Zn3P2 Twinning Superlattice Nanowires Grown on ... - ACS Publications

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Zn3P2 Twinning Superlattice Nanowires Grown on Fluorine-Doped Tin Oxide Glass Substrates Seon Bin Choi, Man Suk Song, and Yong Kim* Department of Physics, Dong-A University, Hadan-2-dong, Sahagu, Busan 49315, Korea

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S Supporting Information *

ABSTRACT: Zn3P2 twinning superlattice nanowires with diameters of 100−300 nm were grown under Sn catalysis on fluorine-doped tin oxide glass by physical vapor transport. The nanowires grew along the [101] direction with nonparallel {101} side facets. The Zn3P2 twinning superlattices had no noticeable crystallographic defects except periodic twin defects. A nonlinear relationship between the twin plane spacing and nanowire diameter was observed. The twin plane formation energy (4.0 × 10−2 to 4.3 × 10−2 J/m2) was estimated by fitting the relationship from the nucleation model with a hexagonal nucleus with monolayer height at the triple-phase boundary. The unexpected nonlinear behavior despite the relatively high twin plane formation energy was ascribed to the strong interaction of P atoms dissolved in Sn droplets with the growth interface. P atoms in the droplets may have acted as a surfactant to reduce the liquid−solid surface energy.

1. INTRODUCTION Semiconductor nanowires, when used as building blocks for bottom-up syntheses, have unique characteristics applicable to novel optoelectronic devices.1 The intriguing characteristics of these quasi-one-dimensional structures stem from their extremely high surface/volume ratio. Nanowires with the zinc blende (ZB)/diamond crystal structure generally grow along the ⟨111⟩ direction according to the vapor−liquid−solid (VLS) mechanism involving catalysis by foreign metals, which form a suitable eutectic phase diagram with the host materials. Straight ZB nanowires grown along the ⟨111⟩ direction are surrounded by six {112} side facets with a hexagonal cross section.2 The {112} side facets can be replaced by alternating {111} microfacets with lower surface energy.2−4 The microfacets lead to sawtooth-like faceting of the sidewall.5,6 Periodic twinning of ZB nanowires with alternating {111} side facets along the ⟨111⟩ growth direction could occur when enough energy is saved by microfacet formation to compensate for the loss because of twin plane formation. Periodically twinned ZB nanowires, which are called twinning superlattices (TSLs), have attracted much attention recently owing to their potential applications. Different from conventional superlattices with compositional or structural periodicity, a TSL is perfectly lattice-matched across the twin plane. Nevertheless, theoretical calculations predict that TSLs can have very different properties to bulk semiconductors. Periodic twinning may induce an electronic miniband structure that could tune the band gap of the host materials.7,8 Further, optical subband transitions may offer an approach toward band gap engineering suitable for novel optoelectronic devices. The © XXXX American Chemical Society

discontinuous electron wavefunction at the twin plane may lead to a reduced mobility of charged carriers. A reduction in thermal conductivity can be useful for thermoelectric devices.9,10 Growth of coherent TSLs with precisely controlled periodicity is a challenging issue. TSLs have been reported for various material systems including GaAs,11 GaP,12,13 InP,14−16 InAs, 17−19 ZnTe, 20 ZnSe, 21 ZnS, 22 ZnO, 23 SiC, 24,25 Zn2SnO4,26 and Zn3P2.18,27−29 Tetragonal Zn3P2 is a crystalline system with c /a = 2 and has a pseudo-face-centered cubic symmetry (Figure S1 in the Supporting Information).30,31 For instance, when a transformation matrix is used,30 the [100] and [101] directions in the tetragonal system correspond to the [110] and [111] directions in the pseudocubic system, respectively. Zn3P2 has an indirect band gap of 1.315 eV, and also, it has a direct band gap of 1.49 eV at room temperature.32 It also has a high optical absorption coefficient (>1 × 104 cm−1) in the visible range and a long minority carrier diffusion length (>5 μm).33 Therefore, Zn3P2 is an ideal material for photovoltaic applications. It is even more promising for scalable thin-film photovoltaic applications owing to its earth abundance of elemental zinc and phosphorus.34 A power conversion efficiency of close to 5% has been achieved for p-Zn3P2/Mg Schottky barrier solar cells.35 Zn3P2 TSLs could be more attractive because a periodically corrugated side wall may increase the light absorption. Although Zn3P2 TSLs have been reported,18,27−29 Received: January 7, 2019 Revised: January 27, 2019 Published: January 28, 2019 A

DOI: 10.1021/acs.jpcc.9b00190 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. (a) FESEM image of the surface obtained at 450−500 °C. (b) XRD pattern. (c) Absorbance spectrum, where the blue line is an extrapolation to determine the band gap.

the profile, the temperature range of the substrates was 500− 210 °C. Before growth, the system was evacuated to a base pressure of ∼10−2 Torr by a mechanical pump for 30 min; then, Ar carrier gas was introduced at a flow rate of 200 sccm. The ramp-up time to 700 °C was 15 min, and the growth time was 1 h, during which the pressure was maintained at 600 Torr. The sample was cooled to room temperature while the carrier gas flow was maintained. The morphology of the as-grown Zn3P2 nanowires was observed by field-emission scanning electron microscopy (FESEM, JEOL JSM-6700F). X-ray diffraction (XRD) patterns were measured by an X-ray diffractometer (Rigaku Ultima IV). Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were measured by a transmission electron microscope (JEOL, JEM-2010). Energydispersive X-ray spectra were obtained using an energydispersive X-ray spectrometry (EDXS) system (Oxford Instruments, INCA) attached to the TEM. TEM specimens were prepared by immersing samples in an ethanol-filled Eppendorf tube, which was sonicated for 10 s to separate the Zn3P2 nanowires from their glass substrates; the nanowires were then dispersed onto a holey carbon grid using a micropipette. The UV−visible absorption of FTO glass with the as-grown Zn3P2 nanowires was measured using a Cary 50Bio UV−visible spectrophotometer.

a detailed study of the relationship between the twinning period and nanowire diameter is needed to exploit the full potential of Zn3P2 for various applications. Shen et al. observed twin-free zigzag nanowires under the vapor−solid (VS) growth mechanism. In contrast, TSL nanowires were obtained under indium catalysis by the VLS mechanism.27 Similarly, Im et al. observed straight nanowires under catalysis by Au, a common catalyst. In contrast, TSL nanowires were observed under catalysis by indium.28 Wu et al. observed Zn3P2 TSL formation using ZnO and InP powders as source materials.29 According to the literature,36−38 Aucatalyzed or catalyst-free growth produces only twin-free Zn3P2 nanowires. Because TSL formation requires liquid catalysts, it is worth comparing the surface energy of the liquid−vapor interface of a metal, γLV. The γLV values of Sn, In, and Au are 0.570, 0.560, and 1.130 J/m2, respectively.39 Catalysts with low surface energy may play a dominant role in forming the Zn3P2 TSL. We have reported various nanowires under Sn catalysis on fluorine-doped tin oxide (FTO) substrates, which are widely used as transparent conducting substrates.40−42 Nanostructures on transparent conducting glass substrates would have practical advantages for use in thin-film optoelectronic devices. Considering the similar material properties of Sn and In (their melting temperatures are 505 and 430 K, respectively), it is essential to explore the possibility of TSL formation in Zn3P2 on FTO substrates.

3. RESULTS AND DISCUSSION Figure 1a shows the surface morphology obtained at 450−500 °C. A high density of TSL nanowires is visible, but the temperature window for TSL nanowire growth is rather narrow. A Sn droplet forms on the FTO substrate and the droplet acts as a catalyst for forming nanowires via VLS mechanism. Sn vapor may be transported from the hotter zone and condensed on the side wall. In this case, secondary growth will occur from the droplet on the side wall and results in a nanotree structure with a trunk and secondary branches as reported earlier for ZnTe nanotrees.41 However, TSL nanowires were formed on the hottest zone of the substrate.

2. EXPERIMENTAL METHODS Zn3P2 nanowires were grown by physical vapor transport employing a single-zone furnace with a 1 in. diameter quartz tube (Lindberg/Blue M Mini-Mite, Thermo Scientific). A quartz boat containing the Zn3P2 source powder (50 mg, Alfa Aesar, 99.99%) was placed in the central zone, where the temperature was set to 700 °C. FTO glass substrates (25 mm × 12 mm × 1.1 mm, AGC Asahi Glass) were cut, cleaned, and placed 15.5−18 cm downstream from the center. Figure S2 in the Supporting Information shows a schematic of the physical vapor transport system and temperature profile. According to B


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Figure 2. (a) Low-magnification TEM image of a Zn3P2 nanowire. (b) TEM image of the region near the catalyst. (c) HRTEM image of the region in the red box in (b). (d) Power spectrum of the lattice image of (c).

Therefore, there is no transported Sn vapor for secondary branch growth. Figure 1b shows the XRD patterns; the diffraction peaks agree well with the tetragonal Zn3P2 phase (JCPDS 74-1156, space group P42/nmc) with a = 0.8097 nm and c = 1.145 nm. The tetragonal Zn3P2 phase has a 40-atom unit cell, in contrast to the 8-atom unit cell of ZB group III−V semiconductors (Figure S3 in the Supporting Information). P atoms are arranged with a perfect ZB phase (Figure S4 in the Supporting Information), but the Zn atoms are distorted and less symmetric.43 Crystallographic indexing based on the pseudoface-centered cubic symmetry is sometimes preferred owing to easy recognition of the crystal symmetry.28 However, in this study, we use tetragonal crystal indexing. Figure 1c shows the absorbance spectrum; the blue line is an extrapolation to determine the band gap. The estimated band gap is 1.49 ± 0.01 eV, which is in good agreement with that of Zn3P2.33 We observed two types of nanowires: exceptionally long but straight nanowires, as shown in Figure 2, and TSL nanowires. Figure 2a shows a low-magnification TEM image of a straight Zn3P2 nanowire. The nanowire is not tapered, indicating no appreciable sidewall growth due to the VS mechanism. The TEM image shown in Figure 2b is a magnified image of the region near the catalyst. Zn3P2 nanowires grew along the [101] direction, which is the [111] direction in pseudocubic indexing, as determined from the high-resolution TEM (HRTEM) image in Figure 2c and the power spectrum of the lattice image by fast Fourier transform (FFT) in Figure 2d. A high density of stacking faults is clearly visible in the power spectrum as a streak pattern extending along the [001] direction (i.e., the c axis). Figure 3a shows an FESEM image of a TSL nanowire with a diameter D of 260 ± 3 nm. Growth proceeds under the VLS mechanism from the catalyst at the top of the nanowire (Figure 3b). The periodicity of the twinning is evident (Figure 3c). During growth, the shape of the top facet which is in contact with the liquid metal droplet changes continuously from a regular hexagon to a truncated triangle (Figure 3d). This changes the contact angle of the liquid droplet with respect to the top facet of the nanowire. Twin plane formation can be energetically favorable when the change exceeds a

Figure 3. (a) FESEM image of a TSL. (b) Magnified view of the region near the catalyst [red box in (a)]. (c) Magnified view of the TSL region [green box in (b)]. (d) Schematic of the TSL nanowire with periodic twin planes. Note that the cross section changes periodically from a hexagonal (dash-dotted line) to truncated triangular shape. Arrows indicate the twin planes.

certain level. Then, the crystal structure is inverted, and a periodic TSL is formed. The exposed side surfaces of the TSL, which are nonparallel to the growth direction, are the {111} surfaces in the pseudocubic indexing system. It is noteworthy that the TSL again becomes a straight nanowire near the catalyst (Figure 3a). We infer that this morphological change is due primarily to additional growth during the cooling stage. Twinning can be viewed as insertion of an atomic bilayer with a wurtzite (WZ) structure.3,17,44 The stacking sequence of atomic layers in the ZB structure is AaBbCc along the ⟨111⟩ direction. Here, letters indicate each atomic layer of cation and anion elements. The segments between the twin plane rotate 60° each other and the sequence changes to CcBbAa. The mirror twins, where the stacking sequence is completely reversed as cCbBaA, have not been reported in nanowires.44 The competition between the ZB and WZ phases depends critically on supersaturation.44−46 Low supersaturation favors the ZB crystal structure.45 The morphological change is associated with a decrease in the source supply rate under low supersaturation during the cooling process. C


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Figure 4. (a) TEM image of a long, periodic TSL with D = 138 ± 2 nm. (b) Magnified TEM image of (a). The angle between adjacent facets is 141 ± 1°, indicating {111} facets in the pseudocubic indexing system. (c) HRTEM image of the convex intersection with the twin plane. (d) Power spectra of lattice images of the regions indicated in (c).

The high level of periodicity of the TSL nanowire grown in the [101] direction is clearly confirmed in the TEM image shown in Figure 4a. The angle between adjacent facets is 141 ± 1°, which is in good agreement with the (111) facet angle of 19.5° with respect to the growth direction (Figure 4b). The HRTEM image shows that the TSL nanowire is defect-free except for the twin plane. High crystallinity is again confirmed by the well-defined spots in the power spectrum of the lattice image (Figure 4d) obtained in selected regions of Figure 4c. The power spectrum of region 2, which includes a twin plane, confirms the mirror reflection of individual spots. When an intersection of adjacent facets is concave, it is very close to the position of the twin plane. However, an obvious offset between the positions of the convex intersection and twin plane is observed (Figure 4b). Emergence of the (001) transition plane from (1̅01) to (1̅01)T is also observed. Such an offset has also been observed11,12 in III−V nanowires. The offset has been attributed to the difference in the overgrowth rate between the A (cation-terminated) and B (anion-terminated) facets.11,47 Burgess et al.11 reported that the facet intersection appeared below the nearest twin plane, and the degree of the offset increased systematically from the catalyst tip to bottom owing to overgrowth of the sidewall. However, the offset we observed may have a different origin, for several reasons. No noticeable tapering due to radial overgrowth is observed in our TSL nanowire. Further, the segments with a noticeable offset occur randomly between periodic TSL segments. The positions of convex intersections within a single segment are rather symmetric (Figure 5a). If one convex intersection appears above the twin plane within a single segment, another convex intersection appears below the next twin plane. The separation between the intersection and the nearest twin plane along the growth direction is almost the same. When the TSL returns to the original periodicity, this offset in the convex intersections disappears, as shown in Figure 5b. At present, we have only a tentative model. When there is a sudden volume expansion of the liquid catalyst, presumably because of pressure fluctuation during growth, twinning starts at a concave intersection and propagates to a convex intersection, but the extra source reactants still participate in the growth even after twinning is complete.

Figure 5. (a) TEM image of a part of a TSL nanowire with D = 204 ± 6 nm. Red arrows indicate the twin plane positions. Green arrows indicate the convex intersections. (b) HRTEM image of the convex intersection [red box in (a)].

EDXS spectra were obtained at various positions on the TSL nanowires (Figure 6a) and catalyst (Figure 6b). There is no trace of Sn within the TSL nanowires (Figure 6c) with a detection limit of 0.1−1 at. %.48 The TSL nanowires are slightly off-stoichiometric and phosphorus-rich. The amorphous layer covering the side wall (Figures 4b and 5b) may be condensed phosphorus. The presence of Zn (∼3.2 at. %) and P (∼3.3 at. %) from the catalyst is detected. For gold catalysts, the contribution of the group III reactant to the chemical D


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Figure 6. (a) TEM image of a TSL nanowire (D = 207 ± 3 nm). Numbers indicate EDXS sampling positions. (b) Magnified TEM image near the catalyst. The number indicates the sampling position. (c) EDXS spectra at the sampling positions.

potential is considered only for nucleation modeling of III−V nanowire growth owing to the extremely low solubility of the group V reactant.13,46 The group V reactant is diffused into the liquid-phase droplet from the triple-phase boundary. Therefore, the group V reactant will not play any role to modify the surface property of the top facet in contact with the liquid droplet. In contrast, the solubility of P in Sn is appreciable (∼6−11 at. % at 450−500 °C) from the Sn−P phase diagram.49 Considering remaining P atoms observed from EDXS spectra after growth, the significant amount of P was dissolved during growth. Therefore, the contribution of P dissolved in the liquid droplets should be considered. Figure 7a shows a TEM image of a straight nanowire near the catalyst; the SAED image confirms that it is ZB with high Figure 8. Twin plane spacing as a function of nanowire diameter. The red line fits the model equation proposed by Algra et al.12,14 The blue line is data for InP TSL nanowires14 using the parameters A = 0.35 nm−1, Δ = 4.5 to stress the striking difference between the two materials.

counting more than 25 segments of a single TSL nanowire with various diameters. The apparent nonlinearity is similar to the reported behavior of GaP,12 InP,14 and InAs.17 In contrast, linear dependence has been reported for GaAs,11 ZnSe,21 and SiC.24 There are two approaches to analyzing the relationship. The first is to compare the total energy after the completion of a monolayer with height h on top of the facet. As stated earlier, the cross-sectional shape of the top facet in contact with the liquid droplet changes from a hexagonal to truncated triangular shape. In facet-conserving growth of a monolayer, the decrease in the interface area changes the contact angle β with respect to the interface, assuming conservation of the total droplet volume (see Figure S5 in the Supporting Information). At a critical height from the position of the hexagonal cross section (Figure 3d), twin plane formation accompanied by an increase in the solid−liquid interface area becomes more energetically favorable than continued facet-conserving growth. Ross et al.5 introduced an energy barrier to edge formation for sawtoothfaceted twin-free Si nanowires. Similarly, Shim et al.25 introduced the edge energy while neglecting the contribution of the twin energy per unit area, γT. In the analytical derivation of the relationship, the shape of the top facet was approximated as a circle with the effective radius, and the theory predicted a linear relationship between the twin period and diameter. In

Figure 7. (a) TEM image of a TSL nanowire (D ≈ 226 nm) near the catalyst showing the contact angle δ0 with respect to the side facet. SAED images of the (b) nanowire and (c) catalyst.

crystallinity and growth along the [101] direction (Figure 7b). The SAED image of the Sn catalyst in Figure 7c shows that it has a tetragonal structure (β-phase Sn, space group I1/amd).50 The period of a TSL nanowire is the key parameter for designing and using various devices employing the TSL structure. Figure 8 shows the twin plane spacing as a function of TSL diameter. The data were obtained and averaged by E


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B facet nucleation is highly preferred and the length of the short side edge (B facets) decreases. As observed in eq 5, Γc increases for facet-conserving nucleation and the inequality may hold beyond Hc the critical axial distance. Finally, facetnonconserving nucleation with twin plane formation can be energetically preferred over further increases in the tautness. Algra et al.12,14 derived the equation considering individual nucleation events to explain the nonlinear relationship between the twin plane spacing and diameter. Considering the statistical aspects, Algra et al. ultimately arrived at an expression that explains the nonlinear relationship between the twin plane spacing (i.e., the twin period) 2H and the nanowire diameter D, as follows É ÅÄ l o o ij −Δ yzÑÑÑÑ| o o ij 1 yz ÅÅÅÅ j z j z 2H = 2Hco 1 + ln 1 − exp m j zÑÑÑo } Å j z j z Å j z Å Ñ o Δ / H h k { ÅÅÇ o o k c {ÑÑÖo (6) n ~

reality, the liquid catalyst is highly asymmetric. It is stretched taut at the short side edge and balled up at the long side edge when a droplet is located on the top facet with a truncated triangular shape. During facet-conserving growth, the tautness increases with decreasing contact angle at the short side edge.14,45 In an entirely different approach, Glas et al.51 proposed a monolayer nucleation model to explain the preference for the WZ phase over the ZB phase. Once a critical nucleus forms, growth occurs in the monocentric or mononuclear regime, in which a single nucleation event controls the entire monolayer.44,45,51,52 The change of Gibbs free energy ΔG for a nucleation event is given as ΔG = − ShΔμ + Ph Γ + SγSN

(1)

where S is the nucleus base area, P is the nucleus perimeter, and γSN is the interface energy between the nucleus and the top facet. First term represents Gibbs free energy change due to chemical reaction, which is described by the chemical potential difference per unit volume Δμ (i.e., supersaturation). Second term represents the contribution from lateral side facets of the nucleus, where Γ is an effective sidewall energy of the nucleus. For facet-nonconserving nucleation, γSN is equal to twin formation energy γT and γSN = 0 for facet-conserving nucleation. For a hexagonal nucleus with height h and facet length ρ, critical length ρ* and critical barrier energy ΔG* by differentiation with respect to ρ are given as 2 Γ 3 Δμ −

ρ* =

ΔG* =

γSN h

Here, Hc is described by ADh introducing a parameter A. A is a prefactor which can be evaluated when the probability of facetnonconserving nucleation exceeds that of conserving nucleation. Fitting of the model equation with the parameters A = 3.31 ± 0.46 nm−1, Δ = 7.99 ± 0.56 is successful (Figure 8). After transformation to A̅ (=A/(1 − 4.46hA/Δ)), A̅ (=5.28 ± 0.72 nm−1) is related to the physically relevant parameters A̅ =

γSN h

(3)

If ΔGt* < ΔGc*, twin plane formation is energetically favorable. The inequality is equivalent to Γt2/(Δμ − γT/h) < Γc2/Δμ. The subscript t and c denote the facet-nonconserving and conserving nucleation, respectively. The probability for nucleating with twin plane formation is given by45 ij ΔG*j yz ji ΔGt* zyz zz zz/∑ expjjjj− Pt = expjjj− z j kT z j kT zz k { j { k

1.982 ijj Γ0 yzz Δ= γj z kBT Tjk Δμ z{

5 i 1 yz zz(γ − γLV cos δ) γ + jjj 6 SL k 6 cos θ { SV

2

(8)

Unfortunately, the material parameters of Zn3P2 are largely unknown. By combining eqs 2 and 3, A̅ can be expressed as (4)

A̅ =

where k and T are the Boltzmann constant and absolute temperature, respectively. j represents all possible orientations of nucleating facets. Effective sidewall energy is given by Γ=

(7)

where Γ0 = 5/6γSL + 1/6[γSV − γLV cos δ0], which is the effective sidewall energy per unit area of the nucleus for the hexagonal interface. A̅ is another prefactor which determines the energetic preference of facet-nonconserving nucleation and it can be obtained under the condition (ΔGt* = ΔGc*). Δ, the free energy difference between conserving and nonconserving nucleation events at the hexagonal cross section divided by thermal energy is expressed as12

(2)

2 3 Γ 2h Δμ −

ij Γ0 yz γT 1.16 jj zz 2 h γLV sin δ0 jk Δμ z{

0.59 kBTγTΔ h2 γLV sin δ0

(9)

Using δ0 = 16 ± 1° (Figure 7a) and γLV sin δ0 = 0.16 ± 0.01 J/m2, the twin formation energy γT is estimated to be 4.0 × 10−2 to 4.3 × 10−2 J/m2, considering the growth temperature uncertainty (=±25°). Compared to the γT values of several TSL semiconductors, including GaAs (∼2.75 × 10−2 J/ m2),11,54 GaP (∼2.10 × 10−2 J/m2),12,54 InP (∼9.00 × 10−3 J/ m2),14,54 ZnSe (∼6.50 × 10−3 J/m2),55 and ZnTe (∼8.00 × 10−3 J/m2),55 γT is quite high. The critical nuclear radius ρ* [=(2/ 3 )(Γ0/Δμ) for a hexagonal nucleus] is 0.85 ± 0.04 nm, which is within a reasonable range.12,45 A high γT value can correspond to difficulty in forming the TSL, as shown in eq 2. The parameter Δ controls the nonlinear dependence of the twin plane spacing. Burgess et al.11 noted that high γT and γLS values result in a high Δ parameter and produce linear dependence. Despite the high γT value of the Zn3P2 TSL nanowires, the obvious nonlinearity can be explained only by the relatively low γLS value, which

(5)

where γSL and γSV are the surface energies of the solid−liquid and solid−vapor interfaces, respectively. θ (θB = −θA = 19.5°) and δ are the facet angle of the nucleus and the contact angle between the droplet and the external side facet of the nucleus, respectively. They are related with δ = β − θ − π/2 (see Figure S5 in the Supporting Information). Algra et al.53 have found that the contact angle δ depends linearly on the deformation of the top facet of the nanowire as δ = δ0 − 4H tan θ/1.1D. Here, δ0 is the contact angle between the droplet and the external side facet of the nucleus for a hexagonal interface, and nanowire height H is evaluated from the position of the hexagonal cross section. F



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effectively reduces Γ. Algra et al.14 observed the onset of an InP TSL while introducing Zn doping species into Au droplets. This phenomenon was ascribed to lowering of the liquid−solid step energy owing to the strong interaction of zinc atoms with the growth interface.14,56 Yuan et al.57 also noted the role of the surfactant in TSL formation. Similarly, P atoms dissolved in Sn droplets (as demonstrated in Figure 6c) may play a decisive role in reducing γLS. This fact may explain the absence of TSL nanowires under catalysis by Au owing to the extremely low solubility of P. The occurrence rate of unusually large segments with a significant twin plane offset (Figure 5a) tends to increase with nanowire diameter and affects the width of the distribution of segment lengths, as shown in Figure 8.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b00190. Relationship between pseudocubic and tetragonal unit cells, physical vapor transport system and the temperature profile, atomic structures of ZB InAs and Zn3P2 for comparison, atomic structures of phosphorus in Zn3P2, and relationship between various angles presented in the text (PDF)

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REFERENCES

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4. CONCLUSIONS We successfully grew Zn3P2 TSL nanowires with diameters of 100−300 nm on FTO glass, which is widely used as a transparent conducting substrate, by physical vapor transport. Catalysis by Sn supplied from the substrate produced the TSL nanowires. The Zn3P2 TSL nanowires grew along the [101] direction with nonparallel {101} side facets. The nanowires had no noticeable crystallographic defects and a high degree of periodic twinning. An obvious nonlinear relationship was observed between the twin plane spacing and TSL nanowire diameter. The twin plane formation energy γT (4.0 × 10−2 to 4.3 × 10−2 J/m2) was deduced by fitting the relationship with the equation suggested by Algra et al.12,14 The unexpected occurrence of the nonlinear behavior even though the twin formation energy was higher than those of other TSL nanowires was ascribed to the strong interaction of P atoms dissolved in Sn droplets with the growth interface. P atoms in the droplets may effectively reduce γLS and thus Γ/Δμ. This may explain the absence of Zn3P2 TSL nanowires under catalysis by Au, which has extremely low solubility with P.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +82-51-200-7276. ORCID

Yong Kim: 0000-0002-8318-2094 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (no. NRF-2018R1A2B6001011). G


Article

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