materials

Article Toward the Growth of Self-Catalyzed ZnO 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 (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 has 5 been reported 6.5 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 and 5 does not use6.5 any metal 360 ± 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. seed layer (SL) of ZnO at high background pressure. Unlike what has been reported in the literature, this 2. Materials andmethod Methods requires only the nanotextured ZnO template for nucleation, and does not use any metal catalyst layer. We demonstrate the growth of vertically-oriented ZnO nanorods on both silicon and An ablationglass-ITO/ source ZnO of substrates. KrF excimer laser (248 nm) with a repetition rate of 10 Hz and pulse laser energy of 400 mJ/pulse (energy density of ~8 J/cm2), and a high purity ZnO target were used (CompexPro 205F,2. Materials Coherent and Methods Inc., Santa Clara, CA, USA). The target was prepared by uniaxial pressing of ZnO commercialAn ablation powder source (99.99% of KrF purityexcimer fromlaser (248 Sigma nm) with Aldrich, a repetition St. Louis,rate of 10 MO, Hz and USA) pulse followed laser by energy of 400 mJ/pulse (energy density of ~8 J/cm2), and a high purity ZnO target were used sintering at 1150 ◦C for two hours. Prior to deposition; p-type Si (100), and Glass-Indium Tin Oxide (CompexPro 205F, Coherent Inc., Santa Clara, CA, USA). The target was prepared by uniaxial (ITO) substrates of 1 1.5 cm2 were ultrasonically cleaned with a consecutive bath of acetone and pressing of× ZnO commercial powder (99.99% purity from Sigma Aldrich, St. Louis, MO, USA) isopropanol followedfollowed by by sintering a drying at 1150 step °C usingfor two compressedhours. Prior to deposition; nitrogen. p-type The samples Si (100), and were Glass-Indium totally covered by a texturedTin thin Oxide layer (ITO) of ZnO; substrates experimental of 1 × 1.5 cm process2 were isultrasonically presented cleaned elsewhere with [a58 consecutive], and then bath introduced of acetone and isopropanol followed by a drying step using compressed6 nitrogen. The samples were in a high vacuum chamber evacuated to a base pressure of about 10− Torr. The target-to-substrate totally covered by a textured thin layer of ZnO; experimental process is presented elsewhere [58], and distance wasthen maintained introduced atin 6.5a high cm vacuum due to chamber equipment evacuated restrictions. to a base pressure Experimental of about 10 setup−6 Torr. detailsThe are presented intarget-to-substrate Figure2. The substratedistance was was maintained heated at to6.5 380cm due◦C to (measuredequipment restrictions. temperature) Experimental at a rate of 30 ◦C/min. Thesetup temperature details are presented was maintainedin Figure 2. The constant substrate duringwas heated deposition to 380 °C (measured in thepresence temperature) of argon (>99.99% purity).at a rate Structural of 30 °C/min. properties The temperature of the was as-grown maintained NWs constant were during characterized deposition in the using presence Bruker D8 of argon (>99.99% purity). Structural properties of the as-grown NWs were characterized using Discover highBruker resolution D8 Discover X-ray high diff resolutionractometer X-ray (XRD, diffractometer Tokyo, (XRD, Japan) Tokyo, with Japan) CuK αwithand CuKλ =α and1.5406 λ = Å and transmission1.5406 electron Å microscopy—FEI—TEMand transmission electron Tecnaimicroscopy (Hillsboro,—FEI—TEM OR, Tecnai USA). (Hillsboro, Morphological OR, USA). properties were examinedMorphological with FEI Nova properties Nano were SEM examined 630 (Hillsboro, with FEI Nova OR, Nano USA) SEM and 630 Zeiss (Hillsboro, Ultra OR, 55 USA) (Hillsboro, and OR, USA), and photoluminescenceZeiss Ultra 55 (Hillsboro, properties OR, USA), wereand photolum studiedinescence using properties Raman Lab were with studied the using samples Raman excited Lab with the samples excited using HeCd laser at 325 nm. using HeCd laser at 325 nm.

Figure 2. (a) PLD setup illustration, (b) diagram presenting the measured substrate temperatures in function of the setting temperature.

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Figure 2. (a) PLD setup illustration, (b) diagram presenting the measured substrate temperatures in function of the setting temperature.

3. Results

MaterialsParametric2020, 13 study, 4427 was performed to optimize the ZnO NWs growth on various 4types of 14 of substrates with different NWs length, diameter, and density. Morphological, structural and optical properties were investigated. 3. Results 3.1. MorphologicalParametric studyProperties—Effect was performed of to optimizeSeed Layer the (SL) ZnO NWs growth on various types of substrates with different NWs length, diameter, and density. Morphological, structural and optical properties wereZnO investigated.NWs were grown directly on Si (100) substrates at 5 Torr (argon pressure). Under deposition conditions presented in the experimental section without ZnO SL, the grown ZnO nanostructures3.1. Morphological showed Properties—E nail-needle-shapeffect of Seed Layermorpho (SL)logy with different dimensions (Figure 3) and random orientation.ZnONWsweregrowndirectlyonSi(100)substratesat5Torr(argonpressure). Underdeposition conditionsA thin layer presented of ZnO innanowall the experimental network section with hone withoutycomb ZnO structure SL, the grown [58] was ZnO deposited nanostructures as SL on Si (100).showed nail-needle-shape morphology with different dimensions (Figure3) and random orientation.

FigureFigure 3. SEM 3. SEMimage image top view top view of ZnO of ZnO nanowires nanowires (NWs) (NWs) grown grown at at 5 5Torr Torr in in argon argon environment, environment, T =

380 °CT = on380 silicon◦C on substrate silicon substrate..

The AZnO thin textured layer of ZnOSL is nanowall highly crystalline network with (c-direction), honeycomb grown structure by [ 58PLD;] was growth deposited parameters as SL on are Si (100). presented elsewhere [32]. Under deposition conditions of 5 Torr argon pressure and a deposition time The ZnO textured SL is highly crystalline (c-direction), grown by PLD; growth parameters are of 30 min, NWs with a perfect vertical orientation were grown on ZnO SL/Si (100) having an average presented elsewhere [32]. Under deposition conditions of 5 Torr argon pressure and a deposition time diameterof 30 min,of about NWs 50 with ± 4 anm, perfect a length vertical of orientation1.3 ± 0.12 µm, were and grown spacing on ZnO of SL46/Si ± (100)8 nm having (Figure an 4). average diameter of about 50 4 nm, a length of 1.3 0.12 µm, and spacing of 46 8 nm (Figure4). ± ± ±

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Figure 4. (a) 40◦ tilted view SEM image of perpendicular ZnO NWs arrays grown on ZnO SL/Si (100) substratesFigure at 4. 5((a) mTorr,) 40°40° tiltedtilted (b view)view top SEMSEM view imageimage SEM ofof image perpendicularperpendicular of the as-grown ZnOZnO NWsNWs ZnOarraysarrays NWs, growngrown ( oncon) correspondingZnOZnO SL/SiSL/Si (100)(100) size distributionsubstratessubstrates histogram atat 55 mTorr,mTorr, of the((b)) top ZnOtop viewview NWs SEMSEM arrays. imageimage ofof thethe as-grownas-grown ZnOZnO NWs,NWs, ((cc)) correspondingcorresponding sizesize distribution histogram of the ZnO NWs arrays. As expected, the geometrical dimensions of the NWs are affected by the deposition conditions. As expected, the geometrical dimensions of the NWs are affected by the deposition conditions. For example,As expected, changing the the geometrical chamber dimensions pressure to of 10 the Torr NWs for are 15 affected min results by the in deposition 600 30 conditions. nm long NWs For example, changing the chamber pressure to 10 Torr for 15 min results in 600 ± 30± nm long NWs and havingFor example, diameter changing 30 3the nm, chamber with a pressure spacing to of 10 75 Torr5 for nm 15 (Figure min results5). It in is noteworthy600 ± 30 nm long that NWs no NWs and having diameter ±30 ± 3 nm, with a spacing of 75 ± 5 nm (Figure 5). It is noteworthy that no NWs were obtained at 2.5 Torr background argon gas pressure. were obtained at 2.5 Torr background argon gas pressure.

Figure 5. Cont.

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Figure 5. ((aa)) 40° 40◦ tiltedtilted view view SEM SEM image image of of perpendicular perpendicular ZnO ZnO NWs NWs arrays arrays grown grown on on ZnO ZnO SL/Si SL/Si (100) (100) substrates at 10 mTorr, (b) top view SEM image of the as-grown ZnO NWs, (c) corresponding size Figuresubstrates 5. (a) 40° at tilted10 mTorr, view ( bSEM) top image view SEM of perpendicular image of the as-grown ZnO NWs ZnO arrays NWs, grown (c) corresponding on ZnO SL/Si size (100) distribution histogram of the ZnO NWs arrays. substrates at 10 mTorr, (b) top view SEM image of the as-grown ZnO NWs, (c) corresponding size distributionUnder thehistogram same deposition, of the ZnO ZnO NWs NWs arrays. grew in a pencil-like morphology with 2.6 0.4 µm length Under the same deposition, ZnO NWs grew in a pencil-like morphology with 2.6 ±± 0.4 µm length and 360 20 nm diameter nearly perpendicular to the surface of glass-ITO/ZnO SL (Figure6). and 360 ± 20 nm diameter nearly perpendicular to the surface of glass-ITO/ZnO SL (Figure 6). Under the same deposition, ZnO NWs grew in a pencil-like morphology with 2.6 ± 0.4 µm length and 360 ± 20 nm diameter nearly perpendicular to the surface of glass-ITO/ZnO SL (Figure 6).

Figure 6. ((aa)) Cross-section Cross-section SEM SEM image image of of nearly nearly perpendicular perpendicular ZnO ZnO NWs NWs arrays arrays grown grown on ZnO SL/Glass/ITOSL/Glass/ITO substrates at 5 mTorr, ((bb)) toptop viewview SEMSEM imageimage ofof thethe as-grownas-grown ZnO ZnO NWs. NWs.

3.2. Structural Structural PropertiesProperties Figure 6. (a) Cross-section SEM image of nearly perpendicular ZnO NWs arrays grown on ZnO A typical XRD pattern of the ZnO NWs array at 5 and 10 Torr is shown in Figure7. Only main SL/Glass/ITOA typical substrates XRD pattern at 5 of mTorr, the ZnO (b) topNWs view array SEM at 5image and 10 of Torrthe as-grown is shown ZnO in Figure NWs. 7. Only main diffractiondiffraction lines from (002) and (004) planes can be observed having the highest peak shown at 34.58° 34.58◦ and 34.47 for the NWs at 5 and 10 Torr, respectively. It is constructive to note that the NWs array has a 3.2. Structuraland 34.47°for◦ Properties the NWs at 5 and 10 Torr, respectively. It is constructive to note that the NWs array has ac-axis c-axis orientation. orientation. The The other other di diffractionffraction peaks peaks shown shown inin FigureFigure7 7 are are due due to to the the siliconsilicon substratesubstrate andand substrateA typical holder. XRD pattern A A slight slight ofshift shift the can can ZnO be seenNWs between array at the 5 andtwo peaks 10 Torr of isthe shown (002 (002)) plane in Figure direction 7. Only of the main diffractionZnO NWs lines grown from at(002) different diff erentand (004) pressures. planes This can might be observed be caused having by the low the highestoxidation peak of the shown ZnO NWs at 34.58° and 34.47°fordue to the the background NWs at 5 argon and 10 environment. Torr, respectively. It is constructive to note that the NWs array has a c-axis orientation. The other diffraction peaks shown in Figure 7 are due to the silicon substrate and substrate holder. A slight shift can be seen between the two peaks of the (002) plane direction of the ZnO NWs grown at different pressures. This might be caused by the low oxidation of the ZnO NWs due to the background argon environment.

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Figure 7. XRD pattern 2θ scan of ZnO NWs grown on ZnO SL at Tsub = 380 C and argon pressure at Figure 7. XRD pattern 2θ scan of ZnO NWs grown on ZnO SL at Tsub = 380 °C and◦ argon pressure at (a) 5 Torr(a) 5 and Torr ( band) 10 (b Torr.) 10 Torr. Figure 7. XRD pattern 2θ scan of ZnO NWs grown on ZnO SL at Tsub = 380 °C and argon pressure at (Thea) 5 Torr structureThe and structure (b) 10 of Torr ZnOof ZnO. NWs NWs on ZnO ZnO SL SL was was furthe furtherr investigated investigated by TEM. byFigure TEM. 8 shows Figure a low-8 shows a low-resolutionresolution image (Figure (Figure 8a),8 a),HRTEM HRTEM image image (Figure (Figure 8b) and 8selectedb) and area selected electron area diffraction electron (SAED) di ff raction Thepattern structure of a singleof ZnO ZnO NWs NW on (Figure ZnO SL 8a). was It furtheis clearr thatinvestigated the ZnO NWsby TEM. are relativelyFigure 8 showsstraight a withlow- a (SAED) pattern of a single ZnO NW (Figure8a). It is clear that the ZnO NWs are relatively straight resolutiondiameter image of (Figure about 50 8a), ± 4HRTEM nm. SAED image pattern (Figure and 8b)HR TEMand selected suggest areathat the electron NWs havediffraction a single (SAED) domain with a diameter of about 50 4 nm. SAED pattern and HRTEM suggest that the NWs have a single patternwurtzite of a single structure ZnO NWwith (Figurehigh± crystal 8a). quality.It is clea Ther that HRTEM the ZnO image NWs shows are arelatively lattice distance straight of with0.52 nma domaindiameterconsistent wurtziteof about with 50 structure ±the 4 nm. c-axis SAED with of wurtzite pattern high crystalZnO and crystal. HR quality.TEM The suggest SAED The pattern HRTEM that the reveals NWs image thehave showsexact a single growth a lattice domain of NWs distance of 0.52wurtzite nmalong consistentstructure the ZnO with with [0002] high the direction, crystal c-axis quality. ofcons wurtziteistent The with HRTEM ZnO the crystal. XRD image result The shows of SAED Figure a lattice pattern 7. The distance growth reveals of of 0.52the the exactnmZnO growth ofconsistent NWsNWs along with is done thethe c-axis ZnOon the [of 2concave] wurtzite direction, tip ZnO near consistent crystal.the grain The withboundaries SAED the pattern XRD between result reveals two of ZnO Figurethe exact thin7 .grains. growth The growth of NWs of the ZnO NWsalong isthe done ZnOon [0002] the concavedirection, tipcons nearistent the with grain theboundaries XRD result of between Figure 7. two The ZnO growth thin of grains. the ZnO NWs is done on the concave tip near the grain boundaries between two ZnO thin grains.

Figure 8. TEM (a) & High-Resolution Transmission Electron Microscopy (HRTEM) (b) of ZnO NWs grown on Si (100)—ZnO SL at 5 Torr. Materials 2020, 13, x FOR PEER REVIEW 8 of 14

Figure 8. TEM (a) & High-Resolution Transmission Electron Microscopy (HRTEM) (b) of ZnO NWs grown on Si (100)—ZnO SL at 5 Torr. Materials 2020, 13, 4427 8 of 14 When ZnO is viewed along < 1120> direction, all the Zn and O atoms are aligned at separate atomic columns,When and ZnO there is viewed is no along mixing< 11 20between> direction, Zn and all the O Zn atoms and O in atoms the column. are aligned This at separate is an ideal case for usingatomic STEM columns, (either and high-angle there is no mixing annular between dark Zn field and O (HAADF) atoms in the column.or annular This isbright an ideal field case for(ABF)), to study theusing polarity STEM of (either ZnO high-angle film. However, annular dark as Zn field and (HAADF) O atoms or annular are only bright 0.112 field nm (ABF)), apart to in study the the < 1120> projection,polarity a probe of ZnO corrector film. However, has asto Zn be and used O atoms to areachieve only 0.112 such nm a apart resolution. in the < 11 Here20 > projection, HAADF is not a probe corrector has to be used to achieve such a resolution. Here HAADF is not applicable, as the applicable, as the oxygen light atom cannot be seen due to a weak signal. In this case, ABF is more oxygen light atom cannot be seen due to a weak signal. In this case, ABF is more suitable to study suitable theto study polarity the of thepolarity film (Figure of the9). film Based (Figure on the contrast, 9). Based the positionon the contrast, of Zn and Othe can position be accurately of Zn and O can be accuratelyidentified. Asidentified. the As isthe pointing nanowire upward, is pointing the polarity upward, was identified the polarity based on was the identified common based on the commondefinition definition of polarity ofof ZnO polarity (the nanowire of ZnO is (the Zn terminated nanowire [59 is]). Zn terminated [59]).

FigureFigure 9. STEM 9. STEM image image of of ZnO ZnO NW NW withwith annular annular bright bright field field (ABF). (ABF).

3.3. Optical Properties 3.3. Optical Properties Figure 10 depicts photoluminescence (PL) measurements of ZnO NWs grown by PLD at 5 Torr Figureand 10 Torrdepicts argon photoluminescence pressure. Different peak (PL) positions measurements of the band of edge ZnO emission NWs in grown the UV by region PLD as at 5 Torr and 10 Torrwell asargon defect-induced pressure. emissionsDifferen int peak the visible positions region of can the be band seen. edge The PL emission spectra exhibit in the normal UV region as well as defect-inducedband-gap emission emissions in the UV regionin the at visible ca. 379.4 re nmgion (3.268 can eV), be and seen. 379.2 The nm PL (3.27 spectra eV) for samplesexhibit normal band-gapgrown emission at 5 Torr in and the 10 UV Torr, region respectively. at ca. A 379.4 slight shiftnm to(3.268 lower eV), wavelength and 379.2 is noticed nm which(3.27 eV) might for be samples caused by quantum confinement of thinner NWs. The emission in the visible region is namely green grown at 5 Torr and 10 Torr, respectively. A slight shift to lower wavelength is noticed which might (541 nm (2.29 eV), and 539 nm (2.3 eV) for 5 Torr and 10 Torr samples, respectively) and yellow (585 nm be caused(2.12 by eV) quantum for both cases).confinement Several types of thinner of defects NWs. in ZnO The can emission induce emission in the in visible the visible region region. is namely green (541Table nm2 presents (2.29 eV), the and intensity 539 nm ratio (2.3 of UV eV)/visible for 5 Torr emission. and It10 shows Torr thatsamples, the UV respectively) to green emission and yellow (585 nm and(2.12 UV eV) to yellow for both emission cases). for ZnOSeveral NWs types are higher of defects for working in ZnO pressure can of induce 10 Torr, emission indicating fewerin the visible region. Tabledefects 2 than presents the 5 Torr the case. intensity ratio of UV/visible emission. It shows that the UV to green emission and UVTable to 2. yellowIntensity ratioemission UV/visible for of ZnO NWs NW depositeds are higher at 5 Torr andfor 10working Torr, respectively. pressure of 10 Torr, indicating fewer defects than the 5 Torr case. Sample UV/Green UV/Yellow NWs at 5 Torr 11.868 22 NWs at 10 Torr 30.6 45.4

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FigureFigure 10. 10. Room-TemperatureRoom-Temperature photoluminescence photoluminescence (RT (RT–PL)–PL) spectra spectra of ofZnO ZnO NWs NWs grown grown at at5 5Torr Torr (dashed(dashed line) line and) and 10 10 Torr Torr (solid (solid line) line )argon argon enviro environment.nment. NBE isis thethe near-bandnear-band edge edge emission emission and and the thedefect defect level level emission emission is DLE.is DLE.

4. DiscussionTable 2. Intensity ratio UV/visible of ZnO NWs deposited at 5 Torr and 10 Torr, respectively.

NWs were grown perpendicularlySample to theUV surface/Green with UV a high-density/Yellow distribution over the entire substrate. The crystal structureNWs of at the 5 SLTorr had a considerable 11.868 effect 22 on the crystallographic orientation of the ZnO NWs. NWs at 10 Torr 30.6 45.4 Many PLD process parameters affect the growth of the ZnO NWs, such as substrate temperature, 4.gas Discussion pressure, and the substrate–target distance. The deposition temperature has a critical effect on surface diffusion [26,51,60–62]. If it is too low <200 ◦C, the deposited ZnO will not have enough mobility to reachNWs nucleationwere grown sites, perpendicularly and would rather to the increase surface thewith roughness a high-density of the distribution surface [63]. over An appropriate the entire substrate.high temperature The crystal would structure allow of the the deposited SL had a speciesconsiderable to migrate effect to on energetically the crystallographic favorable orientation sites where ofthe the nucleation ZnO NWs. energy barrier is lower. This is due to the higher sticking coefficient of ZnO on the nuclei sites.Many These PLD are process likely toparameters be the vicinity affect ofthe grain growth boundaries of the ZnO as NWs, was demonstratedsuch as substrate by temperature, TEM analysis. gas pressure, and the substrate–target distance. The deposition temperature has a critical effect on It is worth mentioning that the temperature used in our work (400 ◦C) is less than in the previously surfacepublished diffusion literature, [26,51,60 as well–62]. as If the it is target–substrate too low <200 °C, distance the deposited (6.5 cm) ZnO being will larger not [ 64have]. The enough lower mobilitysubstrate to temperaturereach nucleation was sites, found and to bewould suffi cientrather for increase activating the roughness surface diff ofusion. the surface In this [63]. research, An appropriatethe pressure high used temperature for the growth would of ZnOallow nanowires the deposi wasted 5species Torr–10 to Torr,migrate recommended to energetically by S. favorable Lemlikchi siteset al. where [65] andthe nucleation R.S.Ajimsha energy et al. barr [66].ier The is lower. average This diameter is due to of the the higher NWs grown sticking at coefficient 10 Torr is lessof ZnO than onthat the of nuclei the 5 Torr.sites. TheThese increase are likely of pressure to be the from vicinity 5 Torr of to grain 10 Torr boundaries caused the as NWs was spacingdemonstrated to increase by TEMfrom analysis. 46 8 nm It is to worth 75 5 mentioning nm, respectively, that the as temperature the argon gas used pressure in our influences work (400 both°C) is the less deposition than in ± ± therate previously and the energypublished of ejected literature, species. as well When as the the target deposition–substrate is processeddistance (6.5 under cm) highbeing pressure, larger [64]. the Theablated lower species substrate undergo temperature a large numberwas found of collisionsto be suff withicient background for activating gas surface molecules diffusion. (argon In atoms), this research,which reduces the pressure the energy used offor the the particles growth arrivingof ZnO nanowires at the substrate–Seed was 5 Torr– Layer10 Torr, (SL) recommended and decreases by the S.size Lemlikchi of the ablatedet al. [65] plume and R.S.Ajimsha [67]. That iset why al. [66]. it is The recommended average diameter to reduce of the the NWs distance grown between at 10 Torr the is less than that of the 5 Torr. The increase of pressure from 5 Torr to 10 Torr caused the NWs spacing to increase from 46 ± 8 nm to 75 ± 5 nm, respectively, as the argon gas pressure influences both the deposition rate and the energy of ejected species. When the deposition is processed under high

Materials 2020, 13, x FOR PEER REVIEW 10 of 14

Materialspressure,2020, the13, 4427ablated species undergo a large number of collisions with background gas molecules10 of 14 (argon atoms), which reduces the energy of the particles arriving at the substrate–Seed Layer (SL) and decreases the size of the ablated plume [67]. That is why it is recommended to reduce the distance targetbetween and substratethe target whileand substrate working while at higher working pressure at higher in order pressure to maintain in order to the maintain optimum the energy optimum of the ablatedenergy species. of the ablated The decrease species. ofThe the decrease kinetic of energy the kinetic is likely energy to beis likely the reason to be the why reason thinner why NWs thinner with lowerNWs density with lower were density obtained were over obtained the surface over the of thesurf substrateace of the atsubstrate 10 Torr. at Furthermore, 10 Torr. Furthermore, ZnO nanowires ZnO werenanowires successfully were grownsuccessfully on glass-ITO-ZnO grown on glass-ITO-ZnO SL substrate SL at 5substrate Torr. The at thickness 5 Torr. The of thethickness glass substrateof the alsoglass affects substrate the morphology also affects the of NWs,morphology due to of heat NWs, transfer due to phenomenon. heat transfer phenomenon. OnOn the the other other hand, hand, for for the the metalmetal oxideoxide such as ZnO, gold gold (Au) (Au) or or silver silver (Ag) (Ag) catalysts catalysts are are not not neededneeded if if Zn Zn can can be be decomposed decomposed ofof ZnOZnO duringduring th thee growth of of NWs. NWs. Having Having a high a high melting melting point, point, ZnOZnO might might have have been been decomposed decomposed and and created created a self-catalytica self-catalytic Zn Zn nano-dot nano-dot from from the the vapor vapor liquid liquid solid processsolid process (VLS). ZnO(VLS). NWs ZnO can NWs be growncan be grown just above just theabove melting the melting point ofpoint Zn. of The Zn. morphology, The morphology, density, anddensity, uniformity and uniformity of the NWs of depend the NWs on depend the surface on the and surface surface and migration surface energiesmigration of energies the substrate. of the substrate.In order to understand the growth process, the deposition of ZnO NWs on ZnO SL was performedIn order using to PLD understand at different the depositiongrowth process, durations. the deposition Figure 11 revealsof ZnO theNWs schematic on ZnO illustration SL was ofperformed growth of using ZnO PLD NWs at growndifferent by deposition PLD on duration Si substratess. Figure having 11 reveals a ZnO the SL. schematic It is suggested illustration that growthof growth rate of of ZnO ZnO NWs along grown the normal by PLD direction on Si substrat is higheres having than a theZnO rate SL. atIt theis suggested different that index growth planes rate of ZnO along the normal direction  is higher than the rate at the different index planes (𝑉(0001) > (V(0001) > V 1010 > V 1011 > V 1011 > V 0001 )[68]. The presence of ZnO SL can efficiently 𝑉(1010) > 𝑉(1011 ) > 𝑉(1011) > 𝑉 (0001)) [68]. The presence of ZnO SL can efficiently lower the lower the nucleation energy barrier leading to nucleation of ZnO NWs. Moreover, the continuous nucleation energy barrier leading to nucleation of ZnO NWs. Moreover, the continuous supply of supply of ZnO assists the growth of NWs in a favorable direction [2]. The NW’s length increases with ZnO assists the growth of NWs in a favorable direction [0002]. The NW’s length increases with growth time, and the density of the NWs varies with the nucleation sites on the surface of the SL and growth time, and the density of the NWs varies with the nucleation sites on the surface of the SL and thethe argon argon pressure pressure in in the the PLD PLD chamber. chamber.

FigureFigure 11. 11.SEM SEM images images revealing revealing thethe growthgrowth process of of ZnO ZnO NWs NWs on on ZnO ZnO SL SL by by PLD. PLD. Ablated Ablated ZnO ZnO speciesspecies are are adsorbed adsorbed on on the the seed seed layer layer (SL)(SL) nanostructurednanostructured surface surface (a (a).). ZnO ZnO species species migrate migrate to tothe the nucleationnucleation points points that that are are energeticallyenergetically favorablefavorable sites sites for for growth growth of of ZnO ZnO NWs NWs (b (),b ),followed followed by by continuouscontinuous growth growth (c ().c).

5.5. Conclusions Conclusions InIn summary, summary, we we have have demonstrated demonstrated thatthat vertically-oriented ZnO ZnO NWs NWs could could be be grown grown on onSi Si andand glass glass/ITO/ITO substrates substrates withwith an intermediate nano nanostructuredstructured ZnO ZnO SL SLby bypuls pulseded laser laser deposition deposition at atrelatively low low temperature temperature under under high high argon argon pressure. pressure. Since Since no no intentional intentional metal metal catalyst catalyst was was introduced,introduced, the the incorporation incorporation of a a textured textured ZnO ZnO SL SL was was a key a keyfor the for growth the growth of the ofdesired the desired NWs. The NWs. Thegrain grain boundaries boundaries of the of used the usedZnO ZnOSL were SL found were foundto be the to most be the favorable most favorable nucleation nucleation sites. The as- sites. Thesynthesized as-synthesized NWs, NWs, found found to present to present Zn polarity, Zn polarity, were were c-axis c-axis oriented oriented in agreement in agreement with with thethe SL SL crystallinity. This is a promising substrate-independent growth method for fabricating aligned NWs crystallinity. This is a promising substrate-independent growth method for fabricating aligned NWs on a large scale to be applied in photovoltaic, light emitting diodes, electronic devices for improved on a large scale to be applied in photovoltaic, light emitting diodes, electronic devices for improved light trapping, and other electronic devices. light trapping, and other electronic devices. 6. Patents 6. Patents The patent number “US 2015/0280017 A1” resulted from the work reported in this manuscript. The patent number “US 2015/0280017 A1” resulted from the work reported in this manuscript.

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Author Contributions: All authors contributed equally to the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors would like to thank Anastasia Khrenova (KAUST) for her assistance on the drawings. Special thanks to Didier Decoster, Zahia Bougrioua (IEMN), Brigitte Sieber, Ahmad Addad (UMET), and P Tuami Lasri (IEMN) for fruitful discussion. Our gratitude goes to Bei Zhang, Lan Zhao, Rachid Sougrat, and Xixiang Zhang (KAUST) for all their support during the acquisition of the images. Conflicts of Interest: The authors declare no conflict of interest.

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