidenced from the photoluminescence spectra which will be discussed later.
Table 2: Lattice parameters and unit cell volume from XRD pattern
Sample 0.2 M hydrophilic glass
Lattice parameter (Å) a 4.75
c 3.10
Volume (Å3) a×a×c 69.94
Page 6 of 19
0.2 M hydrophobic glass
4.82
3.12
72.49
0.4 M hydrophilic glass
4.74
3.17
71.22
0.4 M hydrophobic glass
4.82
3.12
72.49
ip t
The average crystallite size values calculated from the XRD data using Scherrer’s
cr
formula [35] are given in Table 3. As SnO2 films grown over hydrophobic glass substrates have higher crystallite sizes, the assumption that the hydrophobic substrates may have more sites for
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the nucleation process and the formation of grains is reinforced. This can be explained on the fact that a hydrophobic surface obtained by siliconization generally has a rough surface and
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consequently hydrophobicity will increase with roughness. The increase in roughness contributes to the enhancement of surface area which geometrically boosts hydrophobicity (Wenzel model) [36]. Further, air may be trapped between hydrophobic solid-liquid interface which will increase
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the hydrophobic nature as explained by the Cassie model [36]. At any instance, there will be more nucleation sites remaining active on such a rough surface [37]. So adatoms acquiring
d
energy from the hot substrate will migrate into nearby nucleation sites, get nucleated and then coalesced to form larger and better oriented grains.
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FESEM images (Fig. 2) show that SnO2 thin films are composed of nanoparticles in the
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case of samples deposited over hydrophilic glass substrates (precursor molarity 0.2 and 0.4 M) and over hydrophobic glass substrate with a precursor molarity 0.2 M (Fig. 2(a), 2(b) and 2(c)). Whereas, SnO2 thin film composed of clusters of nanoparticles were formed over hydrophobic glass substrate with a precursor molarity 0.4 M (Fig. 2(d)). The surface of the films is found to be fairly uniform and homogenous. The size of nanoparticles were determined from the FESEM images and showed as histograms in Fig. 2. The size of nanoparticles is found to be in the range 2-7 nm while that of nanoparticle clusters are in the range 11-20 nm. The average crystallite size of the sample deposited over hydrophobic glass substrate with a precursor molarity of 0.4 M obtained from XRD data is 24 nm which is close to the cluster size values determined from FESEM image (Fig. 2(d)). The higher value of crystallite size obtained from XRD data suggests that nanoparticles constituting a cluster are coherently oriented (i.e., under the same Bragg diffraction conditions) [13]. SnO2 thin films grown over hydrophilic glass substrates show a high density distribution of smaller particles while films grown over hydrophobic surface have larger particles and their Page 7 of 19
density is comparatively lesser than that of films on hydrophilic glass substrates. The comparably lesser population and increase in size of nanoparticles on hydrophobic glass surface is attributed to the coalescence process driven by the so-called Ostwald ripening phenomenon. It is a thermodynamically driven spontaneous process in which larger particles grow at the expense of smaller particles by the diffusion of adatoms in order to decrease the total energy of the
ip t
system [38]. As a result, average particle size increases while density of particles decreases. The rate of adatom diffusion from smaller particles to larger particles decreases as particle size
cr
increases and finally ceases once all small particles are consumed [39]. Due to the difference in surface wettability, a hydrophobic surface prefers drop-wise condensation while a hydrophilic
us
surface prefers film-wise condensation [40]. The heat transfer rate yielded by drop-wise condensation is one order of magnitude higher than that given by the film-wise condensation because in the latter case the film spans the entire surface and acts as a thermal insulator
an
reducing the heat transfer rate [41]. As Ostwald ripening is a thermodynamically driven process the feasibility for its occurrence is much higher on the hydrophobic samples due to an increased
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heat transfer rate through the substrate. Nanoparticle clusters are observed only on the hydrophobic glass substrate sample (precursor molarity 0.4 M). This phenomenon is attributed to the increase in the rate of growth of nanoparticles due to the increase of precursor molarity
d
which reduces the rate of Ostwald ripening [42]. In addition, mobility of the adatoms decreases
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with increasing size which in turn decreases the probability of coalescence by collision. So, larger particles get hydrophobically accumulated around the nucleation centers to form clusters
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of nanoparticles with gaps between the clusters. Similar hydrophobic organization of monolayer protected Au clusters, on functionalised gold substrates, has been reported by Aslam et al [43]. Such hydrophobic organization will be important in designing complicated semiconducting nanoclusters possessing interesting optical properties on transparent substrates [43].
Page 8 of 19
ip t cr us an M d te Ac ce p Fig. 2. FESEM images of spray pyrolytically grown SnO2 thin films composed of nanoparticles synthesized by using 0.2 M precursor solution over (a) hydrophilic glass (b) hydrophobic glass and by using 0.4 M precursor solution over (c) hydrophilic glass (d) hydrophobic glass (nanoparticle clusters can be observed in this sample as marked in the image) substrates.
Page 9 of 19
ip t cr us an
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Fig. 3. Reflectance spectra of spray pyrolytically grown SnO2 thin films composed of nanoparticles synthesized by using 0.2 M precursor solution over (a) hydrophilic glass (b)
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hydrophobic glass substrates.
d
hydrophobic glass and by using 0.4 M precursor solution over (c) hydrophilic glass (d)
Fig. 4. Tauc plot of spray pyrolytically grown SnO2 thin films composed of nanoparticles synthesized by using 0.2 M precursor solution over (a) hydrophilic glass (b) hydrophobic glass and by using 0.4 M precursor solution over (c) hydrophilic glass (d) hydrophobic glass substrates. Page 10 of 19
There is a small shift in optical reflectance edges which leads to the corresponding variation in the optical band gap (Fig. 3). Optical band gaps can be calculated from Tauc plot as shown in Fig. 4 using Kubelka-Munk relation [44] and the obtained values are given in Table 3. SnO2 nanoparticles are expected to exhibit quantum confinement effects when their particle
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radius is of the order of the corresponding exciton Bohr radius, 2.7 nm [45]. So, there will be strong confinement if the particle size is lesser than the Bohr radius and a weak confinement
cr
effect will be observed if the particle size is slightly larger than the exciton Bohr radius [46]. Samples deposited on both hydrophilic and hydrophobic glass substrates with 0.2 M precursor
us
solution show weak confinement since their particle sizes are slightly larger than 2.7 nm and as a result the band gaps get blue shifted as the particle size decreases. Similar result is observed from films grown with 0.4 M solution on hydrophilic substrates used in the present investigation. No
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confinement effect is observed from films grown with 0.4 M solution on hydrophobic substrate since their size, 4.2 nm, is higher than 2.7 nm. The higher band gap observed in these films is
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ascribed to Burstein Moss shift [47]. Oxygen vacancies induced during the film growth increase the electrons near the conduction band edge producing donor levels which will increase the band
d
gap [47].
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composed of nanoparticles.
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Table 3: Crystallite size, particle size and band gap of spray pyrolytically grown SnO2 thin films
Sample
Average crystallite
Average particle
size (nm) from
size (nm) from
XRD
FESEM
Band gap (eV)
0.2 M hydrophilic glass
4.2
3.2
3.72
0.2 M hydrophobic glass
7.4
3.3
3.62
0.4 M hydrophilic glass
7
3.9
3.47
0.4 M hydrophobic glass
24
4.2
3.91
Page 11 of 19
ip t cr us
Fig. 5. Photoluminescence spectra of spray pyrolytically grown SnO2 thin films
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composed of nanoparticles synthesized by using 0.2 M precursor solution over (a) hydrophilic glass (b) hydrophobic glass and by using 0.4 M precursor solution over (c)
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hydrophilic glass (d) hydrophobic glass substrates.
Fig. 5 shows the room temperature photoluminescence spectra of as prepared SnO2
d
samples measured with an excitation wavelength of 325 nm. Typically SnO2 has a near band
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edge emission (NBE) in the UV region which could be due to excitons and a defect level emission (DLE) in the visible region which can be ascribed to intrinsic defects such as oxygen
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vacancies, tin interstitials and dangling bonds [23,46]. Gaidi et al have reported room temperature photoluminescence of SnO2 nanoparticles, but only after annealing the samples at 700 °C [13]. In our study we have obtained room temperature luminescence without any post deposition annealing treatment. The PL spectra of as prepared samples have mainly two peaks, one broad peak in the UV region centred at 380 nm (3.26 eV) from samples grown over hydrophilic glass substrates and a second broad peak in the visible range 495-594 nm (2.5-2.1 eV) from all samples. The intensity of peaks in the visible region is found to be increased with increasing molarity of the precursor solution irrespective of the nature of substrates used for the deposition of thin films. The broad NBE emission from 0.2 M hydrophilic sample centred at 380 nm is attributed to defects close to the conduction band since the positions of emission are lower than the corresponding band gap of the sample [13]. Similar emission with lesser intensity is observed for 0.4 M hydrophilic glass samples which may be due to the electronic transition from the conduction band to the valence band and also from defects close to the conduction band. All the samples have broad defect level emissions in the range 495-594 nm with varying intensity. Page 12 of 19
This could be attributed to electronic transitions due to various defect levels present in the band gap such as oxygen vacancies. Oxygen vacancies are considered to be the most prominent defects in semiconductor oxides acting as luminescent centres by creating defect levels inside the band gap and trapping electrons from the valence band [48]. The oxygen vacancies do not contribute to the formation of occupied donor levels which are just below the conduction band,
ip t
but the occupied levels associated to the vacancy appear at around 1 eV above the top valence band (i.e., 2.6 eV below the conduction band) [49]. So electrons coming back from the
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conduction band after excitation may get trapped in the shallow defects close to the band edge. They either recombine with holes in the valence band or continue to relax to deeper defect states present in three different charge states in the oxides:
us
before recombining, contributing to a broad visible PL emission [48]. Oxygen defects can be ,
and
states [50]. As
is
assumed to be a very shallow donor, most oxygen vacancies will be in their paramagnetic
an
states under flat band conditions [50]. It has been reported that visible broad emissions are due to the recombination of electrons in singly occupied oxygen vacancies (
) with photo excited
M
holes in the valence band [48]. In addition, the interactions between oxygen vacancies and interfacial tin vacancies cause the formation of a significant number of trapped states, which
te
region [51].
d
form a series of metastable energy levels within the band gap resulting in emission in the visible
One can observe that intensity of NBE emission is suppressed and DLE emission is
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enhanced from films grown over hydrophobic glass substrates. Hydrophobic glass substrates, having roughened surface will increase possibility of lattice mismatch between the substrate and the film which will induce the accumulation of strain during the film growth. This strain will promote the formation of lattice defects within the crystal. Such defects could be both radiative and nonradiative and will reduce the excitonic recombination resulting in poor UV emission efficiency. Radiative defects such as oxygen vacancies formed during the film growth are responsible for visible PL emission. Weak visible PL emission from 0.2 M hydrophobic glass sample indicates that defects formed during the film growth are mainly nonradiative. Nonradiative defects are formed in a material where lattice distortion is high [47]. This is true for this sample as evident from the XRD pattern which shows its poor crystalline nature. Increased crystallite size in 0.4 M hydrophobic glass sample decreases the concentration of nonradiative recombination centres enhancing visible emission [23]. Another factor regarding films on hydrophobic glasses is the presence of isolated nanoparticles, which will have large surface area and may have large number of defects such as oxygen vacancies resulting in the enhancement of Page 13 of 19
trap emission in the visible region. A red shift in the visible PL peak position is observed from samples on hydrophobic glass substrates, which can be attributed to the combined effect of tensile strain and larger crystallite size observed in these samples [52]. The present PL observation suggests that SnO2 thin films grown on hydrophobic substrates can be used for the quenching of UV emission and enhancement of
ip t
photoluminescence in the visible region. Such substrate dependent tuning of photoluminescence properties of SnO2 is important for the fabrication of white LEDs and other optoelectronic
cr
devices. Isolated nanoparticles in SnO2 thin films grown over hydrophobic substrates have large exposed surface area and high surface energy due to more number of surface atoms which
us
accelerates adsorption of oxygen in ambient atmosphere and are good for the fabrication of gas sensors similar to that of ZnO nanorods reported in the literature [29]. Interestingly, the presence states enhances the adsorption of oxygen without lowering the expansion
an
of large amount of
level of depletion layer which may also contribute to gas sensing efficiency [29].
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Conclusions
We report the self-assembled growth of spray pyrolytically grown SnO2 thin films composed of nanoparticles on hydrophilic and hydrophobic glass substrates by using the spray
d
pyrolysis technique. SnO2 thin films deposited on hydrophobic substrates are more oriented
te
compared to hydrophilic substrate samples and the growth of larger grains is attributed to the availability of more nucleation sites. SnO2 thin films grown on hydrophobic substrates have
Ac ce p
larger particles with low density distribution compared to films grown on hydrophilic substrates owing to Ostwald ripening phenomenon. Band gap of samples having particle sizes slightly larger (3.2, 3.3 and 3.9) than the exciton Bohr radius are blue shifted due to quantum confinement effect. The increase in band gap observed from 0.4 M hydrophobic glass sample as compared to other samples is attributed to Burstein Moss shift. Defect level emission is enhanced in the PL spectra from films on hydrophobic substrates with the suppression of near band edge emission. The enhanced visible emission from SnO2 thin films grown on hydrophobic surfaces is attributed to unintentionally induced lattice defects in the films during their growth on these surfaces. Thus SnO2 thin films composed of nanoparticles grown on hydrophobic substrates may find useful in optoelectronic applications and also as gas sensors owing to the occurrence of isolated nanoparticles and large amount of
states.
Page 14 of 19
Acknowledgments The authors are thankful to KSCSTE, Government of Kerala for the financial support of this work (No: 023/SRSPS/2007/CSTE). The authors acknowledge Prof. M. R. N. Murthy, I.I.Sc. Bangalore for providing facilities for the preparation of hydrophobic glass substrates and
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Prof. M. K. Jayaraj, Dept. of Physics, CUSAT, for providing PL measurements under DST, India nano mission initiative program. We also thank the SCSIE–University Valencia for providing the
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FESEM facilities related with (MINECO) projects TEC2011-28076-C02-01/02 and TEC201460173-C2-1-R, Generalitat Valenciana projects PrometeoII/20115/004 and ISIC/2012/008.
us
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Page 17 of 19
Table 1: Texture coefficients and degree of preferred orientation from XRD pattern
Sample
(110) (101) (200) (211) (310) sigma 2.000
Perfectly oriented
1.185 0.752 1.344 0.693 1.026 0.248
ip t
0.2 M hydrophilic glass
0.2 M hydrophobic glass 1.023 0.487 1.392 0.769 1.329 0.340 1.178 0.651 1.369 0.756 1.045 0.265
cr
0.4 M hydrophilic glass
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0.4 M hydrophobic glass 1.170 0.260 1.857 0.385 1.329 0.600
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Table 2: Lattice parameters and unit cell volume from XRD pattern
Ac ce p
0.4 M hydrophilic glass 0.4 M hydrophobic glass
c
Volume (Å3) a×a×c
3.10
69.94
4.82
3.12
72.49
4.74
3.17
71.22
4.82
3.12
72.49
d
0.2 M hydrophobic glass
4.75
te
0.2 M hydrophilic glass
Lattice parameter (Å) a
M
Sample
Page 18 of 19
Table 3: Crystallite size, particle size and band gap of spray pyrolytically grown SnO2 thin films composed of nanoparticles. Average crystallite
Average particle
size (nm) from
size (nm) from
XRD
FESEM 3.2
0.2 M hydrophobic glass
7.4
3.3
0.4 M hydrophilic glass
7
3.9
0.4 M hydrophobic glass
24
3.72
ip t
4.2
(eV)
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0.2 M hydrophilic glass
Band gap
3.47
3.91
an
4.2
3.62
cr
Sample
Ac ce p
te
d
M
Page 19 of 19