One-Step Synthesis of Enhanced Band-Edge Emission ZnSe Nanowires Assisted by SnO2

Zinc selenide (ZnSe) is a direct band gap semiconductor material with a band gap of 2.7 eV at room temperature, and is a very promising blue light emitting material. However, the ZnSe nanomaterials grown by the usual Au-catalyzed chemical vapor deposition (CVD) method are prone to cause defects in the material, and its luminescence spectra generally have strong yellow light emission, which seriously affects its application in blue light devices. The paper proposes a simple method for preparing blue light-emitting ZnSe nanowires. Adding a small amount of SnO2 to the source material ZnSe powder, when using the CVD method to grow ZnSe nanomaterials, can obtain ZnSe nanomaterials with only band edge blue emission. SnO2 can not only act as a catalyst, promote the growth of ZnSe nanowires and nanoribbons, reduce the growth temperature of nanomaterials, but also avoid the occurrence of deep level defects in ZnSe nanomaterials, and obtain pure blue light emitting ZnSe nanowires. The emission peak of the nanowire grown by this method can be shifted from 460 to 500 nm. By precisely regulating the growth of the nanowire, it can make it emit light to cover the entire blue region, which can promote the application of inorganic semiconductor nanomaterials in the field of lighting and display.


Introduction
Zinc selenide (ZnSe) is an important II-VI semiconductor material with a direct band-gap of ~2.7 eV at room temperature, which is widely regarded as a very good candidate for blue-violet light emitting devices [1] [2] [3] [4]. With the How to cite this paper: Hou, L.P., Guo, Y.C. and Zhang, L.J. (2020) One-Step Synthesis of Enhanced Band-Edge Emission ZnSe Nanowires Assisted by SnO2. Advances in Materials Physics and Chemistry, miniaturization of devices, many research groups have begun to study the nanostructures of ZnSe materials and have done a lot of work in this area. In 2011, Liang et al. fabricated vertically aligned ZnSe nanowire arrays by metal-organic chemical vapor deposition (MOCVD) using Ga and Ag catalysts, while SnO 2 , it was found that there are strong deep level luminescence peaks in its photoluminescence (PL) spectra [5]. In 2014, Zannier et al. reported optimized photoluminescence of ZnSe nanowires grown by molecular beam epitaxy. Even with this expensive growth method, the deep-level defect emission peak in its PL spectrum is still significant [6]. In 2019, Zhang and Fand synthesized all kinds of ZnSe nanostructures by surfactant assisted electrodeposition, including nanorods, nano-spheres, and nano-syringes, which all exist ~2.2 eV deep level emission peak in their PL spectra [7]. During this period, a lot of work was devoted to improving the ratio of near-band edge luminous intensity of ZnSe nanostructures. Although the research of ZnSe nanostructures has gone through nearly thirty years, many researchers are still very concerned about its research. In recent years, the development of blue light devices based on ZnSe nanostructures is also in full swing.

Experimental Method
ZnSe nanowires were synthesized by a one-step CVD method. High-purity ZnSe and SnO 2 mixed powders with mass ratio 20:1, load in a ceramic boat, were placed in the center of the horizontal tube furnace. Silicon substrates were located about 10 cm downstream away from the center of the tube furnace to receive the grown ZnSe nanowires. The tube furnace is supplied with Ar(90%)/H 2 (10%) carrier gas to purify the growth atmosphere of nanowires in the tube furnace. Then the mixed powders placed in the middle of the tube furnace were heated at the rate of 120˚C/min to 1050˚C and hold for 30 minutes at this temperature for nanowires growth. When the deposition finished and furnace was naturally cooled to room temperature, yellow colored ZnSe nanowires can be obtained on silicon wafers.
The morphology of the nanowires was characterized by SEM (Zeiss SUPRA 55) equipped with EDS. Photoluminescence (PL) spectra were carried out by a laser cofocal photoluminescence microscope system (Princeton Instruments Acton SP2500) using continuous wavelength 405 nm laser as the excitation source. Temperature-dependent PL spectra (77 -300 K) were measured by liquid-nitrogen-cooled cryostat system (Janis ST-500, Microscope Cryostat). Figure 1 shows the morphology and element content analysis of ZnSe nanowire. Figure 1(a) is an SEM photograph of the nanowire. It can be seen from Figure  1(a) that there is a large ball drop at the top of the nanowire, which is a sign of the gas-liquid-solid growth process. Figure 1(b) and Figure 1(c) are the analysis of the element content at position S1 and S2 in Figure 1(a), respectively. It can be seen from Figure 1(b) that the main elements at position S1 of the nanowire are Zn and Se, while they are Sn and O at position S2. It indicates that the SnO 2 is used as a catalyst to assist the growth of ZnSe nanowires.

Luminescence Properties Analysis
In order to characterize the luminescence properties of the ZnSe nanowires, the steady state PL spectra of the nanowires were tested by continuous wave 405 nm laser. Figure 2 exhibits typically luminescence property of ZnSe nanowires grown by SnO 2 -assisted CVD method. The PL spectrum of the nanowire in Figure 2(a) shows that the nanowires only have near band edge emission, and the center of the PL peak is at 475 nm, which is different from the nanowires grown by an Au-catalyzed CVD method [9] [10], and there is no obvious deep-level defect emission peak at the low energy levels of its PL spectrum. During the growth or synthesis of ZnSe nanostructures, point defects such as Zn vacancies and Zn interstitials and structural defects such as dislocations and stacking faults are easily generated, which making their PL spectrum appear strong deep-level luminescence peaks between 500 and 700 nm [11] [12] [13]. Figure 2(b) is its corresponding emission image in dark field of optical microscope. The spot in  the middle of the nanowire is the excitation position of the laser, and it can be seen from the image that the nanowire emits blue light. Figure 3 shows the PL spectra of a single ZnSe nanowire at different excitation positions. Figure 3(a) exhibits its bright-field photo under the microscope, in which the red dots mark the different excitation positions of the laser on the nanowire. The inset in the upper right corner demonstrates the emission image of nanowires in the dark field at the excitation position P5. Figure 3(b) exhibits their corresponding PL spectra. It can be seen from the PL spectra that as the excitation position of the nanowire changes, the emission peak of the nanowire shifts from 473 to 503 nm. The shift of emission peak indicates that there may be defect-related bound state inside the nanowire due to the doping of SnO 2 , which needs further research. Figure 4 demonstrates PL spectra of ZnSe nanowire at different temperature ranging from 77 -300 K. Figure 4(a) shows the PL spectrum of individual ZnSe nanowire at 77 K, which exhibits blue light emission of nanowire at near band-edge and doesn't exist deep level yellow light emission. The emission peak, center at 456 nm, is assigned to the bound exciton (BX) emission of ZnSe nanowire, Compared with the emission peak of free excitons in ZnSe nanowires [14], the band edge emission energy has shifted to a lower energy level by 24 meV. Obviously, there are other bound state emissions at low energy side of BX, as shown in Figure 4(a). Figure 4(b) shows the PL spectra of ZnSe nanowire at different temperatures ranging from 77 -300 K. With temperature increasing, near band-edge emission peaks produce large redshift from 456 to 487 nm. This is because as the temperature rises, the nanowire lattice expands, and the electron-phonon coupling interaction is enhanced, which changes the band gap of the ZnSe nanowire, causing the emission peak of BX is red-shifted [15] [16]. Furthermore, the redshift of free excitons does not produce such a large movement [17], which also just proves that the emission peak originates from the emission of bound excitons. Although the content of Sn or O is not measured by  EDS, the generation of bound state is related to the doping of SnO 2 , and their relationship will be further studied in the next work.

Conclusion
In conclusion, enhanced near band-edge emission ZnSe nanowires have been grown by SnO 2 assisted CVD method. When using the CVD method to grow ZnSe nanomaterials, adding a small amount of SnO 2 to the source material ZnSe powder can obtain ZnSe nanomaterials with only blue edge emission. From its optical characterization, it can be found that there is only 475 nm near-band edge emission peak at room temperature, and no deep-level defect emission. At the same time, it can be seen from its emission spectra that the emission wavelength of nanowires can be shifted to 503 nm by changing the growth conditions of the ZnSe nanowires and adjusting the doping of SnO 2 . The ZnSe nanowires obtained by the simple one-step method can improve its application in blue light-emitting devices, and solve the problem that the blue light emission of ZnSe nanomaterials is accompanied by strong deep energy level yellow light emission.