NANO LETTERS
Simultaneous Growth of Pure Hyperbranched Zn3As2 Structures and Long Ga2O3 Nanowires
2009 Vol. 9, No. 5 1764-1769
Jianye Li,* Lung-Shen Wang, D. Bruce Buchholz, and Robert P. H. Chang* Department of Materials Science & Engineering and Materials Research Center, Northwestern UniVersity, EVanston, Illinois 60208 Received November 20, 2008; Revised Manuscript Received March 8, 2009
ABSTRACT Through a facile and highly repeatable chemical vapor method, pure three-dimensional hyperbranched Zn3As2 structures and ultralong Ga2O3 nanowires were simultaneously grown with controllable locations in the same experiment. The hyperbranched Zn3As2 consists of coneshaped submicro-/nanowires and has a single-crystalline tetragonal structure. This is the first report of nano Zn3As2 and hyperbranched Zn3As2 structures. The as-grown Ga2O3 nanowires are monoclinic single crystals. A vapor-solid-solid mechanism is suggested for the growth of the Ga2O3 nanowires, and a vapor-solid mechanism, for the Zn3As2 structures.
Semiconductor nanostructures are promising low-dimensional materials with a wide range of applications, including memory, sensing, logic, light emission, and solar-cell technology.1 In the past decade, extensive attention has been focused on the study of II-VI and III-V semiconductor nanomaterials for understanding fundamental physical properties and developing novel nanotechnology applications. Up to now, a number of II-VI and III-V nanostructures, such as ZnO, ZnS, ZnSe, CdS, CdSe, GaN, AlN, InN, GaP, GaAs, and InP, have been reported.2 II3-V2 group semiconductors are of scientific and technological importance for their narrow band gaps. They have large excitonic radii and are expected to exhibit pronounced size quantization effects. II3-V2 semiconductors have complicated crystal structures and large unit cells. The electrons in a semiconducting II3-V2 compound will be confined in crystals that are much larger than if they were in analogous II-VI or III-V semiconductors. II3-V2 semiconductors have potential for application in infrared detectors, lasers, solarcell technology, ultrasonic multipliers, and Hall generators.3,4 However, compared with the significant progress in semiconducting II-VI and III-V nanomaterials, research on II3-V2 group semiconductors has lagged far behind owing to the lack of general synthetic methods. 3 To date, there are only a few reports on the growth of II3-V2 group semiconductor nanostructures, such as Zn3N2,5 Zn3P2,3,4,6,7 Cd3P2,3,7 and Cd3As2.8 Tetragonal structure Zn3As2 is an important II3-V2 semiconductor with a bandgap of approximately 1 eV at * Corresponding authors. E-mails:
[email protected]; jianyeli@ hotmail.com. 10.1021/nl8035228 CCC: $40.75 Published on Web 03/30/2009
2009 American Chemical Society
room temperature.9-11 It is currently attracting strong interest due to its close structural similarities with InP and InGaAs. With high electronic mobility, low electron effective mass, and small direct band gap, it is a promising candidate in long wavelength optoelectronic devices, solar cells, and spintronics when doped with Mn.10,11 Furthermore, the energy band gap of Zn3As2 can be easily tailored from 1 to 0 eV by alloying it with Cd, making it a good candidate for application in wavelength-tunable infrared detection and photovoltaics.11-13 Hence, the study of the growth of nano Zn3As2 structures is important to future applications of this II3-V2 compound in nanophotonics and nanoelectronics. The study of Zn3As2 nanostructures has not been reported thus far. Branched (or dendritic) structures offer an approach for increasing structural complexity and enabling greater function for nanoelectronic and nanophotonic systems.14 In this report, we demonstrate that through a facile chemical vapor deposition (CVD) procedure, pure three-dimensional hyperdendritic Zn3As2 structures and ultralong β-Ga2O3 nanowires can be grown simultaneously but at different locations in the CVD furnace. The hyperbranched Zn3As2 consists of cone-shaped submicro-/nanoscale branches and has a single-crystalline tetragonal structure. To the best of our knowledge, this is the first report of nano Zn3As2 and branched Zn3As2 structures. By this facile technique, the growth of the hyperdendritic Zn3As2 structures and long Ga2O3 nanowires is reliable and highly repeatable. The successful growth of three-dimensional hyperdendritic Zn3As2 structures will enrich the family of II3-V2 group nano-/microstructures available for potential technological applications.
The simultaneous growth of the Zn3As2 structures and the Ga2O3 nanowires was carried out by a system consisting of a horizontal, fused-quartz tube inside a tube furnace, as previously reported.15,16 First, a precursor mixture of ZnO (99.99%, Alfa Aesar) and graphite-carbon powders (99.9995%, Alfa Aesar) with a weight ratio of 2:1 was loaded into a boat. The boat was placed inside the horizontal, fused-quartz tube with the mixed material located at the furnace center, the highest temperature zone of the tube furnace. Two substrates were placed into the horizontal tube downstream from the precursor. One of the substrates was placed ∼10 cm downstream from the center and will henceforth be referred to as substrate #1; the other substrate was placed ∼16 cm downstream from the center and will henceforth be referred to as substrate #2. Substrate #1 was a catalyst patterned thermal silicon oxide/ silicon (or silicon) substrate; the catalyst was nickel oxide and patterned by physically writing using an iron rod coated with nickel (II) nitrate solution.15 Substrate #2 was a blank thermal silicon oxide/silicon (or silicon) substrate. A GaAs flake was placed between substrates #1 and #2 (see Figure S1 in the Supporting Information). The furnace was then heated under a steady flow of argon (99.99%, National Specialty Gases) of about 40 standard cubic centimeters per minute (sccm). When 980 °C was reached, the temperature was kept constant for ∼6-10 h. Finally, the furnace was switched off and allowed to cool to room temperature quickly. The Ga2O3 nanowires were densely grown only on substrate #1, and Zn3As2 structures were large-scale grown on substrate #2 as well as the inner tube surface around substrate #2. The as-grown products were characterized by field emission scanning electron microscopy (FESEM, FEI Quanta 600F), X-ray powder diffraction (XRD, Rigaku X-ray diffractometer with Cu KR radiation at room temperature), energy dispersive X-ray (EDX), and transmission electron microscopy (TEM, JEM-2100F FAST TEM). Figure 1a shows a low-magnification FESEM image of an as-grown, three-dimensional hyperdendritic Zn3As2 structure. Figure 1b is a high-magnification FESEM image taken from a region of a three-dimensional hyperbranched Zn3As2 structure. The FESEM image shown in Figure 1c is a hyperdendritic Zn3As2 structure. It clearly reveals that the as-grown structure is composed of cone-shaped submicro-/ nanowires. The diameter of the cone-shaped branches is from several tens to hundreds of nanometers, and the trunks (backbones), hundreds of nanometers. EDX analysis reveals the hyperdendritic structures contain only element zinc and arsenic with an atomic ratio of 3:2, consistent with stoichiometric Zn3As2. The EDX results indicate the hyperdendritic structures are composed of pure Zn3As2 (see Figure S2 in the Supporting Information). Figure 1d is a low-magnification FESEM image of the Ga2O3 nanowires grown on substrate no.1 (catalyst patterned thermal silicon oxide/silicon). From the image, it can be seen that the product consists almost exclusively of nanowires. The nanowires are very long, with the longest length of up Nano Lett., Vol. 9, No. 5, 2009
Figure 1. (a) Low-magnification FESEM image of a threedimensional hyperdendritic Zn3As2 structure; (b) high-magnification FESEM image from a region of a three-dimensional hyperdendritic Zn3As2 structure; (c) FESEM image of a hyperdendritic Zn3As2 structure; and (d) low-magnification FESEM image of the ultralong β-Ga2O3 nanowires. Inset: High-magnification FESEM image of the Ga2O3 nanowires with catalyst tips.
to 1 mm. The nanowires were grown with controllable position and only on the sites with catalyst. The inset of Figure 1d is a high-magnification FESEM image of the Ga2O3 nanowires. It reveals that the nanowires have diameters from 20 to 200 nm and nanoparticles attached at the tips. EDX analysis indicates the local atomic composition of the nanowires is only Ga and O with an atomic ratio of 2:3. The nanoparticles at the nanowire tips contain catalyst (NiO) (see Figure S3 in the Supporting Information). The phase compositions and structures of the products were examined by powder XRD. The top and lower patterns of Figure 2a are the experimental XRD spectrum of the hyperdendritic Zn3As2 structures grown on a thermal silicon oxide substrate and the standard XRD data of tetragonal Zn3As2 from 2000 Joint Committee on Powder Diffraction Standards (JCPDS) card no. 30-1472, respectively. A detailed 1765
mental diffraction patterns of Figure 2a, the intensity of 332 and 312 peaks are approximately equal. In Figure 2a, the intensity order of the five strongest peaks in the experimental spectrum match the order of them in the standard JCPDS pattern. The peak 413 overlaps with peak 332 in the experimental pattern. There are two reasons to account for the overlapping: (1) the difference of the crystal plane’s distances is small; (2) comparing with bulk crystalline materials, the nanomaterial’s diffraction peaks widened.
Figure 2. (a) Room temperature experimental XRD spectrum of the hyperdendritic Zn3As2 structures (top) and the standard XRD data from 2000 JCPDS 30-1472 (lower). (b) Room temperature experimental XRD pattern of the Ga2O3 nanowires (top), and the standard XRD data from 2000 JCPDS 41-1103 (lower).
Table 1. Detailed Comparison of the Experimental 2θ (Exp 2θ) from All Diffraction Peaks with the Standard Data of JCPDS Card no. 30-1472a 312
215
224
321
322
323
exp 2θ JCPDS 2θ
25.0 25.042
OL/NO 25.302
26.2 26.179
NO 27.538
28.3 28.323
29.5 29.589
exp 2θ JCPDS 2θ
400 30.3 30.329
402 31.2 31.287
332 33.1 33.130
413 OL 33.346
326 35.7 35.686
336 39.7 39.688
exp 2θ JCPDS 2θ
408 43.4 43.378
516 NO 45.489
536 NO 50.763
624 51.4 51.403
41 11 53.3 53.328
633 NO 53.425
exp 2θ JCPDS 2θ
634 NO 54.468
552 55.7 55.709
33 12 57.3 57.331
722 NO 57.452
723 58.2 58.190
724 59.2 59.152
a
NO ) not observed; OL ) overlapped.
comparison of the experimental 2θ from all diffraction peaks with the standard JCPDS data is shown in Table 1. All the observed experimental diffraction peaks are in good agreement with the standard data from JCPDS card no. 30-1472, and no extra peaks are observed. The XRD result indicates that the as-synthesized Zn3As2 structures are pure, singlephased tetragonal Zn3As2 (space group P42/nbc). If the intensity of the strongest XRD peak (408) is set to a relative intensity of 100, there are six peaks with a relative intensity of more than 5 in the standard XRD data of Figure 2a. From strong to weak, the six strongest peaks are 408, 224, 624, 332, 312, and 413, respectively. Compared with the standard diffraction peaks, the three strongest peaks in the experimental pattern of Figure 2a are also 408, 224, and 624 and with the same intensity order. In both standard and experi1766
As shown in Figure 2a, there are 18 weak peaks with a relative intensity of 5 or less in the standard pattern, and only 11 of them are observed in the experimental diffraction spectrum. From left to right, the 11 weak peaks are 322, 323, 400, 402, 326, 336, 4111, 552, 3312, 723 and 724, and they match well to the corresponding standard values of JCPDS 30-1472. Seven weak peaks in the standard XRD pattern are not observed in the experimental spectrum of Figure 2a. Among them, the relative intensity of 321, 516, 633, 634, and 722 in the standard XRD pattern is only 1, and 536 is 2. Due to the continuous scanning mode of the XRD measurement, these six very weak peaks were not resolved in the experimental spectrum. With a standard relative intensity of 2, the peak 215 overlapped with 312 or too weak to be observed in the experimental spectrum. Ga2O3 (gallia) has five structures: R-, β-, γ-, δ-, and ε-Ga2O3. Among the five structures, only β-Ga2O3 is stable; the other structures are metastable and transform into the β form at sufficiently high temperatures.17 β-Ga2O3 is a transparent, conducting oxide with an extremely wide band gap of 4.8 eV. It has very promising applications in optoelectronics and could be used in flat panel displays, excimer laser, and solar energy conversion devices.18 The top and lower patterns of Figure 2b are the experimental XRD spectrum of the Ga2O3 nanowires grown on the thermal silicon oxide substrate and the standard data of monoclinic structure Ga2O3 (or β-Ga2O3) from the 2000 JCPDS card (no. 41-1103), respectively. Table 2 is a detailed comparison of all the experimental diffraction peaks with the standard JCPDS data. It is obvious that all the observed experimental diffraction peaks match well to the reported standard values of β-Ga2O3 from JCPDS card no. 41-1103, and no extra peaks appeared. The XRD result reveals that the Ga2O3 nanowires have a pure monoclinic structure phase. Due to the existence of some very weak diffraction peaks and some overlapping peaks, there are nine diffraction peaks listed in the JCPDS card that are not observed from the experimental spectrum of Figure 2b. Among them, five peaks overlapped with others: -110 with -401, -310 with 401, 311 with -112, 003 with -510, and 511 with 203. If the intensity of the strongest peak (002) in the JCPDS 41-1103 is set to a relative intensity of 100, there are four peaks with a relative intensity of less than 5 in the standard pattern of Figure 2b. The four weak peaks are -203, -511, 601, and -113, and they have a relative intensity of only
