Letter pubs.acs.org/NanoLett
Solid−Solution Semiconductor Nanowires in Pseudobinary Systems Baodan Liu,*,†,‡,§ Yoshio Bando,† Lizhao Liu,§ Jijun Zhao,§ Mitome Masanori,† Xin Jiang,‡ and Dmitri Golberg† †
World Premier International (WPI) Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Namiki 1−1, Tsukuba, Ibaraki, 305−0044 Japan ‡ Shenyang National Laboratory for Materials Science, Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS), No. 72 Wenhua Road, Shenyang 110016 China § Key Laboratory of Materials Modification by Laser, Ion and Electron Beams (Dalian University of Technology), Ministry of Education, Dalian 116024, China S Supporting Information *
ABSTRACT: Pseudobinary solid−solution semiconductor nanowires made of (GaP)1−x(ZnS)x, (ZnS)1−x(GaP)x and (GaN)1−x(ZnO)x were synthesized based on an elaborative compositional, structural, and synthetic designs. Using analytical high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDS), we confirmed that the structure uniformity and a lattice match between the two constituting binary components play the key roles in the formation of quaternary solid−solution nanostructures. Electrical transport measurements on individual GaP and (GaP)1−x(ZnS)x nanowires indicated that a slight invasion of ZnS in the GaP host could lead to the abrupt resistance increase, resulting in the semiconductor-to-insulator transition. The method proposed here may be extended to the rational synthesis of many other multicomponent nanosystems with tunable and intriguing optoelectronic properties for specific applications. KEYWORDS: Pseudobinary system, solid−solution nanowires, synthesis, lattice matching, structure homology
Q
The challenging synthesis of multicomponent solid−solution nanowires is inspired by the formation of a variety of onedimensional (1D) heterostructures3,4,8,9 in which the two major constituting domains stick together on a shared crystalline plane along the long structure axis, or its radius, or forming a core−shell ensemble due to lattice matching (Figure 1a−c). The two components comprising a heterostructure can be either of the same structure (similar structural symmetry, space group, close lattice constants etc.) or of completely different ones but having a common crystalline plane with close lattice distances. Additionally, the two components creating a heterostructure can be either elemental or made of binary compounds.10 If the two components are confined to the same symmetry, space group, and possess decently close lattice constants, a solid−solution structure can in principle be obtained through adjusting the specific growth conditions (schematically illustrated in Figure 1d).1,2 In fact, such structures were experimentally demonstrated in a variety of binary and ternary alloying nanostructures in the past years11−14 but such structures still remain challenging in the quaternary material systems.15
uaternary semiconductor nanowires in pseudobinary systems comprised of two compound semiconductors may possess exciting optoelectronic properties and some unexpected phenomena valuable for technological applications in diverse fields.1,2 Fabrication of such complex alloyed nanocompounds generally requires rigid composition range of the constituting components and a delicate growth control under critical conditions.1,2 Materials at the nanoscale have exhibited numerous advantages over their bulky counterparts and revealed rich potentials in prospective building blocks due to their large aspect ratios and size-dependent properties. Various nanostructures ranging from heterostructures to complex multicomponent nanoarchitectures have been produced using a number of methods.3−9 However, the synthesis of nanoscaled multicomponent solid−solution structures still remains a challenge, and their properties are still unknown. Here we show that some pseudobinary nanostructures ((GaP)1−x(ZnS)x, (ZnS)1−x(GaP)x, and (GaN)1−x(ZnO)x), so-called solid−solution nanowires, that are composed of groups II−VI or III−V binary compound semiconductors can be fabricated based on an elaborative compositional, structural, and synthetic designs. Different from previously reported heterostructures,3,4,8,9 all solid−solution nanowires exhibit a single phase with all four elements mixed mutually and completely in the investigated composition range. © 2012 American Chemical Society
Received: September 19, 2012 Revised: November 27, 2012 Published: December 3, 2012 85
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The absence of any peak splitting and impurity peaks indicates that the as-synthesized (GaP)1−x(ZnS)x nanowires are of high phase purity. This has been also confirmed by detailed structural analyses on numerous nanowires using highresolution transmission electron microscopy (HRTEM). Figure 2b shows a typical low-magnification TEM image of an individual (GaP)1−x(ZnS)x nanowire with an Au catalyst particle at its tip-end. The nanowire diameter is preciously guided by the catalyst particle size, proving the characteristic VLS growth.16 The HRTEM images of the (GaP)1−x(ZnS)x nanowire (Figure 2c,d), taken along the [11̅4] and [112̅] zone axes, clearly demonstrate that it is a single crystal. No any structural defects such as stacking faults or microtwins were observed, and all the atoms are regularly arranged within the fcc-type lattice. In addition, perfect ordering and succinctness of the two electron diffraction (ED) patterns (Figure 2e,f) taken from the same nanowire (these correspond to the HRTEM images in Figure 2c,d) further confirm the perfect crystallinity of the material. In order to investigate the possibility of diffraction spot overlapping from the separated cubic GaP and ZnS phases, which have quite close lattice constants (5.45 Å for GaP (see JCPDS No 89-5238) and 5.41 Å for ZnS, (see JCPDS No 65-1691)), the ED spots are ultimately magnified in Figure 2e,f. Also, no spot splitting (typical for crystalline heterostructures4) due to the independent GaP and ZnS phases is seen (Figure 2e,f). In addition, no visible interfaces under atomic resolution TEM (these have commonly been observed in a variety of heterostructures3,4,8,9) were noticed in either axial or radial nanowire directions. All the experimental evidence suggests that the two starting binary components/phases are mutually and completely merged/mixed after alloying. The absence of any structural defect that may occur during invasions of S and Zn atoms into GaP host lattices (resulting in the corresponding strain−stress fields) is possibly because of the lower solubility (x < ∼0.07) of ZnS in a GaP matrix and a relatively small lattice mismatch (∼0.7%). Atomic resolution HRTEM and energy dispersive X-ray spectroscopy (EDS) techniques convincingly document the formation of (GaP)1−x(ZnS)x solid−solution nanowires from a viewpoint of crystallinity. EDS techniques paired with spectral imaging and line-scanning functions, which have been extensively used in determining the compositional interface of heterostructures,4 were then additionally utilized to get further insights into elemental spatial distribution in both axial and radial directions of the alloying nanowires. Figure 3a depicts the representative scanning transmission electron microscopy (STEM) image of a (GaP)1−x(ZnS)x nanowire with a diameter of ∼90 nm and an Au catalyst particle at its tip-end. The corresponding elemental maps of Ga, P, Zn, and S (displayed in Figure 3b−e) clearly reveal that each individual element has a uniform spatial distribution within the nanowire (Au map, Figure 3f, verifies the Au catalytic function). No compositional interface among four constituting elements is observed. Such homogeneity is in a good agreement with the HRTEM data (Figure 2c,d). In addition, the elemental axial and radial linescan profiles (Figure 3g,h) further demonstrate the regarded chemical uniformity. The color contrast in elemental maps and the normalized X-ray signal intensity in the line-scan profiles verify that the nanowires are mainly made of Ga, P (as the host lattice elements) and are slightly doped with Zn, S species. Especially, the lowest intensity of Au element in the (GaP)1−x(ZnS)x nanowire body along the axial profile together with the observable intensity differences between Au and S, and
Figure 1. (a) Symbols used for two binary components; (b) schematic of AB-type heterostructures with the interface perpendicular or parallel to the axial direction; (c) the model for a 1D core−shell structure; (d) schematic of a pseudobinary 1D nanostructure. Right side images show the corresponding cross-sectional views of the regarded structures.
On the basis of binary and ternary solid−solution nanostructures, the aim of this work is to explore a general routine that governs the formation of quaternary or even more complex alloying systems. Our approach is based on the crystallography considerations and the rational synthesis design. A vapor−liquid−solid (VLS)16 process using Au nanoparticles as catalysts was chosen for all syntheses. Typically, the required types of individual atoms (constituting the desired quaternary solid−solutions) were obtained from the specific precursors at the high-temperature zone of a reaction chamber; these were then accumulated by the Au catalyst droplets to generate multicomponent chemical mixtures through a self-assembling process. Such mixtures were further catalyzed to finally produce one-dimensional pseudobinary solid−solution nanowires during the subsequent growth. (GaP)1−x(ZnS)x nanowires (x < 0.07) were grown on an Aucoated Si substrate through the regarded process using highpurity GaP and ZnS powders as the precursors. The nanowires densely covered the Si substrate and possessed diameters ranging from tens of nanometers to several hundreds of nanometers and a length up to dozens of micrometers (Figure 2a,b). Comparative X-ray diffraction (XRD) patterns of starting
Figure 2. (a) Low-magnification SEM image of (GaP)1−x(ZnS)x nanowires; (b) representative TEM image of a (GaP)1−x(ZnS)x nanowire with an Au catalyst particle; (c,d) HRTEM images of the (GaP)1−x(ZnS)x nanowire shown in b; (e,f) corresponding ED patterns taken along the [11̅4] and [112̅] zone axes.
GaP and as-synthesized (GaP)1−x(ZnS)x nanowires demonstrated no peak splittings or impurity peak appearances for the quaternary (GaP)1−x(ZnS)x nanowires in spite of the expected invasion of the ZnS dopants into the GaP host lattice, and all the diffraction peaks could be nicely indexed as a zinc-blendetype face-centered cubic (fcc) structure with the F4̅3m space group (Supporting Information, Figure S1). 86
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Figure 3. (a) STEM image of a (GaP)1−x(ZnS)x nanowire with the Au catalyst particle attached to its tip; (b−f) P, S, Ga, Zn, and Au elemental maps, respectively; (g,h) elemental profiles of a (GaP)1−x(ZnS)x nanowire taken along the radial and axial directions. The scale bar is 100 nm.
nanostructures, rather than cubic ZnS phase.19 The problem could be partially solved through a design of a complementary reaction routine. The cubic (ZnS)1−x(GaP)x (x < ∼0.05) nanowires with ZnS as the host were obtained through the in situ doping technique during the formation of cubic ZnS nanowires on a cubic GaP substrate. However, the assynthesized quaternary solid−solution nanowires containing some structural defects such as microtwins and a minor fraction of impurity nanowires were also simultaneously formed in this case. By controlling the growth temperature approaching to the melting/sublimation point of GaP, surface P and Ga atoms can escape from the substrate surface and become incorporated into the ZnS lattice and then form (ZnS)1−x(GaP)x solid− solution nanowires, as presented in Supporting Information, Figure S3. The gained insight into the formation of cubic GaP−ZnS solid−solution nanowires also allowed us to realize such alloying structures in pseudobinary GaN−ZnO system, which crystallizes in wurtzite-type hexagonal structure and also has a small lattice mismatch (1.7% along the a-axis and 0.3% along the c-axis). The phase purity and uniformity of the spatial distribution of each element in quaternary (GaN)1−x(ZnO)x (x < ∼0.05) nanowires were also confirmed using the same methods as for (GaP)1−x(ZnS)x nanowires. Elemental mapping of individual elements and HRTEM images of randomly selected (GaN)1−x(ZnO)x nanowires (Figure 4) demonstrated the complete mixing of Ga, N, Zn, O and single-crystal characteristics in the investigated composition range. The clear and regularly arranged ED spots further confirmed the high p hase purit y and crystallinity of as-synt hesized (GaN)1−x(ZnO)x alloying nanowires. The successful formation of (GaN)1−x(ZnO)x nanowires (made of the III−V and II−VI
Zn elements serves as a direct evidence of the successful incorporation of Zn and S atoms into GaP lattices (Figure 3h). These results were randomly and carefully checked by taking numerous EDS spectra, elemental maps and line-scan profiles (not shown here) on dozens of wires. These all confirmed the uniform solidification of GaP and ZnS pseudobinary compounds. Unlike the bulky GaP−ZnS system,1,2 in which the ratio of the ZnS component in the GaP host could be selectively tuned in a wide range, (GaP)1−x(ZnS)x nanowires with high x values were found to be difficult to fabricate. An increase of the ZnS ratio (to ∼50 wt %) in initial reactants (with other growth parameters, e.g., temperature, gas flowing rate, etc., unchanged) leads to the formation of beadlike Ga−P−O nanoparticles periodically connected with thin nanowires (Supporting Information, Figure S2). EDS measurements confirmed that these nanowires have the same overall chemical compositions as the above-described (GaP)1−x(ZnS)x nanowires, but phase separation3,4 was found in the latter case. Both elemental mapping results and radial line-scan profiles reveal that the nanowires are composed of GaP (cubic phase) core and ZnS (cubic phase) shell (see Supporting Information, Figure S2). In addition, a large amount of structural defects, such as periodic twins (which were not observed in the homogeneously structured (GaP)1−x(ZnS)x solid−solution nanowires) were generated to release the structural strains between the two phases.17 This suggests that an optimal solubility of ZnS in a GaP matrix host exists for the sake of structural stability, as has been demonstrated under doping of a variety of materials.18 When the ZnS ratio in the starting reactants becomes predominant (>50 wt %), the product reveals hexagonal ZnS nanowires and nanobelts mixed with some impurity Ga−P−O 87
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Figure 5. Formation energy of the (GaP)1−x(ZnS)x nanowire at different doping ratios.
Figure 4. (a) STEM image of a (GaN)1−x(ZnO)x nanowire; (b−e) N, Ga, O, and Zn elemental maps, respectively; (f) HRTEM image of a (GaN)1−x(ZnO)x nanowire. Inset is the corresponding ED pattern. The scale bar is 100 nm.
be understood why a continuous composition variation in the GaP−ZnS or GaN−ZnO solid−solution nanowire systems is not possible. It is noted that even in many bulk pseudobinary systems, the consecutive composition tuning has also been a challenge so far.2 The lower solubility in nanoscaled quaternary systems is possibly related to the self-generated processes in a confined space. It should also depend on the lattice constants mismatch for each particular pseudobinary system. However, it is also envisaged that the composition ratio in the quaternary nanowires can be finely tuned under some critical conditions.1 (2) Chemical similarity. Similar to a doping process, solubility of a dopant in the host matrix is not only determined by the lattice mismatch but is also governed by the chemical similarity/difference.18 As demonstrated here for the three groups studied, the anions or cations for the two components are quite close and exhibit similar physical and chemical characteristics. Therefore, the newly formed covalent bonds in a quaternary structure approach the character of those in the starting binary components. Too large of a difference in the chemical properties makes the formation of quaternary structures energetically costly and thus unlikely. In addition, the more chemical elements are involved, the more complex and new chemical bonds form, which leads to the difficulties in formation of a pure phase compound and results in the appearance of phase separation. For example, compared with quaternary solid−solution nanostructures, the syntheses of binary and ternary alloying structures are more favorable and have been reported for a variety of compounds such as Cu−Fe,11 Bi−Sb,12 CdyZn1−yS,13 and ZnxCd1−xSe.14 (3) Proper growth conditions. As discussed in the previous21 and the present work, a wide range of experimental parameters including growth temperature, pressure, catalysts choice, substrate type, and gas flowing rates may strongly affect the nanostructure size, morphology, shape, growth direction, and the phase purity. Therefore, a specific nanostructure with versatile functions can be synthesized through a proper control of all these parameters during a selected process. For example, the formation of (GaP)1−x(ZnS)x quaternary nanowires can be achieved on an Au-coated Si substrate through a selfassembled process, whereas the (ZnS)1−x(GaP)x nanowires require the GaP substrate and a critical growth temperature for the realization of the in situ lattice penetration.
group binary components) provides an additional solid evidence for the universality of the solid−solution nanostructures that are made of quaternary multicomponents. Undoubtedly, the peculiar pseudobinary solid−solution nanostructures can also be expected in different groups and a variety of systems, such as individual element−element11,12 and compound−compound ones,15 provided that their crystal structures and lattice constants are the same or at least are very close. All three material groups analyzed by us have some characteristics in common, that is, single-crystallinity and lower solubility of the minor component in the host matrix. These similarities imply that existence of such quaternary solid−solution nanostructures is likely a general phenomenon in a multielemental nanosystem. Formation of such complex nanostructures in two or more binary component systems (whose consisting elements belong to different groups) should basically meet the following requirements, as we can state now: (1) Structure compatibility. From the point of crystallography, a stable structure requires fewer internal strains and higher long-range atomic ordering. If the two components in a quaternary system tend to form a single crystal, it requires similarity of their structures and the same space group. Especially, the lattice constants of the two units should be quite close to each other and their mismatch should be below some threshold value. Thus the partial substitution of one component to another, that is, the substitution of anions/cations in the host by the corresponding doped minor components at the predefined sites becomes possible, as exampled for a fcc-type GaP−ZnS system (Supporting Information, Figure S4). It should be pointed out that such substitution cannot surpass a sort of solubility limit for the sake of structural stability and avoidance of the phase separation.20 In order to further elaborate the phase evolution of solid−solution nanostructures in the GaP− ZnS system, the quaternary system formation energies were calculated using the first-principles methods to search the critical ratio which initiates the phase separation. It has been found that the critical doping ratios for the formation of uniform solid−solution nanostructures are about 4.5 at.% for GaP as a host and 3.2 at.% for ZnS as a host, as depicted in Figure 5. While the critical ratios are exceeded these values, phase separation should occur in the GaP−ZnS system from a viewpoint of the formation energy. Therefore, it can now 88
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were synthesized through a chemical vapor deposition (CVD) process using Au nanoparticles as catalysts, as reported in our previous work.26 Typically, a thin layer of Au (5−10 nm) was deposited on the corresponding substrates (Si for (GaP)1−x(ZnS)x and (GaN)1−x(ZnO)x nanowires and GaP for (ZnS)1−x(GaP)x) and then the substrates were placed downstream in a reactor furnace. Mixed precursor powders of GaP/ZnS or (Ga2O3/Zn/ZnO) with varied ratios were put in the center of the furnace. All the growths were carried out at 1150 °C and ambient pressure, and high purity Ar (200−400 mL/min) and NH3 (200 mL/min) gases were used for the formation of GaP−ZnS and GaN−ZnO nanostructures, respectively. After growing for 30 min, the nanowires deposited on substrates were collected for the subsequent characterizations and analyses. Structural Characterization and Composition Analysis. The as-grown nanowires were examined by means of an X-ray powder diffractometer (XRD, Rigaku RINT 2000) operating at 40 kV and 40 mA by using Cu Kα radiation (λ = 1.54056 Å), a scanning electron microscope [(SEM) JEOL, JSM-6700F], and a high-resolution field-emission transmission electron microscope [(TEM) JEOL, JEM-3000F] equipped with an X-ray energy dispersive spectrometer (EDS). The elemental maps and line-scan profiles were recorded in a JEM-3100FEF fieldemission transmission electron microscope (Omega Filter) in the STEM mode. Electrical Transport Measurements. The nanowires dispersed in ethanol solution were first transported to a 15 × 15 SiO2/Si substrate and then a designed electrode patterns composed of 10 nm Ti and 100 nm Au layers were stamped on the previously deposited nanowires using a mask template, followed by a photolithography process, as reported in our previous work.20 The I−V curves were recorded using a Keithley 4200-SC S apparatus under a bias voltage of 1 V. Structural Models and First-Principles Calculations. Firstprinciples calculations were performed by employing the Vienna ab initio simulation package (VASP) with PW91 function for the exchange-correlation interaction and ultrasoft pseudopotential for ion-electron interaction. Cutoff energy for the planewave basis was chosen as 360 eV. For the cubic zincblende lattice, the optimized lattice constant was 5.49 Å for GaP and 5.47 Å for ZnS. The GaP−ZnS random solid− solution was modeled by a 2 × 2 × 2 or 3 × 3 × 2 cell of zinc blende lattice with P (Ga) atoms randomly substituted by S (Zn) atoms. To represent the case of phase separation, GaP/ ZnS superlattice models with 128 or 288 atoms were constructed from GaP and ZnS slabs of different thicknesses to meet the doping ratio from 1.39 to 50%.
In conclusion, our results provide a solid ground for the controlled synthesis of pseudobinary/quaternary solid−solution nanostructures based on the principle of lattice matching and structure uniformity. Different from previous heterostructures,3,4,8,9 the presently synthesized quaternary nanowires represent a valuable member of the nanomaterials family. We envisage that their synthesis represents an important breakthrough in the controllable growth of a variety of multicomponent functional nanomaterials. The mechanism proposed here is likely to be universal and thus can be extended to the practical formation of diverse complex nanoscaled pseudobinary systems. Because of notable peculiarities in structure and composition, such quaternary nanostructures may exhibit some unexpected and interesting properties needed for the fabrication of specific nanooptoelectronic blocks or devices. For example, the present (GaP)1−x(ZnS)x alloy nanowires start to exhibit high resistivity characteristics under only a minor invasion of ZnS into GaP lattice matrix, whereas the intrinsic GaP nanowires are semiconducting (Figure 6). In
Figure 6. Electrical transport measurements of (a) a GaP nanowire and (b) a (GaP)1−x(ZnS)x nanowire.
addition, the nanoscaled solid−solution nanostructures may possess predominant advantages over their bulky counterparts. For example, bulk (GaN)1−x(ZnO)x solids are important photocatalytic materials that promote effective water splitting into O2 and H2 under irradiation with a visible light22,23 for a clean and recyclable energy production. By contrast, standard binary GaN24 or ZnO25 can only do so under UV irradiation. Nanoscaled (GaN)1−x(ZnO)x nanowires are suggested to be even more profitable in water-splitting due to their larger aspect ratios and perfect crystallization. Therefore, it is believed that the method proposed in this work may open new opportunities for rational synthesis of many alloying nanostructures with intriguing properties and these nanomaterials may find interesting applications in a variety of fields such as optoelectronic sensors, single wire transistors, and visiblelight-driven photocatalysts. Methods. Nanowire Synthesis. All the quaternary nanowires ((GaP)1−x(ZnS)x, (ZnS)1−x(GaP)x, (GaN)1−x(ZnO)x)
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ASSOCIATED CONTENT
S Supporting Information *
XRD pattern, TEM images, elemental mappings and line-scan profiles of (GaP)1−x(ZnS)x nanowires, (ZnS)1−x(GaP)x nanowires, and GaP@ZnS core−shell nanostructures and schematic of the (GaP)1−x(ZnS)x solid−solution nanowire formation. 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]. 89
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by the World Premier International Center for Materials Nanoarchitectonics (MANA) tenable at the National Institute for Materials Science (NIMS), Tsukuba, Japan. B. D. Liu thanks the Fundamental Research Fund for the Central Universities of China (grant No. DUT11NY07) and the Knowledge Innovation Program of Institute of Metal Research (grant No. Y2NCA111A1) for the support of this work. J. J. Zhao and L. Z. Liu thank the National Natural Science Foundation of China (11134005). All authors contributed equally to this work.
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