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Volume 2012 (2012), Article ID 839612, 6 pages
Chemical Synthesis and Properties of Novel Nanostructures and Microplatelets
1Department of Physics, University of Botswana, Private Bag UB 0022 Gaborone, Botswana
2African Materials Science and Engineering Network (AMSEN), A Carnegie IAS-RISE Network, UB Node, Private Bag UB 0022 Gaborone, Botswana
Received 2 August 2012; Accepted 29 August 2012
Academic Editors: S.-S. Lin, K. Yamashita, and K. Zupan
Copyright © 2012 Cosmas M. Muiva et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Bi2Se3 nanostructures, microsised fibre bundles, and platelets have been synthesised through a simple chemical spray pyrolysis route on glass substrates. Low substrate temperatures () favoured growth of nanostructures while elevated was accompanied by increased grain growth and formation of microsised platelets. The nanostructures were identified as nanorods, nanofibers, nanotubes, nanoflowers, and nanoneedles of various aspect ratios. There was an observed red shift of the optical band gap () with increased from 2.40 eV at 95°C to 1.25 eV at 360°C. Thermo probe measurements confirmed n-type conductivity with dark resistivities which followed a near exponential decline with elevated . The observed changes in optoelectronic properties existing in the different regimes were explained on the basis of microstructural and textural changes in the pristine clusters.
Recently, research attention geared toward chemical and electrochemical synthesis of nanostructured colloidal chalcogenide semiconductors has increased several folds [1–10]. In this group, Bi2Se3 nano- and microsised clusters, isolated or well-adhered morphological orientations have been reported in literature. Ota et al.  has described synthesis and characterization of Bi2Se3 nanorods through a hydrothermal method. Nearly spherical grains by chemical bath deposition (CBD) were reported recently by Bari and Patil . Other reports include hexagonal rod like nanostructures by convectional solvothermal method  and microplatelets by single-source precursor metal-organic chemical vapour deposition (MOCVD) . A number of interesting nanostructures such as hexagonal nanosheets and nanotubes through hydrothermal coreduction  and hexagonal nanoflakes through a novel solvothermal method  were recently fabricated and characterised. In addition, nanocrystalline thin films by electrochemical atomic layer epitaxy (ECALE)  and successive ionic layer adsorption and reaction (SILAR)  were reported. Li et al.  have synthesised and traced shape evolution of novel bismuth selenide hollow nanospheres through precipitation in a capping agent while thermally evaporated structures have been discussed by Manjulavalli et al. .
Reduced dimensions and different shapes in the nanoregime are quite an attractive recipe for applications owing to reduced dimensions or surface area catalysis of the well-known properties. Beside applications such as photosensitive devices, Hall-effect magnetometer, high-frequency power sensor thermopiles, solar selective decorative coating, wide band radiation detectors, and humidity sensors using the Seebeck and Peltier effects, addition of a third element such as Te to Bi2Se3 is known to produce the best thermoelectric properties so far reported . In this paper we report cheap synthesis and characterisation of Bi2Se3 nanostructures (nanorods, nanofibres, nanotubes, nanoflowers, and nanoneedles) and microplatelets grown on glass substrates through a simple chemical spray pyrolysis technique.
2. Experimental Details
Bi2Se3 nanostructures and microplatelets were prepared using a chemical spray pyrolysis set up described elsewhere [12, 13]. The starting analytical grade compounds were purchased from Sigma-Aldrich and used without further processing. The precursor solution was a mixture of 0.1 M selenourea (SeC(NH2)2) and 0.1 M bismuth (III) chloride (BiCl3) dissolved in 85% acetone and 15% water. The solution was sprayed on 3 cm × 7 cm precleaned glass substrates held 12 cm below the spray nozzle. The spray rate for all the samples was 2 mL/min making a total of 30 mL for each spray cycle which lasted 15 min. A bimetallic thermometer that could measure the temperature to an accuracy of ±1°C was used to measure the substrate temperature (). A set of samples were deposited at each operating temperature (95°C, 165°C, 237°C, 310°C, and 360°C).
The crystallographic phases and structure of the material were examined with a Philips PW-3710 X-ray diffraction (XRD) system using Cu K radiation () operating at 40 kV, 30 mA. The surface morphology of the sample was studied using a Philips XL 30 scanning electron microscope (SEM) fitted with an energy dispersive spectroscopy (EDS) detector for elemental composition analysis. A double beam Cary 500 UV/VIS/NIR spectrophotometer was used for the optical characterisation of the clusters by collecting the incident transmittance () and reflectance () spectra in the spectral range 300–2500 nm. A Keithley-based four-point probe set-up was used to collect data.
3. Results and Discussion
Figure 1 shows XRD diffractographs of Bi2Se3 structures prepared at different . The observed Bi2Se3 diffraction peaks correspond to hexagonal crystallographic lattice (JCPDS 33-0214) with space group: (166).
The XRD pattern shows that samples prepared at low are not of high purity because of too many reflections that cannot be indexed to a pure phase of Bi2Se3. Conspicuously, a sample prepared at 95°C reveal other crystallographic presentations which could not be assigned to Bi2Se3. These unidentified peaks were assigned to impurities and included BiOCl, BiOCl4, and Bi2(SeO5). An EDS scan confirmed presence of oxygen and chlorine in the samples prepared at 95°C and 165°C. At higher temperatures, both XRD and EDS showed presence of Bi2Se3 only. It is anticipated that owing to low temperature there was partial decomposition of the precursor leading to formation of some by products and undecomposed metalorganics. At higher temperature, there was complete decomposition of the precursor and vaporisation of the volatile by products. The oxygen may have formed SeO which sublimed at higher temperatures.
The dislocation density () of the crystallites in the Bi2Se3 films at varied was obtained from (2)  while grain size () was calculated using the Deby-Scherrer’s  formula where is the wavelength of the X-rays used, the full width at half maximum of the peak in radian, the Bragg’s angle of diffraction, and is usually a constant ≈ unity. The values of and are shown in Table 1.
SEM images (Figure 2(a)) reveals nanoneedles embedded in an amorphous matrix. In the low regimes, nanoparticles of different morphologies and shapes are formed while the growth shifts towards microplatelets at higher substrate temperatures. The nanoparticles formed at 95°C (Figure 2(a)) are not well defined. They were identified as nanoneedles with random orientations along all directions. This random orientation implies low adatom surface energies and lifetimes on the substrate surface.
At 165°C, the observed surface was too warped with dislocations and cavities. Nanoparticles of different shapes, sizes, aspect ratios, and morphologies were observed in different cavities of the same substrate. The distinct ones were identified as nanoneedles, nanofibers, nanorods, nanotubes, and microflowers. Although pure clusters of nanoneedles were observed in some cavities, SEM images reveal that these microflowers and nanotubes are build up through aggregation of nanoneedles in regular patterns. Similarly, the nanofibers clung together to form bundles of irregular lengths and widths. The aspect ratio of the nanoneedles (), Figure 2(g), was found to be in the range of 14.2. The cavities may be due to gases evaporating coupled with lack of enough thermal energy for reorganisation due to low lifetime of adatoms. The difference shapes, sizes, and morphologies may be due to thermal gradients along the substrate which was possibly the optimum temperature for nanoparticle formation. At these low , the slowly evaporating solvent may have acted as a capping agent leading to formation of isolated nanoparticles. The observed nanoparticles were oriented length wise parallel to the substrate surface. The grain sizes () were also estimated by random sampling and averaging from SEM images. The values were entered in Table 1.
SEM images (Figures 3(a) and 3(b)) reveal that at 310°C and 360°C, growth shifted towards irregularly shaped microplatelets with cross-sectional spans of up to 10 μm. There was enhanced increase in size as the changed from 310°C to 360°C. High surface energy indicates high adatom mobility and increased lifetime on the substrate surface . It can therefore be concluded that nuclei yielding platelets represent the lowest surface energy. The same microplatelets were observed by others  using single source MOCVD at precursor temperature of 275°C and of 425°C.
The lattice parameters were calculated using the expression [10, 17] where , , and are the miller indices of the crystallographic planes, and are the lattice parameters, and is the lattice constant. For hexagonal structure the lattice parameters and are related to the lattice angles and through [18, 19] The observed values of , , , , and are shown in Table 2. The values have been compared to standard values of Bi2Se3. The calculated values are corresponding to standard values of hexagonal Bi2Se3 confirming that the samples are hexagonal Bi2Se3. The deviation of the experimental cell parameters and angles from the cell parameters in JCPDS 33-214 may be due to strain in the material resulting to elongated bonds and consequently higher cell parameters ( and ) .
The percentage of incident radiation transmitted through the samples was determined from spectrophotometric measurements and presented in Figure 4. An absorption edge corresponding to the spectral wavelengths (), 480–750 nm, was observed. Interference fridges observed for Bi2Se3 prepared at 95°C in the NIR region was explained on the basis of the difference in the refractive index of the smoother sample and substrate. There was no outright absorption edge and the observed curves were seen to rise gradually which may have been due to impurities and mixed clusters each presenting a distinct absorption edge. The recorded transmission of a sample fabricated at 165°C was low due to small grains, rough surface, and higher dislocation densities, all contributing elevated scattering of photons.
The optical absorption coefficient is determined from transmittance spectra and film thickness using the relation: The optical absorption is related to the incident photon energy () by the following equation  where is the optical band gap, the frequency of the photons, is a constant referred to as the band tailing parameter, and can take values 1/2, 3/2, 2, and 3 depending on the mechanism of transition across the energy gap. From the obtained plots, offers the best fit which is consistent with allowed direct transitions. The value of can be obtained by plotting versus as shown in Figure 5.
The observed plots have a linear portion from where can be obtained by extrapolation towards . In this region, the absorption coefficient is of the order of >104 cm−1. The observed values of are shown as an inset in Figure 5 and the range is consistent with those reported earlier through other deposition methods 2.25 eV , 1.5 eV , 2.35 eV , and 1.7–1.41 eV . There was a red shift towards lower energies with increasing substrate temperature for the samples. Lower values have been reported in thin films, 0.35 eV  and 0.55 eV . This wider band gap values in nanostructures may be due to quantum confinement effects . For our case the largest band gap value 2.40 eV was observed for the lowest of 95°C. We observed a similar widening of the band gap at lower in spray-pyrolysed In2Se3  thin films, which was attributed to presence of impurities such as oxides which are known to have wide band gap values. Presence of these impurities in the present case was detected during both XRD and EDS analysis.
Thermo probe studies established that all the samples synthesized in this study showed n-type dark conductivity. The resistivity () values were calculated from the characteristics measured at room temperature (300 K) from the expression , where is the film thickness and is the slope of a against plot. Sampling and averaging techniques were used since the clusters prepared at 165°C were inhomogeneous. The resistivity values as a function of are shown in Figure 6. The resistivity curve displayed a monotonic decline (Figure 6) with from an order of 103 Ω cm at 95°C to 100 Ω cm at 360°C. Improved conductivity was attributed to better crystallinity with increased substrate temperature resulting from better stacking of the film. It is anticipated that at lower temperature the adatoms are having very low surface energies and their mobility over the substrate is very low with consequent random stacking of the grains resulting in high porosity . Moreover, at these low volatile by products which would have been vaporised at higher temperatures get trapped in grain boundaries and pores acting as carrier traps or sources of potential barriers increasing the resistivity. Increased grain sizes at higher temperatures imply lower dislocation densities (Table 1) and hence lower densities of grain boundaries.
Nano- and microsised structures of Bi2Se3 thin films were prepared by CSPT. The optical band gap was observed to decrease with with a minimum value of 1.25 eV at 360°C. The morphology of growth was influenced by the with low favouring growth of nanoparticles while high resulted to microplatelets. XRD studies confirmed formation of hexagonal Bi2Se3 with typical lattice parameters, , , and for a sample prepared at 310°C. An n-type dark conductivity was observed for all the samples with resistivities that decreased with elevated and grain size.
The authors are grateful for the financial support of African Materials Science and Engineering Network (AMSEN), a Carnegie IAS-RISE network, and the University of Botswana through research Grant R 907.
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