Superlattices and Microstructures 85 (2015) 849–858

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Superlattices and Microstructures

journal homepage: www.elsevier .com/locate /super lat t ices

Improved structural features of Au-catalyzed siliconnanoneedles

http://dx.doi.org/10.1016/j.spmi.2015.07.0210749-6036/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, 81310 Skudai, Johor Baharu, ME-mail address: yasirphd@yahoo.com (Y.H. Mohammed).

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, Malaysiab Department of Physics, Faculty of Education, University of Mosul, 41002 Mosul, Iraqc Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, 81310 Skudai, Johor Baharu, Malaysia

a r t i c l e i n f o a b s t r a c t

Article history:Received 7 July 2015Accepted 8 July 2015Available online 9 July 2015

Keywords:VHF-PECVDAu NSsVLS growthRF sputteringSiNNs

Nanometer sized silicon (Si) needles (nanowires) offer a vehicle for varieties of applicationsat nanoscale. We grow Si nanoneedles (SiNNs) using very high frequency plasma enhancedchemical 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 areprepared on Si(100) substrates via radio frequency magnetron sputtering. Au catalyst sizedependent surface morphology and structural features of these SiNNs are determined.Samples are characterized via imaging and spectroscopic measurements. Controlledgrowth of such SiNNs structure with reproducibility that is achieved via Au NPs size tun-ability is attributed to the vapor–liquid–solid (VLS) growth mechanism. SiNNs with diam-eters between 20 and 120 nm and length up to 5 lm are acquired. SiNNs diameter is foundto increase with the increase of Au NPs size. Processing parameters optimization is demon-strated to play a critical role in nucleating Au NPs and thereby achieving high density SiNNsmorphology. X-ray diffraction patterns authenticated an enhanced SiNNs crystallinity withincreasing catalyst size. Raman spectra of SiNNs revealed a red-shift (�8.26 cm�1) in thefirst-order transversal band as the average diameter of NNs are decrease from 69 to57 nm. Our systematic method for synthesis and characterization may contribute towardthe development of SiNNs based optoelectronics.

� 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Tunable growth of high quality SiNNs and their subsequent characterizations are essential for optoelectronic device fab-rication. Recently, researches in Si nanowires (SiNWs) are fuelled by their enormous potential in broad arrays of technolog-ical 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 dueto their easy structural modification, biocompatibility and environmental affability [6,7]. Compare to NWs, SiNNs are distinc-tively noteworthy because their sharp curvilinear tips are suitable for high sensitive probing with superior spatial resolutionand augmented field emission enhancement factor [8–10]. Unlike NWs, SiNNs are difficult to grow owing to the unavailabil-ity of accurate synthesis techniques to successfully prepare NNs with ultra-sharp tips.

alaysia.

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Traditionally, catalyst assisted VLS process is used to synthesize SiNWs [11]. In the VLS process, growth catalysts such asAu, tin (Sn) or Indium (In) are exploited as energetically effective site for vapor-phase Si adsorption. Moreover, they requirelow eutectic temperatures of 363 �C (Au/Si), 232 �C (Sn/Si) and 157 �C (In/Si). This process being growth catalyst size andposition dependent offers a good capacity to tune the diameter and arrangement of NWs [12,13]. Consequently, the size con-trol of the metal catalyst is pre-requisite because it decides the NWs growth. The effect phonon confinement on the opticaland 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 sundryapplications. 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 duringgrowth 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 ofNWs in different geometries (diameter, height, shape) with varying density and regularity. Most of the previous attemptsfor 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 fre-quency [22]. It is demonstrated that VHF plasma enhances the growth rate as much as 10 Å/s [23] than 0.1 Å/s for standardplasma [24,25]. Reduced dust formation, less intrinsic stress [26] and enhanced growth rate [27] are some notable advan-tages of the VHF plasma over the standard plasma system. To the best of our knowledge, only few preliminary efforts arededicated to prepare SiNNs with controlled dimensions [21,28,29]. SiNNs growth via VHF-PECVD method is not yet exploredextensively.

We report the effects of Au-catalyst size on the growth morphology and structural evolution of VHF-PECVD synthesizedSiNNs. The samples morphologies are characterized using field emission scanning electron microscopy (FESEM), X-raydiffraction (XRD), Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM) measurements.

2. Experimental

SiNNs are grown on Au-coated Si(100) substrates. Substrates are cleaned in an ultrasonic bath using acetone, isopropanolalcohol, and deionized (DI) water over a period of 5 min each. Next, these cleaned substrates are immersed in 10% hydroflu-oric acid for 2 min to remove the surface oxide layer followed by a rinse in DI water. Finally, they are dried using nitrogen gasbefore being transferred into the sputtering chamber. High-purity Au films are deposited at room temperature via RF mag-netron sputtering operated at frequency of 13.56 MHz and base pressure of 8.5 � 10�6 Torr in pure Ar atmosphere under10 sccm. A fixed RF power of 50 W having different growth time is employed to obtain Au NSs of various sizes as summarizedin Table 1. The estimated film thicknesses are in the range of 1–10 nm. SiNNs are grown by loading these substrates intoVHF-PECVD chamber operated at high vacuum (1 � 10�8 Torr) and heated up to 550 �C in the presence of Ar plasma pressureof 20 mTorr. The substrate temperatures are recorded using a K-type (Chromel–Alumel) thermocouple which touched thebottom of the substrate holder. Then, H2 diluted SiH4 gas is inserted into the chamber at constant flow rate between 60and 10 sccm, respectively. Power electrode of the radio frequency generator (150 MHz) produced the plasma. A constantradio 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 foundto be brown dark in color. The optimized growth parameters are obtained through their systematic variation over a broadrange.

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 foundto be the Gaussian. The growth and nucleation of SiNNs are confirmed via HRTEM (HRTEM, JEM-2100; JEOL) operated at200 kV. For TEM measurement, the sample is prepared on a Formvar/Carbon film containing 300 mesh copper grid (LaceyF/C 300 mesh Cu) by dipping the substrate in pure ethanol solution and sonicated for 15 min to remove the NNs from thesubstrate. Subsequently, the suspension containing an individual nanoneedle is places onto the TEM copper grid through

Table 1Average diameter of synthesized Au catalysts and SiNNs, FWHM of the Si(111) XRD peak and c-Si Raman peaks, Raman peaks positions and SiNNs crystallitesize. The indicated errors are mostly instrumental and measurement related.

Average catalyst diameter(nm)

Average NN diameter(nm)

FWHM of Si(111) XRDpeak (�)

FWHM of c-Si Raman peak(cm�1)

Raman peak(cm�1)

DR (nm)

9 ± 1 57 ± 1 0.54 ± 0.02 38 ± 2 510.89 ± 0.05 3.12 ± 0.0514 ± 1 61 ± 1 0.49 ± 0.02 20 ± 1 513.25 ± 0.05 3.62 ± 0.0525 ± 2 67 ± 1 0.31 ± 0.02 18 ± 1 517.97 ± 0.05 6.51 ± 0.05Island-like 69 ± 1 0.18 ± 0.02 16 ± 1 519.15 ± 0.05 10.17 ± 0.05Film 60 ± 1 0.36 ± 0.02 14 ± 1 516.79 ± 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 structureof the grown SiNNs are verified using a PANalytical X-ray diffractometer (Empyrean) with Cu Ka radiation (k = 1.54 Å) oper-ating at 40 kV, 40 mA with 2h from 20� to 60� and scanning angle step size of 0.02�. Raman measurements are performed bythe 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, respec-tively. Spontaneous formation of coherent 3D Au NPs on Si substrate is attributed to hetero-epitaxy lattice mismatch medi-ated Volmer–Weber growth process. A lattice mismatch �25% between the Au and Si allowed the direct formation of Auparticles on the substrate surface [30–32]. Thus, the dispersed spherical particles grew as bigger island-like structuresand finally agglomerated to form continuous film at the later stages of deposition. These FESEM images (Fig. 1) reveal thegrowth time dependent surface morphology of the Au NSs deposited on Si substrate. Initially, at growth time of 5 s very tinynucleation centers (diameter �9 nm) are created and spread over the entire substrate (Fig. 1a). As the growth time isincreased up to 20 s more nucleation sites are formed and the deposited amount of Au is increased. Fig. 1b and c exhibiteda smooth surface with spherical Au NPs of average diameter �14 and 25 nm, respectively. Further increase in growth time to30 s caused agglomeration of some smaller NPs to create larger NPs simultaneously a few of them are coalesced to formisland-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 figuresreveal 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 filmstructure.

The deposited Au films involved several growth stages. Firstly, small nuclei are formed following Volmer–Weber mech-anism. Secondly, bigger islands are created via the coalescence of smaller NPs. Finally, these bigger islands are coalesced toform continuous film. The formation of metal film-like pattern is interpreted in terms of Maxwell Garnett theory. At higherdeposition time with growing film thickness both the Au NPs morphology and particle–particle interaction appeared moreintricate [33,34]. As the coalescence propagates at large-scale, the film appeared more like interpenetrating network struc-ture 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 thefilm can be explained following Bruggeman theory. Even though the comprehensive understanding regarding the role of sur-face electrodynamics remains obscure, the formation of continuous film is believed to be more or less compatible with theDrude theory.

Fig. 2 illustrates the FESEM images of VHF plasma synthesized SiNNs grown on substrates containing Au NSs catalysts ofdifferent types including NPs, islands, and continuous film. The dependence of SiNNs morphology on catalyst Au NSs size isclearly 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 istruly controlled by the catalyst structures. Conversely, sparse and inhomogeneous distribution of NNs obtained fromsmall-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 growthof NNs with large diameter. This observation is attributed to competition of silicon homogeneous nuclei formation in theAu/Si droplet with the adatom attachment at the Au/Si droplet substrate interface [38]. The diameter of the SiNNs is largerthan 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 con-tinuous 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 isfound 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 favorablefor VHF plasma to initiate the growth of SiNNs. The appearances of slight gradual bending like growth defects are ascribed tothe accumulation of elastic strains on these very small diameter nanoneedles [20]. Conversely, some kinks are resulted fromthe high growth rates [40]. Growth rate of SiNNs is estimated to be about 350 nm/min after 15 min of growth. This value iscertainly 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 thegrowth 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 andlength 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 NSsare 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, SiNNscatalyzed 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 LPCVDsystem at 440 �C using varying diameters of colloidal Au NPs as catalyst. They observed that an increase in the catalyst diam-eter 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) continuousfilm.

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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 nanowirediameter 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. Thediameter ranged from 90 to 115 nm is found to enhance between 130 and 150 nm as the Au NPs size is increased from16 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. Theexistence of the spherical Au NPs at the tips of NNs suggests that the SiNNs are formed via VLS growth mechanism, assumingthe liquid phase of Au NP at the growth temperature. However, the disappearance of Au NPs from some of the SiNNs tip isdue to the presence of H2 plasma in VHF-PECVD that etched them away during the growth process [21]. Nevertheless, theFESEM images demonstrated that the typical tips of the SiNNs are scale down to unique sharpness ranging between 3 and10 nm.

Fig. 5 illustrates the XRD spectra of the SiNNs catalyzed using various sizes of the Au NSs. The observed three prominentdiffraction peaks located at 28.376�, 47.259� and 56.084� are assigned to the (111), (220) and (311) 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 growthis highly crystalline in nature. It can be inferred from the X-ray diffraction line shapes that increasing the Au catalyst sizecauses a substantial change in the Si(111) peak intensity and the corresponding full-width at half maxima (FWHM). TheScherrer’s FWHM of the Si(111) diffraction peaks decreased from 0.54� to 0.18� with an increase in the size of the catalystfrom 9 nm to island-like Au NSs. A broadening of 0.36� (Table 1) is observed when samples are catalyzed with continuous Aufilm. The observed decrease in FWHM of the XRD peak with the increase of NNs diameter is majorly attributed to the mod-ification of samples crystallinity [42]. These results are well consistent with the Raman spectral features. In addition, twosharp diffraction peaks including (111) and (200) 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) continuousfilm.

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.

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44.391� respectively, are evidenced. This verified the formation of crystalline Au NPs in the face-centered cubic structure asmatched 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 wellas island-like and film Au NSs revealed sharp peaks centered at 513.25, 517.97, 519.15 and 516.79 cm�1, respectively. Thesharp peaks accurately correspond to the first-order transversal optical (TO) phonon mode of bulk c-Si. Conversely, NNsgrown from 9 nm Au NPs displayed a broad peak around 510.89 cm�1 accompanied by a weak shoulder at a lower frequencyof �490 cm�1. The appearance of weak shoulder is attributed to the presence of amorphous structure in the SiNNs. The loca-tion of TO phonon mode for amorphous-Si (a-Si) film is centered at 480 cm�1 and the same for bulk c-Si is occurred at520 cm�1 [43,44]. Strikingly, a shift and an asymmetric broadening of the TO phonon mode are observed with the decreaseof NNs diameter (Table 1). This asymmetric broadening and shift of the Raman peaks toward lower frequencies are related tothe effect of phonon confinement in SiNNs [45,46] or impact of the local laser beam induced intense heating of the NNs dur-ing the measurement [47,48]. Needless to mention that special care is taken to minimize the heating effects caused laserexcitation. Two broad peaks located around 930–950 cm�1 and 290–300 cm�1 are allocated to the second-order transverseoptical (2TO) and the second-fold transverse acoustic (2TA) phonon modes, respectively [45,49]. These peaks are observed inRaman spectra for all the prepared samples. The shift of Raman peaks from 520 cm�1 is majorly related to the decrease incrystallite 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 showsmagnified 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 therelation [50]:

Fig. 7.image o

DR ¼ 2pffiffiffiffiffiffiffiffiB

Dx

r; ð1Þ

where B is 2.24 cm�1 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 bycontinuous Au film, where SiNNs with low-density is achieved almost devoid of Au NPs. The increase in DR with the increaseof Au catalyst size indeed implies an improvement in SiNNs crystallinity. This enhanced crystallinity of the samples is closelyassociated 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 cm�1 with the increase in Au catalyst size. The reduction in the FWHM of the c-Si peak is related tothe enhancement in crystalline quality to form defect free or less defect structures [52]. This agrees well with the XRD anal-ysis discussed above. The increase in the NNs diameter led to an augmentation in the crystallinity of samples which maycause the shifting (�8.26 cm�1) of the TO phonon mode toward higher frequencies as clearly depicted in the inset ofFig. 6. Al-Taay et al. [53] acknowledged that the Raman peak location depend on the NWs diameter, where NWs are synthe-sized by pulsed plasma enhanced CVD (PPECVD). The Raman peaks are located at 496.5 cm�1 for NWs with diameters rang-ing from 40 to 100 nm and shifted to 513.5 cm�1 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 cm�1 for 10 nm wide NWs synthesized by laser ablation, which is shifted to517.7 cm�1 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 diameter25 nm. It clearly demonstrates that the diameter of the NN is decreasing gradually from bottom to the top with a smoothslope from �65 nm to �10 nm. Furthermore, the Au NPs can be observed clearly at the tip of the NN. The zoomed-in viewof 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 isfound to terminate with �7 nm thick a-Si outer shell which originated mainly from the enhanced side-wall growth ofa-Si via the SiH4 plasma. Consequently, VHF-PECVD provided a technique to produce crystalline–amorphous core–shellNN structure. The a-Si oxide shell may be important for SiNNs since it not just makes the surface passivated but can likewisehelp keep the charge carriers confined in the NNs. On top, HRTEM image clearly revealed the nanocrystalline structure ofSiNN. This agrees well with the Raman spectral analyses. The nanoneedle is highly crystalline with a lattice spacing of0.314 nm corresponding to the (111) plane as per JCPDS# 05-0565, which is also reflected from the most intense XRD peak.This verified that (111) 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 bulkc-Si (�5 nm), suggesting a weak phonon confinement regime. The confinement effect is responsible for the occurrence ofshift and an asymmetric broadening of the Raman bands. The TEM analysis confirms that the NNs growth process occursthrough the VLS mechanism.

VLS mechanism is described as follows. First, the silane (SiH4) molecules get decomposed in the form of Si onto the sur-face of Au catalyst. Second, the Si atoms get dissolved into the liquid Au particles to form Au–Si alloy. Third, the supersat-urated Au–Si alloy via the continuous adsorption of the Si in Au–Si liquid droplets allows the growth of a solid Si core in thedroplet underneath. Fourth, these Au–Si droplets enclosing the Si core remains at the top and kept on accumulating more Siatoms from the silane source. Finally, the solid Si grows in the liquid–solid interface to create NWs. Thus, the VLS reactionentails three phases: (a) Si atoms diffusion from the vapor phase to the droplet interface of vapor/Au–Si; (b) Si atoms diffu-sion into the liquid droplet; and (c) Si atoms precipitation at the droplet/NW interface. Moreover, the required thermal

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 HRTEMf 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 isworth noting that the plasma deposition being a thermodynamic non-equilibrium process the SiNWs growth via plasmaactivation to CVD system does not obey VLS mechanism. Besides, in VHF-PECVD procedure silane gets pre-dissociated toan ionized gas. Thus, in VHF-PECVD Si atoms possess higher diffusivity and adsorption capacity than in CVD. Unlike CVD pro-cess, NNs growth in the present case is not restricted by the decomposition of silane. Several other factors contribute to thegrowth process.

Another advantage of using the VHF plasma system for NWs growth is associated to the non-catalytic deposition of Si onthe sides of NWs (VS mechanism), resulting the tapering of NWs. Generally, this effect is undesirable for cylindrical NWsgrowth, but it allows the production of cone-shaped or needle-like SiNWs with the ultra-sharp tips. Synthesis of suchSiNNs with well controlled dimensions may effectively contribute in building futuristic nanodevices useful for diverse appli-cations. For example, these NNs can be utilized as sensitive typical probe-tips in atomic force microscopy (AFM) due to theirvery 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 applica-tions [56–58]. In short, present findings attest to the validity of size-controlled growth process, where SiNNs dimension canprecisely 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 theVHF-PECVD system. The influence of Au catalyst size on the improved surface morphology and structural properties ofSiNNs are reported. The utilization of very high frequency (150 MHz) plasma is established to open interesting perspectivesfor swift processing of accurate size controlled SiNNs with minimal effort at low-cost. The metal catalyst played a significantrole in growing high quality NNs with controlled morphology. It is shown that the size and morphology of Au NSs can easilybe controlled using RF magnetron sputtering technique, where manipulation of growth time is important. FESEM imagesconfirmed the enhancement of SiNNs diameter with the increase of catalyst size, with the exception for those catalyzedusing continuous film. Small-size Au catalysts are determined to be easier for VHF plasma activation to grow long and denseNNs than that with the continuous Au film. The existence of Au NPs at the tips of the SiNNs verified the VLS mechanismassisted growth of NNs. XRD spectra of SiNNs verified their high crystallinity with preferred growth direction along(111), (220) and (311) crystallographic planes. The achievement of highly crystallinity makes these NNs potential for solarcell applications. Raman spectral analyses of SiNNs confirmed their increased crystallinity, where Raman peaks are observedto be shifted from 510.89 cm�1 to 519.15 cm�1 with increasing catalyst size. The HRTEM images authenticated the SiNNscore–shell structure which comprised of a-Si shell surrounded by a crystalline Si core. Furthermore, the core diameter isdetermined to be small enough for the observation of phonon quantum confinement effects. Excellent features of the resultssuggested that very high frequency plasma enhanced chemical vapor deposition is a promising method for the synthesis ofhigh-quality needle-like SiNWs.

Acknowledgments

Authors are thankful to the Malaysian Ministry of Education (MoE), Universiti Teknologi Malaysia for Research UniversityGrant (Vote No. 06H69) and Ibnu Sina Institute for Fundamental Sciences. Yasir is grateful Ministry of Higher Education andScientific Research, Iraq for providing PhD scholarship.

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