Improved structural features of Au-catalyzed silicon nanoneedles

Superlattices and Microstructures 85 (2015) 849–858

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Improved structural features of Au-catalyzed silicon nanoneedles Yasir Hussein Mohammed a,b,⇑, Samsudi Bin Sakrani a, Md Supar Rohani c a

Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, 81310 Skudai, Johor Baharu, Malaysia Department of Physics, Faculty of Education, University of Mosul, 41002 Mosul, Iraq c Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, 81310 Skudai, Johor Baharu, Malaysia b

a r t i c l e

i n f o

Article history: Received 7 July 2015 Accepted 8 July 2015 Available online 9 July 2015 Keywords: VHF-PECVD Au NSs VLS growth RF sputtering SiNNs

a b s t r a c t Nanometer sized silicon (Si) needles (nanowires) offer a vehicle for varieties of applications at nanoscale. We grow Si nanoneedles (SiNNs) using very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) method with distinct gold (Au) nanostructures (NSs) as catalyst. Au NSs in the form of nanoparticles (NPs) and continuous thin film are prepared on Si(1 0 0) substrates via radio frequency magnetron sputtering. Au catalyst size dependent surface morphology and structural features of these SiNNs are determined. Samples are characterized via imaging and spectroscopic measurements. Controlled growth of such SiNNs structure with reproducibility that is achieved via Au NPs size tunability is attributed to the vapor–liquid–solid (VLS) growth mechanism. SiNNs with diameters between 20 and 120 nm and length up to 5 lm are acquired. SiNNs diameter is found to increase with the increase of Au NPs size. Processing parameters optimization is demonstrated to play a critical role in nucleating Au NPs and thereby achieving high density SiNNs morphology. X-ray diffraction patterns authenticated an enhanced SiNNs crystallinity with increasing catalyst size. Raman spectra of SiNNs revealed a red-shift (8.26 cm1) in the first-order transversal band as the average diameter of NNs are decrease from 69 to 57 nm. Our systematic method for synthesis and characterization may contribute toward the development of SiNNs based optoelectronics. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Tunable growth of high quality SiNNs and their subsequent characterizations are essential for optoelectronic device fabrication. Recently, researches in Si nanowires (SiNWs) are fuelled by their enormous potential in broad arrays of technological applications such as lithium batteries [1], sensors [2], field-effect transistors [3], catalysts [4] and photovoltaics [5]. Furthermore, new possibilities are opened up for SiNWs biological applications in biosensors and tissue-engineering due to their easy structural modification, biocompatibility and environmental affability [6,7]. Compare to NWs, SiNNs are distinctively noteworthy because their sharp curvilinear tips are suitable for high sensitive probing with superior spatial resolution and augmented field emission enhancement factor [8–10]. Unlike NWs, SiNNs are difficult to grow owing to the unavailability of accurate synthesis techniques to successfully prepare NNs with ultra-sharp tips.

⇑ Corresponding author at: Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, 81310 Skudai, Johor Baharu, Malaysia. E-mail address: [email protected] (Y.H. Mohammed). http://dx.doi.org/10.1016/j.spmi.2015.07.021 0749-6036/Ó 2015 Elsevier Ltd. All rights reserved.

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Traditionally, catalyst assisted VLS process is used to synthesize SiNWs [11]. In the VLS process, growth catalysts such as Au, tin (Sn) or Indium (In) are exploited as energetically effective site for vapor-phase Si adsorption. Moreover, they require low eutectic temperatures of 363 °C (Au/Si), 232 °C (Sn/Si) and 157 °C (In/Si). This process being growth catalyst size and position dependent offers a good capacity to tune the diameter and arrangement of NWs [12,13]. Consequently, the size control of the metal catalyst is pre-requisite because it decides the NWs growth. The effect phonon confinement on the optical and electronic structure properties is inherently related to NWs dimensions as well as alignments [14]. In this view, VHF-PECVD method is used to nucleate SiNNs with desired crystallographic and physical properties promising for sundry applications. Amongst all metals, Au is advantageous due to its excellent chemical stability (high resistant to oxidation), low eutectic temperature of Au/Si liquid alloy with high Si solubility, optimum growth temperature above 450 °C [15], low vapor pressure at high temperatures which reduces the chance of the re-evaporation of the catalyst material during growth and large surface tension of Au/Si liquid alloy [16]. Numerous techniques are developed for the fabrication of SiNWs: laser ablation (LA) [17], chemical vapor deposition (CVD) [18], rapid thermal CVD (RTCVD) [13], inductively coupled plasma CVD (ICP-CVD) [19], low-pressure CVD (LPCVD) [20] and standard plasma enhanced chemical vapor deposition (PECVD) [21]. Such techniques often lead to the growth of NWs in different geometries (diameter, height, shape) with varying density and regularity. Most of the previous attempts for the growth of SiNWs are centered toward standard PECVD at high frequency of 13.56 MHz. Conversely, VHF plasma (150 MHz) represents a unique alternative, where growth rate can remarkably be enhanced by increasing the excitation frequency [22]. It is demonstrated that VHF plasma enhances the growth rate as much as 10 Å/s [23] than 0.1 Å/s for standard plasma [24,25]. Reduced dust formation, less intrinsic stress [26] and enhanced growth rate [27] are some notable advantages of the VHF plasma over the standard plasma system. To the best of our knowledge, only few preliminary efforts are dedicated to prepare SiNNs with controlled dimensions [21,28,29]. SiNNs growth via VHF-PECVD method is not yet explored extensively. We report the effects of Au-catalyst size on the growth morphology and structural evolution of VHF-PECVD synthesized SiNNs. The samples morphologies are characterized using field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM) measurements.

2. Experimental SiNNs are grown on Au-coated Si(1 0 0) substrates. Substrates are cleaned in an ultrasonic bath using acetone, isopropanol alcohol, and deionized (DI) water over a period of 5 min each. Next, these cleaned substrates are immersed in 10% hydrofluoric acid for 2 min to remove the surface oxide layer followed by a rinse in DI water. Finally, they are dried using nitrogen gas before being transferred into the sputtering chamber. High-purity Au films are deposited at room temperature via RF magnetron sputtering operated at frequency of 13.56 MHz and base pressure of 8.5  106 Torr in pure Ar atmosphere under 10 sccm. A fixed RF power of 50 W having different growth time is employed to obtain Au NSs of various sizes as summarized in Table 1. The estimated film thicknesses are in the range of 1–10 nm. SiNNs are grown by loading these substrates into VHF-PECVD chamber operated at high vacuum (1  108 Torr) and heated up to 550 °C in the presence of Ar plasma pressure of 20 mTorr. The substrate temperatures are recorded using a K-type (Chromel–Alumel) thermocouple which touched the bottom of the substrate holder. Then, H2 diluted SiH4 gas is inserted into the chamber at constant flow rate between 60 and 10 sccm, respectively. Power electrode of the radio frequency generator (150 MHz) produced the plasma. A constant radio frequency power (25 W), growth pressure (650 mTorr) and time (15 min) is maintained during the NNs growth. At last, all the samples are removed from the chamber and characterized at room temperature. Samples containing SiNNs are found to be brown dark in color. The optimized growth parameters are obtained through their systematic variation over a broad range. The surface morphology of as-synthesized Au NSs and SiNNs are determined using FESEM imaging (FESEM, SU8020; Hitachi). The statistical analyses of these are performed through software Image J 1.47v, where NPs size distribution is found to be the Gaussian. The growth and nucleation of SiNNs are confirmed via HRTEM (HRTEM, JEM-2100; JEOL) operated at 200 kV. For TEM measurement, the sample is prepared on a Formvar/Carbon film containing 300 mesh copper grid (Lacey F/C 300 mesh Cu) by dipping the substrate in pure ethanol solution and sonicated for 15 min to remove the NNs from the substrate. Subsequently, the suspension containing an individual nanoneedle is places onto the TEM copper grid through

Table 1 Average diameter of synthesized Au catalysts and SiNNs, FWHM of the Si(1 1 1) XRD peak and c-Si Raman peaks, Raman peaks positions and SiNNs crystallite size. The indicated errors are mostly instrumental and measurement related. Average catalyst diameter (nm)

Average NN diameter (nm)

FWHM of Si(1 1 1) XRD peak (°)

FWHM of c-Si Raman peak (cm1)

Raman peak (cm1)

DR (nm)

9±1 14 ± 1 25 ± 2 Island-like Film

57 ± 1 61 ± 1 67 ± 1 69 ± 1 60 ± 1

0.54 ± 0.02 0.49 ± 0.02 0.31 ± 0.02 0.18 ± 0.02 0.36 ± 0.02

38 ± 2 20 ± 1 18 ± 1 16 ± 1 14 ± 1

510.89 ± 0.05 513.25 ± 0.05 517.97 ± 0.05 519.15 ± 0.05 516.79 ± 0.05

3.12 ± 0.05 3.62 ± 0.05 6.51 ± 0.05 10.17 ± 0.05 5.25 ± 0.05

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a clean pipette. The TEM grid is dried in a cabinet for three hours before the imaging is carried out. The crystalline structure of the grown SiNNs are verified using a PANalytical X-ray diffractometer (Empyrean) with Cu Ka radiation (k = 1.54 Å) operating at 40 kV, 40 mA with 2h from 20° to 60° and scanning angle step size of 0.02°. Raman measurements are performed by the Renishaw InVia PL/Raman spectrophotometer equipped with a He–Cd laser operated at a wavelength of 514 nm.

3. Results and discussion Fig. 1a–e displays the FESEM images of the Au NSs deposited with different growth time of 5, 10, 20, 30, and 50 s, respectively. Spontaneous formation of coherent 3D Au NPs on Si substrate is attributed to hetero-epitaxy lattice mismatch mediated Volmer–Weber growth process. A lattice mismatch 25% between the Au and Si allowed the direct formation of Au particles on the substrate surface [30–32]. Thus, the dispersed spherical particles grew as bigger island-like structures and finally agglomerated to form continuous film at the later stages of deposition. These FESEM images (Fig. 1) reveal the growth time dependent surface morphology of the Au NSs deposited on Si substrate. Initially, at growth time of 5 s very tiny nucleation centers (diameter 9 nm) are created and spread over the entire substrate (Fig. 1a). As the growth time is increased up to 20 s more nucleation sites are formed and the deposited amount of Au is increased. Fig. 1b and c exhibited a smooth surface with spherical Au NPs of average diameter 14 and 25 nm, respectively. Further increase in growth time to 30 s caused agglomeration of some smaller NPs to create larger NPs simultaneously a few of them are coalesced to form island-like NSs (Fig. 1d). Finally at growth of 50 s a continuous Au film is achieved (Fig. 1e). The growth time dependent

Fig. 1. FESEM images of Au NSs deposited at different growth durations of (a) 5 s, (b) 10 s, (c) 20 s, (d) 30 s, and (e) 50 s. The insets in the first three figures reveal the corresponding Au NPs size-distributions.

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structural evolution clearly displayed the transformation of morphology from Au NPs to island-like NSs to continuous film structure. The deposited Au films involved several growth stages. Firstly, small nuclei are formed following Volmer–Weber mechanism. Secondly, bigger islands are created via the coalescence of smaller NPs. Finally, these bigger islands are coalesced to form continuous film. The formation of metal film-like pattern is interpreted in terms of Maxwell Garnett theory. At higher deposition time with growing film thickness both the Au NPs morphology and particle–particle interaction appeared more intricate [33,34]. As the coalescence propagates at large-scale, the film appeared more like interpenetrating network structure and continuously percolated over the entire substrate [35]. At optical frequencies, the diffusion of conduction electrons (from the metal surface) over the fractal lattice structure may be evidenced [36,37]. The manifested non-uniformity in the film can be explained following Bruggeman theory. Even though the comprehensive understanding regarding the role of surface electrodynamics remains obscure, the formation of continuous film is believed to be more or less compatible with the Drude theory. Fig. 2 illustrates the FESEM images of VHF plasma synthesized SiNNs grown on substrates containing Au NSs catalysts of different types including NPs, islands, and continuous film. The dependence of SiNNs morphology on catalyst Au NSs size is clearly manifested. Both the length and density of SiNNs are strongly varied depending on the catalysts morphology (Fig. 2b– d). Interestingly, the NNs density is found to be almost proportional to the Au NPs density, indicating that SiNNs nucleation is truly controlled by the catalyst structures. Conversely, sparse and inhomogeneous distribution of NNs obtained from small-sized catalyst of average diameter 9 nm (Fig. 2a) may be due to the minimum size of Au/Si droplet (liquid alloy) to initiate the growth of small-diameter SiNNs. However, Au NPs after attaining a critical size had facilitated the growth of NNs with large diameter. This observation is attributed to competition of silicon homogeneous nuclei formation in the Au/Si droplet with the adatom attachment at the Au/Si droplet substrate interface [38]. The diameter of the SiNNs is larger than the NPs because of the flow of Si into the NPs and alloy formation during the synthesis. The catalyst droplets swell in

Fig. 2. FESEM images for SiNNs synthesized using different Au NSs (a) 9 nm, (b) 14 nm, (c) 25 nm, (d) island-like, and (e) continuous film. The inset in (e) shows the high-magnification FESEM image taken from a dense spot as indicated by a box.

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size until the critical supersaturation concentration is reached [39]. Low-magnification FESEM image of NNs formed on continuous Au film (Fig. 2e) show that NNs are concentrated only in a small surface area as dense patches (localized pockets). The inset in Fig. 2e shows the magnified view of NNs morphology obtained from a dense spot. The density of the NNs is found to significantly decrease. The SiNNs acquired with Au NPs diameter of 14 nm and 25 nm as well as island-like Au NSs, revealed long and dense NNs than those obtained on continuous Au film. This indicates that small-size catalysts are favorable for VHF plasma to initiate the growth of SiNNs. The appearances of slight gradual bending like growth defects are ascribed to the accumulation of elastic strains on these very small diameter nanoneedles [20]. Conversely, some kinks are resulted from the high growth rates [40]. Growth rate of SiNNs is estimated to be about 350 nm/min after 15 min of growth. This value is certainly higher than the conventional CVD (12 nm/min) [41] and RTCVD (80 nm/min) [13] assisted growth rates of SiNWs. High growth rate is primarily related to the usage of VHF plasma power that caused the absence of ion bombardment in the growth region and decomposed the SiH4 precursor more efficiently. Fig. 3 depicts the size (diameter) distribution of the synthesized SiNNs estimated from Fig. 2a–e. The mean diameter and length of SiNNs grown with Au NPs of average diameter 9 nm, 14 nm, 25 nm are found to be 57, 61, 67 nm and 1.1 ± 0.1, 1.55 ± 0.1, 3.18 ± 0.15 lm, respectively. Alternatively, the mean diameter and length of SiNNs grown on island-like Au NSs are discerned to 69 nm and 3.1 ± 0.15 lm, respectively. Au NSs size has greatly affected the dimension (diameter and length) of SiNNs. Increase in catalyst NSs size led to an increase in both the diameter and the length of the NNs. However, SiNNs catalyzed using continuous Au film exhibited a decrease in both average diameter (60 nm) and length (1.5 ± 0.1 lm). Similar kind of structural evolution is acknowledged by Cui et al. [20], where mono-disperse SiNWs are grown by LPCVD system at 440 °C using varying diameters of colloidal Au NPs as catalyst. They observed that an increase in the catalyst diameter from 5 to 30 nm resulted in an increase in the average NW diameter from 6 to 31 nm. Furthermore, Hochbaum et al. [18]

Fig. 3. Diameter-distribution for SiNNs synthesized using different Au NSs of diameter (a) 9 nm, (b) 14 nm, (c) 25 nm, (d) island-like, and (e) continuous film.

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synthesized SiNWs using CVD system at 800 °C from Au colloids catalyst of diameters 20, 30, and 50 nm. Again, the nanowire diameter is found to increase from 39 to 93 nm with the increase of Au colloids diameter from 20 to 50 nm. Qin et al. [19] used ICP-CVD system to grow SiNWs on Si substrates at 380 °C via two different size (16 and 40 nm) of Au colloids. The diameter ranged from 90 to 115 nm is found to enhance between 130 and 150 nm as the Au NPs size is increased from 16 to 40 nm. These overall reviews validate our observation. Fig. 4 shows the typical FESEM images cross-sectional view of SiNNs synthesized using various Au NSs morphology. The existence of the spherical Au NPs at the tips of NNs suggests that the SiNNs are formed via VLS growth mechanism, assuming the liquid phase of Au NP at the growth temperature. However, the disappearance of Au NPs from some of the SiNNs tip is due to the presence of H2 plasma in VHF-PECVD that etched them away during the growth process [21]. Nevertheless, the FESEM images demonstrated that the typical tips of the SiNNs are scale down to unique sharpness ranging between 3 and 10 nm. Fig. 5 illustrates the XRD spectra of the SiNNs catalyzed using various sizes of the Au NSs. The observed three prominent diffraction peaks located at 28.376°, 47.259° and 56.084° are assigned to the (1 1 1), (2 2 0) and (3 1 1) lattice planes of Si, respectively. The XRD patterns for crystallites SiNNs growth correspond to diamond cubic structure with JCPDS# 05-0565 (same phase as Si). The occurrences of sharp diffraction Si peaks for all samples clearly authenticate that the SiNNs growth is highly crystalline in nature. It can be inferred from the X-ray diffraction line shapes that increasing the Au catalyst size causes a substantial change in the Si(1 1 1) peak intensity and the corresponding full-width at half maxima (FWHM). The Scherrer’s FWHM of the Si(1 1 1) diffraction peaks decreased from 0.54° to 0.18° with an increase in the size of the catalyst from 9 nm to island-like Au NSs. A broadening of 0.36° (Table 1) is observed when samples are catalyzed with continuous Au film. The observed decrease in FWHM of the XRD peak with the increase of NNs diameter is majorly attributed to the modification of samples crystallinity [42]. These results are well consistent with the Raman spectral features. In addition, two sharp diffraction peaks including (1 1 1) and (2 0 0) corresponding to the Au crystallographic planes at 2h of 38.155° and

Fig. 4. FESEM images cross-sectional view for SiNNs synthesized using different Au NSs (a) 9 nm, (b) 14 nm, (c) 25 nm, (d) island-like, and (e) continuous film.

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Fig. 5. The XRD patterns for SiNNs synthesized using different Au NSs (a) 9 nm, (b) 14 nm, (c) 25 nm, (d) island-like, and (e) continuous film.

44.391° respectively, are evidenced. This verified the formation of crystalline Au NPs in the face-centered cubic structure as matched with JCPDS# 04-0784 of Au. Fig. 6 presents the Raman spectra of as-synthesized SiNNs. The NNs catalyzed using Au NPs of 14 nm, and 25 nm as well as island-like and film Au NSs revealed sharp peaks centered at 513.25, 517.97, 519.15 and 516.79 cm1, respectively. The sharp peaks accurately correspond to the first-order transversal optical (TO) phonon mode of bulk c-Si. Conversely, NNs grown from 9 nm Au NPs displayed a broad peak around 510.89 cm1 accompanied by a weak shoulder at a lower frequency of 490 cm1. The appearance of weak shoulder is attributed to the presence of amorphous structure in the SiNNs. The location of TO phonon mode for amorphous-Si (a-Si) film is centered at 480 cm1 and the same for bulk c-Si is occurred at 520 cm1 [43,44]. Strikingly, a shift and an asymmetric broadening of the TO phonon mode are observed with the decrease of NNs diameter (Table 1). This asymmetric broadening and shift of the Raman peaks toward lower frequencies are related to the effect of phonon confinement in SiNNs [45,46] or impact of the local laser beam induced intense heating of the NNs during the measurement [47,48]. Needless to mention that special care is taken to minimize the heating effects caused laser excitation. Two broad peaks located around 930–950 cm1 and 290–300 cm1 are allocated to the second-order transverse optical (2TO) and the second-fold transverse acoustic (2TA) phonon modes, respectively [45,49]. These peaks are observed in Raman spectra for all the prepared samples. The shift of Raman peaks from 520 cm1 is majorly related to the decrease in crystallite size. Furthermore, the presence of micro/nanocrystalline Si structures can also contribute to the downshift

Fig. 6. Raman spectra for SiNNs synthesized using different Au NSs (a) 9 nm, (b) 14 nm, (c) 25 nm, (d) island-like, and (e) continuous film. The inset shows magnified TO phonon mode.

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because of the quantum confinement effect [45]. The NNs crystallite size (DR) is obtained from the Raman spectra using the relation [50]:

DR ¼ 2p

rffiffiffiffiffiffiffiffi B ; Dx

ð1Þ

where B is 2.24 cm1 nm2 for Si and Dx is the shift of the TO phonon mode from the bulk c-Si peak location. Value of DR is found to increase from 3.12 nm to 10.17 nm for the NNs catalyzed using NPs with the average diameter of 9 nm and by island-like Au NSs, respectively (Table 1). Furthermore, it is radically decreased to 5.25 nm for NNs catalyzed by continuous Au film, where SiNNs with low-density is achieved almost devoid of Au NPs. The increase in DR with the increase of Au catalyst size indeed implies an improvement in SiNNs crystallinity. This enhanced crystallinity of the samples is closely associated with reduction of FWHM of the TO phonon mode [51]. The FWHM of the c-Si peak is found to gradually decline (Table 1) from 38 to 14 cm1 with the increase in Au catalyst size. The reduction in the FWHM of the c-Si peak is related to the enhancement in crystalline quality to form defect free or less defect structures [52]. This agrees well with the XRD analysis discussed above. The increase in the NNs diameter led to an augmentation in the crystallinity of samples which may cause the shifting (8.26 cm1) of the TO phonon mode toward higher frequencies as clearly depicted in the inset of Fig. 6. Al-Taay et al. [53] acknowledged that the Raman peak location depend on the NWs diameter, where NWs are synthesized by pulsed plasma enhanced CVD (PPECVD). The Raman peaks are located at 496.5 cm1 for NWs with diameters ranging from 40 to 100 nm and shifted to 513.5 cm1 when the diameter of the NWs is increased in the range of 160–220 nm. Wang et al. [17] observed a Raman peak at 509.8 cm1 for 10 nm wide NWs synthesized by laser ablation, which is shifted to 517.7 cm1 as the diameter of the NWs increased to 21 nm. These observations validate our Raman spectral analyses. Fig. 7 depicts the zoomed-out view of TEM image of an individual Si nanoneedle grown with a Au NPs catalyst of diameter 25 nm. It clearly demonstrates that the diameter of the NN is decreasing gradually from bottom to the top with a smooth slope from 65 nm to 10 nm. Furthermore, the Au NPs can be observed clearly at the tip of the NN. The zoomed-in view of the NN stem (within the white square area in Fig. 7) is shown as the inset (top left) of Fig 7. The surface of the SiNN is found to terminate with 7 nm thick a-Si outer shell which originated mainly from the enhanced side-wall growth of a-Si via the SiH4 plasma. Consequently, VHF-PECVD provided a technique to produce crystalline–amorphous core–shell NN structure. The a-Si oxide shell may be important for SiNNs since it not just makes the surface passivated but can likewise help keep the charge carriers confined in the NNs. On top, HRTEM image clearly revealed the nanocrystalline structure of SiNN. This agrees well with the Raman spectral analyses. The nanoneedle is highly crystalline with a lattice spacing of 0.314 nm corresponding to the (1 1 1) plane as per JCPDS# 05-0565, which is also reflected from the most intense XRD peak. This verified that (1 1 1) direction of NN growth is preferred (Fig. 5). The calculated lattice parameters from XRD patterns is 0.315 nm. The crystalline silicon core of diameter about 16 nm is approximately equal to the exciton Bohr radius of bulk c-Si (5 nm), suggesting a weak phonon confinement regime. The confinement effect is responsible for the occurrence of shift and an asymmetric broadening of the Raman bands. The TEM analysis confirms that the NNs growth process occurs through the VLS mechanism. VLS mechanism is described as follows. First, the silane (SiH4) molecules get decomposed in the form of Si onto the surface of Au catalyst. Second, the Si atoms get dissolved into the liquid Au particles to form Au–Si alloy. Third, the supersaturated Au–Si alloy via the continuous adsorption of the Si in Au–Si liquid droplets allows the growth of a solid Si core in the droplet underneath. Fourth, these Au–Si droplets enclosing the Si core remains at the top and kept on accumulating more Si atoms from the silane source. Finally, the solid Si grows in the liquid–solid interface to create NWs. Thus, the VLS reaction entails three phases: (a) Si atoms diffusion from the vapor phase to the droplet interface of vapor/Au–Si; (b) Si atoms diffusion into the liquid droplet; and (c) Si atoms precipitation at the droplet/NW interface. Moreover, the required thermal

Fig. 7. The zoomed-out view TEM image of the Au-catalyzed Si nanoneedle prepared using VHF-PECVD. The top left inset is the zoomed-in view HRTEM image of the NN stem taken from white square area. The dashed white lines indicate the crystalline silicon core diameter.

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activation energy of Au-catalyzed SiNWs is in consistent with the thermally activated decomposition of silane [54]. It is worth noting that the plasma deposition being a thermodynamic non-equilibrium process the SiNWs growth via plasma activation to CVD system does not obey VLS mechanism. Besides, in VHF-PECVD procedure silane gets pre-dissociated to an ionized gas. Thus, in VHF-PECVD Si atoms possess higher diffusivity and adsorption capacity than in CVD. Unlike CVD process, NNs growth in the present case is not restricted by the decomposition of silane. Several other factors contribute to the growth process. Another advantage of using the VHF plasma system for NWs growth is associated to the non-catalytic deposition of Si on the sides of NWs (VS mechanism), resulting the tapering of NWs. Generally, this effect is undesirable for cylindrical NWs growth, but it allows the production of cone-shaped or needle-like SiNWs with the ultra-sharp tips. Synthesis of such SiNNs with well controlled dimensions may effectively contribute in building futuristic nanodevices useful for diverse applications. For example, these NNs can be utilized as sensitive typical probe-tips in atomic force microscopy (AFM) due to their very small apex angle (h  5°), allowing a superior scanning of the sample surface profile with high spatial resolution [55]. Furthermore, these ultra-sharp SiNNs possessing high absorption coefficient are prospective for efficient solar cell applications [56–58]. In short, present findings attest to the validity of size-controlled growth process, where SiNNs dimension can precisely be improved by optimizing the catalyst morphology and other processing parameters. 4. Conclusion Different sizes of the catalyst Au NSs ranging from 9 nm to continuous thin film are used to synthesize SiNNs via the VHF-PECVD system. The influence of Au catalyst size on the improved surface morphology and structural properties of SiNNs are reported. The utilization of very high frequency (150 MHz) plasma is established to open interesting perspectives for swift processing of accurate size controlled SiNNs with minimal effort at low-cost. The metal catalyst played a significant role in growing high quality NNs with controlled morphology. It is shown that the size and morphology of Au NSs can easily be controlled using RF magnetron sputtering technique, where manipulation of growth time is important. FESEM images confirmed the enhancement of SiNNs diameter with the increase of catalyst size, with the exception for those catalyzed using continuous film. Small-size Au catalysts are determined to be easier for VHF plasma activation to grow long and dense NNs than that with the continuous Au film. The existence of Au NPs at the tips of the SiNNs verified the VLS mechanism assisted growth of NNs. XRD spectra of SiNNs verified their high crystallinity with preferred growth direction along (1 1 1), (2 2 0) and (3 1 1) crystallographic planes. The achievement of highly crystallinity makes these NNs potential for solar cell applications. Raman spectral analyses of SiNNs confirmed their increased crystallinity, where Raman peaks are observed to be shifted from 510.89 cm1 to 519.15 cm1 with increasing catalyst size. The HRTEM images authenticated the SiNNs core–shell structure which comprised of a-Si shell surrounded by a crystalline Si core. Furthermore, the core diameter is determined to be small enough for the observation of phonon quantum confinement effects. Excellent features of the results suggested that very high frequency plasma enhanced chemical vapor deposition is a promising method for the synthesis of high-quality needle-like SiNWs. 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