A single-source precursor route to anisotropic halogen-doped zinc oxide particles as a promising candidate for new transparent conducting oxide materials

Abstract

Numerous applications in optoelectronics require electrically conducting materials with high optical transparency over the entire visible light range. A solid solution of indium oxide and substantial amounts of tin oxide for electronic doping (ITO) is currently the most prominent example for the class of so-called TCOs (transparent conducting oxides). Due to the limited, natural occurrence of indium and its steadily increasing price, it is highly desired to identify materials alternatives containing highly abundant chemical elements. The doping of other metal oxides (e.g., zinc oxide, ZnO) is a promising approach, but two problems can be identified. Phase separation might occur at the required high concentration of the doping element, and for successful electronic modification it is mandatory that the introduced heteroelement occupies a defined position in the lattice of the host material. In the case of ZnO, most attention has been attributed so far to n-doping via substitution of Zn(2+) by other metals (e.g., Al(3+)). Here, we present first steps towards n-doped ZnO-based TCO materials via substitution in the anion lattice (O(2-) versus halogenides). A special approach is presented, using novel single-source precursors containing a potential excerpt of the target lattice 'HalZn·Zn3O3' preorganized on the molecular scale (Hal = I, Br, Cl). We report about the synthesis of the precursors, their transformation into halogene-containing ZnO materials, and finally structural, optical and electronic properties are investigated using a combination of techniques including FT-Raman, low-T photoluminescence, impedance and THz spectroscopies.

Keywords: chemical doping; metal oxides; semiconductor nanoparticles; single-source precursors.

Scheme 1

Synthesis of halogen-substituted alkylzinc–alkoxide precursors.

Figure 1

¹H NMR spectra (Zn–CH₃ region) for the reaction of I₂ with [MeZnOiPr]₄ before (a) and after purification (b), and for the reaction with [MeZnOt-Bu]₄ (c). The assignment of the signals to different ZnMe → ZnI substitution degrees is shown schematically (a). The grey bar indicates the peak for the respective starting compound 1. (d) Excerpt from the EI-MS spectrum (black line) of compound 2a and calculated signals for the fragment [I(CH₃)₂Zn₄(Ot-Bu)₄]⁺ (m/z = 710.9).

Figure 2

(a) TGA traces (black) and its first derivative (grey) of the thermal decomposition of the molecular precursor compound [Cl(Et)₃Zn₄(OiPr)₄] in nitrogen atmosphere (squares) and artificial air (circles), heating rate: 5 K/min. The dashed grey line indicates the remaining mass expected for complete removal of any organic constituents. (b) EDX spectra for pure ZnO as a reference (grey graph) and ZnO_1−xCl_x materials with different chlorine content: 3.6% (black graph), 1.8% (blue graph) and 1.4% (red graph). (c) XPS spectrum for the Cl-2p region.

Figure 3

(a) Experimental PXRD pattern for materials prepared via thermolysis of [Cl(Et)₃Zn₄(OiPr)₄] (black graph) and [EtZnOiPr]₄ (grey graph). The pattern of ZnO (wurtzite) is shown as black bars. (b) Enlargement of the 2θ region 31–35° for better visibility of the (100) and (002) diffraction signals. (c) TEM (scalebar 100 nm) and HRTEM (scalebar 10 nm) of the ZnO_1−xCl_x material. See also Figure S4 (Supporting Information File 1).

Figure 4

(a) Absorption spectra in diffuse reflection modus, room temperature photoluminescence spectra; overview (b) and band gap region (c). (d) PL spectra recorded at T = 7 K. Hashes (blue): ZnO (D_cryst = 22 nm); triangles (green): ZnO_0.986Cl_0.014 (D_cryst = 25 nm); circles (red): ZnO_0.982Cl_0.018 (D_cryst = 27 nm); squares (black): ZnO_0.964Cl_0.036 (D_cryst = 37 nm).

Figure 5

(a) Raman spectra of ZnO_1−xCl_x: x = 0.0% (black), 1.4% (dark grey), 1.8% (grey) and 2.5% (light grey); (b) Raman spectra in the range of the non-polar E₂ (high) and polar LO modes.

Figure 6

Impedance spectra (Nyquist Plots) of ZnO_0.986Cl_0.014 (hashes) and pure ZnO (squares) prepared in an analogous way.

Figure 7

Theoretical model for dielectric function ε(w) for different carrier concentrations N = 1 × 10¹⁴ (black lines), N = 1 × 10¹⁵ (dark gray lines), N = 5 × 10¹⁵ (gray lines), and N = 1 × 10¹⁶ (light gray lines). The triangles mark the real parts ε₁ and the circles the imaginary parts ε₂; (a) Drude model; (b) Drude–Lorentz model with a resonance at 2 THz.

Figure 8

Measured dielectric function of ZnO_1−xCl_x (a) real part; (b) imaginary part with x = 0.0% (black lines), 1.4% (dark gray lines), 1.8% (gray lines) and 2.5% (light gray lines); An infrared-active phonon at about about 2 THz is scaling in intensity with increasing Cl concentration.