Guided Growth of Horizontal ZnS Nanowires on Flat and Faceted Sapphire Surfaces

The surface-guided growth of horizontal nanowires (NWs) allows assembly and alignment of the NWs on the substrate during the synthesis, thus eliminating the need for additional processes after growth. One of the major advantages of guided growth over post-growth assembly is the control on the NWs direction, crystallographic orientation and position. In this study, we use the guided growth approach to synthesize high-quality, single-crystal, aligned horizontal ZnS NWs on flat and faceted sapphire surfaces, and show how the crystal planes of the different substrates affects the crystal structure and orientation of the NWs. We also show initial results of the effect of Cu doping on their photoluminescence. Such high-quality aligned ZnS NWs can potentially be assembled as key components in phosphorescent displays and markers due to their unique optical properties. The ZnS NWs have either wurtzite or zinc-blende structure depending on the substrate orientations and contain intrinsic point defects such as sulfur vacancies, which are common in this material. The crystallographic orientations are consistent with those of guided NWs from other semiconductor materials, demonstrating the generality of the


Introduction
Semiconductor nanowires (NWs) are important building blocks for future nanotechnology 1 . Owning to their controlled size and properties, NWs are especially promising for potential applications in nanoelectronics 2-3 , photonics 4-5 and sensing devices [6][7] . Growing NWs requires addition of material during the growth process along only one direction. One of the most common methods to synthesize NWs is the vaporliquid-solid (VLS) method, which was first described in 1964 by Wagner and Ellis 8 . The VLS growth mechanism is based on the directional growth of the semiconductor material from a metallic nanoparticle acting as a catalyst. The grown NWs can be formed either vertically, horizontally or tilted relative to the substrate, and can grow in oriented arrays or randomly. One of the major challenges of NWs is their integration into practical devices.
Several techniques offered large-scale aligning and assembling of NWs on the substrate, such as assembly with liquid flows 9 , electric fields 10 , mechanical shearing 11 and Langmuir-Blodgett compression 12 . Yet, these post-growth routes can be problematic because the alignment of the NWs is subject to thermal and dynamic fluctuations, and the brittle NWs are usually damaged and contaminated during the process.
A different approach for integrating the NWs is based on their horizontally guided growth directed by the substrate 13 , as previously shown also for carbon nanotubes [14][15][16][17][18] .
Various semiconducting materials such as GaN 13, 19-21 , ZnO [22][23][24] , ZnSe 25-26 , ZnTe 27 , CdSe 28 , CdS [29][30] and CsPbBr3 31 were grown into aligned NW arrays, guided by epitaxial and graphoepitaxial relationships with the substrate. The epitaxial growth is usually driven by 3 the minimization of the mismatch between the NW and the substrate, which controls the growth along specific lattice directions and crystallographic orientation ( Figure. 1a-d), while the graphoepitaxial growth is driven by maximization of the interface area between the NW and substrate and the NWs are directed and grow along the faceted surface of the substrate, such as nanosteps or nanogrooves ( Figure. 1e-f). The guided NWs growth enabled fabrication of aligned arrays of high-performance electronic and optoelectronic devices, including transistors 27 , logic circuits 23 , photodetectors 28 and photovoltaic cells 26 .
ZnS is an important II-VI semiconductor with direct wide-bandgap and two stable polymorphs: zinc-blende (ZB) with a bandgap of 3.72 eV and wurtzite (WZ) with 3.77 eV [32][33] . ZnS has been extensively studied due to its unique optical properties and is used in photonic applications such as cathode ray tubes (CRT) 34 [38][39] . Recent studies demonstrated the growth on ZnS NWs on silicon 38,[53][54][55] , sapphire 56 , and zinc foils 57 , focusing on their crystal structure and optical properties, but so far only vertical NWs were observed, and not their horizontally guided growth has 4 not been reported. Since it is not obvious that every material should grow as horizontally guided NWs, it is important to demonstrate that ZnS can also do it. Expanding the generality of the guided growth phenomenon to different materials is important for the understanding of the phenomenon. Moreover, every material has different epitaxial relationships with different substrates, leading to NWs with often unexpected crystal structures and crystallographic orientations for each specific plane of a single-crystal substrate. A survey of these epitaxial and graphoepitaxial relations and crystallographic data for guided NWs of a specific composition on different surfaces of a substrate is important for their subsequent integration into devices 23,25,[27][28][29][30] , and more complex structures such as core-shell guided NWs 26 .
Herein we report for the first time the surface-guided growth of horizontal singlecrystal ZnS NWs. The NWs were grown epitaxially and graphoepitaxlly on various sapphire substrates, including C (0001) ,R (11 � 02) and annealed M (11 � 00), where they display different crystal phases and crystallographic orientations In addition, Cu-doped ZnS powder was used to grow Cu-doped ZnS guided NWs. The latter results could be used to design other nanostructures (e.g. core-shell heterostrucures, doped NWs, etc.) , a basis for devices (e.g. LEDs, transistors, photodetectors, photovoltaic cells, etc.) and for further applications as phosphorescent displays and markers.

Nanowires synthesis
The NWs were synthesized on sapphire wafers (Roditi International, Inc.) with three different orientations (C-, R-and M-planes). The M-plane wafer was annealed before the synthesis at 1600°C in air for 10 hours using high-temperature tube furnace. The NWs were grown from dispersed gold nanoparticle solution and gold catalyst pattern in order to achieve long arrays. The patterning was done by a conventional photolithography process (MA/BA6 Karl-Suss mask aligner) with negative photoresist (NR-9 1000PY) and suitable masks. A gold (Holland Moran, 99.999%) thin film of 5Å thickness was deposited using electron-beam physical vapor deposition system (Telemark), followed by lift-off in acetone. Prior to the synthesis, the substrates were heated to 550°C for 7 min.
The ZnS NWs were synthesized in a three-zone tube furnace (Lindberg Blue M, Thermo Scientific). The ZnS source was ZnS powder (Sigma Aldrich, 99.99%). The tube was purged with N2 (Gordon Gas, 99.999%) and H2 (Parker Dominic Hunter H2 generator, 99.99995%) 490:12 sccm mixture and 400 mbar and the sapphire substrates were placed downstream on a quartz slide. The temperature of the ZnS source powder was held at 960 °C and the sapphire substrates were held at 780 °C. The typical growth time was 15-20 minutes.
Cu-doped ZnS powder was used for synthesizing Cu-doped ZnS guided NWs with the same procedure as for undoped ZnS NWs. The Cu-doped ZnS powder was prepared from 0.1 gr of CuCl2•H2O powder (BDH Laboratory Reagents, 98%) was heated to 70° C for 10 minutes and later was mixed with 0.5 gr of ZnS powder (Sigma Aldrich, 99.99%).
The mixture was cooled down to room temperature prior to the NWs synthesis.

Structural characterization
The morphology of the ZnS NWs was studied using Scanning Electron Microscope (SEM, Zeiss Supra 55VP FEG LEO). In order to study the crystallographic structures and orientations, focused-ion-beam (FIB, FEI Helios 600 dual beam microscope) was used to cut thin lamellae across the NWs. The NWs cross-sections were examined by high-

Optical characterization
Photoluminescence (PL) measurements were conducted using a micro-Raman/micro-PL system (Horiba LabRAM HR Evolution). A 325 nm He−Cd laser was focused on the NWs through a 40x objective lens. 8

Results and Discussion
The study of undoped ZnS guided NWs was carried out on three different sapphire surfaces: the flat planes C (0001) and R (11 � 02) to demonstrate the epitaxially guided growth and the faceted plane M (11 � 00) to demonstrate the graphoepitaxial guided growth.
On all the planes, the NWs are guided and well aligned having a nearly squared crosssection with rounded corners (Figure 2b, 2e, 2h and 2j). The NWs have a gold droplet at their ends, as can be seen in Figure 2g, which a strong indication of VLS tip-growth mechanism. The NWs have a typical thickness of 10-60 nm with a length of 2-50 µm. The crystallographic orientation and the growth direction of the grown NWs varied with the orientations of the sapphire substrates, as summarized in Table 1  ZnS NW lattice constant difference and the sapphire lattice constant in the direction transversal to the NW long axis, was calculated to be 0.51%, which is also compatible with the observation of misfit dislocations. and ±[022 � 1] of the sapphire substrate, separated by 94° and 86° angles, as can be seen in Figure 1d. Similar to the NWs that grow on the C-plane, these NWs also have WZ structure, but with growth axis along the polar [0001] crystallographic orientation. The observed growth directions on the sapphire substrate are similar to those that were demonstrated for ZnSe 25 and CdSe 28 guided NWs but different from those that were found on GaN 13 and ZnO 22 guided NWs. For ZnTe 27 NWs no guidance was observed on R-plane Sapphire at all. This comparison indicates that the guided growth phenomenon is not only substratedependent but may be affected by the grown material itself and its relation to the substrate.
In general, each combination of NW material and substrate has been found to prefer to grow along a specific lattice direction of the substrate, the NWs having one or a few specific crystallographic orientation 13 . The epitaxial relationship between the NW material and the substrate is not sufficient to determine the preferred growth direction. For instance, ZnO NWs on M-plane sapphire can grow in two perpendicular directions of the substrate with respectively perpendicular crystallographic orientations both keeping the same epitaxial relationship, depending on the NW diameter 22 . This thickness dependence has been attributed to the minimum thickness for misfit dislocations, which are an efficient mechanism of strain relaxation. The surface energy of the NW facets may also play a role in determining the preferred direction and crystallographic orientation of guided NWs.
These different orientations have been studied in detail by means of second-harmonic generation polarimetry 24 . In the case of guided ZnS NWs on R-plane sapphire, the transversal mismatch was calculated to be 2.62%, which is also compatible with the observation of misfit dislocations.

Guided growth of ZnS NWs on faceted sapphire surfaces. Flat M (11 � 00)
sapphire is a thermodynamically unstable plane with relatively high surface energy 59 . Upon thermal treatment at elevated temperature (1400-1600 °C), the M-plane surface undergoes restructuration and present the more thermodynamically stable S-plane (101 � 1) and Rplane (11 � 02) in periodically faceted V-shaped nanogrooves (Figure 2h , 2i, 2j and 2k).   However, the EELS quantitative analysis from the NW itself showed less than 5 at. % of Oxygen.

Conclusions
In this paper, we demonstrated the guided growth of aligned VLS ZnS NWs on flat and faceted sapphire substrates by epitaxy and graphoepitaxy, respectively. The guided NWs are all single-crystal with a nearly squared cross-section. Depending on the substrate orientation, the NWs show polar or non-polar crystallographic orientations, and exhibited WZ structure when grown epitaxially on C-and R-plane sapphire, and either WZ or ZB structure when grown graphoepitaxially on annealed M-plane sapphire. Chemical analysis revealed high purity ZnS NW with slightly less sulfur than zinc. The optical characterization confirmed the WZ structure and correlated to the chemical analysis results of sulfur vacancies and a thin oxide layer of the NW facets that are exposed to ambient air after growth. Preliminary doping of the ZnS NWs was conducted using Cu, presenting additional emissions peaks. The guided growth of Cu-doped ZnS NWs with additional optical transitions is promising toward potential applications of phosphorescent NW arrays. The overall results, combined with similar ones for other materials, demonstrate the generality of the guided growth approach for the large-scale integration of NWs into devices.

Supporting Information
Cross-section TEM images and crystallographic orientation of the examined NWs, EELS compositional analysis of the guided ZnS NWs.