materials Article Toward the Growth of Self-Catalyzed ZnO Nanowires Perpendicular to the Surface of Silicon and Glass Substrates, by Pulsed Laser Deposition Basma ElZein 1,2,* , Yingbang Yao 3 , Ahmad S. Barham 4 , Elhadj Dogheche 5 and Ghassan E. Jabbour 6 1 Electrical Engineering Department, College of Engineering, University of Business and Technology (UBT), Jeddah 21361, Saudi Arabia 2 Institute of Electronics, Microelectronics and Nanotechnology, CNRS and University Lille Nord de France- Avenue Poincaré, CEDEX, 59652 Villeneuve d’Ascq, France 3 Faculty of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China; [email protected] 4 General Subjects Department, College of Engineering, University of Business and Technology (UBT), Jeddah 21361, Saudi Arabia; [email protected] 5 Campus Le Mont Houy, IEMN CNRS, Polytechnic University Hauts de France, CEDEX, 59309 Valenciennes, France; [email protected] 6 Canada Research Chair in Engineered Advanced Materials and Devices, Faculty of Engineering, University of Ottawa, Ottawa, ON K1N 6N5, Canada; [email protected] * Correspondence: [email protected] Received: 7 September 2020; Accepted: 29 September 2020; Published: 5 October 2020 Abstract: Vertically-oriented zinc oxide (ZnO) nanowires were synthesized on glass and silicon substrates by Pulsed Laser Deposition and without the use of a catalyst. An intermediate c-axis oriented nanotextured ZnO seed layer in the form of nanowall network with honey comb structure allows the growth of high quality, self-forming, and vertically-oriented nanowires at relatively low temperature (<400 ◦C) and under argon atmosphere at high pressure (>5 Torr). Many parameters were shown to affect the growth of the ZnO nanowires such as gas pressure, substrate–target distance, and laser energy. Growth of a c-axis-crystalline array of nanowires growing vertically from the energetically favorable sites on the seed layer is observed. Nucleation occurs due to the matching lattice structure and the polar nature of the ZnO seed layer. Morphological, structural, and optical properties were investigated. X-ray diffraction (XRD) revealed highly c-axis aligned nanowires along the (002) crystal plane. Room temperature photoluminescence (PL) measurements showed a strong and narrow bandwidth of Ultraviolet (UV) emission, which shifts to lower wavelength with the increase of pressure. Keywords: zinc oxide; seed layer; vertically oriented nanowires; polar nanowires; glass/ITO substrates; pulsed laser deposition 1. Introduction One-dimensional nanometer-sized electrically conducting and semiconducting nanowires (NWs), nanotubes, and nanorods have attracted much attention due to many exciting attributes including a direct path for charge transport and a large surface area for light harvesting. Such characteristics make them excellent candidates for many applications including solid-state lighting and photovoltaics. Freestanding NWs array morphology is favorable to light trapping where the incident light scatters within its open interiors. The scattering improves the efficiency of light absorption by increasing the Materials 2020, 13, 4427; doi:10.3390/ma13194427 www.mdpi.com/journal/materials Materials 2020, 13, 4427 2 of 14 Materials 2020, 13, x FOR PEER REVIEW 2 of 14 2 1 1 photonabsorption path length. by increasing Due to thethe photon high electron path length. mobility Due (reachingto the high tenselectron cm mobilityV− S− (reaching)[1], photo-generated tens cm2 chargesV−1 areS−1) transported[1], photo-generated quickly charges to the electrode, are transported especially quickly when to the the electrode, NWs are especially vertically-oriented when the with respectNWs to are it (Figure vertically-oriented1). with respect to it (Figure 1). FigureFigure 1. Illustration 1. Illustration of (a )of light (a) trappinglight trapping in nanowires in nanowires arrays arrays and ( band) electron (b) electron transport transport in vertical in vertical nanowires. nanowires. Due to the unique properties of ZnO such as a large direct band gap of 3.37 eV and exciton binding energy ofDue 60 meV to the [2– unique8], it has properties been employed of ZnO insuch numerous as a large applications direct band such gap asof solar3.37 eV cells, and light exciton emitting diodesbinding (LED), energy optical of 60 switches, meV [2–8], and it has waveguides, been employed to mention in numerous a few. applications For example, such ZnOas solar NWs-based cells, solarlight cell researchemitting diodes has become (LED), a optical hot topic switches, in science and waveguides, and engineering to mention [9–19]. a Devicefew. For architecture example, ZnO having radial,NWs-based axial, or substrate solar cell junctionresearch hashas alsobecome been a exploredhot topic [in20 ].science These and architectures engineering have [9–19]. been Device employed in solararchitecture cells using having homogeneous radial, axial, andor substrate heterogeneous junction has NWs. also been explored [20]. These architectures have been employed in solar cells using homogeneous and heterogeneous NWs. There are many approaches for the growth of ZnO nanostructures such as catalytic growth via There are many approaches for the growth of ZnO nanostructures such as catalytic growth via vaporvapor liquid liquid solid solid (VLS) (VLS) mechanism mechanism [ 21[21,22],,22], thermalthermal evaporation evaporation [23 [23–25],–25 pulsed], pulsed laser laser deposition deposition (PLD)(PLD) [26,27 [26,27],], hydrothermal hydrothermal growth growth [ 28[28––3131],], rapidrapid hydrothermal growth growth [32 [–3234],–34 and], andwet wetchemical chemical processingprocessing [35 –[3537–],37], etc. etc. TheThe choice of of the the growth growth techni techniqueque is dictated is dictated by the byrequirements the requirements of the of the application.application. PLDPLD has beenhas been recognized recognized as a as powerful a powerful technique technique in thinin thin film film growth. growth. It canIt can produce produce high high quality epitaxialquality materials epitaxial as materials well as as amorphous well as amorphous layers at layers low temperature.at low temperature. It is also It is usedalso used to produce to produce various nanostructuresvarious nanostructures like nanorods like [ 38nanorods–42], nanoparticles [38–42], nanoparticles [41,43–45 [41,43], and–45], nanowalls and nanowalls [46]. A [46]. survey A survey [26,47 –57] [26,47–57] of the synthesis parameters of ZnO NWs by PLD is presented in Table 1 presenting the of the synthesis parameters of ZnO NWs by PLD is presented in Table1 presenting the growth growth parameters such as the type of seed layer, temperature, pressure, and distance between parameters such as the type of seed layer, temperature, pressure, and distance between substrate substrate and target. It is noticed that growth temperature varies between 500 and 900 °C, pressure > and target.1 Torr, and It is relatively noticed short that growthtarget–substrate temperature distance varies <3 cm. between 500 and 900 ◦C, pressure > 1 Torr, and relatively short target–substrate distance <3 cm. Table 1. Survey of ZnO nanowires and nanorods by pulsed laser deposition (PLD). Table 1. Survey of ZnO nanowires and nanorods by pulsed laser deposition (PLD). Distance Between Temp Pressure Diameter Length Substrate Target and Substrate Ref (°C) (Torr) Distance Between (nm) (µm) Temp Pressure Diameter Length Substrate Target and(cm) Substrate Ref ( C) (Torr) (nm) (µm) Sapphire (0001) ◦ 600–700 1–5 (cm)2 300 6 [47] Si (100) 450–500 5 2.5 120–200 12 [48] Sapphire (0001) 600–700 1–5 2 300 6 [47] SiO2/Si/Au 900 400 - 20 10 [49] Si (100) 450–500 5 2.5 120–200 12 [48] Sapphire (0001) 600 5 2 300 6 [50] SiO2/Si/Au 900 400 - 20 10 [49] Sapphire (0001)Si (100) 600600–850 4.8 5–6.3 2.5 2 20–50 300 0.5–2 6[51] [50] a-Sapphire Si (100) 600–8501000 4.8–6.3260 1.5 2.5 200 20–50 0.5–3 0.5–2[52] [51] a-Sapphirec-Sapphire 1000 260 1.5 200 0.5–3 [52] c-Sapphirec-Sapphire 500–800 0.15–0.50 2.5 50–90 Few µm [26] c-Sapphire ZnO SL 500–800 0.15–0.50 2.5 50–90 Few µm [26] ZnO SLSapphire Sapphire - 260 1.2–2.5 130–200 1.5–4 [53] (0001) - 260 1.2–2.5 130–200 1.5–4 [53] (0001) c-Sapphire 600 0.1–0.2 5 150–200 0.9 [54] c-Sapphire 600 0.1–0.2 5 150–200 0.9 [54] Sapphire 650 10−2 5 - - [55] Sapphire 650 10 2 5 - - [55] a-Sapphire − a-Sapphire 870–950870–950 18–15018–150 0.5 0.5–3.5–3.5 150 1501.5–20 1.5–20 [56] [56] c-Sapphirec-Sapphire+ Au + Au n-dopedn-doped 500–600 0.225 3 - - [57] 500–600 0.225 3 - - [57] 400 µm Si (111) Materials 2020, 13, 4427 3 of 14 Table 1. Cont. Distance Between Temp Pressure Diameter Length Substrate Target and Substrate Ref ( C) (Torr) (nm) (µm) ◦ (cm) Si(100) + ZnO Seed Layer 380 5 6.5 50 4 1.3 0.12 This work ± ± Si (100) + ZnO Seed Layer 380 10 6.5 30 3 0.6 0.03 This work ± ± Glass/ITO + ZnO Seed Layer 380 5 6.5 360 20 2.6 0.4 This work ± ± Materials 2020, 13, x FOR PEER REVIEW 3 of 14 In this paper, we400 report µm Si the(111) growth of vertically-oriented ZnO NWs on nanotextured seed layer (SL) of ZnO at high backgroundSi(100) + pressure.ZnO Seed Layer Unlike 380 what has5 been reported6.5 in the literature,50 ± 4 1.3 this ± 0.12 method This work requires only Si (100) + ZnO Seed Layer 380 10 6.5 30 ± 3 0.6 ± 0.03 This work the nanotexturedGlass/ITO ZnO template + ZnO Seed for Layer nucleation, 380 and5 does not use6.5 any metal360 ± catalyst20 2.6 ± layer.0.4 This We work demonstrate the growth of vertically-orientedIn this paper, we report ZnO the nanorods growth of on vertic bothally-oriented silicon and ZnO glass-ITO NWs on nanotextured/ZnO substrates.