Lu3+/Yb3+ and Lu3+/Er3+ co-doped antimony selenide nanomaterials: synthesis, characterization, and electrical, thermoelectrical, and optical properties
© Hanifehpour et al.; licensee Springer. 2013
Received: 18 February 2013
Accepted: 11 March 2013
Published: 27 March 2013
Lu3+/Yb3+ and Lu3+/Er3+ co-doped Sb2Se3 nanomaterials were synthesized by co-reduction method in hydrothermal condition. Powder X-ray diffraction patterns indicate that the Ln x Ln′ x Sb2−2xSe3 Ln: Lu3+/Yb3+ and Lu3+/Er3+ crystals (x = 0.00 − 0.04) are isostructural with Sb2Se3. The cell parameters were increased for compounds upon increasing the dopant content (x). Scanning electron microscopy and transmission electron microscopy images show that co-doping of Lu3+/Yb3+ ions in the lattice of Sb2Se3 produces nanorods, while that in Lu3+/Er3+ produces nanoparticles, respectively. The electrical conductivity of co-doped Sb2Se3 is higher than that of the pure Sb2Se3 and increases with temperature. By increasing the concentration of Ln3+ions, the absorption spectrum of Sb2Se3 shows red shifts and some intensity changes. In addition to the characteristic red emission peaks of Sb2Se3, emission spectra of co-doped materials show other emission bands originating from f-f transitions of the Yb3+ ions.
KeywordsCo-doped Nanomaterial Luminescent Electrical conductivity Hydrothermal
Nanosized semiconductor materials have drawn much research attention because their physical and chemical properties, due to size quantization effect, dramatically change and, in most case, are improved as compared with their bulk counterparts [1–3]. Rare earth-substituted compounds with various compositions have become an increasingly important research topic in diverse areas, such as luminescent device, light-emitting displays, biological labeling, and imaging [4–6], due to the introduction of dopant levels within the bandgap and modification of the band structure. In addition, significant efforts have been devoted to enhance the activity of wide bandgap photocatalysts by doping for environmental remediation [7, 8]. Semiconductor selenides find applications as laser materials, optical filters, sensors, and solar cells. Antimony selenide, an important member of these V 2 VI 3 compounds, is a layer-structured semiconductor of orthorhombic crystal structure and exhibits good photovoltaic properties and high thermoelectric power, which allows possible applications for optical and thermoelectronic cooling devices [9–11]. The research of impurity effects or doping agents on the physical properties of Sb2Se3 is interesting both for basic and applied research. Doping of some transition metal and lanthanide to the lattice of metal chalcogenides has been investigated [12–20]. The incorporation of large electropositive ions such as lanthanides into metal chalcogenide frameworks is expected to affect the electronic properties of that framework. In this work, we report the preparation, structural, electrical, and optical properties of Lu3+/Yb3+ and Lu3+/Er3+ co-doped antimony selenide via co-reduction method at hydrothermal condition.
All chemicals were of analytical grade and were used without further purification. Gray selenium (1 mmol) and NaOH (5 mmol) were added to distilled water (60 mL) and stirred well for 10 min at room temperature. Afterwards, hydrazinium hydroxide (2 mL, 40 mmol), SbCl3 (1.98, 1.96, 1.94, and 1.92 mmol) and Ln2O3 (0.00, 0.01, 0.02, and 0.04 mmol) (Ln: Lu3+, Yb3+, Er3+) based on the molecular formula Ln x Ln′ x Sb2−2xSe3 (0 ≤ x ≤ 0.04) were added, and the mixture was transferred to a 100-mL Teflon-lined autoclave. The autoclave was sealed, maintained at 180°C for 48 h, and then cooled to room temperature. The optimum conditions for this reaction are pH = 12, temperature = 180°C, and reaction time = 48 h. The black precipitate obtained was filtered and washed with ethanol and water. It was dried at room temperature. Yields for the products were 75% to 85%. Phase identification was performed by powder X-ray diffraction (XRD, D5000 Siemens AG, Munich, Germany) with Cu Kα radiation. Cell parameters were calculated using the Celref program (CCP14, London, UK) from powder XRD patterns, and reflections have been determined and fitted using a profile fitting procedure with the WinXPOW program (STOE & CIE GmbH, Darmstadt, Germany). The reflections observed in 2θ = 4° to 70° were used for the lattice parameter determination. The morphology of materials was examined by scanning electron microscopy (SEM, Hitachi S-4200, Hitachi High-Tech, Minato-ku, Tokyo, Japan). A linked ISIS-300 Oxford EDS detector (Oxford Instruments plc, Oxfordshire, UK) was used for elemental analyses. The high-resolution transmission electron microscopy (HRTEM) image and selected area electron diffraction (SAED) pattern were recorded by a Cs-corrected HRTEM (JEM-2200FS, JEOL Ltd., Akishima, Tokyo, Japan) operated at 200 kV. Photoluminescence measurements were carried out using a Spex FluoroMax3 spectrometer (HORIBA Jobin Yvon Inc., Edison, NJ, USA) after dispersing a trace amount of sample via ultrasound in distilled water. Four-point probe method was used for the measurement of electrical and thermoelectrical resistivity of samples. A small oven was needed for the variation of temperature of the samples from the room temperature to about 200°C (maximum). A small chip with 1-mm thickness and 7-mm length was used for this analysis.
Results and discussion
The temperature dependence of the electrical resistivity for co-doped Sb2Se3 nanomaterials between 290 and 350 K is shown in Figure 8b. Electrical resistivity decreases linearly with temperature, and the minimum value for Lu0.04Yb0.04Sb1.92Se3 was measured as 0.0006 Ω·m and for Lu0.04Er0.04Sb1.92Se3 as 0.005 Ω·m. Two factors that include the overlapping of wave functions of electrons in doped Sb2Se3 and that acting as a charge carrier due to lanthanide atomic structure (having empty f orbitals) are important reasons for decreasing electrical resistivity. The obtained data shows higher electrical resistivity for co-doped samples in comparison with doped samples in the case of Lu3+, Yb3+ and Er3+ doped Sb2Se3[16, 17]. The measurements indicate that the co-doping materials have higher electrical and thermoelectrical conductivity than the doped compounds in spite of lower lanthanide content [16–20]. Comparing both doped and co-doped data, the combining energy levels of the two lanthanides and the overlapping of wave functions of electrons in two different lanthanides are responsible for the difference between the obtained results. Among the co-doped compounds, Lu3+/Yb3+-doped Sb2Se3 has the higher electrical conductivity.
For Lu0.04Er0.04Sb1.92Se3, the transition of the Er3+ ions is not observed because of instrument limitation. The peaks between 500 and 620 nm can then be assigned to the lattice of Sb2Se3 (Figure 9b). The difference between absorption patterns of compounds is related to various defects created in the lattice. There is a red shift in the doped materials in comparison with pure Sb2Se3 because of the smaller nanoparticles of Sb2Se3, in which the bandgap is higher than the doped nanomaterials [24, 25]. It is well known that the fundamental absorption can be used to determine the nature and value of the optical bandgap of the nanoparticles. The bandgap energies of samples were estimated from the absorption limit. The calculated bandgap is 2.43 eV for Lu0.04Yb0.04Sb1.92Se3 and 2.36 eV for Lu0.04Er0.04Sb1.92Se3.
In case the of Lu0.04Er0.04Sb1.92Se3, intra-4f Er3+ transitions of the 4I11/2 and 4I13/2 levels to the ground state (4I15/2) are expected around 1.54 μm. These could, however, not be determined due to equipment limitations . Therefore, emission bands at 550 to 700 nm are related to the crystal structure of Sb2Se3 (Figure 10b). The optical properties of co-doped compounds considering absorbance and photoluminescence spectra show similar f-f transitions in the case of Yb-doped materials and similar results for Lu- and Er-doped materials as obtained for Ln-doped Sb2Se3. We expect that these materials can be good candidates as novel photocatalysts due to their modified bandgaps by doping with lanthanides. Indeed, doping is the best way for the modification of semiconductors for special uses such as photocatalysts in order for the degradation of azo dye and organic pollutant to take place.
New thermoelectric Ln2xSb2−2xSe3 (Ln: Lu3+/Yb3+ and Lu3+/Er3+)-based nanomaterials were synthesized by a simple hydrothermal method. The cell parameters were increased for compounds upon increasing the dopant content (x). According to the SEM and TEM images, different morphologies were seen in co-doped Sb2Se3. The HRTEM image and SAED pattern show similar growth  directions for Lu3+/Yb3+ co-doped like Sb2Se3 nanorods. Lanthanide doping promotes the electrical conductivity of Sb2Se3 as well as thermoelectrical conductivity. UV–vis absorption and emission spectroscopy reveals mainly the electronic transitions of the Ln3+ ions in the case of Yb3+-doped nanomaterials.
This work is funded by the World Class University grant R32-2008-000-20082-0 of the National Research Foundation of Korea.
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