Structural Studies and Optical and Electrical Properties of Novel Gd 3 +-Doped Sb 2 Se 3 Nanorods

1 Department of Inorganic Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz 51664, Iran 2 World Class University Nano Research Center, School of Mechanical Engineering, Yeungnam University, Gyongsan 712-749, Republic of Korea 3 Center for Research Facilities, Yeungnam University, Gyongsan 712-749, Republic of Korea 4 Department of Nano, Medical and Polymer Materials, Yeungnam University, Gyongsan 712-749, Republic of Korea


Introduction
Investigations on semiconductor nanostructures have recently been in the focus of intensive research activities because of intrinsic fundamental interest and manifold possibilities for applications.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 (TEP) which allows possible applications for optical and thermoelectronic cooling devices [1][2][3][4].Inorganic nanomaterials doped by lanthanide (Ln 3+ ) ions with various compositions have become an increasingly important research subject, and opened up opportunities for creating new applications in diverse fields, including biological labelling and imaging and light-emitting displays, owing to their distinct optical properties [5][6][7].Over the past two decades, many methods have been employed to prepare Sb 2 Se 3 nanotubes, nanowires, nanosheets, nanorods, nanobelts, nanospheres, and nanoflakes including thermal decomposition [8], microwave irradiation [1,9], complex decomposition approach [10], solvothermal reaction [11][12][13][14], hydrothermal method [15,16], vacuum evaporation [17], colloidal synthetic method [18], and other chemical reaction approaches.Studies of impurity effects or doping agents on the physical properties of Sb 2 Se 3 are interesting both for basic and applied research.Doping of trivalent cations such as Sb 3+ [19], In 3+ [20], Fe 3+ [21], Mn 3+ [22], and a number of further trivalent 3d elements [23] to the lattice of Bi 2 Se 3 has been investigated, also EPR spectra of Gd-doped bulk Bi 2 Se 3 [24].Also, new Ln x Bi 2−x Se 3 (Ln: Sm 3+ , Eu 3+ , Gd 3+ , Tb 3+ , Nd 3+ ) based nanomaterials were synthesized by Alemi et al. [25,26].Recently, we have reported novel luminescent nanomaterials based on doping of lanthanide (Ln: Ho 3+ , Nd 3+ , Lu 3+ ) into the lattice of Sb 2 S 3 and (Ln: Ho 3+ , Nd 3+ , Lu 3+ , Sm 3+ , Er 3+ , Yb 3+ ) into the lattice of Sb 2 Se 3 [27][28][29][30].The incorporation of large cations such as lanthanides into antimony chalcogenide frameworks is expected to lead to materials with different optical and electrical properties.The incorporation of lanthanide ions  into a Sb-Se framework could dramatically affect the electronic properties of that framework.In this research, nanorods of Gd x Sb 2−x Se 3 crystals (x = 0.00-0.04)were synthesized by introducing small amounts of Gd 3+ to the Sb 2 Se 3 lattice.Structural, spectroscopic properties and electrical conductivity of the synthesized materials are reported.

Experiment
All chemicals were of analytical grade and were used without further purification.Grey selenium (1 mmol) and NaOH (5 mmol) were added to distilled water (60 mL) and stirred well for 10 min at room temperature.Afterwards,    The HRTEM image and SAED pattern were recorded by a Cs-corrected high-resolution TEM (JEM-2200FS, JEOL) operated at 200 kV.Photoluminescence measurements were carried out using a Spex FluoroMax3 spectrometer after dispersing a trace amount of sample via ultrasound in distilled water.The Four-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 (max.).Small chip with 1 mm thickness and 7 mm length was used for this analysis.This chip was obtained by pressing of 10 mg of sample under 30 kpa pressing device.

Results and Discussion
Sb 2−x Gd x Se 3 samples were prepared by a hydrothermal coreduction method.SbCl 3 (99.99%),selenium powder, and Gd 2 O 3 were used as starting materials, and hydrazinium hydroxide was used as the reducing agent.The powder Xray diffraction (P-XRD) patterns (see.e.g. Figure 1) indicate that the Gd 3+ -doped powders have the same orthorhombic structure as Sb 2 Se 3 and that single phase Sb 2 Se 3 is retained at lower doping concentrations of Gd 3+ .All the peaks in the Figure 1  The EDX analysis of the product confirms the ratio of Sb/ Se/Gd as expected (Figure 2).Also, ICP analysis confirms the exact amount of doping.
The cell parameters of the synthesized materials were calculated from the XRD patterns.With increasing dopant content (x), the a and b parameters for Gd 3+ increase and the c decreases (Figure 3).The trend for lattice constants can be correlated to the effective ionic radii of the Gd 3+ ions; assuming that the radius of Gd 3+ is larger than that of Sb 3+ results in greater amount of lattice parameters for Gd 3+doped materials.Scanning electron microscopic (SEM) images of Sb 2 Se 3 nanorods show rods up to 5 μm lengths and diameter of 25-120 nm (Figure 4).Also, Figures 5(a   UV-Vis spectra of Gd 3+ doped Sb 2 Se 3 are shown in Figure 7.The absorption bands of Gd 3+ doped Sb 2 Se 3 exhibit maxima at 850, 860, and 870 nm were assigned to gadolinium electronic transition of 8 S 7/2 → 6 I 5/2 , 13/2,11/2 , 8 S 7/2 → 6 I 17/2 , 9/2 and 8 S 7/2 → 6 I 7/2 [31,32]. There is also a shift in the onset of absorption to lower energies (red shift) in Gd-doped samples compared to Sb 2 Se 3 (see Figure 8).Band gap energies (E g ) for Sb 2 Se 3 and Sb 1.96 Gd 0.04 Se 3 are calculated from the UV-Vis absorption spectra as E g = 1.55 eV for pure Sb 2 Se 3 and E g = 1.49eV for Sb 1.96 Gd 0.04 Se 3 [29,30].
The sharp emission peak at 840 nm on excitation with the 835 nm in Figure 9 can be assigned to 6 P 7/2 → 8 S 7/2 transitions of Gd 3+ [31,32].
In doped semiconductors, the two types of emissions are responsible for the dopant (impurity) luminescence.One can be observed only upon direct excitation of the dopant.The second type is obtained if energy transfer from host to dopant occurs.Scheme 1 shows the band-gap model for the luminescence of Gd-doped Sb 2 Se 3 .The excitation of electrons across the host lattice band gap from the valence to conduction band is followed by a radiative recombination with a deeply trapped hole at a defect state or a nonradiative relaxation into the lanthanide ion 5d level.Radiative transition from the excited state to the ground state of Gd 3+ ions occurs, leading to luminescence of the Gd 3+ ions [26].
Binary compounds such as Sb 2 Se 3 and its alloys are thermoelectric materials with layered crystalline structures.These materials have been investigated for direct conversion of thermal energy to electric energy and they specially are used for electronic refrigeration [33][34][35][36].The Four-Probe Method was used for the measurement of electrical and thermoelectrical resistivity of samples (Scheme 2).
The electrical resistivity of the compounds is shown in Figure 10.With the increase in the lanthanide cation concentration, the electrical resistivity of synthesized nanomaterials decreased obviously.At room temperature the electrical resistivity of pure Sb 2 Se 3 was of the order of 0.2 Ω • m and for Sb 1.96 Gd 0.04 Se 3 was 6 × 10 −2 Ω • m, respectively.
The temperature dependence of the electrical resistivity for Sb 1.96 Gd 0.04 Se 3 between 290-350 K is shown in Figure 11  in which electrical resistivity decreases with temperature.As a result, the electrical conductivity of Gd-doped Sb 2 Se 3 materials is higher than pure Sb 2 Se 3 at room temperature, and increases with temperature.Two factors including overlapping of wavefunctions of electrons in doped Sb 2 Se 3 and acting as a charge carrier due to Gd atomic structure (having empty f orbitals) are important reasons for decreasing electrical resistivity.

Conclusions
Coreduction synthesis is a simple and efficient method for preparing luminescent nanomaterials of Gd x Sb 2−x Se 3 (x = 0.00 to 0.04).SEM images show that doping of Gd 3+ into the sites of the Sb 3+ does not change the morphology of Sb 2 Se 3 .HRTEM images show that the growth direction is the same for pure Sb 2 Se 3 and Gd-doped samples.Emission and absorption spectra of doped materials, in addition to the characteristic red emission peaks of Sb 2 Se 3 , show other emission bands originating from f-f transitions of the Gd 3+ ions.The electrical conductivity of Gd-doped materials is higher than pure Sb 2 Se 3 at room temperature, and increases with temperature.

Figure 5 :
Figure 5: SEM image of Sb 1.96 Gd 0.04 Se 3 nanorods (a) low and (b) high magnification synthesized at 180 • C and 48 h.

Figure 8 :
Figure 8: Absorption spectra of Sb 2 Se 3 at room temperature.
hydroxide (2 mL, 40 mmol), SbCl 3 (2, 1.99, 1.98, 1.96 mmol) and Gd 2 O 3 (0.00, 0.01, 0.02, 0.04 mmol) 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 85-90%.Phase identification was performed with an X-ray powder diffractometer (XRD D5000 Siemens) with Cu-K α radiation.Cell parameters were calculated with Celref program from powder XRD patterns, and reflections have been determined and fitted using a profile fitting procedure with the Winxpow program.The reflections observed in 2θ = 4-70 • were used for the lattice parameter determination.The morphology of materials were examined by a scanning electron microscope SEM (Hitachi S-4200).