Metal-Catalyst-Free Synthesis and Characterization of Single-Crystalline Silicon Oxynitride Nanowires

Large quantities of single-crystal silicon oxynitride nanowires with high N concentration have been synthesized directly on silicon substrate at 1200◦C without using any metal catalyst. The diameter of these ternary nanowires is ranging from 10 to 180 nm with log-normal distribution, and the length of these nanowires varies from a few hundreds of micrometers to several millimeters. A vapor-solid mechanism was proposed to explain the growth of the nanowires. These nanowires are grown to form a disordered mat with an ultrabright white nonspecular appearance. The mat demonstrates highly diffusive reflectivity with the optical reflectivity of around 80% over the whole visible wavelength, which is comparable to the most brilliant white beetle scales found in nature. The whiteness might be resulted from the strong multiscattering of a large fraction of incident light on the disordered nanowire mat. These ultra-bright white nanowires could form as reflecting surface to meet the stringent requirements of bright-white lightemitting-diode lighting for higher optical efficiency. They can also find applications in diverse fields such as sensors, cosmetics, paints, and tooth whitening.


Introduction
In recent years, silicon oxynitride (SiO x N y ) has received considerable attention as an important dielectric material, due to its superior electrical and physical properties [1].The fully compatibility of this material with conventional Si processing technology greatly accelerates its application in electronic and optoelectronic devices [2].SiO x N y has a static dielectric constant that increases linearly with the nitrogen concentration from SiO 2 (ε = 3.8) to Si 3 N 4 (ε = 7.8) [3], and a large refractive index ranging from 1.45 up to 2.0 [4,5].A high concentration of nitrogen in SiO x N y is generally desired: by increasing N/O ratio, a higher dielectric constant and refractive index can be obtained [6,7].However, most of the silicon oxynitride materials studied up to now have a nitrogen concentration less than 15 at.% [8,9], and primarily are amorphous SiO x N y films.As a result, the properties of silicon oxynitride materials with high N concentration remain largely unexplored [10].
On the other hand, one-dimensional nanostructures have attracted steadily growing interests due to their peculiar properties and applications superior to their bulk counterparts [11,12].For example, it has been demonstrated that nanowires actually can be grown to form one of the most strongly scattering materials in a recent study [13].Light scattering in nanowire materials has also been shown significant importance to optimize the external efficiency of nanowire-based photovoltaic devices [14,15].Furthermore, multielement alloy nanowires possess the characteristics of variety by adjusting their chemical composition and physical structure.Therefore, it is important to synthesize 1D SiO x N y nanostructures and explore their properties.However, ternary SiO x N y nanowires, especially N-rich singlecrystal SiO x N y nanowires, have been seldom reported.Recently, Gu et al. [16] reported the successful fabrication of N-rich single crystalline SiO x N y nanowires using both Fe(NO 3 ) 3 9H 2 O and Co(NO 3 ) 2 4H 2 O as catalyst precursors, while little information is available regarding their optical properties.It is desirable that nanowries could be synthesized without metal catalyst since catalyst contaminations might affect the characteristics of the nanowire, especially their optical properties.
In this work, we report the catalyst-free synthesis of bulk single-crystal SiO x N y nanowires with high N concentration.The chemical composition and structural characterization of these nanowires have been investigated, and the growth mechanism is discussed.The optical properties of the disordered SiO x N y nanowire mat have also been studied.

Experimental Details
In a typical synthesis procedure, the single-crystalline Si wafer [100] with a native oxidation layer (5 nm) was first dipped into acetone under ultrasonic agitation for 10 min to erase the surface contaminations.Then, the Si wafer was loaded to a quartz crucible boat and placed in the central region of an alumina tube furnace (GSL-1400X).Before the experiment, the furnace was evacuated to 10 −3 Torr and then flushed with N 2 (99.999%) to eliminate the oxygen and moisture in the chamber.The crucible boat was heated to 1200 • C at the heating rate of 15 • C/min under a N 2 flow of 50 standard cubic centimeters per minute (sccm).When the temperature reached the preset value, the gas flux was switched to mixture gas flow of H 2 (5%)/N 2 (99.999%) at a rate of 100 sccm.The growth time can be varied to obtain nanowire mats with different thickness.During the cooling process, only N 2 (99.999%) was used as the feed gas.When the furnace was naturally cooled down to room temperature, ultrabright white products were observed on the wafer.
The structure, morphologies, and composition of the products were characterized with scanning electron microscope (SEM, JEOL JSM-5510LV), high-resolution transmission electron microscope (HRTEM, FEI-F20 with EDS attachment and JEOL 2010), and selected-area electron diffraction (SAED).X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD) and Fourier transform-infrared Spectroscope (FT-IR, VERTEX 70) were also used to investigate their chemical composition.The optical reflectivity was measured under the wavelength from 400 to 780 nm using UV-Vis spectrophotometer (Lambda 35) with a 50 mm integrating sphere.Photoluminescence (PL) property was also studied using PL instrument (FP-6500) at room temperature with a xenon lamp line of 258 nm as the excitation source.

Morphologies and Structures.
The SEM image see Figure 1(a) shows that the synthesized nanowires are up to several millimeters in length, with high density, and random directions to form a disordered thick mat.For each individual nanowire, the diameter is uniform along its entire length, and the surface of these nanowires is smooth and clear without any residues.To pursue detailed structural information on a single nanowire, TEM investigations were applied.A higher magnification TEM image of the Y-shaped SiO x N y nanowire is shown in Figure 2(a), which is obtained from one of the junctions between the stem and extended branch marked in Figure 1(b).From the picture, it appears that the diameter near the root of the Y-branch is almost the same with that of the stem, while close to the tip, its diameter gets smaller.To better understand the formation of the branch, a HRTEM image of the junction is obtained as shown in Figure 2(b).It is found that the SiO x N y nanowire consists of a single crystalline phase throughout the whole structure with a clean backbone-to-branch junction, indicating the possibly epitaxial growth of the branch from the stem.EDS analyses reveal that the chemical composition of the nanowire consists of three elements including Si, N, and O. Detailed composition result shows that the nanowire has the high concentration of N of around 54.27 at.% and the O concentration around 7.44 at.%.These N-rich nanowires may inherit the high refractive index of their bulk materials, which renders strong photonic property [15].Figures 2(c XPS has been employed to analyze the chemical bonding within the nanowires and to give insight into their composition, where monochromatic Al Kα X-ray source was operated at 150 W. Figure 3(a) shows the XPS spectra of the obtained nanowires with Si 2p, N 1s, and O 1s signals.Three strong XPS signals display direct evidence of the existence of Si, N, and O elements.The peak of N 1s, located at 397.5 eV, shifts to a higher binding energy compared with the 397.0 eV peak which is attributed to N combined with three Si atoms [17], due to the formation of N-O bond [18].Deconvolution of the 2p Si spectrum yields three Gaussian-Lorentzian components centered at 101, 102, and 103.2 eV which are assigned to, respectively, Si-N and Si-O-N and Si-O bond configurations [19].
The chemical composition of these nanowires is further confirmed by FT-IR spectrometer.The broad FT-IR spectrum from 400 to 4000 cm −1 is shown in Figure 3(b).The absorption band at 492 originates from the vibrations of the Si-O-Si bonds.Peaks corresponding to the vibration modes of Si-N bonds appear at 846, 887, and 924 cm −1 [20].The band of 1048 cm −1 indicates the asymmetrical stretching vibration of Si-O bonds.The 3440 cm −1 and 1632 cm −1 adsorption bands are attributed respectively to the stretching and bending vibrations of O-H bond for adsorbed water, which may come from its adsorption on the SiO x N y nanowire surface [21].

Growth Mechanism.
Since the nanowires are synthesized without the aid of metal catalysis, vapor-solid (VS) growth mechanism is more plausible than the vapor-liquid-solid (VLS) growth model in this case.There was no other silicon species introduced into the system, so the silicon wafer was the only source of Si element.Sustained reaction of solid silicon or silicon dioxide (SiO 2 ) with N 2 is not allowed since solid silicon and SiO 2 are barriers for nitrogen diffusion [22]; therefore, gaseous SiO x (x ≈ 1) is the only source to directly supply Si element for Si-related nanowire growth [23].In our experiment, hydrogen is introduced into the system as mixed gas, which can lead to the reaction of (1) and generate gaseous SiO x .Meanwhile, the residual oxygen in the furnace due to the low vacuum condition and leakage can also lead to the generation of gaseous SiO x through the reaction (2): The reaction of SiO x with introduced N 2 lead to the formation of SiO x N y , which is illustrated by (3): ( When we reduced the chamber temperature to 1100 • C, the growth of nanowire was not observed, while etched pits were found randomly distributed on the substrate shown in Figure 4(a), indicating that the temperature is critical to initiate the growth of nanowire.The EDS spectrum acquired from the pit showing only Si signal (Figure 4(b)) indicates that the silicon surface was etched, which also validate the reaction of ( 1) and ( 2).Thus, the formation process of the nanowires can be explained as follows: through the reactions (1) and ( 2), gaseous SiO x was generated, and silicon oxide (SiO x ) nanoclusters were formed at the surface of the silicon substrates as illustrated in Figure 4(c).Then, SiO x reacted with nitrogen to form SiO x N y nuclei according to the reaction indicated by (3) (Figure 4(d)).With continuous supply of Si, O, and N elements, the growth of SiO x N y , nanowire was initiated and maintained Figure 4(e).
A similar experiment has been reported by Ran et al. [24], in which crystalline Si 3 N 4 /amorphous SiO 2 nanocables were synthesized on silicon substrates under N 2 atmosphere at 1250 • C without using catalyst and H 2 .We believe that the difference is due to the different atmosphere and the temperature control curve.Another experiment reported by Farjas et al. [25] shows that oxygen partial pressure and the control of temperature affect the formation of nanowires, in which Si 3 N 4 nanowires were obtained through a CVD mechanism.

Optical Properties.
To show the brightness and whiteness of obtained nanowire mat, a thin carbon film has been patterned on Si wafer for comparison as shown in Figure 5(a), where black area is carbon film, and white rings are grown SiO x N y nanowires.Visual inspection of the samples shows no specular reflection and no obvious angular anisotropy.The optical properties of the nanowire mats were further characterized as shown in Figures 5(b) and 5(c).
The reflectivity was measured from 400 to 780 nm to quantitatively characterize the optical reflectance of SiO x N y nanowire mats.The reflectivity of two nanowire mats, defined as thicker mat of sample A and thinner mat of sample B, is compared and shown in Figure 5(b).At the whole wavelength range, the reflectivity of sample A is maintained above 70%, and the highest point reaches out to 82%, while sample B has a lower reflectivity with an obvious similar appearance.The high optical reflectivity and its slight variations in whole visible-light wavelength result in ultrabright whiteness of the nanowire mat.The results are comparable to the optical performance of the most brilliant white beetle scales reported by Vukusic et al., in which around 60% of incident visible-light is reflected [26].
Strongly diffusive reflectivity of the disordered nanowire mat arises mainly from multiple scattering [27], unlike ordered vertical nanowire mat which has low reflectivity, and its optical interactions can be calculated by solving Maxwell's equations [28,29].Disordered SiO x N y nanowire mats have an ultrabright appearance due to the interaction between scattering and absorption in the nanowires.The smallscatterer (Rayleigh) limit is valid for diameters d < λ/(πn) with n = 3.5, which sets the critical diameter well below λ/10, where λ is the optical wavelength.For nanowires with the thicker diameter than the critical diameter, light scattering strongly contributes to the nanowire optical reflectivity, which for a dense collection results in diffuse transport of light [15].As characterized above, the diameter of the obtained nanowires follow a log-normalized distribution from 10-180 nm, showing the existence of large amount of nanowires thicker than λ/10 in the visible light range, which keeps the diffuse reflectivity at a high level.Moreover, for a sufficiently thick nanowire mat, photons experience a finite number of scattering events before being reemitted from the surface, corresponding to a total path length for the photon.This length [30] is much less than the mat thickness (∼20 μm in sample A), allowing the neglect of significant absorption by the silicon substrate, and therefore contribute to the high reflectivity.Meanwhile, the strong photonic nanowires with high refractive index may enhance their overall scattering efficiency [15,26].
In sample B, the reflectivity is lower than that of sample A probably because of stronger substrate absorption.Thus, the enhancement of the optical reflectivity could be further Figure 1(b) shows a TEM image of SiO x N y nanowires, where different diameters are observed.Y-shaped nanowires are also observed as marked in the figure.A histogram of the diameter distribution of 285 nanowires from TEM measurements is shown in Figure 1(c).It is shown that the diameter distribution follows a log-normal distribution function (solid line in the figure) with the peak distribution of around 42 nm.
) and 2(d) are the typical HRTEM images of the prepared nanowires, and the insets correspond to SAED patterns of the nanowires.HRTEM images show that the SiO x N y nanowires grew along two different directions with zone axes of [221] (Figure 2(c)) and [111] (Figure 2(d)), respectively, and the lattice spacings of the single crystal nanowire were measured to be 0.679 and 0.415 nm, respectively.

Figure 1 :
Figure 1: (a) SEM image of the as-grown SiO x N y nanowires.(b) TEM image of the as-grown SiO x N y nanowires.Circles mark the backboneto-branch junctions of Y-shaped nanowire formation.(c) histogram of the nanowire diameter distribution with the solid line representing a log-normal fit.

Figure 2 :
Figure 2: (a) and (b) are TEM image and HRTEM micrograph of Y-shaped SiO x N y nanowire obtained from one region marked in Figure 1(b), respectively, EDS spectrum as the inset in (b).(c) and (d) are typical HRTEM images of SiO x N y nanowires, each with the corresponding SAED pattern as an inset.

Figure 3 :Figure 4 :
Figure 3: (a) XPS spectra of the SiO x N y nanowire around Si 2p, N 1s and O 1s binding energies; the inset shows the fitting to the original Si 2p peak.(b) FT-IR spectra of the SiO x N y nanowire.