Growth of Bimodal Sn-Catalyzed CdS Nanowires by Using Tin Sulfide

Feb 25, 2014 - Sn-catalyzed CdS nanowires were grown by a physical vapor transport method by using tin sulfide as a precursor. The bimodal distributio...
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Growth of Bimodal Sn-Catalyzed CdS Nanowires by Using Tin Sulfide Man Suk Song and Yong Kim* Department of Physics, Dong-A University, Hadan-2-dong, Saha-gu, Busan 604-714, Korea S Supporting Information *

ABSTRACT: Sn-catalyzed CdS nanowires were grown by a physical vapor transport method by using tin sulfide as a precursor. The bimodal distribution consisting of tall/thin and short/thick nanowires is found at the growth temperatures of 250−350 °C. The tall/thin nanowires grow along ⟨0001⟩ while the short/thick nanowires grow along ⟨011̅0⟩ and all have wurtzite crystal structure. The relationships, valid in the wide range 25−150 nm in nanowire diameters, of dc = (2.64 ± 0.08)dNW for the tall/thin nanowires and of dc = (1.27 ± 0.02)dNW for the short/thick nanowires, are obtained where dc and dNW are the diameters of catalyst and nanowire, respectively. Using nucleation theory, we calculate that the energies of nucleation are quite similar for both types of nanowires, which may explain the bimodality. Both types of nanowires show prominent photoluminescence without any noticeable trace of impurity-related bands, indicating their high optical quality.

1. INTRODUCTION II−VI nanowires catalyzed by Au or other metals have drawn much attention because these nanowires provide unique characteristics highly desirable for novel optoelectronic devices.1 The material systems investigated so far include ZnO, ZnS, CdS, and ZnTe, to name a few.2−6 These II−VI nanowires usually have wurtzite (WZ) crystal structure and show morphological variations including nanowires, nanobelts, nanosprings, nanosheets, and nanosaws due to the nature of the inherently polar surface.7−10 As one of the most notable and promising II−VI nanowires, CdS nanowires have been extensively investigated for the realization of nanowire lasers, photodetectors, and other optoelectronic devices.11−14 While Au nanoparticles have been widely employed as a catalysts via the vapor−liquid−solid (VLS) mechanism,15 alternative catalysts have been pursued, since Au is known to form nonradiative recombination centers that degrade the optical properties of the nanowires.16 Sn is a possible candidate for catalysts and facilitates low-temperature growth because Sn has a low melting temperature of 232 °C. Several research groups have investigated the growth of nanowires using Sn catalysts.17−21 SnO2 powder with graphite powder or SnO2 thin films have been used for forming Sn catalysts. In this study, however, tin sulfide (SnS) is utilized due to a couple of reasons: (1) the vapor pressure of SnS is close to that of CdS and thereby SnS may be suitable source material specifically for the process of physical vapor transport; (2) SnS is anticipated to induce sulfur passivation of the CdS nanowire, in addition to forming Sn catalyst droplets. Interestingly, a distinct bimodal distribution consisting of two kinds of nanowires is observed. The two types of nanowires have different aspect ratios (i.e., height/diameter), crystalline orientations, and contact angles to the catalyst. Further, the two types of nanowires coexist even if their catalyst sizes are similar. This bimodality has been rarely © 2014 American Chemical Society

observed in previous studies, since the diameter of nanowires is usually defined by the size of nanoscale catalysts.22,23 Furthermore, the stable growth of nanowires using Sn catalysts, a low melting point metal, is also curious, since Sn may not meet the Nebol’sin stability criteria due to its low surface energy (∼0.575 J/m2).24 Probably the most important issue in nanowire research area is the control of nanowire parameters including diameters, defect density, junction abruptness, and so forth. The controllability may be obtained from the better understanding of liquid-phase (or solid-phase) catalyst behavior. Therefore, the investigation of the bimodality may provide insight on the behavior of specifically low melting catalysts. In this paper, we explore the possible explanation for the bimodal behavior of SnS-catalyzed CdS nanowires by discussing the Nebol’sin stability criteria and the free energy change during the formation of a CdS nucleus in the liquid catalyst.

2. EXPERIMENTAL METHODS CdS nanowires were grown by a physical vapor transport method employing a conventional single zone furnace with a 1 in. diameter quartz tube (Lindberg Blue M Mini-Mite). A quartz boat containing CdS powder (100 mg, Alfa Aesar 99.999%) was placed in the center zone and another quartz boat containing SnS(II) powder (20 mg, Alfa Aesar 99.5%) at 13.5 cm upstream from the center. In the conventional methods, source and catalyst materials are in the same boats, or catalysts are already deposited on substrates. In our case, the source and catalyst boats are separated, and this allows the independent control of both temperatures. We believe this Received: January 21, 2014 Revised: February 24, 2014 Published: February 25, 2014 5988

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Figure 1. Side-view FESEM images of as-grown bimodal CdS nanowires at the substrate temperatures of (a) 250 °C, (b) 300 °C, and (c) 350 °C and (d) two types of CdS nanowires: thin/tall (type 1) and thick/short (type 2) nanowires. (e) X-ray diffraction pattern of the bimodal CdS nanowires grown on Si substrates.

emitter filter (bandwidth < 1.9 nm), and dichroic beam splitter. The laser beam was focused to a spot (spot diameter ∼3.2 μm) by an objective lens (Olympus ×50, N.A. = 0.75). By using a commercial dual-port attachment, imaging by a charge-coupled device (CCD, Imaging source DBK41AF02) and spectroscopic measurements were done simultaneously. For spectroscopic measurements, the μ-PL signal was introduced into a multimode fiber using a fiber-launching module. Then, μ-PL was dispersed by 0.5 m monochromator (Dongwoo Optron Monora 500i) and detected by a Peltier-cooled spectroscopic CCD camera (Andor, iDus DV401A). For single nanowire measurements, nanowires were transferred to SiO2-coated Si substrates by casting and drying of a liquid drop containing CdS nanowires. All measurements were done at room temperature.

technique is important specifically for the growth of low melting point metal catalyzed nanowires. The CdS source temperature was set to 700 °C. Long Si substrates (3.5 cm × 1.2 cm), cleaned and hydrogen-terminated by diluted HF were placed at 14.5−18 cm downstream from the center. Figure S1 in the Supporting Information shows the schematic of the physical vapor transport system and its temperature profile, which is obtained by calibration with a thermocouple. According to the profile, the SnS source temperature was 600 °C, and the temperature range of the Si substrates was 250− 550 °C. First, the system is evacuated by a mechanical pump for 0.5 h (base pressure ∼10−2 Torr), and then, N2 carrier gas balanced with 10% H2 was introduced at a flow rate of 200 standard cubic centimeters per minute. The ramp-up time to 700 °C was 15 min, and another 1 h was used as the growth time while keeping the pressure at 100 Torr. The sample was cooled to room temperature while maintaining the flow of carrier gas. The morphologies of as-grown CdS nanowires were observed by a field-emission scanning electron microscope (FESEM, JEOL JSM-6700F). X-ray diffraction (XRD) patterns were measured by an X-ray diffractometer (PANalytical X’Pert Pro). Transmission electron microscope (TEM) specimens were prepared in the following manner. The samples were immersed in a vial with ethanol. The vial was sonicated for 20 s to separate CdS nanowires from their substrates. These nanowires were then dispersed onto a holey carbon grid. Low- and high-resolution TEM and selected area electron diffraction (SAED) patterns were measured by a TEM (JEOL JEM 2010). Energy-dispersive X-ray (EDX) spectra were obtained by an EDX system (INCA, Oxford Instruments) attached to the TEM. Microphotoluminescence (μ-PL) measurements of individual CdS nanowires were carried out by employing a home-built μPL system. An Ar+ laser (488 nm) was band-pass filtered and guided into a modified commercial microscope (Olympus BX60M) with a commercial Raman filter cube (Semrock). The filter cube consists of exciter filter (edge steepness < 1.0 nm),

3. RESULTS AND DISCUSSION Figure 1a−c shows the side-view FESEM images of as-grown CdS nanowires at the temperature regions of 250, 300, and 350 °C, respectively. Though the growth temperatures are calibrated, the temperatures presented here are nominal, rather than absolute, due to the steep temperature gradient in our furnace system (Figure S1, Supporting Information). Two different kinds of nanowires are clearly observable in Figure 1a−c: thin/tall nanowires (type 1) and thick/short nanowires (type 2). Figure 1d is a magnified image of Figure 1b to highlight the morphological difference between type 1 and type 2 nanowires. The length of nanowires tends to increase with the increase of substrate temperature. The ratios of the occurrence of type 1 to type 2 nanowires are estimated by counting numbers of nanowires through the inspection of planview FESEM images (Figure S2, Supporting Information). The occurrence ratios are 0.34, 0.43, and 0.41 at the temperatures of 250, 300, and 350 °C, respectively. Figure 1e shows an XRD pattern of the bimodal CdS nanowires. Most diffraction peaks in the pattern can be indexed to the hexagonal WZ and cubic zinc blende (ZB) crystallographic structures of CdS. The other diffraction peaks 5989

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correspond to Sn and SnS-related peaks. As we inspect hundreds of nanowires by TEM, all nanowires have WZ crystal structure, and therefore, ZB-related peaks may originate from CdS thin film on the substrate. Figure 2 shows the TEM images and the SAED pattern of the thin/tall CdS nanowires (type 1). The high-resolution

Figure 3. (a) High-resolution TEM image showing defects (arrows), (b) low-magnification TEM image, and (c) the SAED pattern image of the type 2 CdS nanowire.

pattern in Figure 3c proves that the growth direction is [0110̅ ], which is perpendicular to the [0001], in the WZ structure. From the EDX measurements as shown in Figure S3 in the Supporting Information, only Sn was detected in the catalyst, and only Cd and S elements were detected in the nanowire. Examination of phase diagrams, together with our postgrowth EDX measurements, provide insight into the catalyst composition during growth. To the best of our knowledge, the ternary Cd−Sn−S is as yet unknown, so we must rely on binary phase diagrams. The rounded shape of the catalyst indicates that, for both type 1 and type 2 nanowires, the catalyst was liquid during growth, consistent with the VLS mechanism. The low melting point of Sn (232 °C) also confirms the catalyst should be liquid at all growth temperatures relevant to this study. The Sn−S phase diagram indicates that, across the range of growth temperatures utilized (250−550 °C), the solubility of S is low in liquid Sn.27 This is consistent with the EDX results showing no trace of S in the postgrowth Sn catalysts for type 1 and type 2 nanowires. We note that the Cd−Sn phase diagram indicates the possibility of a Cd−Sn molten alloy at the growth temperatures studied.28 For this to be the case, the Cd must leave the catalyst during cooling from growth temperature to room temperature, because no Cd was detected in postgrowth EDX measurements of the Sn catalyst. The precipitating Cd would form either Cd deposits or a “neck” region on the CdS nanowire. No Cd deposits were detected around the Sn catalyst in postgrowth analysis, eliminating the former possibility. To investigate the latter possibility, we examined the region of the nanowires immediately beneath the catalyst. If significant CdS precipitation occurs during cooling, a neck should form that does not resemble the rest of the nanowire, analogous to III−V

Figure 2. (a) High-resolution TEM image, (b) low-magnification TEM image, and (c) the SAED pattern of the type 1 CdS nanowire.

TEM (HRTEM) image near the interface region between nanowire and catalyst (shown in Figure 2a and the corresponding area is shown as a yellow rectangle in Figure 2b) demonstrates that the diameter of the nanowire is ∼45 nm. As shown in Figure 2b, the tapering along the nanowire is negligible, and the diameter of the catalyst at the tip is much larger than that of the nanowire. Figure S3 in the Supporting Information shows EDX spectra obtained from the regions in Figure 2a. Only the Sn peak is detected from the tip area (area 1) while only Cd and S peaks are detected in the nanowire area (area 2). The SAED pattern in Figure 2c shows that the type 1 nanowire grows along the [0001] direction, which is the c axis in the WZ structure. Stacking faults are observed on the (0001) planes. The stacking faults appear to be randomly distributed along the entire nanowire length. The presence of stacking faults is evidence that nucleation occurs at the triple phase boundary (TPB).25,26 This also indicates that the catalyst only wets the top facet of the nanowire and eliminates the possibility of the catalyst wetting the sidewalls as observed for low surface energy Ga catalysts on GaAs nanowires.25 Figure 3 shows the TEM images and the SAED pattern of the thick/short CdS nanowires (type 2). The HRTEM image (Figure 3a and the corresponding area is shown in a yellow rectangle in Figure 3b) exhibits that type 2 nanowires grow along a different crystallographic direction from type 1 nanowires. They feature WZ structure, and a few defects (stacking faults) exist on (0001) planes (arrows in Figure 3a). The type 2 nanowire in Figure 3b has a diameter of ∼140 nm that strikingly contrasts to the type 1 nanowires. The SAED 5990

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Since the catalyst shape perfectly fits to a truncated sphere, the linear fit data allows us to evaluate the contact angles, β(= (π/2) + cos−1(dNW/dc)), for type 1 and type 2 nanowires. This is valid for nanowire diameters within the range from 20 to 150 nm. The contact angle, β, is approximately 157.7° ± 0.7° and 128.1° ± 1.1° for type 1 and type 2 nanowires, respectively. Though there is wire-to-wire variation in contact angles, we use β to simplify our analysis. As discussed earlier, the size and Sn composition of the catalyst droplet do not change significantly after growth, so we assume that postgrowth β values are good estimates for β values during growth. First, it is important to confirm the stability of nanowires. Suppose that the liquid−

nanowires.29 Type 1 nanowires show only a short neck region, and type 2 nanowires show no neck region. The absence of this neck region suggests that Cd precipitation from the catalyst is insignificant, so that the catalyst composition was predominantly Sn during growth. Furthermore, liquid Cd has a higher surface energy than liquid Sn,30 so its incorporation into the molten Sn nanoparticle would be energetically unfavorable. We infer that the size and composition of the Sn catalyst are change little after growth, so that postgrowth analysis gives a reliable indication of the size and composition of the catalyst during growth. This is important for the calculations that follow. A bimodal/trimodal distribution of Sn-catalyzed ZnO nanowires was already observed by Ding et al.17 The growth direction of nanowires was [0001] while the growth directions of nanobelts were either [011̅0] or [21̅1̅0]. Hence, the growth behavior is somewhat different from that of our CdS nanowires. At first glance, the bimodal/trimodal distribution seems to be one of the characteristics of nanowires catalyzed by low melting point metals. However, it is noteworthy that no evidence of bimodal behavior was found in Sn-catalyzed Si nanowires.19,20 This means not only the surface energy of the liquid-phase catalyst but also the surface and interface energies of the nanowire itself are the important parameters for the bimodality. As shown in Figure 4a, although the catalyst diameters are very

Figure 5. Schematic illustrating the stability of the catalyst droplet. (a) Surface energy balance when the catalyst rests on the top of nanowire. (b) Perturbation of the catalyst, such that the catalyst wets the nanowire sidewalls, must be counteracted by a restoring force to return the catalyst to the top of the nanowire.

solid interface is flat (Figure 5a), the horizontal force balance should be satisfied as follows:

γl cos β + γls = 0

(1)

where γl is the surface energy of the liquid-phase catalyst and γls is the interface energy between the catalyst and the top facet of the nanowire. If the liquid Sn catalyst experiences some random perturbation and temporarily wets the nanowire side wall, a restoring force is required to move the catalyst upward to sit on the top facet of the nanowire. Otherwise, the catalyst is unstable and may slide off the top of the nanowire (Figure 5b). This means that an inequality should be satisfied as follows:

Figure 4. (a) Low-resolution TEM image showing both the thin/tall CdS nanowire (type 1) and the thick/short CdS nanowire (type 2). Note that they have Sn catalysts with similar diameters. (b) Plot of the diameter of catalyst as a function of the diameter of CdS nanowires.

similar (100 nm in type 1 and 108 nm in type 2 nanowires), the two nanowires have very different nanowire diameters. This indicates two kinds of nanowires have different growth scenarios though their growth environments are essentially identical. Initially, we expect that such bimodality may be limited within the certain narrow range of nanowire diameter. However, from the plot of the catalyst diameter (dc) as a function of nanowire diameter (dNW) prepared through the TEM inspection of 140 nanowires (Figure 4b), the bimodal behavior is quite universal in the wide range of nanowire diameters from 20 to 150 nm. This means that there is no prevalence of a particular nanowire type at least within this range. Linear fittings for the type 1 and type 2 nanowires yield the results of dc = (2.64 ± 0.08)dNW and dc = (1.27 ± 0.02)dNW, respectively.

γl sin β + γ ′ls > γs

(2)

where γ′ls is the interface energy between the catalyst and the side wall of the nanowire and γs is the surface energy of the nanowire side wall . Introducing a parameter k (= γ′ls/γls) and combining eqs 1 and 2, an inequality24 for the stable growth of nanowire is derived as follows: γl(sin β − k·cos β) > γs

(3)

When k = 1, the inequality reduces to the well-known Nebol’sin criteria.24 The successful growth of nanowires, coupled with our postgrowth TEM analysis showing catalysts sitting on the nanowire tips, indicates that the Nebol’sin stability criteria are indeed satisfied. It is instructive to consider the Nebol’sin 5991

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πr 2hΔμ + rhγeff (4) 2Ω 3 where r and Ω (= 0.0499 nm ) are the radius of the nucleus and the molecular volume, respectively. Δμ, the change in chemical potential per molecule is given by

criteria theoretically using published surface energies. To our best knowledge, the only available data of the surface energies of CdS on the crystallographic orientations were from the report by Barnard and Xu.31 On the basis of their estimated values γ (0001) (∼0.90 J/m 2 ), γ (011̅ 0 ) (∼0.28 J/m 2 ), and γ(21̅1̅0)(∼0.29 J/m2) with the surface energy of liquid Sn (γl ∼ 0.575 J/m2), the inequality can be tested for type 1 and type 2 nanowires. For type 1 nanowires, substituting the typical (= 0.53 J/m2). contact angle β = 157.7° into eq 1 yields γ(0001) ls For type 2 nanowires, substituting the typical contact angle β = ̅0) (= 0.35 J/m2). To assess the 128.1° into eq 1 yields γ(011 ls Nebol’sin criteria, we also require estimates of γ′ls for both types of nanowires. Fortunately, estimates of γ′ls for can be made based on our calculations of γ ls(0001)and γ ls(011̅ 0 ) . Considering the type 1 nanowires feature (011̅0) side facets, we assume that the nuclei also feature (011̅0) side facets. 0) Therefore, for type 1 nanowires, γ′ls corresponds to γ(011̅ (= ls 0.35 J/m2) (calculated above). This yields k = γ′ls/γls = 0.66, and the inequality in eq 3 is safely satisfied for γs = γ(011̅0)(≈ γ(211̅ 0̅ )), the surface energy of the side facets of type 1 nanowires exposed to vapor. In type 2 nanowires, the nucleus side facets (= are assumed to be (0001), so that γ′ls corresponds to γ(0001) ls 0.53 J/m2). Thus, k ≈ 1.52, and the inequality is also satisfied for type 2 nanowires with γs = γ(0001). Therefore, as far as Nebol’sin criteria are concerned, both types of nanowires are stable. According to the VLS mechanism, the reactant materials in the supersaturated liquid-phase catalyst start to precipitate at the solid/liquid interface.15 It is generally believed that the growth direction is determined for minimizing the total free energy of the nanowire taking γs and γls into account.32 The crystal plane with the lowest surface energy is (111) in the cubic crystal structure. Therefore, the nanowire with the diameter exceeding a critical diameter (for instance ∼20 nm for Au-catalyzed Si nanowires32) favors the ⟨111⟩ growth direction, since the direction minimizes the total free energy. However, if the size of the catalyst limits the nanowire diameter below the critical diameter, nanowires favorably grow along ⟨110⟩ due to the increasing contribution of the surface energies of the (111) side facets to free energy. In this model, the liquid-phase catalyst provides preferential adsorption sites and does not play an important role in determining the growth direction of the nanowire. However, the coexistence of completely different kinds of nanowires in spite of their similar catalyst sizes indicates the important role of the catalyst itself. The existence of atomic ordering at the Sn catalyst/nanowire interface, despite the growth temperature exceeding the melting point of Sn, has been suggested.17 The lattice mismatch between the ordered interface and the nanowire determined the direction of the nanowire. The idea may be applicable to explain the bimodality of our nanowires. However, in this study, the bimodality is studied by comparing the nucleation energetics between two types of nanowires. The growth would be in the monocentric or mononuclear regime in which a single nucleation event controls the whole monolayer growth.33−35 Several shapes of nuclei including triangular and hexagonal shapes have been modeled.26,33−35 In our work, a semicircular nucleus with monolayer height, h, similar to the model nucleus investigated by Johansson et al., is adopted as a model nucleus.35 Following the approaches previously discussed to explain the prevalence of WZ nanowires over ZB nanowires,26,33−35 the free energy change due to the formation of a semicircular nucleus can be described as follows:

ΔG = −

⎛ C ⎞ ⎟⎟ Δμ = kBT ln⎜⎜ ⎝ Ceq ⎠

(5)

where kB and T are Boltzmann’s constant and the absolute temperature, respectively, and C and Ceq are the concentration and the equilibrium concentration of constituent elements in the catalyst, respectively. The effective surface energy is given by γeff = 2(γn − γl sin β) + πγnl

(6)

where γnl and γn are the lateral surface energies of the nucleus in contact with liquid and vapor, respectively (Figue 6). γlsin β is

Figure 6. (a) Center and triple phase boundary nucleation. (b) Formation of semicircular nucleus at the triple phase boundary in type 1 nanowires. (c) Formation of semicircular nucleus at the triple phase boundary in type 2 nanowires.

due to the fact that the nucleus partially eliminates the surface of liquid catalyst when it is nucleated at the TPB. This term, which reduces γeff, is the main reason for the prevalence of the nucleation at the TPB compared to the center nucleation (Figure 6a). From ∂ΔG/∂r = 0, critical radius rc is obtained as rc =

Ωγeff πΔμ

(7)

Also, the critical free energy change for the nucleation is given by ΔGc =

hΩγeff 2 2πΔμ

(8)

Type 1 nanowire that grows along ⟨0001⟩, as illustrated in Figure 6b, has a hexagonal cross section enclosed by {21̅1̅0} and/or {011̅0} facets.36 In general, all possible combination of 5992

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r2 2L1Δμ + r2 2(γ2 + γ2′) + 4r2L 2γ1″ (11) Ω where γ2, γ′2, γ″2 , r2, and L2 have the same meanings as in the case of column-shape nuclei. From the similar differentiation procedure, the critical free energy change for the nucleation is given by the following:

facets of nucleus on the pre-existing facet should be carefully examined.35 However, for the nucleation at the TPB, γn is equal to γ(011̅0) ≈ 0.28 J/m2, since the nucleation is purely homoepitaxial in this study. γnl ≈ γn is usually assumed in other material systems,26 but γnl for CdS has not been documented. Noting that type 1 and type 2 nanowires are ̅0) grown under the equivalent conditions and γ(011 can be ls obtained from type 2 nanowire contact angle analysis, γnl may 0) 1̅0) be approximated to γ(011̅ ≈ γ(21̅ ≈ 0.35 J/m2. By using these ls ls type 1 2 (= 0.155/Δμ) are obtained data, γeff (= 1.22 J/m ) and ΔGc considering h = 0.336 nm along the c axis (Δμ in eV unit). The type 2 nanowire has a rectangular cross section enclosed by {21̅1̅0} and {0001} facets.36 The nucleus can form at the TPB of either the side facet of (21̅1̅0) or (0001). The nucleation at the TPB of the (211̅ 0̅ ) facet (Figure 6c) is only considered because the surface energy is lower compared to that of the (0001) facets. Again considering h (= 0.359 nm) from h = ((√3)/2)a (a = 0.414 nm) and using the fact of γn = 2 γ(21̅1̅0) ≈ 0.29 J/m2, γeff (= 1.06 J/m2) and ΔGtype (= 0.125/ c Δμ) are obtained. The side of the semicircular nucleus in contact with the liquid catalyst consists of {21̅1̅0} and {0001} facets. Thus, in this estimation, the average γnl (≈ 0.44 J/m2) of ̅1̅0) ̅0) γ(0001) ≈ 0.53 J/m2 and γ(21 ≈ γ(011 ≈ 0.35 J/m2 is used. ls ls ls ΔGc is the barrier energy for the nucleation, and the nucleation rate is proportional to ∼e(−ΔGc/kBT). Therefore, type 2 nanowire growth is somewhat more energetically favorable, consistent with the occurrence observation (Figure S2, Supporting Information). Considering the number of the equivalent facets (six facets and two facets in type 1 and type 2 nanowires, respectively), we can equate the ratio of nucleation rates to the occurrence ratios. We calculate Δμ (= 280 meV) type 1

ΔG2 = −

ΔGc2 = 16(γ2″Ω/Δμ)2 (γ2 + γ2′)

The rectangular nucleus model may correspond to type 2 ̅0) nanowires. Again, noting that γ2 = γ(011̅0), γ′2 = γ(011 , and γ″2 = ls 2 γnl, ΔGc2 = 2.0(Ω/Δμ) is obtained. The critical free energies in two nucleus shapes are quite similar. Therefore, the bimodality and the occurrence ratio are well explained using this model. Although the thermodynamic argument may give insight concerning the stability of the nucleus, the growth kinetics determines the final products of the nanostructures. Following Wang et al.,38 the final diameter ratio of the nanowires is approximately equal to the ratio of the critical sizes based on 2 type 1 their kinetics study. Therefore, dtype NW /dNW = r2*/r1* = 2γ2″/γ1″ ≈ 2.5 is obtained, and the value is in good agreement with the experimentally observed value of ∼2.1 (Figure 4). Figure 7 shows a μ-PL spectrum from a single CdS nanowire with the peak at 503 nm corresponding to band edge emission.

type 2

from the relation (3e(−(ΔGc − ΔGc )/ΔμkBT) ≈ 0.41) at a growth temperature of 350 °C. In spite of a number of the assumptions mainly due to the lack of available data and the possible uncertainty in the growth temperatures, Δμ is still in the valid range (230−1570 meV), but rc (= 0.4 nm) is rather small.26,37 However, the energetic analysis presented here at least qualitatively explains the bimodality and occurrence ratios of the two types of nanowires. It is worthwhile to examine the different approaches.38−40 In the approach, the free energies between two kinds of nuclei are compared. First, column-shape nuclei in contact with vapor and liquid are considered. Following the notations presented by Wang et al.,38 the free energy change can be expressed as ΔG1 = −

(12)

πr12L1Δμ + πr12(γ1 + γ1′) + 2πr1L1γ1″ Ω

Figure 7. Representative μ-PL spectrum of the CdS nanowire with the main peak at 503 nm,a and μ-PL image of a CdS nanowire with strong green-light emission (the inset).

Recently Roder et al.12 have reported the 1−2% inclusion of Sn in CdS nanowires from the EDX observation when a mixture of CdS, SnO, SnO2, and graphite power was used as the source material. In their work, Sn doping gave rise to an impurityrelated PL band in the longer wavelength region. However, in our case, there is no trace of such a PL band except strong band edge emission in consistent with EDX observations (Figures S3 and S4, Supporting Information). In addition, the strong greenlight emission from the entire nanowire region was observed as shown in μ-PL imaging (the inset of Figure 7). Such a bright photoluminescence with exceptional quantum efficiency may indicate a sulfur passivation effect from the SnS source during growth. It was hard to differentiate type 1 nanowires from type 2 nanowires from the microscope inspections during μ-PL measurements. Therefore, we have measured the spectrum from hundreds of nanowires by moving the microscope stage. Considering the occurrence ratio of type 1 to type 2 nanowires, at least one-third of the observed spectrum should be from type 1 nanowires. However, no noticeable differences in the spectra

(9)

where γ1, γ′1, and γ″1 are the nucleus/vapor, the nucleus/liquid, and the side facet of the nucleus/liquid interface energies, respectively, and r1 and L1 are the radius and the height of the nucleus, respectively. The only difference to Wang’s model is the consideration of the side wall interface energy of the nucleus. From ∂ΔG1/∂r1 = 0 and ∂ΔG1/∂L1 = 0, the critical free energy change for the nucleation is given by the following: ΔGc1 = 4π (γ1″Ω/Δμ)2 (γ1 + γ1′)

(10)

This model may explain the critical free energy for type 1 ̅0) , and γ″1 = γ(011 , nanowires. Noting that γ1 = γ(0001), γ′1 = γ(0001) ls ls 2 ΔGc1 = 2.2(Ω/Δμ) is obtained. Second, for rectangular-shape nuclei, the free energy change for the nucleation is given by 5993

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(8) Kim, Y. L.; Jung, J. H.; Kim, K. H.; Yoon, H. S.; Song, M. S.; Bae, S. H.; Kim, Y. The Growth and Optical Properties of CdSSe Nanosheets. Nanotechnology 2009, 20, 095605−095605−7. (9) Kong, X. Y.; Wang, Z. L. Spontaneous Polarization-Induced Nanohelixes, Nanosprings, and Nanorings of Piezoelectric Nanobelts. Nano Lett. 2003, 3, 1625−1631. (10) Ma, C.; Ding, Y.; Moore, D.; Wang, X.; Wang, Z. L. SingleCrystal CdSe Nanosaws. J. Am. Chem. Soc. 2004, 126, 708−709. (11) Duan, X.; Huang, Y.; Agarwal, R.; Lieber, C. M. SingleNanowire Electrically Driven Lasers. Nature 2003, 421, 241−245. (12) Roder, R.; Wille, M.; Geburt, S.; Rensberg, J.; Zhang, M.; Lu, J. G.; Capasso, F.; Buschlinger, R.; Peschel, U.; Ronning, C. Continous Wave Nanowire Lasing. Nano Lett. 2013, 13, 3602−3606. (13) Guo, P.; Zhuang, X.; Xu, J.; Zhang, Q.; Hu, W.; Zhu, X.; Wang, X.; Wan, Q.; He, P.; Zhou, H.; Pan, A. Low-Threshold Nanowire Laser Based on Composition-Symmetric Semiconductor Nanowires. Nano Lett. 2013, 13, 1251−1256. (14) Hayden, O.; Agarwal, R.; Lieber, C. M. Nanoscale Avalanche Photodiodes for Highly Sensitive and Spatially Resolved Photon Detection. Nat. Mater. 2006, 5, 352−356. (15) Wagner, R. S.; Ellis, W. C. Vapor−Liquid−Solid Mechanism of Single Crystal Growth. Appl. Phys. Lett. 1964, 4, 89−91. (16) Schmidt, V.; Wittermann, J. V.; Senz, S.; Gosele, U. Silicon Nanowires: A Review on Aspects of Their Growth and Their Electric Properties. Adv. Mater. 2009, 21, 2681−2702. (17) Ding, Y.; Gao, X.; Wang, Z. L. Catalyst-Nanostructure Interfacial Lattice Mismatch in Determining the Shape of VLS Grown Nanowires and Nanobelts: A Case of Sn/ZnO. J. Am. Chem. Soc. 2004, 126, 2066−2072. (18) Zhang, J.; Yang, Y.; Jiang, F.; Li, J.; Xu, B.; Wang, S.; Wang, X. Fabrication of Semiconductor CdS Hierarchical Nanostructures. J. Cryst. Growth 2006, 293, 236−241. (19) Yu, L.; Fortuna, F.; O’Donnell, B.; Patriache, G.; Roca i Cabarrocas, P. Stability and Evolution of Low-Surface-Tension Metal Catalyzed Growth of Silicon Nanowires. Appl. Phys. Lett. 2011, 98, 123113−123113−3. (20) Yu, L.; O’Donnell, B.; Maurice, J.-L.; Roca i Cabarrocas, P. Core-Shell Structure and Unique Faceting of Sn-Catalyzed Silicon Nanowires. Appl. Phys. Lett. 2010, 97, 023107−023107−3. (21) Gao, P. X.; Wang, Z. L. Substrate Atomic-Termination-Induced Anisotropic Growth of ZnO Nanowires/Nanorods by the VLS Process. J. Phys. Chem. B 2004, 108, 7534−7537. (22) Cui, Y.; Lauhon, L. J.; Gudiksen, M. S.; Wang, J.; Lieber, C. M. Diameter-Controlled Synthesis of Single-Crystal Silicon Nanowires. Appl. Phys. Lett. 2001, 78, 2214−2216. (23) Hochbaum, A. I.; Fan, R.; He, R.; Yang, P. Controlled Growth of Si Nanowire Arrays for Device Integration. Nano Lett. 2005, 5, 457−460. (24) Nebol’sin, V. A.; Shchetinin, A. A. Role of Surface Energy in the Vapor−Liquid−Solid Growth of Silicon. Inorg. Mater. 2003, 39, 899− 903. (25) Dubrovskii, V. G.; Cirlin, G. E.; Sibirev, N. V.; Jabeen, F.; Harmmand, J. C.; Werner, P. New Mode of Vapor−Liquid−Solid Nanowire Growth. Nano Lett. 2011, 11, 1247−1253. (26) Glas, F.; Harmand, J.-C.; Patriache, G. Why Does Wurtzite Form in Nanowires of III-V Zinc Blende Semiconductors. Phys. Rev. Lett. 2007, 99, 146101−146101−4. (27) Sharma, R. C.; Chang, Y. A. The S-Sn (Sulfur-Tin) System. Bull. Alloy Phase Diagrams 1986, 7, 269−273. (28) Dutkiewicz, J.; Zabdyr, L.; Moser, Z.; Salawa, J. The Cd-Sn (Cadmium-Tin) System. Bull. Alloy Phase Diagrams 1989, 10, 223− 229. (29) Persson, A. I.; Larsson, M. W.; Stenstrom, S.; Ohlsson, B. J.; Samuelson, L.; Wallenberg, L. R. Solid-Phase Diffusion Mechanism for GaAs Nanowire Growth. Nat. Mater. 2004, 3, 677−681. (30) Hogness, T. R. The Surface Tensions and Densities of Liquid Mercury, Cadmium, Zinc, Lead, Tin and Bismuth. J. Am. Chem. Soc. 1921, 43, 1621−1628.

were found indicating no qualitative difference between the two types of nanowires.

4. CONCLUSIONS We have investigated the bimodal behavior of Sn-catalyzed CdS nanowires grown by physical vapor transport utilizing a SnS source. The tall/thin and the short/thick CdS nanowires grow simultaneously at the substrate temperatures of 250−350 °C. The tall/thin nanowires grow along ⟨0001⟩ while the short/ thick nanowires grow along ⟨0110̅ ⟩. By inspecting hundreds of nanowires, the bimodal distribution is universal, and the relationships between dc and dNW, valid in the dNW range from 20 to ∼150 nm, were obtained for two kinds of nanowires. We discussed the free energy change for forming nuclei for these nanowires. The free energy changes for the nucleation of two kinds of nanowires are similar to each other. Therefore, this energetics discussion may explain the bimodality. The nanowires, regardless of their types, show prominent PL emission without the trace of impurity band emission, indicating their high optical quality.

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ASSOCIATED CONTENT

S Supporting Information *

Physical vapor transport system and temperature profile; histograms of type 1 and type 2 nanowires with various substrate temperatures; EDX spectra of type 1 and type 2 nanowires from the catalyst and the nanowire area. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012R1A1A2002076). The authors thank Prof. Hannah J. Joyce at the University of Cambridge for fruitful discussion.

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REFERENCES

(1) Lieber, C. M.; Wang, Z. L. Functional Nanowires. MRS Bull. 2007, 32, 99−108. (2) Wang, Z. L. ZnO Nanowire and Nanobelt Platform for Nanotechnology. Mater. Sci. Eng., R 2009, 64, 33−71. (3) Ma, C.; Moore, D.; Li, J.; Wang, Z. L. Nanobelts, Nanocombs, and Nanowindmills of Wurtzite ZnS. Adv. Mater. 2003, 15, 228−231. (4) Zhai, T.; Fang, X.; Li, L.; Bando, Y.; Golberg, D. OneDimensional CdS Nanostructure: Syntheis, Properties, and Applications. Nanoscale 2010, 2, 168−187. (5) Gu, F.; Yang, Z.; Yu, H.; Xu, J.; Wang, P.; Tong, L.; Pan, A. Spatial Bandgap Engineering along Single Alloy Nanowires. J. Am. Chem. Soc. 2011, 133, 2037−2039. (6) Moon, S. R.; Kim, J. H.; Kim, Y. ZnTe Nanowires with Oxygen Intermediate Band Grown by Bismuth-Catalyzed Physical Vapor Transport. J. Phys. Chem. C 2012, 116, 10368−10374. (7) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Nanobelts of Semiconducting Oxides. Science 2001, 291, 1947−1949. 5994

dx.doi.org/10.1021/jp500696r | J. Phys. Chem. C 2014, 118, 5988−5995

The Journal of Physical Chemistry C

Article

(31) Barnard, A. S.; Xu, H. First Principles and Thermodynamic Modeling of CdS Surfaces and Nanorods. J. Phys. Chem. C 2007, 111, 18112−18117. (32) Schmidt, V.; Senz, S.; Gosele, U. Diameter-Dependent Growth Direction of Epitaxial Silicon Nanowires. Nano Lett. 2005, 5, 931−935. (33) Dubrovskii, V. G. Physical Consequences of the Equivalence of Conditions for the Steady-State Growth of Nanowires and the Nucleation on Triple Phase Line. Tech. Phys. Lett. 2011, 37, 53−57. (34) Joyce, H. J.; Wong-Leung, J.; Gao, Q.; Tan, H. H.; Jagadish, C. Phase Perfection in Zinc Blende and Wurtzite III-V Nanowires Using Basic Growth Parameters. Nano Lett. 2010, 10, 908−915. (35) Johansson, J.; Karlsson, L. S.; Dick, K. A.; Bolinsson, J.; Wacaser, B. A.; Deppert, K.; Samuelson, L. Effects of Supersaturation on the Crystal Structure of Gold Seeded III-V Nanowires. Cryst. Growth Des. 2009, 9, 766−773. (36) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Nanobelts of Semiconducting Oxides. Science 2001, 291, 1947−1949. (37) Algra, R. E.; Verheijen, M. A.; Feiner, L.; Immink, G. G. W.; van Enckevort, W. J. P.; Vlieg, E.; Bakkers, E. P. A. M. The Role of Surface Energies and Chemical Potential during Nanowire Growth. Nano Lett. 2011, 11, 1259−1264. (38) Wang, B.; Yang, Y.; Xu, N.; Yang, G. W. Mechanism of SizeDependent Shape Evolution of One-Dimensional Nanostructure Growth. Phys. Rev. B 2006, 74, 235305−235305−6. (39) Wang, C. X.; Wang, B.; Yang, Y. H.; Yang, G. W. Thermodynamic and Kinetic Size Limit of Nanowire Growth. J. Phys. Chem. B 2005, 109, 9966−9969. (40) Liu, Q. X.; Wang, C. X.; Xu, N. S.; Wang, G. W. Nanowire Formation during Catalyst Assisted Chemical Vapor Deposition. Phys. Rev. B 2005, 72, 085417−085417−6.

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