Article pubs.acs.org/JPCC
Crystal Phase and Facet Effects on the Structural Stability and Electronic Properties of GaP Nanowires Xiaodong Yang,† Haibo Shu,*,†,‡ Pei Liang,† Dan Cao,§ and Xiaoshuang Chen‡ †
College of Optical and Electronic Technology and §College of Science, China Jiliang University, 310018 Hangzhou, China ‡ National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Science, 200083 Shanghai, China, and S Supporting Information *
ABSTRACT: The control of electronic properties of GaP nanowires is of particular importance for their applications in nanoelectronics and optoelectronics. However, a fundamental understanding is still lacking of atomic and electronic properties of GaP nanowires due to the diversity of the crystal phase in the fabricated nanowires. Here we reveal the crucial role of the crystal phase and nanowire facets in the structural and electronic properties of zinc-blende, wurtzite, and polytypic GaP nanowires by using the first-principles calculations. The stability mechanism of GaP nanowires depends on the competition between the crystal phase and facet effects: the former dominates the stability of larger-sized nanowires, but the stability of ultrathin nanowires is mainly determined by the latter. This mechanism can be applied to explain a large amount of experimental observations about the formation of stacking faults and twin defects during the growth of III−V semiconductor nanowires. Likewise, electronic properties of GaP nanowires, including band gap values and the band structure characteristic, are sensitive to not only the nanowire size but also the crystal phase. Moreover, the quantitative relationship between the band gap of GaP nanowire polytypes and their diameter has been established, and the calculated band gaps agree well with the experimental data. Our results can provide a theoretical guidance for engineering electronic structures of GaP nanowires as well as their applications in optoelectronic devices.
I. INTRODUCTION III−V semiconductor nanowires have attracted much attention in the past decade due to their outstanding physical and chemical properties that potentially lead to a wide variety of applications, in particular, for nanoelectronics and optoelectronics.1−3 As a key class of III−V semiconductor nanowires, gallium phosphide (GaP) nanowires have drawn special research interest since they exhibit many advantages of properties including of medium band gap (∼2.3 eV),4,5 high absorption coefficients,6 as well as their compatibility with the Si technology,7 which makes them a potential candidate for the applications in nanoscale field effect transistor, photodiodes,8 photodetectors, solar cells,9 and photocatalysts.10,11 The application of GaP nanowires in the nanodevices requires the fabricated nanowires with well-controlled crystal structures, sizes, and morphology to induce the reliable manufacture of practical devices. Experimentally, GaP nanowires generally adopt a so-called vapor−liquid−solid (VLS) mechanism12 to induce nanowire growth.13−15 This mechanism was also widely used in synthesizing other III−V nanowires, such as GaAs,16 InP,17 and InAs.18 Interestingly, although most of the III−V semiconductors in their bulk phase have the zincblende (ZB) crystal structure, VLS-grown III−V nanowires often exhibit wurtzite (WZ) structure or mixed ZB and WZ polytypic structures.18−20 The GaP bulk with cubic ZB structure has an indirect band gap which limits their applications in optoelectronic devices, such as light-emitting devices.8 In contrast, one-dimensional GaP nanowires offer a © 2015 American Chemical Society
chance to produce a direct band gap by controlling their crystal phase during their growth. For instance, Assali et al. reported the fabrication of GaP nanowires with a hexagonal WZ structure by metal−organic vapor phase epitaxy (MOVPE).8 The WZ GaP nanowires were found to have high green emission efficiency due to their direct band gap. In comparison, III−V semiconductor nanowires more often exhibit spontaneous ZB-WZ polytypism and stacking faults19,20 along the growth direction of nanowires. The transition of the crystal phase will result in a change of electronic structure. The abundant crystal phases in III−V semiconductor nanowires lead to their sidewalls featured with different crystal facets. For the ZB nanowires, several different facets including {11̅ 1̅ }A, {111}̅ B, {112}A, {112}B, and {110} have been observed in experiment, while {11̅00} and {112̅0} facets have been found in the WZ nanowires.8,19−22 Owing to the large surface-to-volume ratio, the crystal facet effect becomes a key factor for electronic and optical properties of GaP and other III−V semiconductor nanowires. From previous theoretical investigations, the surface effect was proved to be important for structural and electronic properties of InP, GaAs, InAs, ZnO, and SiC nanowires.23−26 Our recent theoretical studies have also demonstrated the crystal-facet effect on electronic structures of GaAs and InP nanowires.27,28 Received: March 21, 2015 Revised: May 7, 2015 Published: May 8, 2015 12030
DOI: 10.1021/acs.jpcc.5b02738 J. Phys. Chem. C 2015, 119, 12030−12036
Article
The Journal of Physical Chemistry C
Figure 1. (a) Top and side views of atomic structures of four types of GaP nanowires with (a) zinc-blende phase (3C), (b) 6H phase, (c) 4H phase, and (d) wurtzite phase (2H), respectively. The shadow regions in the side-view atomic structures represent the WZ section in these nanowires. The distance between two neighboring dash lines in the side-view atomic structures denote the range of unit cell along the axial direction of nanowires. The large and small balls denote Ga and P atoms, respectively. The surface of all nanowires is passivated by pseudo-hydrogen atoms.
been established. The calculated results suggest that structural stability and electronic properties of GaP nanowires are determined by not only the quantum confinement effect but also the crystal phase and facet effects.
Although previous studies have established a solid theoretical basis for understanding structural and electronic properties of GaP and other III−V nanowires, there are still several key issues that have not been clarified. First of all, most of the previous theoretical studies focused on the single-phase nanowires (either WZ or ZB phase), but little about the polytypic III−V nanowires which contain mixed ZB-WZ structures,29,30 in particular, for GaP nanowires. The extensive experiments15,20,31 have demonstrated that electronic and optical properties of III−V nanowires were strongly related to their polytypic structures, but atomic-scale studies about the crystal-phase effect on structural and electronic properties of these nanowires are still lacking. Second, the surface effect was considered as a crucial factor for the geometry and electronic properties of III−V nanowires, but it has not been discriminated carefully based on the consideration of crystal facets. Especially for polytypic III−V nanowire structures,19−22 they often present {1̅1̅1}A and {111̅}B facets that have been little discussed in previous theoretical studies. Third, it is still difficult for the quantitative description of the size dependence of band gap in III−V nanowires by the experimental methods. So far, there is no report about the size dependence of band gap in GaP nanowires. Therefore, it is strongly desired to establish the quantitative relationship between the band gap of GaP nanowires and the nanowire diameter. In this work, we present for the first time geometry and electronic properties of GaP nanowires with different sizes and crystal structures (ZB, WZ, and polytypic ZB-WZ structures) using accurate first-principles calculations. We find that the structural stability of GaP nanowires originates from the competition between the crystal phase and facet effects. This result can be applied to explain a large number of experimental observations about the formation of stacking faults and twin defects in the nanowires. Our results demonstrate that electronic structures of GaP nanowires including their bandstructure characteristic and band gap values are strongly affected by the wire diameter and the crystal phase. Furthermore, the quantitative relationship between the band gaps of various GaP nanowire polytypes and their diameter has
II. COMPUTATIONAL DETAILS GaP nanowires are created on the basis of the optimized bulk crystal structures. Here we consider four types of nanowire polytypes (see Figure 1), including ZB (3C), WZ (2H), 4H, and 6H phases. The calculated lattice constant of cubic ZB structure (a = b = c) is 5.545 Å. For 2H, 4H, and 6H structures, they have the hexagonal unit cell. The calculated lattice parameters are a = 3.910 Å and c/a = 1.647 for the 2H structure, a = 3.917 Å and c/a = 3.296 for the 4H structure, and a = 3.918 Å and c/a = 4.941 for the 6H structure, respectively. As shown in Figure 1a, the 3C nanowires with six {112} sidefacets are oriented along the [111] direction and the nanowire diameter ranges from 1.35 to 2.71 nm. The 2H nanowires with six {11̅00} side-facets are oriented along the [0001] direction (Figure 1d), and the range of diameter is from 1.17 to 3.52 nm. The polytypic 4H and 6H nanowires include alternating WZ and ZB segments along the axial direction of nanowires (see Figure 1b,c). The proportion of WZ in 4H and 6H nanowires is 50% and 33.3%, respectively. The sidewall of 4H and 6H nanowires contains {11̅00}, {1̅1̅1}A and {111̅}B microfacets, which results in a twisted shape. The planar atomic models of all GaP nanowires considered in the present study are shown in Figure S1 of Supporting Information (SI). The nanowire sidewalls are passivated by artificial hydrogen atoms32 with fractional charges of 1.25 e and 0.75 e for Ga and P atoms, respectively. The distance between the neighboring nanowires is set to be larger than 12 Å to prohibit the interactions between adjacent wires. All calculations are performed within the framework of density functional theory (DFT) as implemented in the Vienna ab initio Simulation Package (VASP).33,34 The exchangecorrelation energy is treated in the generalized-gradient approximation (GGA) using the Perdew−Burke−Ernzehof (PBE) functional.35 The projector augmented wave (PAW) 12031
DOI: 10.1021/acs.jpcc.5b02738 J. Phys. Chem. C 2015, 119, 12030−12036
Article
The Journal of Physical Chemistry C
Figure 2. Formation energies of GaP nanowires as a function of (a) nanowire diameter d and (b) surface atom ratio χ, respectively. The inset shows formation energies of 3C and 2H nanowires in the diameter range from 8−16 nm. The DFT data of GaP nanowires with 3C, 2H, 4H, and 6H phases are indicated by rectangles, circles, triangles, and stars, respectively. The solid lines are the fitting curves on the basis of DFT data.
potentials36 are used to describe the interaction between valence electrons and ion cores. A kinetic energy cutoff of 500 eV is adopted to get reliable results. The Monkhorst−Pack grids of k-point are used to sample the surface Brillouin zone. The k-point mesh of 1 × 1 × 6 is found to provide adequate accuracy for the calculations. In the band structure calculations, a total of 15 k-points are used along the K vector direction from Γ (0, 0, 0) to Z (0, 0, 0.5). For the geometry optimization, the energy convergence criteria for electronic and ionic iterations are 10−3 eV and 10−2 eV/Å, respectively. In order to provide an accurate prediction for the nanowire band gaps, the Heyd− Scuseria−Ernzerhof (HSE)37 screened hybrid functional is used to calculate the band structures of four polytypic GaP bulk phases. The k-point mesh is set to 8 × 8 × 8 and 8 × 8 × 6 for cubic and hexagonal GaP polytypes, respectively.
but their formation energies are lower than that of 3C-type nanowires in the smaller diameter. Hence, in addition to the crystal-phase effect, the nanowire side-facets must play an important role in the stability of GaP nanowires. In order to understand the facet effect on the nanowire stability, we plot formation energies of four GaP nanowire polytypes as a function of surface atom ratio χ (that is, the ratio of surface and total atoms) in Figure 2b. It can be divided into two regions to evaluate the stability of GaP nanowires on the basis of χ value. When χ is larger than 35%, formation energies of GaP nanowires with the same χ follow the trend, Ef(6H) < Ef(4H) < Ef(2H) < Ef(3C), while formation energies of 3Ctype nanowires are correspondingly lower than that of 2H nanowires for χ < 35%. There is no data for the polytypic 4H and 6H nanowires in the range of 0 < χ < 35% because the calculation of large-sized nanowires in the two structures is beyond the ability of DFT calculation. Generally, the energy of a nanowire (ET) can be described by ET = EC + ES, where EC and ES are the cohesive energy of bulk atoms and surface energy of the nanowire, respectively. In the region of 0 < χ < 35%, the nanowires mainly consist of bulk atoms, and their stability is thus dominated by their crystal phase (or EC). Our calculated result indicates that the energy of Ga−P pair in the ZB phase is 14 meV lower than that of WZ phase. Therefore, the 3C-type nanowires are more stable than the 2H-type nanowires in larger size (or smaller χ). In contrast, the surface effect (or ES) will become a main factor for the nanowire stability when χ is beyond 35%. Owing to different crystal facets on nanowire sidewalls, the stability of GaP nanowires depends on the stability of crystal facets. In other words, the surface effect of nanowires can be evaluated by ES = AECF, where ECF is the surface energy of specified crystal facets, and the computational details of surface energies of various nanowire crystal facets are presented in Figure S1 of SI. The calculated result indicates that the surface energy of a GaP thin film with a (111)A and a (111)B surface is 95.8 meV/Å2 which is lower than that of GaP(11̅00) and GaP(112) films by 11.8 and 54.2 meV/Å2, respectively. The lowest-energy {1̅1̅1}A and {111̅}B facets are responsible for the higher stability of the polytypic 4H and 6H nanowires at the χ > 35% region. It needs to be mentioned that the surface energy of a nanowire (ES) is determined by not only the energetics of its crystal facets but also its surface area (A). It is the reason why the formation energies of polytypic 4H and 6H structures are higher than that of the 2H one for the ultrathin GaP nanowires even though the polytypic structures include the lowest-energy crystal facets, as shown in Figure 2a.
III. RESULTS AND DISCUSSION We first evaluate the structural stability of GaP nanowires with different sizes and crystal structures. All structures are relaxed to the minimum energy configuration, and the formation energies (Ef) per Ga−P pair in GaP nanowires are calculated by the following formula, Ef = (E T − nGaPμGaP − nHμH )/nGaP
(1)
where ET is the total energy of the nanowire, μGaP and μH are the total energy of Ga−P pair in ZB-GaP bulk and half of the energy of hydrogen molecule respectively, and nGaP and nH are the number of Ga−P pairs and pseudo-hydrogen atoms in a nanowire, respectively. The formation energies of GaP nanowires as a function of nanowire diameter (d) are shown in Figure 2a. We can see that Ef values of all nanowires are positive, implying that GaP nanowires are less stable than their bulk. Moreover, the Ef values of the nanowires decrease with increasing nanowire diameter, suggesting the size dependence of nanowire stability. The inverse relationship between formation energies and the nanowire diameter (d) can be described by Ef = K/dn, where K and n are fitting parameters. By fitting the DFT data points (see Figure 2a), we obtain that the parameters K and n are 482 (in meV·nm) and 1.0 for 3Ctype nanowires and 395 (in meV·nm) and 0.9 for 2H-type nanowires, respectively. It can be seen from fitting curves that the 2H-type nanowires are more stable than the 3C-type nanowires in ultrathin nanowires, but the 3C-type nanowires are energetically more favorable when the diameter exceeds 8 nm. A similar result has also been found in other III−V nanowires.23,38 It seems that the polytypic 4H and 6H nanowires should have the largest formation energies due to their twisted crystal shape that results in the large surface area, 12032
DOI: 10.1021/acs.jpcc.5b02738 J. Phys. Chem. C 2015, 119, 12030−12036
Article
The Journal of Physical Chemistry C
nanowires have an indirect band gap with the CBM at the Γ-Z valley. In contrast, the band structures of the nanowires at d = 2.84 and 3.55 nm (Figure 3g,h) have a direct band gap with the CBM at the Γ point. Therefore, the shift of CBM with increasing diameter causes an indirect-to-direct band gap transition in the band structure of 2H-type nanowires. As a result, the 2H nanowires with large and small nanowire sizes should display very different electrical and optical properties. A similar phenomenon about size-induced indirect-to-direct band gap transition has also been observed in wurtzite GaAs nanowires.39,40 For polytypic ZB-WZ GaP nanowires, we find that the 4Htype nanowires indicate a direct band gap characteristic, but the 6H-type nanowires show an indirect band gap feature. The result suggests that the 3C/2H phase ratio is also an important parameter for tuning the band structure of polytypic GaP nanowires. On the one hand, the distribution of band-edge electronic states of the nanowires can be adjusted by the 3C/ 2H ratio. On the other hand, the 3C/2H ratio changes the axial length of unit cell by the axial atomic relaxation, and then the band structure of the nanowires can be indirectly adjusted due to the axial strain effect. The above results suggest that the electronic properties of GaP nanowires are controlled by not only the quantum size effect but also the crystal phase. To quantitatively describe the size and crystal-phase effects on the band gap of GaP nanowires, we plot the band gap increment (ΔEg) of different crystal-phase nanowires as a function of diameter (d) in Figure 4a. The ΔEg is defined as the band gap difference between GaP nanowires with a certain crystal phase and their bulk. It can be found that all types of nanowires indicate size dependence of band gap. Nevertheless, four GaP nanowire polytypes display slightly different variation amplitude of band gap with the nanowire diameter. According to the quantum mechanical particle-in-box model,1 the band gap increment ΔEg of semiconductor nanowires is inversely proportional to the square of nanowire diameter (d2). Therefore, the size dependence of ΔEg can be fitted by ΔEg = A/d2, where A is a scale factor that corresponds to the band gap difference for a 1 nm nanowire relative to its bulk value. The fitting parameter A is 1.90 and 1.75 for the 3C and 2H nanowires, respectively. The result implies that the ZB-type nanowires have a relatively larger band gap increment than the 2H-type nanowires with the change of nanowire diameter. We find that the size dependence of ΔEg in 4H and 6H nanowires also slightly deviate from the fitting ΔEg curves of 3C and 2H nanowires (see Figure 4a), which mainly originates from the crystal-phase effect. On the basis of the above analysis, the band gap of an arbitrary sized GaP nanowire can be expressed by
On the basis of above analysis, the stability mechanism of GaP nanowires can be ascribed to the competition between the crystal phase and facet effects. Such a result can be applied to explain the formation of crystal defects (e.g., stacking faults and twin planes) in III−V semiconductor nanowires. For example, the formation of twin defects in III−V nanowires will introduce the lowest-energy {1̅1̅1}A and {111̅} B facets.19−22 As mentioned above, the energy of a nanowire (ET) consists of the cohesive energy of bulk atoms (EC) and surface energy (ES). The EC of a nanowire with twin planes is close to that of its ZB phase, but its surface energy is lower than that of the 3C nanowire. Taking ∼10 nm nanowires with the same axis length as an example, the calculated energy of a GaP nanowire with a twin structure is lower than that of 3C and 2H nanowires by ∼26 and ∼29 meV/Ga−P pair, respectively. Hence, massive stacking faults and twin planes in III−V nanowires observed in experiments refer to not only the kinetic factor but also the thermodynamic one. We now turn to electronic properties of four GaP nanowire polytypes. Figure 3 shows electronic band structures of 3C, 2H,
Figure 3. Band structures of GaP nanowires with different crystal phases. (a−d) ZB phase (3C) nanowires with the diameters of 1.35, 1.80, 2.26, and 2.71 nm. (e−h) WZ phase (2H) nanowires with diameters of 1.17, 1.95, 2.74, and 3.52 nm. (i−j) 4H nanowires with the diameters of 1.23 and 1.61 nm. (k−l) 6H nanowires with the diameters of 1.32 and 1.69 nm. The position of Fermi level is set to energy zero.
Eg (d) = Eg ‐ bulk + A /d 2
(2)
where Eg‑bulk is the band gap of GaP bulk. Owing to the wellknown fact that the DFT/GGA method underestimates the band gap of semiconductors by about 30−40%, we employed the HSE method to calculate the band gap of GaP bulk. Using the HSE method, the calculated band gaps of GaP bulk in 3C and 2H phases are 2.28 and 2.21 eV respectively, which is in good agreement with the experimental band gap values (2.24 eV in 3C phase8,41 and 2.09 eV in 2H phase8,15). According to this equation, the band gap of four types of GaP nanowire polytypes as a function of diameter is plotted in Figure 4b. The details for the theoretical prediction of band gap of GaP
4H, and 6H GaP nanowires with different diameters. It can be seen that the 3C-type nanowires indicate an indirect band gap characteristic with the valence band maximum (VBM) at the Γ point and the conduction band minimum (CBM) at the Γ-Z valley (see Figure 3a−d), which follows the indirect band gap nature of zinc-blende GaP bulk. With the increase of diameter, the band gap of the nanowires decreases due to the well-known quantum confinement effect. The 2H-type nanowires exhibit distinct features at band-edge electronic states with increasing diameter, as shown in Figure 3e−h. It is clearly seen that at d = 1.42 and 2.12 nm (Figure 3e,f), the band structures of the 12033
DOI: 10.1021/acs.jpcc.5b02738 J. Phys. Chem. C 2015, 119, 12030−12036
Article
The Journal of Physical Chemistry C
Figure 4. (a) Variation of band gap ΔEg and (b) predicted band gap Eg of GaP nanowire polytypes as a function of nanowire diameter d. The data points of 3C, 2H, 4H, and 6H phases are indicated by squares, circles, triangles, and stars, respectively. The solid lines represent the fitting curves. The experimental band gap of GaP nanowires is extracted from refs 5, 7, and 8.
Figure 5. Top and side views of charge-density isosurface distributions of conduction band minimum (CBM) and valence band maximum (VBM) of ∼1.8 nm GaP nanowires with (a) 3C, (b) 2H, (c) 4H, and (d) 6H phases.
theoretically predicted band gaps of both 3C and 2H nanowires match experimental data5,7,8 very well. To further understand the crystal phase effect on electronic structures of GaP nanowires, charge-density isosurface (CDI) distributions of valence band maximum (VBM) and conduction band minimum (CBM) in four nanowire polytypes have been investigated, as shown in Figure 5. The CDI distributions of GaP nanowires exhibit the following three features: (i) The VBM and CBM states of all four nanowire polytypes localize at the core region of nanowires, implying that the band gap of GaP nanowires is not affected by the surface effect. Meanwhile, it is the reason why the size dependence of band gap in all four nanowire polytypes indicates similar trend, while the crystal phase effect only leads to a very small band gap difference (∼0.1 eV) among the nanowire polytypes. Although some previous studies have reported the importance of surface effect in electronic structures of III−V semiconductor nanowires,24−28 most of their results were obtained on the basis of bare or partly passivated nanowires. In the present study, all
nanowires are indicated in S2 of Supporting Information. The size effect leads to the band gap reduction of GaP nanowires from ∼4.0 to ∼2.2 eV with an increase of diameter from 1 to 100 nm. In contrast, the crystal phase does not significantly change the band gap values of GaP nanowires. For example, the band gap of 3C nanowires is 0.3−0.1 eV larger than that of 2H nanowires in the diameter range of 1−100 nm. Moreover, their band gap differences basically maintain a constant (∼0.1 eV) when the nanowire diameter is beyond 3 nm. As shown in Figure 4b, the variation of band gap with the diameter in polytypic 4H and 6H nanowires nearly follows the trend of band gap in 3C nanowires. Hence, the band gap magnitude of GaP nanowires is mainly determined by the size effect (or quantum confinement effect), and the crystal phase has a relatively smaller effect on the band gap of the nanowires. In order to examine the rationality of theoretically predicted band gap of GaP nanowires, we make a comparison between our calculated results and experimental values that are extracted from previous literatures. As shown in Figure 4b, the 12034
DOI: 10.1021/acs.jpcc.5b02738 J. Phys. Chem. C 2015, 119, 12030−12036
Article
The Journal of Physical Chemistry C Present Address
considered nanowires are passivated by hydrogens, and surface states of nanowires are eliminated by the hydrogen passivation. Therefore, the electronic structures of GaP nanowires are not sensitive to the surface effect. (ii) The VBM and CBM of the nanowire polytypes mainly arise from the contribution of Ga-4s and P-3s states and that of the P-3p states, respectively. The result is very similar to the cases of InP and GaAs nanowires.27,28 (iii) The VBM state in all four nanowire polytypes spatially spreads along their axis direction, but the CBM state in these nanowires indicates a different distribution. The CBM state displays a sawtooth-like distribution along {111}A facets in the 3C structure (Figure 5a) and an armchair-like distribution along the [0001] direction in the 2H structure (Figure 5b). For polytypic 4H and 6H structures, the CBM states show nonuniform distribution along the nanowire axial direction (see Figure 5c,d). We find most of electronic states localized in the WZ segments and part of them in the ZB segments. The result suggests that the electron transport of 4H and 6H nanowires along the axial direction will be affected by their polytypic structures. Nevertheless, the polytypic 4H and 6H nanowires are not enough to create a quantum well to confine electrons due to the very small axis size of WZ segments. Previous studies29,31 have demonstrated the type-II band alignment of ZB-WZ polytypism in some typical III−V nanowires. Hence, increasing the axis length of both WZ and ZB segments in polytypic GaP nanowires may lead to the confinement of electrons and holes in different crystal-phase segments due to the band-offset formation.
(H.S.) College of Optical and Electronic Technology, China Jiliang University, 310018 Hangzhou, China. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported in part by the National Natural Science Foundation of China (Grant Nos. 11404309, 11347109, 51402275), Zhejiang Provincial Natural Science Foundation of China (Grant Nos. Y1110777 and LQ13A040001). Computational resources from the Shanghai Supercomputer Center are acknowledged.
■
IV. CONCLUSIONS In summary, we have performed detailed first-principles DFT calculations to reveal the structural and electronic properties of GaP nanowires with different sizes and crystal phases. The structural stability of GaP nanowires depends on the competition between the crystal phase and facet effects. The mechanism indicates that large-size GaP nanowires prefer the formation of the ZB structure with the stacking faults and twin planes, and the WZ structure is energetically favorable in the ultrathin nanowires, which is in good agreement with the experimental observations for VLS-grown III−V nanowires. We also found that electronic properties of GaP nanowires including the band-structure characteristic and band gap values are affected by not only the nanowire size but also the crystal phase. Moreover, we have provided a quantitative description for the relationship between the band gap of GaP nanowire polytypes and the nanowire size, and the predicted band gaps match the available experimental data very well. Our results suggest the possibility to engineer electronic properties of GaP nanowires by controlling the crystal phase and facets during the growth.
■
ASSOCIATED CONTENT
S Supporting Information *
Planar atomic configurations of GaP nanowires, computational details of surface energies of GaP films, and the details for calculating the band gap of GaP nanowires. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b02738.
■
REFERENCES
(1) Yu, H.; Li, J. B.; Loomis, R. A.; Wang, L.-W.; Buhro, W. E. Twoversus Three-Dimensional Quantum Confinement in Indium Phosphide Wires and Dots. Nat. Mater. 2003, 2, 517−520. (2) Wernersson, L.-E; Thelander, C.; Lind, E.; Samuelson, L. III-V NanowiresExtending a Narrowing Road. Proc. IEEE 2010, 98, 2047−2060. (3) Johansson, J.; Dick, K. A. Recent Advances in Semiconductor Nanowire Heterostructures. CrystEngComm 2011, 13, 7175−7184. (4) Yeh, C. Y.; Wei, S. H.; Zunger, A. Relationships between the Band Gaps of the Zinc-blende and Wurtzite Modifications of Semiconductors. Phys. Rev. B 1994, 50, 2715−2718. (5) Panda, J. K.; Roy, A.; Gemmi, M.; Husanu, E.; Li, A.; Ercolani, D.; Sorba, L. Electronic Band Structure of Wurtzite GaP Nanowires via Temperature Dependent Resonance Raman Spectroscopy. Appl. Phys. Lett. 2013, 103, 023108. (6) Dean, P. J.; Kaminsky, G.; Zetterstrom, R. B. Intrinsic Optical Absorption of Gallium Phosphide between 2.33 and 3.12 eV. J. Appl. Phys. 1967, 38, 3551−3556. (7) Zhang, Z.; Senz, S.; Zhao, F.; Chen, L.; Gao, X.; Liu, J. M. Phase Transition Induced Vertical Alignment of Ultrathin Gallium Phosphide Nanowire Arrays on Silicon by Chemical Beam Epitaxy. RSC Adv. 2012, 2, 8631−8636. (8) Assali, S.; Zardo, I.; Plissard, S.; Kriegner, D.; Verheijen, M. A.; Bauer, G.; Meijerink, A.; Belabbes, A.; Bechstedt, F.; Haverkort, J. E. M.; et al. Direct Band Gap Wurtzite Gallium Phosphide Nanowires. Nano Lett. 2013, 13, 1559−1563. (9) Holm, J. V.; Jørgensen, H. I.; Krogstrup, P.; Nygård, J.; Liu, H. Y.; Aagesen, M. Surface-Passivated GaAsP Single-Nanowire Solar Cells Exceeding 10% Effciency Grown on Silicon. Nat. Commun. 2013, 4, 1498. (10) Sun, J. W.; Liu, C.; Yang, P. D. Surfactant-Free, Large-Scale, Solution-Liquid-Solid Growth of Gallium Phosphide Nanowires and Their Use for Visible-Light-Driven Hydrogen Production from Water Reduction. J. Am. Chem. Soc. 2011, 133, 19306−19309. (11) Liu, C.; Sun, J. W.; Tang, J. Y.; Yang, P. D. Zn-Doped p-Type Gallium Phosphide Nanowire Photocathodes from a Surfactant-Free Solution Synthesis. Nano Lett. 2012, 12, 5407−5411. (12) Wagner, R. S.; Ellis, W. C. Vapor-Liquid-Solid Mechanism of Single Crystal Growth. Appl. Phys. Lett. 1964, 4, 89−90. (13) Husanu, E.; Ercolani, D.; Gemmi, M.; Sorba, L. Growth of Defect-Free GaP Nanowires. Nanotechnology 2014, 25, 205601. (14) Gu, Z. J.; Paranthaman, M. P.; Pan, Z. W. Vapor-Phase Synthesis of Gallium Phosphide Nanowires. Cryst. Growth Des. 2009, 9, 525−527. (15) Kriegner, D.; Assali, S.; Belabbes, A.; Etzelstorfer, T.; Holý, V.; Schülli, T.; Bechstedt, F.; Bakkers, E. P. A. M.; Bauer, G.; Stangl, J. Unit Cell Structure of the Wurtzite Phase of GaP Nanowires: X-Ray Diffraction Studies and Density Functional Theory Calculations. Phys. Rev. B 2013, 88, 115315. (16) Soci, C.; Bao, X.-Y.; Aplin, D. P. R.; Wang, D. L. A Systematic Study on the Growth of GaAs Nanowires by Metal-Organic Chemical Vapor Deposition. Nano Lett. 2008, 8, 4275−4282.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 12035
DOI: 10.1021/acs.jpcc.5b02738 J. Phys. Chem. C 2015, 119, 12030−12036
Article
The Journal of Physical Chemistry C (17) Paiman, S.; Gao, Q.; Tan, H. H.; Jagadish, C.; Pemasiri, K.; Montazeri, M.; Jackson, H. E.; Smith, L. M.; Yarrison-Rice, J. M.; Zhang, X.; et al. The Effect of V/III Ratio and Catalyst Particle Size on the Crystal Structure and Optical Properties of InP Nanowires. Nanotechnology 2009, 20, 225606. (18) 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. (19) Caroff, P.; Dick, K. A.; Johansson, J.; Messing, M. E.; Deppert, K.; Samuelson, L. Controlled Polytypic and Twin-plane Superlattices in III−V nanowires. Nat. Nanotechnol. 2009, 4, 50−55. (20) Bao, J.; Bell, D. C.; Capasso, F.; Wagner, J. B.; Mårtensson, T.; Trägårdh, J.; Samuelson, L. Optical Properties of Rotationally Twinned InP Nanowire Heterostructures. Nano Lett. 2008, 8, 836− 841. (21) Han, N.; Hou, J. J.; Wang, F.; Yip, S. P.; Yen, Y.-T.; Yang, Z.-X.; Dong, G.; Huang, T. F.; Chueh, Y.-L.; Ho, J. C. GaAs Nanowires: From Manipulation of Defect Formation to Controllable Electronic Transport Properties. ACS Nano 2013, 7, 9138−9146. (22) Johansson, J.; Karlsson, L. S.; Svensson, C. P. T.; Mårtensson, T.; Wacaser, B. A.; Deppert, K.; Samuelson, L.; Seifert, W. Structural Properties of B-Oriented III-V Nanowires. Nat. Mater. 2006, 5, 574−580. (23) Akiyama, T.; Nakamura, K.; Ito, T. Structural Stability and Electronic Properties of InP Nanowires: Role of Surface Dangling Bonds on Nanowire Facets. Phys. Rev. B 2006, 73, 235308. (24) Rosini, M.; Magri, R. Surface Effects on the Atomic and Electronic Structure of Unpassivated GaAs Nanowires. ACS Nano 2010, 4, 6021−6031. (25) Shu, H. B.; Liang, P.; Wang, L.; Chen, X. S.; Lu, W. Tailoring Electronic Properties of InAs Nanowires by Surface Functionalization. J. Appl. Phys. 2011, 110, 103713. (26) Pan, H.; Feng, Y. P. Semiconductor Nanowires and Nanotubes: Effects of Size and Surface-to-Volume Ratio. ACS Nano 2008, 2, 2410−2414. (27) Jin, M. T.; Shu, H. B.; Liang, P.; Cao, D.; Chen, X. S.; Lu, W. Role of Chemical Potential in Tuning Equilibrium Crystal Shape and Electronic Properties of Wurtzite GaAs Nanowires. J. Phys. Chem. C 2013, 117, 23349−23356. (28) Yang, X.; Shu, H.; Jin, M.; Liang, P.; Cao, D.; Li, C.; Chen, X. Crystal Facet Effect on Structural Stability and Electronic Properties of Wurtzite InP Nanowires. J. Appl. Phys. 2014, 115, 21430. (29) Akiyama, T.; Yamashita, T.; Nakamura, K.; Ito, T. Band Alignment Tuning in Twin-Plane Superlattices of Semiconductor Nanowires. Nano Lett. 2010, 10, 4614−4618. (30) Li, D. F.; Wang, Z. G.; Gao, F. First-Principles Study of the Electronic Properties of Wurtzite, Zinc-Blende, and Twinned InP Nanowires. Nanotechnology 2010, 21, 505709. (31) Spirkoska, D.; Efros, Al. L.; Lambrecht, W. R. L.; Cheiwchanchamnangij, T.; Fontchberta i Morral, A.; Abstreiter, G. Valence Band Structure of Polytypic Zinc-Blende/Wurtzite GaAs Nanowires Probed by Polarization-Dependenent Photoluminescence. Phys. Rev. B 2012, 85, 045309. (32) Shiraishi, K. A New Slab Model Approach for Electronic Structure Calculation of Polar Semiconductor Surface. J. Phys. Soc. Jpn. 1990, 59, 3455−3458. (33) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953−17979. (34) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169−11186. (35) Payne, M. C.; Teter, M. P.; Allan, D. C.; Arias, T. A.; Joannopoulos, J. D. Iterative Minimization Techniques for Ab Initio Total-Energy Calculations: Molecular Dynamics and Conjugate Gradients. Rev. Mod. Phys. 1992, 64, 1045−1097. (36) Kresse, G.; Joubert, D. From Ultrasoft Pseudopentials to the Projector Augmented-Wave Methods. Phys. Rev. B 1999, 59, 1758− 1775.
(37) Paier, J.; Marsman, M.; Hummer, K.; Kresse, G.; Gerber, I. C.; Á ngyán, J. G. Screened Hybrid Density Functional Applied to Solids. J. Chem. Phys. 2006, 124, 154709. (38) Yamashita, T.; Akiyama, T.; Nakamura, K.; Ito, T. Effects of Facet Orientation on Relative Stability between Zinc Blende and Wurtzite Structures in Group III-V Nanowires. Jpn. J. Appl. Phys. 2010, 49, 055003. (39) Copple, A.; Ralston, N.; Peng, X. H. Engineering Direct-Indirect Band Gap Transition in Wurtzite GaAs Nanowires Through Size and Uniaxial Strain. Appl. Phys. Lett. 2012, 100, 193108. (40) Persson, M. P.; Xu, H. Q. Electronic Structure of NanometerScale GaAs Whiskers. Appl. Phys. Lett. 2002, 81, 1309−1311. (41) Belabbes, A.; Panse, C.; Furthmüller, J.; Bechstedt, F. Electronic Bands of III-V Semiconductor Polytypes and their Alignment. Phys. Rev. B 2012, 86, 075208.
12036
DOI: 10.1021/acs.jpcc.5b02738 J. Phys. Chem. C 2015, 119, 12030−12036
