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J. Phys. Chem. C 2009, 113, 834–838
Formation and Photoelectric Properties of Periodically Twinned ZnSe/SiO2 Nanocables Xia Fan,†,§ Xiang Min Meng,*,† Xiao Hong Zhang,† Ming Liang Zhang,‡ Jian Sheng Jie,‡ Wen Jun Zhang,‡ Chun Sing Lee,‡ and Shuit Tong Lee*,‡ Key Laboratory of Photochemical ConVersion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, CAS, Beijing 100101, P. R. China, and Center of Super-Diamond and AdVanced Films (COSDAF) and Department of Physics and Materials Science, City UniVersity of Hong Kong, Hong Kong SAR, China ReceiVed: July 10, 2008; ReVised Manuscript ReceiVed: September 21, 2008
Nanoscaled coaxial materials with a periodically twinned ZnSe single crystal core and an amorphous silicon dioxide shell were synthesized by a simple thermal evaporation process. As-fabricated ZnSe/SiO2 core/shell nanowires and nanoribbons were studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and a selected area electron diffraction (SAED) pattern. The periodically twinned ZnSe core with a twinning period of about 10-30 nm has a sphalerite crystal structure and a common growth direction of {11-1]. The amorphous SiO2 shell has a thickness of about 5 nm. The formation mechanism can be explained by the surface pressure stress from SiO2 shell to ZnSe core at the solid/liquid interface. The materials show strong cathodoluminescence (CL) related to the mixed light emission composed of band gap emission and defect emission. The single periodically twinned ZnSe/SiO2 nanowire has pronounced a photoconduction effect with a fast response time. The results suggest that the periodically twinned ZnSe nanocables have potential application in nanoscaled photodetectors and optical switches. Introduction Recently, semiconductor nanostructures have been extensively studied for their potential applications as building blocks of nanoscaled photoelectric devices, such as bio/chemical sensors,1 photodetectors,2 and solar cells.3 Photodetection, photoconductivity, and optical switching effects have been reported in several nanostructures, such as InP nanowires,2 ZnO nanowires,4 In2O3 nanowires,5 and CdS nanoribbons.6,7 A piezoelectric nanogenerator based on ZnO nanowire arrays has been demonstrated.8 Since physical and chemical properties are determined from the microstructure of nanomaterials, much effort has been devoted to fabricating nanoscaled semiconductor materials with special morphology.9 Among the wide variety of nanoscaled materials, structurally modulated nanomaterials are of special interest. Current-voltage characteristics10 of Si periodically twinned nanostructures have been studied, which showed that the adjacent pieces in the Si nanowire had different electrical properties. Several reports of high-density twinning nanowires have appeared where periodical fluctuation and self-oscillation were attributed to the formation mechanism of twined nanowires,10-15 and the twinning periodicity was shown to be related to the diameter of the nanowire.13 Most recently, twinning superlattices in III-V semiconductor nanowires also have been prepared,16,17 and the structure and segment thicknesses were discussed, showing that the distance between the twin boundary depended on the temperature in the growth zone. However, the formation mechanism of periodically twinned nanowire with sphalerite structure remains unclear. Although the presence of an amorphous layer in the periodically twinned nanomaterials * To whom correspondence should be addressed. (X.M.M.) e-mail:
[email protected], tel: 86-10-82543557; (S.T.L.) e-mail: apannale@ cityu.edu.hk. † Technical Institute of Physics and Chemistry. ‡ City University of Hong Kong. § Also holds a position at Graduate School of Chinese Academy of Sciences, Beijing, P. R. China.
was not mentioned previously, we consider it very important for twin formation. ZnSe is an important wide band gap II-VI polar semiconductor18 and has been used in optoelectronic devices,19 a blueultraviolet photodetector,20 and blue laser diodes,21,22 etc. Various nanostructures of ZnSe, including nanowires,23-28 nanoribbons,23,29 and nanotubes30 with wurtzite structure and periodically twinned nanowires13 with sphalerite structure, have been synthesized. Some growth models were used to explain the formation of periodically twinned nanowires such as the periodic insertion of stacking faults into the lattice13 and the misfit dislocation in the interface between catalyst and nanowire.15 Here, we selected ZnSe and SiO as the source material powders, prepared the periodically twinned nanostructures, analyzed their formation mechanism, and investigated their photoelectric properties. Experimental Methods Preparation of Periodically-Twinned ZnSe-SiO2 Core/ Shell Nanostructures. The periodically twinned ZnSe/SiO2 core/shell nanostructures were synthesized using a high-temperature horizontal tube furnace, as described previously.31 Briefly, a quartz tube with an outer diameter of 31 mm and a length of 1200 mm was placed inside a fixed alumina tube in the furnace. An alumina boat containing 2 g of SiO powder (Alfa Aesar, 99.5%) was placed at the center of the quartz tube, while 2 g of ZnSe powder (Alfa Aesar, 99.99%) was placed upstream about 7 cm from the SiO powder. Two pieces of goldcoated silicon wafers (70 mm × 10 mm) were put at about 20 cm downstream of the SiO source. After the tube was evacuated to a pressure of 4.5 × 10-4 Torr, argon premixed with 8% hydrogen was fed at a rate of 50 sccm (standard cubic centimeters per minute) from the ZnSe end of the quartz tube. The temperature of the central part of the furnace was then ramped at a rate of 35 °C /min to 1150 °C and kept there for 2 h. Pressure inside the tube was maintained at 100 Torr throughout the whole heating process. The system was then
10.1021/jp806093m CCC: $40.75 2009 American Chemical Society Published on Web 12/30/2008
Periodically Twinned ZnSe/SiO2 Nanocables cooled down naturally to room temperature under the same gas flow and pressure. Sample Characterization. Deposited products were first analyzed with a field-emission scanning electron microscope (FE-SEM, Hitachi S-4300). The samples were further investigated with transmission electron microscope (TEM, JEM-200CX operated at 160 kV; Philips, CM200 FEG and CM20 operated at 200 kV) and an energy-dispersive X-ray spectrometer (EDS) attached to the CM20 TEM. Cathodoluminescence (CL) spectra of the samples were measured with a CL system attached to a Philips XL 30 FEG SEM. Sample for Photoelectric Measurement. For the fabrication of a single periodically twinned nanowire (PTN) device, twinned ZnSe nanowire samples were first sonicated in alcohol for 30 min to disperse into isolated nanowires. Meanwhile, long nanowires were broken and shortened, and the sample would consist of some short nanowires with two freshly exposed ends. A drop of the resulting PTN suspension was then put onto a SiO2 (thickness of 300 nm)-coated Si (p+) wafer. After drying, patterned Ti (80 nm) and Au (15 nm) electrodes were successively deposited on the PTNs in high vacuum by e-beam evaporation through a mesh-grid mask with tungsten wires of 5 µm diameter. The electrodes were able to contact with the ZnSe core through the two ends of the PTNs, which are not covered by the oxide layer. A fast annealing at 350 °C was carried out in H2(5%)/N2 atmosphere for 3 min to form the ohmic contact between the electrodes and the PTNs. A green laser (514.5 nm, output 3 mW) attached to a Raman system was used for the photoconductivity measurement. The laser was focused to a spot size of 100 µm.
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Figure 1. (a) Low-magnification and (b) enlarged FE-SEM images of as-fabricated nanowires.
Results and Discussion The morphology of the prepared product was probed by SEM. Yellowish wool-like product on the silicon wafer in the region of about 850 °C was first transported into SEM without destroying the original morphology of the as-synthesized sample. A typical low-magnification SEM image is shown in Figure 1a. The substrate was covered uniformly up to 40 × 10 mm2 with a high density of wire-like nanostructure with length of several micrometers. A higher magnification image (Figure 1b) shows that the wire-like structure had a diameter of about 100 nm. Fine structure of these nanowires was further investigated with TEM. Figure 2a is a typical low-magnification bright-field (BF) TEM image of the nanowires, which have diameters of 50-150 nm and clean and smooth surfaces. All nanowires show periodic light/dark alternating contrast along the growth direction. Figure 2b is a high-magnification TEM BF image of a typical periodically twinned ZnSe nanowire, showing that the twinning period is about 10-30 nm and has no apparent relationship with the diameter of nanowire, the same as that in reference.13 For example, in the thicker nanowire marked by a dark arrow, the twin period is smaller than that in a thinner one (marked by white arrow). Figure 2c is a high-magnification bright-field TEM image of a fine wire-liked nanocable with an amorphous cladding layer of about 5 nm thick, while the inset shows a typical SAED pattern of a nanowire core. It consists of two sets of diffraction spots, which can be indexed as 1-10 zones of sphalerite-structured ZnSe phase. Some weak and sharp spots were found between two stronger spots in the partial enlarged image (under diffraction pattern), which indicates that twinned nanowire had clear periods. Figure 2d is an HRTEM image corresponding to the SAED pattern, and detailed crystallographic analysis reveals that the dark and light regions have
a twinning relationship and share a common (11-1) plane. The angle between the two (111) planes is 141°. Some stacking faults were found at the interface of the twins. The growth of nanowire is along the [11-1] direction. An amorphous layer was found to wrap around the ZnSe core. Figure 2e shows an EDX spectrum of the nanowire, which reveals the presence of Zn, Se, Si, and O (also the Cu signal comes from the Cu TEM sample grid). The atomic ratio of Zn to Se and Si to O is close to 1:1 and 1:2, respectively. A Si element mapping of the nanocable in Figure 2c is shown in Figure 2f, which is consistent with the proposition that the wire has a cable-like structure with a periodically twin crystalline ZnSe core and a thin amorphous SiO2 sheath. In the SEM studies, some ribbon-shaped structures with a width of 3 to 10 µm and a length of several tens of microns were also found together with the nanowires at the edge of the deposit in the temperature region of about 900 °C (Figure 3a). The volume ratio of nanowire and nanoribbon is about 1:3 in this region. Figure 3b is a TEM image of a single ribbon with a width of about 3 µm. The upper inset is an enlarged image of the nanoribbon, which shows the dark/white contrast and that it consists of sections of alternate multitwins. The growth direction of the ribbon is also preferentially aligned to the [111] directions. A SAED pattern corresponding to the ribbon shows two sets of diffraction spots (Figure 3c), which can be indexed as 1-10 the zone axis of the cubic ZnSe phase having a twin orientation relationship. Figure 3d is an HRTEM image of a typical ZnSe nanoribbon, showing that the core has a periodically twinned ZnSe structure with uniform and clear twin boundaries perpendicular to the long edge. A thin shell about 5 nm thick of lighter contrast can be clearly observed around the nanoribbon. The twinning period is about 10-15 nm. These
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Figure 3. (a) FEG SEM image of a mixture of nanowires and nanoribbons, (b) bright-field TEM image (inset are the enlarged image), (c) SAED pattern, and (d) HRTEM images of a periodically twinned ZnSe/SiO2 nanoribbon.
Figure 2. (a) Low-magnification bright-field (BF) TEM image and (b) high-magnification BF image showing the relationship between the period and cross-section of the periodically twinned ZnSe/SiO2 nanowires, (c) low-magnification HREM image and SAED pattern, (d) enlarged HRTEM images, (e) EDS spectrum, and (f) the Si element mapping of a periodically twinned ZnSe/SiO2 nanowire.
results show that the long sidewall of the nanoribbon core is the nonpolar plane of {110}, which is different from the {111} nanowire. The growth of the periodically twinned ZnSe/SiO2 core/shell nanowires discussed above was accomplished by simple thermal evaporation of ZnSe and SiO powders. All of the nanowires grow along a common 〈111〉 direction of cubic ZnSe. Such a nanowire growth direction is usually dominated by a metalcatalyst VLS growth mechanism. First, it is assumed that the {111} plane of sphalerite-structured ZnSe has the lowest interface energy with the gold liquid droplet on the silicon substrate. When the concentration of ZnSe and SiO2 in the liquid droplet exceeds a certain critical level, silicon dioxide in the liquid metal catalyst would be preferentially precipitated and form a thin shell surrounding the liquid droplet due to its amorphous nature and higher melting point, while solid ZnSe precipitated at the interface between the liquid droplet/Si substrate. Because of the spherical geometry of the liquid metal droplet, most of the precipitated ZnSe crystal would have a geometry close to a circular disk with most of its surfaces being a {111} plane to minimize the surface energy. Thus, in the initial stage, a ZnSe nucleus that precipitated from the liquid metal should assume the shape shown in Figure 4a. A crystal seed that fulfills all these requirements can be visualized as a thin slice of crystal dividing a {111}-faced octahedron into halves
(Figure 4b). As more ZnSe vapor is absorbed into the metal droplet, ZnSe would continuously deposit at the interface of the liquid droplet/ZnSe rod to achieve a dynamic balance. This drives the axial growth of the nanowire and pushes the liquid metal tip away from the substrate. To maintain the minimum surface energy of the crystal, the ZnSe seed would grow along the 〈111〉 direction as shown by the arrow in Figure 4c. Because ZnSe with sphalerite structure is a polar crystal with Zn-terminated {111} being a positive plane and Seterminated {-1-1-1} a negative one, the growing speed of these two planes is different, and the crystal would grow mainly along the positive plane. Therefore, as the growth continues, three alternative edges (1, 3, and 5) (intersection of top plane and positive plane) of the ZnSe crystal would become longer and the other three (2, 4, and 6) (intersection of top plane and negative plane) would shrink. Obviously, this would lead to increasing distortions from the original “nearly-circular” cross-section that fits the spherical metal liquid droplet and the tubular silicon dioxide shell. The three shrinking edges would push onto the silicon dioxide shell, leading to a high compressive stress that opposes the growth. Furthermore, as the growth continues, the compressive stress would increase and the cross-sectional area would decrease. This might also affect the deposition rate of the ZnSe and thus the concentration of ZnSe in the metal liquid droplet. With a small stacking fault energy in the {111} plane of ZnSe, the compressive stress can be easily released by twinning as shown in Figure 4c. After twinning and with the polarity of the liquid droplet/ZnSe solid interface unchanged, edges 1, 3, and 5 will shrink and the other edges will elongate as the wire grows axially. Upon further growth, cross-section of the twinned crystal will be transformed back to an equilateral hexagon (Figure 4d) and release the compressive stress at edges 2, 4, and 6. Again upon further growth, the shrinking edges 1, 3, and 5 will press onto the silicon dioxide shell and cause compressive stress. Eventually, the crystal will twin back to the original orientation to release the stress. By repeating twinning periodically, the system will achieve a balance, thus leading to the formation of periodi-
Periodically Twinned ZnSe/SiO2 Nanocables
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Figure 4. A formation model of periodically twinned core/shell nanowire: (a) a nucleus precipitated from a liquid droplet; (b) a seed visualized as a regular octahedron; (c) single nanorod formed; (d) twinned nanorod formed in the S/L interface; (e) formed periodically twinned nanowire growing alone in the [11-1] direction; (f) formed periodically twinned ribbon-liked nanocable growing alone in the [11-1] direction.
Figure 5. A typical CL spectrum of the periodically twinned ZnSe/ SiO2 core-shell nanomaterials.
cally twinned nanowires (Figure 4e). During the growth stage of the periodically twinned nanowires, the ZnSe concentration in the liquid droplet would periodically change because of the periodic change in the deposition area of the solid/liquid interface. The area is the largest when the interface is a regular hexagon, which is 3/2 times of the triangle upside in the octahedron (Figure 4b). In the growth of a twin slice, the concentration of ZnSe in the droplet undergoes a highlow-high sequence, which would facilitate the periodically twinned nanowire formation. Therefore, the formation mechanism of the twinned nanowires can be attributed the periodically change of the pressure stress from SiO2 shell to ZnSe and the concentration of ZnSe in the drop. The nucleation and growth mechanism of the ribbon-like nanocable is similar with the nanowire. When a long and smooth needle-shaped nucleus is obtained in the droplet, a periodically twinned nanoribbon is formed under perusal stress as shown in Figure 4f. One difference between the growth process of the twinned nanowire and the twinned nanoribbon is that the concentration of ZnSe in the liquid drop during the growth of periodically twinned nanoribbon is stable. The model suggests that periodically twinned nanoribbon can be initiated via stress alone at the ZnSe/SiOx interface. Similar repeating twinning nanowires have been observed in ZnS and GaAs, etc.13,16,17,32 From the HRTEM images of those nanowires, an amorphous layer sheathing the nanowires can clearly be found. Some work indicated no periodical twin was found in ZnS nanowires of the sphalerite structure without an
Figure 6. (a) I-V curves of the single ZnSe periodically twinned nanowire in the dark (0) and under laser irradiation of 38 W/cm2 at 514.5 nm (∆), respectively. The insert shows an SEM image of the single nanowire device. (b) Time response of the periodically twinned ZnSe nanowire to a pulsed incidence laser. The laser was switched on (off) manually, and the bias voltage is 2 V during the measurement.
amorphous layer.31 It appears that the stress-induced mechanism proposed by us can similarly explain such a twinning process provided that (1) the system has a low stacking fault energy, (2) the surface energy of the {111} plane is significantly lower than those of others, and (3) the nanomaterial has a thin shell restraining the lateral growth. Further theoretical work is being performed to study the interplay between these factors. Figure 5 shows a room temperature CL spectrum of the asprepared ZnSe/SiO2 nanostructures. The broad emission from 400 to 650 nm has the peak located at ∼500 nm (2.48 eV). This mixed light emission is composed of a band gap emission
838 J. Phys. Chem. C, Vol. 113, No. 3, 2009 of 459 nm (2.70 eV) of ZnSe and a broad defect emission such as donor-acceptor pairs related to Zn-vacancy and interstitial states.33-35 Figure 6a shows I-V curves of a single ZnSe PTN in the dark and under laser irradiation, respectively. The shape of the curves indicates that the device has ohmic contact at its electrodes. Significantly, the laser irradiation dramatically improves the conductance of ZnSe nanocable from 70.5 pS in dark to 123 pS. Photoelectric responss of another two PTN were measured and showed qualitatively similar results. One sample shows a more than 1 order of magnitude increase in conductance upon irradiation. The same measurements were then conducted with a similar device fabricated with a single crystalline (i.e., no twinning) ZnSe nanowire. However, the current signal in the single crystal device is too low (
