Size-Dependent Localized Phonon Population in Semiconducting Si

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Size Dependent Localized Phonon Population in Semiconducting Si Nanowires Avinash Patsha, and Sandip Dhara Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b03300 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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Size Dependent Localized Phonon Population in Semiconducting Si Nanowires Avinash Patsha,*†‡ and Sandip Dhara*†

†Surface

and Nanoscience Division, Indira Gandhi Centre for Atomic Research,

Kalpakkam-603102, India

‡ Presently

*E-mail:

at Department of Materials Science & Engineering, Tel Aviv University, Israel

[email protected]; *E-mail: [email protected]

ABSTRACT The optical phonons in semiconductor nanostructures play an indispensable role in fundamental phenomenon and device applications based on these nanostructures. We study the Raman spectroscopy of optical phonons in Si nanowires (NWs) whose sizes are beyond the phonon confinement regime. The peak shift and unusual asymmetric

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broadening by one-phonon mode in Si NWs is observed during far-field Raman studies. Using an appropriate thermal anchoring and localized Raman measurements on single NWs by near-field tip-enhanced Raman spectroscopy (TERS), we demonstrate the decoupling of multiple origins responsible for the peak shift accompanied by asymmetric broadening of the one-phonon mode, and appearance of multiple phonon peaks from a single measurement area. A model based on the localized phonon population induced by NW size dependent charge depletion, is proposed to explain the observed dependence of phonon characteristics on NW size. The scanning Kelvin probe force microscopy measurements confirm the size dependent intrinsic semiconductor surface and interface states induced charge depletions in single Si NWs. The study clearly suggests the size dependent phonon characteristics of Si NWs which are crucial for several NW based photovoltaic and thermoelectric devices.

KEYWORDS: Si nanowires, Phonon population, Raman spectroscopy, Size dependent, Kelvin probe force microscopy, depletion layers,


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Semiconductor nanowires (NWs) are the subject of interest for almost two decades, owing to their fundamental quantum confinement characteristics in the onedimensionality, size dependent electrical, optical, thermal, chemical and surface properties.1-4 Among all extensive size dependent studies of elemental (Group IV: Si, Ge) and compound (III-V and II-VI) semiconductor NWs, Si NWs stand forefront, both in fundamental phenomenon and application point of view. The first-principle studies predicted the significant electronic quantum confinement in Si NWs of diameters < 2.3 nm, and the confinement is experimentally confirmed by the observed optical bandgaps of up to 3.5 eV.5-7 Similarly, the phonon confinement is found to be significant in Si NWs of diameters less than 15 nm.8-13 Even the NWs of sizes beyond the confinement regime found to show size dependent electrical and thermal (550 W.m-1.K-1) conductivity, charge carrier mobility (10-3102 cm2.V-1.s-1), ideality factors (27), piezoresistance (7010-11  -3,55010-11 Pa-1), super-hydrophobicity (contact angle ~159), and polarization-independent anti-reflection properties.14-20 Many of these size dependent properties are found to originate from the major contribution of ubiquitous intrinsic and


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extrinsic surface and interface states exists on semiconductor NWs and the resultant scattering processes by these states.16,19,21,22

Phonon confinement effects are observed in Raman spectroscopic studies of several Si nanostructures including nanocrystals (NCs), NWs. The debate on red-shift and asymmetric broadening of one-phonon mode observed in Raman studies of Si NWs is also as old as the topic of semiconductor NWs and several models have been evolved in this process. The nanostructure size and shape dependent characteristics of Raman spectra are modeled by Richter-Wang-Ley (microcrystalline size) and Campbell-Fauchet (shape-dependent) confinement models.8,23 Both models conclude the phonon confinement as the origin of red-shift and asymmetric broadening of one-phonon Raman spectrum of Si nanostructures of sizes < 15 nm, compared to that of bulk c-Si. The confinement is effective in Si NWs of sizes below 20 nm and found to show additional overtone and combination modes owing to finite size effect in this size limit.10,12 In the same size limit of 515 nm of Si NWs, the laser induced Fano resonance scattering is also observed due to the coupling between photon-induced free carriers and optical


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phonons.24 The line shape of the one-phonon Raman spectrum is modeled with Fanoline shape,

I() = A (q+)2 / (1+)2

(1)

where,  = ( ) /  ,  is the shift in phonon wavenumber with respect to that of the bulk c-Si, q is asymmetry parameter, and  is the full width at half maximum (FWHM).

In addition to phonon confinement, laser induced heating at high excitation powers and poor thermal anchoring of nanostructures with substrate is also identified as the origin of the red-shift and broadening of phonon peak in similar size ranges of < 20 nm for both Si NWs and dots.11,25,26 The induced temperatures are estimated by Raman thermometry for studying the heating effects. In most recent studies, the Raman thermometry is found to be inaccurate in estimating the temperatures at high optical excitation on Si NCs for studying the heating effects.27 The observed red-shift and broadening of 520 cm-1 phonon mode is explained by the nonthermal optical phonon population generated by inefficient diffusion of laser-induced charges in inherently poor electrical conducting Si NCs.27 The Raman spectra of very small self-assembled Si quantum dots (QDs) of sizes ~3 nm


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embedded in the matrix of SiCx showed the shifted and broadened phonon peak in the region (400  500 cm-1) of amorphous Si.28 All most all the Raman studies discussed above are conducted on ensemble Si nanostructures, considering the homogeneous size distribution and surface properties of nanostructures. In particular, the majority of the Raman studies on Si NWs are explored for the sizes below 25 nm. However, as discussed above, the NW size dependent electrical properties, carrier mobilities, their scattering by surface and interface states, and coupling among carriers and phonons have not been considered in understating the Raman spectra of optical phonons in Si NWs whose sizes are beyond the confinement regime.

In the present studies, we show that the observed multiple optical phonon peaks, in addition to their peak shift and broadening in semiconductor Si NWs depends on the NW size and intrinsic surface properties. We systematically decouple the multiple origins such as phonon confinement, laser heating, Fano-interference scattering, and intrinsic surface dominated properties for understanding the appearance of multiple peaks and their peak shift accompanied by broadening in a single measurement area. For the first


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time, the localized Raman measurements using tip-enhanced Raman spectroscopy (TERS) on single Si NWs are conducted to study the size depend phonon characteristics. The size dependent intrinsic semiconductor surface and interface states induced charge depletions in single NWs are explored by scanning Kelvin probe force microscopy (SKPM).

Morphological studies by field emission electron microscopy (FESEM) (Figure S1) shows that the Si NWs have length distribution ~1030 µm and diameter distribution ~20110 nm. The aspect ratio of the NWs are in the range of 2001500. A typical low magnification transmission electron microscopy (TEM) micrograph of a NW (Figure S2a) shows a perfect rod-like shape with a diameter ~50 nm. The selected area electron diffraction (SAED) pattern (Figure S2b) reveals that the NW is single crystalline cubic phase of Si. An interplanar spacing of 0.313 nm corresponding to {111} planes of cubic Si is observed in the high resolution TEM (HRTEM) micrograph (Figure S2c). The growth direction of the NW is found to be along [111] direction. HETRM studies along the NWs of different sizes showed the negligible strain in the NWs, considering experimental error


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in the measurement of ‘d’ spacing (Table S1). It is obvious due to the fact that high temperature CVD growth process facilitates in the growth of strain free oriented single crystalline Si NWs.29 A thin layer of 1.8 nm corresponding to a native oxide is also observed on the surface of the Si NW. The structural studies on NW with smaller diameter of ~20 nm (Figure S3) also reveals its perfect single crystalline nature of the Si without any significant defects or presence of other crystalline phases.

Figure 1. Typical Raman spectra of ensemble Si NWs and bulk c-Si. The c-Si peak is fitted with single Lorentzian line profile. A typical Raman spectrum of ensemble Si NWs shows a broad peak (FWHM ~10 cm-1) centered around 517 cm-1, which is red shifted by ~3.5 cm-1 as compared to the 9 ACS Paragon Plus Environment

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zone center optical phonon mode of bulk crystalline (c-) Si (Figure 1). Both spectra were recorded at a maximum excitation power density of 140 kW.cm-2. The c-Si peak is well fitted with one-phonon Lorentzian line profile, centered at 520.6 cm-1 and FWHM () of 3.2 cm-1. However, the Raman peak of Si NWs showed poor fitting towards a single Lorentzian line profile, due to large asymmetric broadening on high-wavenumber side (Figure S4). In general, the peak shift and broadening of the optical phonon mode in Si nanostructures could be due to several factors, which were already well studied and reported as discussed earlier.8,10-12,24-26 The red-shift and asymmetric broadening of a Raman peak due to phonon confinement is ruled out in the present study, as the dimensions of the Si NWs are larger than that of confinement regime which is in the range of 1520 nm.12,24

The red-shift of the peak is possible due to heating by high optical excitation power density on ensemble Si NWs assuming the poor thermal coupling among the NWs, and between the NWs and substrate.24 On the other hand, since the Raman spectra of Si NWs were recorded using an excitation source of 514.5 nm, which is higher than the


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bandgap energy of Si, we expect the asymmetric broadening due to laser induced Fanointerference scattering by photo-excited carriers and optical phonons. One such study was already reported for the ensemble Si NWs of smaller diameters around 515 nm.24 Considering these facts, the Raman spectra of Si NWs was fitted using Eq.1 of Fano-line shape (Figure S4). However, Fano-line fit also deviated from the phonon spectrum of NWs. In general, the asymmetric Fano-line shape depends on the type of carriers (and carrier density) which interact with the optical phonons. In the case of electron-phonon interaction, red-shift accompanied by asymmetric broadening on low-wavenumber side with respect to bulk c-Si mode, should be observed for the resultant optical phonon mode.24 Whereas for the hole-phonon interaction, blue-shift accompanied by asymmetric broadening on high-wavenumber side should be observed.24 In the case of present Si NWs, the unusual characteristics of red-shift and asymmetric broadening on highwavenumber side of phonon mode are observed. As a result, the Fano-line fit deviated from the phonon spectrum. However, we find that the spectrum is well fitted with two Lorentzian curves with peak positions ~517.5, 522.3 cm-1 and FWHMs ~6.6, 7.9 cm-1 (Figure S4). Two Lorentzian curves, having completely different phonon characteristics, 11 ACS Paragon Plus Environment

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mean that there are two distinct phonon population densities within the probed area. If it is the case, then the resultant phonon spectrum of Si NWs is a convolution of two distinct optical phonons; one is red-shifted by 3 cm-1 with a phonon lifetime () of 0.8 ps and the other is blue-shifted by 1.8 cm-1 with a lifetime of 0.67 ps. It may be noted that the phonon peak shift due to stress in the NWs can be ruled out as the HRTEM studies shows negligible presence of strain in NWs. In addition, the stress does not change the shape of the peak nor induces additional phonon peaks. Therefore, the origin of multiple phonon peaks observed in a single measurement spot on the Si NWs, in the present study, cannot be ascribed to the stress. Several reports were already there for the size dependent optical, thermal, electronic, and surface properties of Si NWs. When performed on ensemble nanostructures, all of these size dependent properties can influence the Raman scattering measurements. In this context, it is difficult to ascribe the origin of multiple phonon population densities to the characteristics of either ensemble NWs or a single NW, as the probing area by 100 objective of micro-Raman spectrometer may contain more than one Si NW.


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In order to decouple the multiple origins by size dependent characteristics on optical phonon population densities during the Raman scattering measurements, we performed the localized studies on single Si NWs, using near-field Raman spectroscopy. TERS is greatly significant in enhancing the very low-intense Raman modes from localized measurements on single nanostructures of sizes below the sub-diffraction limit.30,31 The TERS is highly potential and reproducible technique to analyses the phonon peak positions, number of modes, peak intensity ratios and these reproducible phonon characteristics have been well studied using different TERS setups.32 The Si NWs were dry dispersed on to Au coated (150 nm) cover glass to avoid the problem of poor thermal coupling between NW and substrate, and among the NWs as in the case of ensemble NWs. The Si NWs having two different diameters, one ~25 nm and the other ~60 nm, were selected using atomic force microscopic (AFM) measurements, for the localized studies. Both sizes of NWs are beyond the phonon and electronic confinement-regimes and within the surface significant regime. The experimental parameters such as excitation power density and spectral acquisition time were optimized to low values to avoid the unwanted far-field contribution and laser heating during the near-field measurements. 13 ACS Paragon Plus Environment

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Figure 2a shows the topography map of Si NW on which TERS measurements were performed. The measured height of the NW is ~60 nm. A typical near-field and farfield Raman spectra are shown in Figure 2b. The intensity of far-field spectrum is almost in the noise level, whereas the near-field spectrum is enhanced by two orders of magnitude in the intensity. The TERS spectrum clearly shows that there are two distinct intensity maxima under one convoluted peak (Figure 2b). The peak did not show reasonable fitting towards either single Lorentzian or a Fano-line profiles (not shown in figure). However, it is well fitted with two Lorentzian curves. The deconvoluted phonon peaks are centered at 517 and 521.7 cm-1 having line widths () 5.3 and 8.0 cm-1. The TERS measurement confirms that there are two distinct phonon population densities having different lifetimes of ~1.0 ps and 0.6 ps, within the probing area which is around the diameter of the NW. It may be noted that both far-field Raman measurements on ensemble NWs (Figure 1) and near-field measurements on single NW showed red-shifted phonon peak with higher lifetime as compared to that of blue-shifted phonon peak. The results clearly indicate that the observation of more than one optical phonon population densities is a characteristic of a single Si NW rather than ensemble NWs. 14 ACS Paragon Plus Environment

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Figure 2. (a) Topography map (3D) of the single Si NW. Inset shows the line profile across the NW indicating diameter ~60 nm. (b) Typical far-field and near-field Raman spectra collected on the single NW. The dotted lines correspond to fitted curves.

In fact, the multiple optical phonon peaks were observed earlier in the Si NCs of different sizes.33 The origin of those peaks, however, were not explored as the measurements were conducted on ensemble nanostructures. Recent studies on Si NCs of sizes 1034 nm reported the origin of multiple phonon populations along with red-shift and broadening of the peaks at high excitation power densities.27 The origin is identified as the nonthermal phonon population generated by variations in the electrical conductivity of different agglomerates of ensemble NCs present within the same measurement spot. Such agglomerated NCs are found to have sizes in the range of hundreds of nanometers


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within the measurement spot. Although, the measurements in the present study were conducted on single NWs at relatively low (one order less) excitation power density than the reported values, we observed multiple peaks due to near-field enhancement of optical phonon excitations. In addition, since the measurements were conducted on a single NW, the concept of electrical conductivity variation generated by size variation within the measurement spot (few tens of nanometers) can be ruled out.

Another interesting observation in the near-field measurements is that the temporal behavior of the phonon peaks. The localized near-field phonon spectra were collected at different acquisition times during which the sample is under illumination with excitation source. Figure S5 shows the near-field Raman spectra collected with acquisition times of 1, 4 and 10 s on the Si NW. At very low acquisition or exposure times (1 and 4 s), the spectra are well fitted with single Lorentzian profile. The resultant curves are red-shifted by 3 and 4.5 cm-1 and broadened to 4.4 ( = 1.2 ps) and 6.0 cm-1( = 0.9 ps), respectively. Whereas, at high acquisition time (10 s), the multiple phonon peaks at 515.7 ( = 7 cm-1,  = 0.8 ps) and 520.4 cm-1 ( = 9.4 cm-1,  = 0.6 ps) appear with Lorentzian shape. It may be


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noted that high exposure times, in general, may cause heating effects if there is a poor thermal coupling between the sample and substrate (heat sink). In such cases, the redshift with symmetric broadening of the peak is expected due to anharmonic effects in lattice. In the present study, however, Si NW was placed on a proper heat sink (Au/Cr) to minimize the heating effect. This fact can be clearly evident from the negligible shift and asymmetric broadening of peak at ~ 515.7 cm-1 when the acquisition time rises from 4 to 10 s (Figure S5). The lifetimes of observed optical phonons in far-field measurements on ensemble NWs and in near-field measurements on single NW suggest the same origin behind the appearance of multiple phonon populations, which could be a characteristic of Si NW itself. Such characteristic property of the NW can be verified by the localized Raman measurements at different locations along the single NW. The TERS map allows us to access the Raman spectra at different locations along the NW to study the local variations in the phonon characteristics with sub-wavelength spatial resolution. The simultaneous topography and TERS maps were recorded with minimal power density (34 kW.cm-2) and acquisition time (1 s). Figure 3a shows the TERS map overlaid on its corresponding 3D height map of Si NW having diameter ~60 nm. A 17 ACS Paragon Plus Environment

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small portion at the end of NW shows very low Raman intensity due to drift in the position of TERS tip from laser spot during the period of scanning. Few spectra (a  f) were selected at different locations along the NW and were plotted in Figure 3b. It is very clear from the spectra that all the peaks are red-shifted and broadened. All the spectra are well fitted with single curves of Lorentzian line shapes. The fitting parameters are listed in the Table 1. The peak positions and the corresponding lifetimes of the optical phonons are well correlated each other and except at the tip of the NW. Most of the spectra possess similar characteristics at different locations along the NW. The TERS map and the corresponding spectra confirmed that there were no surface non-uniformities along the NW. In order to verify the possible presence of multiple phonon peaks at different locations along the NW, TERS spectra were collected with acquisition time of 10 s (Figure S6) at locations ‘a to d’ on the NW shown in Figure 3. All the spectra are found to fit with two Lorentzian curves and the corresponding curve parameters are tabulated in Table S2. The curve parameters show that, irrespective of the location of spectra collected on NW, there are always two optical phonons having different lifetimes at same measurement point. 18 ACS Paragon Plus Environment

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Figure 3. (a) TERS map overlaid on its corresponding 3D height map of Si NW having diameter ~60 nm. Color scale indicates the relative spectral intensity of TERS map. (b) TERS spectra picked from the recorded map at different locations along the NW. The dashed lines represent the Lorentzian fits of respective curves.

Table 1. The curve parameters and corresponding values obtained by Lorentzian line shaping fitting of TERS spectra at different locations along the NW. Spectral

a

b

c

d

e

f

location along the NW Peak position

515.98 517.52

517.91

517.71 518.11 518.30

 (cm-1 ) FWHM

6.57

5.51

4.09

4.23

 (cm-1 ) 19 ACS Paragon Plus Environment

4.71

4.20

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Lifetime

0.8

1

1.3

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1.2

1.1

1.2

 (ps)

Figure 4. (a) Topography map (3D) of the single Si NW. Inset shows the line profile across the NW indicating diameter ~25 nm. (b) TERS spectra recorded with an acquisition time of 10 s, at different locations (af) along the NW, and (c) the corresponding Lorentzian profile fitted curves at locations ad.


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We further extended the near-field TERS studies on smaller diameter NW to observe the size dependent characteristics. Figure 4a shows the topography (3D) map of single Si NW whose diameter is found be ~25 nm. The diameter of the NW is beyond the phonon confinement regime. The TERS spectra at different locations ( a  f ) along the NW were collected with high acquisition time of 10 s to observe the multiple phonon peaks (Figure 4b). All the spectra are found to be red-shifted by ~4 cm-1. Figure 4c shows the Lorentzian fittings of the curves at locations a – d. Interestingly, the curves are well fitted with single Lorentzian profile and all the resultant spectra possess similar characteristics of peak shift and phonon lifetimes (Table S3). The localized near-field measurements clearly show that there may be size dependent phonon characteristics which influence the appearance of multiple phonon peaks at the same measurement spot in the larger diameter NW. The origin of red-shift and broadening of the single phonon peak in smaller diameter NW and multiple phonon peaks in larger diameter NW can be understood based on the size dependent surface band bending (SBB) and charge depleted layers in semiconductor NWs.


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It is well known that every semiconductor possesses SBB and charge depletion layers due to the poor screening of surface charge and the Fermi level pinning induced by surface and interface states.20 Based on the charge neutrality condition, for an n-type semiconductor, negative surface charge induces positive space charge region near the surface. Similarly, for a p-type semiconductor, the positive surface charge is balanced by the negative space charge region near the surface. The width (Dd) of the depletion layers depends on the surface charge density, doping, and adsorbed molecules in bulk semiconductors and the depletion width is expressed by,34

Dd = (ns / NA) = (2 B  0 / eNA)1/2

(2)

where ns is surface charge density, NA is dopant concentration, B is SBB,  is the static dielectric constant of Si, 0 is the permittivity of free space, and e is electron charge. In semiconductor NWs having large surface-to-volume ratio, the width of SBB and depletion regions further increases due to the enhanced contribution of surface and interface states. The pristine Au-assisted VLS grown Si NWs with native oxide layer are found to show positive surface charge density of the order of 1012 cm-2 due to their p-type nature by Au


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impurities.22 Such space charge regions are found to reduce the effective electrical conducting cross-sectional area along the NW. Further, the effective cross-sectional area of electrical conductance is decreased with the size of the NWs due to the increased width of charge depleted layer. Below a critical NW diameter (dNW < dc), NWs are completely depleted (dNW = Dd) (Figure 5a). In VLS process CVD grown intrinsic Si NWs, the critical diameter (dc) is found to be ~25 nm above which (dNW > dc) NWs are partially depleted (Figure 5a).22 In fact, similar depletion regions in many other elemental (Si, Ge) and compound (III-V and II-VI) semiconductor nanostructures have been shown to strongly influence the wide range of properties such as diode characteristics, photoconducting and photovoltaic, thermoelectric, gas and chemical sensing properties.14-18,35-39 The width of depletion layers in such nanostructures are found to vary from 10 to 50 nm depending on the size and type of semiconductor. The effect of charge depletion and SBB on optical phonons in bulk semiconductors have been studied in the past, using Raman spectroscopy and SKPM.40 In the Raman spectroscopic measurements of Si NWs above the optical bandgap excitation, the photogenerated electron-hole pairs are separated by the surface electric 23 ACS Paragon Plus Environment

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field induced by depletion layer. The photogenerated holes move towards un-depleted core region of the NW and the photoelectrons move towards surface due to the positive surface charge density and eventually trapped by the Si/SiOx interface states, resulting in two distinct local regions. Such charge separated local regions are only significant for the NWs having diameters dNW > dc (partially depleted) (Figure 5a). The thermalization of photo-excited carriers in to optical phonons within and out of the Dd, is a vital process particularly for the indirect bandgap semiconductors like Si and Ge, and the process depends on the local scattering mechanisms. The existence of two local charge regions means that there two distinct optical phonon population densities for dNW > dc due to the multiple scattering mechanisms. Whereas, for NW diameters dNW < dc, fully depleted NWs have uniform distribution of LO phonon populations. Due to the poor electrical conduction of photoelectrons in fully depleted NWs, the phonon peak is red-shifted and broadened with respective to that of the bulk. At low excitation photon flux on partially depleted NWs (dNW > dc) in near-field TERS measurements, the effective density of photo-carriers within the Dd dominate over the core region due to the surface sensitivity of the TERS technique.


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Whereas, at high excitation photon flux, both the contribution of core and depleted regions contribute the optical phonons resulting in multiple phonon populations.

Figure 5. (a) Schematic representation of the depletion regions in NWs having diameters

dNW > dc, and dNW < dc. (b) The plot of diameter Vs surface potentials of the NWs measured using SKPM. The sold line is guide to an eye. In order to confirm the size dependent charge depletions in Si NWs, we have measured the surface potentials (SPs) of the individual NWs using SKPM technique (Figure S7), based on contact potential difference (CPD) of tip and sample and can be calculated by41 SP = VCPD = (M  S) / q

(3)


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where M and S are work function of the metal tip and the sample, respectively, and q is the elementary charge. The SP is related to SBB by

B = (EcEf) +   S

(4)

where  is electron affinity of the Si, Ec and Ef are positions of conduction band minimum and Fermi level, respectively. Figure 5b shows the plot of diameter Vs SPs of Si NWs. The increase in the SP with increasing diameter of NW directly indicates the reduction in SBB and hence in the depletion width (Eqn. 2). Thus, for larger dNW, high value of SP means that NW is partially depleted, while for smaller dNW (< dc) NW is fully depleted and the value of SP is found be low.

In Conclusion, the peak shift and asymmetric broadening of one-phonon mode of Si are observed in the far-field Raman spectroscopy on Au-assisted VLS grown ensemble Si NWs, whose diameters are beyond the electron and phonon confinement regime. The deconvolution of asymmetrically broadened optical phonon mode found to show multiple phonon peaks in NWs. The appropriate thermal anchoring and localized Raman measurements on single NWs by near-field TERS demonstrated the decoupling of


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Nano Letters

multiple origins responsible for the peak shift and broadening of the phonon mode from a single measurement area. Size dependent single to multiple phonon peaks in Si NWs are observed in localized TERS measurements. A model based on the localized phonon populations induced by size dependent charge depletion in the NW, is proposed to explain the red-shift accompanied by asymmetric broadening and the multiple phonon population in larger diameter NWs. The size dependent charge depletion in NWs is further confirmed by SKPM measurements on single NWs of various diameters. Our studies suggest the size dependent phonon characteristics of Si NWs are very important to understand the several optoelectronic and thermoelectric properties which are crucial for the Si NWs based photovoltaic and thermoelectric devices.

METHODS Catalytic Au assisted vapor-liquid-solid (VLS) grown Si nanowires (NW) were procured commercially, for Raman spectroscopy studies. The length and diameter of the NWs were measured using FESEM (AURIGA, Zeiss) and TEM (LIBRA 200FE Zeiss). TEM studies were performed on Si NWs which were dispersed in isopropyl alcohol and transferred to the TEM Cu grids. The crystalline nature and orientation of the nanowires were studied 27 ACS Paragon Plus Environment

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using HRTEM. Raman scattering measurements were performed by exciting the NWs with 514.5 nm Ar+ ion laser and the backscattered signals were collected using a Raman spectrometer

(inVia,

Renishaw)

mounted

with

3000

gr.mm-1

grating

and

thermoelectrically cooled charged coupled device (CCD) detector. The far-field spectra from ensemble NWs were collected using 100 objective having numerical aperture (NA) value of 0.85. The far-field excitation power dependent measurements were carried out in the power density range of 1.4 to 140 kW.cm-2. The localized optical phonons in a single Si NW were studied using near-field (TERS. In order to avoid the surface modifications by solvent dispersions process, the NWs were dispersed using dry dispersion method on to Au (150 nm) / Cr (5 nm) coated high quality fused silica cover glass slide (150 µm thick) for the TERS studies. The TERS measurements were carried out using the scanning probe microscope (SPM) (Nanonics, MultiView 4000) coupled with the same Raman spectrometer (inVia, Renishaw) in the backscattering configuration. An AFM bent glass probe, working in the tuning fork feedback mechanism, attached with an Au NP (diameter

Size-Dependent Localized Phonon Population in Semiconducting Si

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