Controlled growth of 1D and 2D ZnO nanostructures on 4H-SiC using Au catalyst
© Dahiya et al.; licensee Springer. 2014
Received: 23 June 2014
Accepted: 24 July 2014
Published: 3 August 2014
A perfect control of nanostructure growth is a prerequisite for the development of electronic and optoelectronic device/systems. In this article, we demonstrate the growth of various ZnO-derived nanostructures, including well-ordered arrays of high aspect ratio single crystalline nanowires with preferred growth direction along the  axis, nanowalls, and hybrid nanowire-nanowall structures. The growths of the various ZnO nanostructures have been carried out on SiC substrates in a horizontal furnace, using Au thin film as catalyst. From experimental observations, we have ascribed the growth mechanisms of the different ZnO nanostructures to be a combination of catalytic-assisted and non-catalytic-assisted vapor–liquid-solid (VLS) processes. We have also found that the different ZnO nanoarchitectures' material evolution is governed by a Zn cluster drift effects on the SiC surface mainly driven by growth temperature. Au thin film thickness, growth time, and temperature are the parameters to optimize in order to obtain the different ZnO nanoarchitectures.
Extensive research efforts have been recently dedicated to the synthesis of high-quality zinc oxide (ZnO) nanostructures, targeting high-performance electronic and optoelectronic applications [1–6]. Devices such as field-effect transistors , sensors , field emission  photovoltaic , room temperature UV lasers , and light-emitting diodes  have already been investigated in the literature. The interest in ZnO nanomaterials has been largely driven by the material's excellent electrical and optoelectronic properties, including direct wide band-gap (3.37 eV), high exciton binding energy (60 meV), and moderate to high electron mobility (1 to 200 cm2/Vs) [1, 4]. Moreover, ZnO's excellent piezoelectric and pyroelectric properties are finding widespread applications targeting various energy harvesting systems [7–11].
Synthesis strategies, including carbothermal reduction [12–22], pulse laser deposition , and hydrothermal  and electrochemical deposition , have been widely exploited for growing ZnO nanostructures such as nanowires (NWs), nanowalls (NWLs), and/or a hybrid of the two aforementioned nanostructures. Among them, carbothermal reduction of ZnO powder is offering high-quality ZnO nanostructures via the VLS process. In this process, a so-called seed thin layer of metal (such as Au) is first deposited onto the desired substrate. When increasing the temperature, the catalyst seed layer of metal is converted into nanoparticles. The nanoparticles can act as sink sites for vapors of the desired nanomaterial. In some cases, the vapors are efficiently trapped by the metal catalyst islands, and during the growth (‘tip’ growth), the metallic nanoparticle rides atop the nanostructures. In some others, the metal nanoparticle acts only as the nucleation site and not as a catalyst for nanomaterial growth. In this case, the metal nanoparticles remain at the bottom of the nanomaterial during growth (‘base’ growth) [10, 15–17, 21]. In addition to this ‘base’ growth, one may also observe side branches growing from the bottom of the nanostructures. The latter scenario often results in the formation of complete nanostructured networks such as nanowalls (NWLs) . Such structures are quasi-2D nanomaterials with potential applications in emerging technologies, including solar cells , sensors [23, 27], and piezoelectric nanogenerators . It has been shown that NWs and NWLs can also co-exist in a single synthesis batch . Kumar et al.  successfully demonstrated the growth of NWs, NWLs, and hybrid nanowire-nanowall (NW-NWL) in which material morphology was optimized by careful control of the metal layer (Au) thickness. On the other hand, some reports have shown that various ZnO nanostructures can also be produced through precise control of the temperature-activated Zn source flux during a vapor transport and condensation synthesis process . Despite these several reports of different ZnO nanostructure growth processes, the exact mechanism responsible for the evolution of the different nanostructures is still not fully understood.
In this paper, we will present a detailed study of the growth and evolution of a diverse range of ZnO nanostructures that can be grown on Au-coated 4H-SiC substrates. We will emphasize that VLS synthesis and its optimization is driven by Au layer thickness, growth temperature, and time. Finally, we will demonstrate that the diverse nanostructures obtained here can be attributed to the temperature-activated Zn cluster drift phenomenon on the SiC surface and, hence, can be controlled.
Growth parameters for various ZnO nanostructures
msource, ZnO/C (ratio)
Au thickness (nm)
Temperature of growth (°C)
Ar flow (sccm)
Time of growth (min)
Nanowire-Zn cluster drift hybrid
Density and mean radius of Au nanoparticles and ZnO NWs
Au layer thickness (nm)
Mean radius (nm)
Temperature of annealing/growth (°C)
12 ± 1.5
5 ± 1
69 ± 31
5 ± 1
151 ± 71
5 ± 1
207 ± 114
6 ± 1
125 ± 10
21 ± 7
125 ± 10
25 ± 10
125 ± 10
28 ± 12
5 ± 1
42 ± 15
70 ± 10
35 ± 15
Results of the growths have been characterized in three different equipments. First, a dual beam FEI Strata 400 (FEI, Hillsboro, OR, USA), a focused ion beam (FIB) coupled to a scanning electron microscopy (SEM) system, has been used. It is equipped with a flip stage, a scanning transmission electron microscopy (STEM) detector, and an energy-dispersive X-ray spectroscopy (EDX) for sample transfer, observation, and elemental composition characterization, accordingly. Additionally, NW and NWL lamellas have been prepared using the FIB mode and then characterized in STEM mode, but also in a second equipment: a high-resolution transmission electron microscopy (HRTEM) using a JEOL 2100 F (JEOL Ltd., Akishima-shi, Japan) operating at an accelerating voltage of 200 kV. Finally, the ZnO nanostructure crystallinity was studied using X-ray diffraction (XRD) with CuKα1 radiation on the high resolution parallel beam diffractometer Bruker D8 discover (Bruker AXS, Inc., Madison, WI, USA). The scans were performed in the 2θ range from 25° to 85° at a scanning rate of 0. 01° s-1.
Results and discussion
To follow the morphological evolution of the ZnO nanostructures, time-dependent growths were also carried out on the SiC substrates using the different Au nanoparticle densities. For this present investigation, the growth temperature was fixed at 900°C, while the growth times were either 90 or 180 min. Figure 7 presents the experimental results obtained for ZnO nanomaterial synthesis as a function of time. In Figure 7a, b, the growth of the ZnO NW-NWL hybrids and NWLs is obtained by varying time between 90 and 180 min, respectively, for the high-density Au nanoparticle case. Once again, the drifting was effectively halted by Zn clusters merging with other clusters and/or Au seed nanoparticles resulting in the formation of complete ZnO networks over large areas of the SiC substrates, as already shown in Figure 6b. When growing with low-density Au nanoparticles, the following observations can be made: (i) the drift of the Zn cluster results in the formation of vertically oriented ZnO NWs at the Zn cluster drift sites and not at the seed particle site as shown in Figure 7c, and (ii) with increasing growth time (Figure 7d), a new form of nanostructure can be observed, in which NWLs are effectively terminated by NWs at one end. These observations were found to be consistent with the so-called nanofins, reported in . With longer synthesis time (180 min), we observed that the boundaries between ZnO NWs and horizontal trace of the Zn cluster were more favorable nucleation sites, forcing the growth of the observed ZnO nanofin-NW structures.
With increasing growth time, the continual supply of Zn vapors results in an increase in Zn concentration in Au-Zn alloy clusters. The process of Zn condensation/dissolution within the Au-Zn alloy system continues until the supersaturation point, where a solid crystal of ZnO nucleates out of the molten alloy droplet . However, the present experimental work shows that depending on the system (growth) temperature, ZnO nucleation can occur either on the Au-Zn alloy droplets (850°C, Figure 6c) or away from the Au-Zn alloy droplet (900°C). At 900°C, Zn-rich clusters that are precipitated on Au-Zn alloy droplets experience a drift as a result of the high thermal energy . In our system, it was observed that at 700 sccm of Ar flow, the Zn cluster drift phenomenon can be significant above 850°C. As can be seen in Figure 8b (ii), the Zn cluster appears to drift with no preferential direction. The Zn cluster drift was subsequently halted either by (1) merging with other moving Zn cluster traces and/or Au-Zn alloy droplets (Figure 8a (ii) for the high density of Au nanoparticle case), (2) sticking on a substrate defect site, and/or (3) reduction in the local substrate temperature (Figure 8b (ii) for the low density of Au nanoparticle case). With continual supply of Zn vapors and residual oxygen atoms inside the growth chamber, precipitation of ZnO NWs via self-catalyzed VLS process is established (Figure 8 (iii)). Beyond this stage, NW growth is effectively controlled by a non-catalytic-assisted VLS mechanism and the Au nanoparticles play no further role in the evolution of the growth process [16, 22]. In the final step of growth, as the edges between the NW and drifted traces are thermodynamically more favorable sink sites for incoming Zn vapors, ZnO NWL networks are formed. This is clearly demonstrated in the case of high-density Au nanoparticles, as shown in Figure 8a (iv). On the other hand, when the distance between the Au nanoparticles is significantly larger than the drifted Zn length, as in the low-density case, the growth process can also result in the formation of NW-nanofin hybrid structures with prolonged synthesis time (as depicted in Figure 8b (iv)).
In summary, controlled growth of various ZnO nanostructures, including nanowires (NWs), nanowalls (NWLs), and hybrid nanowire-nanowall, was demonstrated through careful control of key experimental parameters, including Au seed thickness, synthesis temperature, and time, via a combination of catalytic-assisted and non-catalytic-assisted VLS processes. A combination of nanomaterial characterization techniques revealed that highly crystalline wurtzite nanostructures were produced. Experimental work presented here suggests that the nanomaterial synthesis temperature effectively controlled the Zn cluster drift phenomenon, responsible for the formation of the various studied ZnO nanostructures. NWs were found to grow at comparatively lower temperatures, and the overall NW density was effectively controlled through the Au seed film thickness. High-density Au clusters and high growth temperatures resulted in NWLs and hybrid NW-NWL formation. The formation of such structures was found also to depend on the synthesis time. These results offer a new prospective towards the development of applications that require various predefined ZnO nanostructures on -oriented SiC as well as other similar compound substrates, including GaN, AlN, and GaN-on-Si substrates targeting future high-performance nanodevices.
The authors gratefully acknowledge the support of the MIND (Multifunctional and Integrated Piezoelectric devices) European Network of Excellence (NoE 515757–2 of the 6th Framework Program) and the Region Centre who supports the CEZnO project (Convertisseur Electromécanique à base de nanofils ZnO, 2011 to 2014). The authors also thank Drs. D. Valente and V. Grimal for their technical assistance in material characterization experiments.
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