GaOx Core−Shell Nanowires and Nanochains and Their

Nov 4, 2008 - The characteristic I−V curves demonstrated good conductivity for the core−shell .... single GaP nanowire and nanochain (the scale ba...
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18588

J. Phys. Chem. C 2008, 112, 18588–18591

GaP/GaOx Core-Shell Nanowires and Nanochains and Their Transport Properties Z. M. Zeng,† Y. Li,† J. J. Chen, and W. L. Zhou* AdVanced Materials Research Institute, UniVersity of New Orleans, New Orleans, Louisiana 70148 ReceiVed: April 30, 2008; ReVised Manuscript ReceiVed: September 30, 2008

GaP/GaOx core-shell nanowires and nanochains have been synthesized in a large quantity by thermal evaporation of mixture of GaP and Ga powders at high temperature. The as-synthesized products were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Microstructure analysis indicates that both products are composed of GaP nanowires and GaOx amorphous shells. Their growth directions are along face-centered-cubic (fcc) GaP direction with high density twins. The transport properties were also investigated by patterning the core-shell nanowires and nanochains on Si/SiO2 substrates. The characteristic I-V curves demonstrated good conductivity for the core-shell nanowires and nanochains, but a field-effect property was only observed in the core-shell nanowires. A distinct response between the core-shell nanowire and nanochain devices was observed under an illumination of an ultraviolet light, which might be attributed to the nature of the nanowires and nanochains. 1. Introduction Recently, wide bandgap semiconducting one-dimensional (1D) nanostructures have attracted much attention due to their novel electrical, optical, and mechanical properties and potential applications in many fields.1-7 Among them, gallium phosphide (GaP) nanostructures are promising nanomaterials for electronic, optoelectronic, and magneto-optical devices because of their wide bandgap (2.26 eV).8-11 So far, GaP nanostructures have been synthesized by various methods, such as laser-assisted catalytic growth method,12 reacting carbon nanotubes,13 laser ablation,14 thermal evaporation,15 etc. In fact, GaOx can be easily formed on the surface of GaP nanowires during the nanostructure growth, which is also a wide bandgap (4.9 eV) material being used as insulating oxide layer for gallium based electronic devices. The core-shell structures could have unique properties and broad applications in nanoelectronics, such as roomtemperature ultraviolet lasing, gas sensor with high sensitivity, etc. The similar work have been performed in ZnO nanowires coated with a thin layer material.17-19 These stimulate the growth of GaP/GaOx core-shell structures as well as the investigation of their transport properties. In present work, we report a synthesis of GaP/GaOx core-shell nanowires and nanochains by high temperature thermal evaporation method. By careful investigation of the microstructures, the growth mechanism was discussed. In addition, electrical properties and photoelectric effect of the core-shell nanowires and nanochains were also studied. 2. Experimental Section A horizontal tubular furnace was used for this synthesis. Powder gallium phosphide and gallium in a mass ratio of 3:1 were mixed and then placed in an alumina crucible. After transferring the crucible to the center of the tube, the tube was evacuated by a mechanical rotary pump to 10 Torr. Then a 150 sccm (standard cubic centimeters per minute) flow of Ar was maintained during the synthesis process. While the mechanical * To whom correspondence should be addressed. E-mail: [email protected]. Tel: 504-280-1068. Fax: 504-280-3185. † These authors contributed equally to this paper.

Figure 1. X-ray diffraction pattern of the as-synthesized products showing the products consist of zinc blende GaP and metal Ga.

pump continually pumped the system and kept the pressure at 300 Torr, the temperature of alumina tube was elevated from room temperature to 1400 °C and held for 60 min. Then the furnace was cooled down to room temperature and yellow-green cotton-like products were obtained on the inner wall of the tube. The morphology of as-synthesized products was investigated by Carl Zeiss 1530 VP field-emission scanning electron microscope (FESEM). Powder X-ray diffraction (XRD) patterns of the products were obtained with an X-ray diffractometer (Phillips X’Pert System with CuKR radiation). Transmission electron microscopy (TEM) analyses were carried out using JEOL 2010 transmission electron microscope and FEI Tecnai F20 transmission electron microscope equipped with electron dispersive spectroscopy (EDS). The electrical and photoelectric properties were measured by Keithley 2400 source meter. 3. Results and Discussions Morphologies and Crystal Structure. The XRD pattern shown in Figure 1 reveals the crystal structure of the assynthesized products. The majority of the diffraction peaks can

10.1021/jp807500x CCC: $40.75  2008 American Chemical Society Published on Web 11/04/2008

GaP/GaOx Core-Shell Nanowires and Nanochains

J. Phys. Chem. C, Vol. 112, No. 47, 2008 18589

Figure 4. (a) TEM image of a nanochain. The insert is a SAED pattern from the area of circled region of the nanochain. (b) Enlarged TEM image of a droplet with small diameter on the nanochain. (c) EDS result of the nanochain.

Figure 2. SEM images showing the typical morphologies of the assynthesized GaP (a) nanowires and (b) nanochains, respectively.

Figure 3. (a) TEM image of a GaP nanowire. The insert is SAED pattern from the nanowire. (b) HREM image of the nanowire. (c) EDX results from the core-shell nanowire.

be indexed as zinc blende GaP with lattice parameter of 5.45 Å. Some of the peaks, indicated by the small rectangles in Figure 1, match to orthorhombic Ga, implying that some remnant Ga exists in the as-synthesized product. The stronger (111), (220), and (311) peaks demonstrate the possible preferential growth along these three lattice planes. Figure 2 presents the SEM image of the as-synthesized products, which were obtained from different reaction zone along the tube. One is gained from the center of the tube, another one is gained from the off-center position. The typical morphologies of nanowires (center of the tube) and nanochains (offcenter position) can be seen in panels a and b of Figure 2, respectively. It is confirmed from the SEM images that the length of the nanowires and nanochains ranges from several tens of micrometers to millimeters. The diameter of the nanowires and nanochains varies from 30 to 100 nm. It is evident that many nanospheres are evenly distributed on the nanowire forming a chain structure as shown in Figure 2b. It was also found the yield of the nanowires is more than that of the nanochains in the products of one experiment from SEM investigation. Therefore, the nanowires are the main products in our experiment. TEM samples were prepared by dispersing the products in alcohol using ultrasonication and then dropping them onto carbon coated TEM copper grids. Figure 3a is a TEM image of a single nanowire and the insert is the corresponding selected area diffraction electron diffraction (SAED) pattern from the nanowire. The SAED pattern can be indexed as GaP with a blende structure and a lattice constant of 5.45 Å, which is coincident with the XRD result. As the SAED pattern was

obtained with the incident beam parallel to direction of GaP, twins can be clearly seen as black bands across the nanowire. Two sets of diffraction spots that appeared in the SAED pattern also confirm the existence of twins. A HRTEM image of the GaP nanowire was also taken as shown in Figure 3b, in which the characteristic (11j1) and (11j1j) planes were labeled. Apparently, the growth direction of the GaP nanowire is along [11j1], which gives the explanation of why the (111) peak is the strongest one in the XRD pattern. From the HRTEM image, a thin amorphous shell layer can be easily seen on the nanowire and formed a core-shell structure. Figure 3c is a corresponding EDS analysis showing the nanowire consist of Ga, P and O element besides C and Cu peaks from the TEM grid. Based on the above TEM results, it can be deduced that the nanowire core is pure GaP and the O peak, in fact, comes from the shell, which was oxidized during the synthesis. The shell is identified as GaOx by putting a nanoprobe at the edge of the shell layer for EDS analysis. Figure 4a is a TEM image of the nanochain and SAED pattern (inset) collected from the denoted circle region. The nanochains also grow along [11j1] with high density twins as seen from the insert of SEAD pattern and are composed of nanowires and sphere-like droplet shell. It is confirmed by HRTEM observation that the droplets on the nanowires are all amorphous as well. By focusing the nanoprobe at the edge of the droplet as shown in Figure 4b, it was found that a stronger O peak can be seen (Figure 4c), implying the oxygen mainly come from the amorphous droplets. Therefore, the droplets are amorphous GaOx too. The growth mechanism of the core-shell nanowires and nanochains can be attributed to VLS growth mechanism.12,14 The Ga metal plays a role as a catalyst due to its low melting point although the melting point for GaP is about 1477 °C. The GaP started to melt at 1400 °C owing to the low pressure in furnace. The evaporated GaP species were carried by Ar flow, and Ga liquid droplets and temperature gradient directed the growth and formed the nanowires. When the temperature went down to room temperature, the surface of the nanowires was oxidized by leaking air, forming GaOx shell structure on GaP nanowires. However, the redundant Ga liquid was preserved as the temperature was decreasing. Similar to the reported results in carbon nanotubes,22 an adhesive wetting interface were formed between partial GaP nanowires and Ga liquid drops by Rayleigh instability.23,24 Capillary forces make the adjacent Ga liquid droplets in similar diameters and spread almost evenly along the nanowires, forming the nanochain structure. As the form was kept to room temperature, the Ga liquid droplets were also oxidized to amorphous spheres along the nanowires, generating core-shell structure nanochains. Electrical Transport Properties. The basic electrical transport properties of the core-shell GaP nanowire and nanochain

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Figure 5. Typical Ids-Vds curve of the GaP nanowire device with nanowire and nanochain at a Vg ) 0 V. The inset in top left and bottom right show corresponding SEM images of the devices with single GaP nanowire and nanochain (the scale bar is 2 µm), respectively.

were investigated in our experiment. Electrical measurements were performed on individual core-shell GaP nanowire and nanochain devices. The single core-shell nanowire/nanochain was first dispersed on Si substrate covered with a 600-nm-thick thermally grown SiO2 layer. The Si substrate was served as a back-gate. Then Cr/Au metal electrodes were patterned on the two ends of the core-shell nanowire/nanochain through electron beam lithography, metal evaporation, and lift-off process, respectively. One thing should be mentioned is that the shell layer of GaOx was removed by chemical etching (dipping in 3% HF solution for 10 s) before metal electrodes deposition. The insets in Figure 5 show the SEM images of typical GaP nanowire (left top) and nanochain (right bottom) devices, respectively. Current-voltage (I-V) measurements of the single GaP nanowire/nanochain devices were shown in Figure 5, which demonstrate typical semiconducting I-V characteristic. It can be seen that the current of nanochain device (channel length ) 6.4 µm, diameter ) 200 nm) is larger than that of nanowire device (channel length ) 5.0 µm, diameter ) 80 nm) when applied same-value voltage. For example, the current of nanochain/nanowire device is around 1.8 and 4 µA at an applied voltage of 8 V, respectively. This difference in I-V curves of the two devices is mainly ascribed to the geometric factors such as channel lengths and diameters although there are other factors contributing to conductance of the device. Meanwhile, Both I-V curves show a good symmetry although they are nonlinear at low applied voltage, which indicates the weak Schottky contact was formed due to the remnant very thin GaOx layer on the surfaces of both kinds of GaP core-shell nanowires. The transport property was further investigated for back-gate geometry. Figure 6 shows the gate-dependence of GaP nanowire device, which indicates the obvious field-effect characteristic. However, the nanochain device does not show gate-effect. The reason for absence of gate-effect may be related to the large nanosphere. The diameter of nanosphere is around 600 nm, which may make nanowire with nanochain to suspend from the substrate, and the dielectric layer with a total of 900 nm (600 + 300 nm) is too thick to offer back-gate effect. The Ids-Vg curve of the corresponding device in right bottom in Figure 6 shows that the device works as n-channel. The mobility can be calculated by fitting Ids-Vg curve with the following equation

Ids ) (WCµ ⁄ 2L)(Vg - Vo)2 where W is the width and L is the length, the source-drain voltage is fixed at +10 V. For the 600 nm SiO2 layer, C ) 5.5

Zeng et al.

Figure 6. Ids-Vds characteristic curves at variable Vg for a typical device bridged with a single GaP nanowire. The inset in top left and bottom right show magnified Ids-Vds curves and Ids-Vg characteristic curve at a Vds ) +10 V for the single nanowire FET, respectively.

Figure 7. UV-dependent transport curves from typical devices with a single nanowire and nanochain, respectively.

nF/cm2. We assumed that the W ) 80 nm and L ) 5.0 µm according to the SEM measurements, the calculated mobility from this device is around 0.24 cm2/Vs, which is lower than the reported results,15 which might be due to the diffusive nature of electrical transport in GaP nanowire devices The photoelectric effect of GaP devices was also studied. In our experiment, the Blak-Ray Ultraviolet (UV) Lamp with a wavelength of 365 nm (Ted Pella Inc.) was employed to investigate photoelectric properties. The intensity we used in experiment is about 8.9 mW/cm2. The source-drain current was measured under exposure to the UV light in air with applying Vg ) 0 V and Vds ) 1.0 V. Figure 7 shows the time-dependent UV-response of the devices with nanowire and nanochain as the UV light was switched ON and OFF. It can be seen that the devices are sensitive to UV illumination, but they show the obvious different behavior. For nanowire device, under illumination, the current rapidly increases to 104 nA. The current suddenly decreases to its original value as the light is turned off. The exact response and recovery time are below the detection limit of the equipment. Therefore, it can only be estimated that the upper limits of the response time and recovery time are 0.1 s. In contrast, the response and recovery is significantly slow in the nanochain device. Notice that the interval of illumination ON/OFF switching is 60 s, which is not enough for response and recovery. So the current increases after 500 s compared to its original value as shown in Figure 7. It has been found that the response of a semiconductor to photon is a complex process of electron-hole generation, trapping and recombination.27 In our case, provided that the

GaP/GaOx Core-Shell Nanowires and Nanochains bandgap of GaOx is ∼4.9 eV, which is transparent to 365 nm, the response of GaP nanowire (bandgap is ∼2.26 eV) is the main contribution to the conductance change of nanowire/ nanochain device. As shown in Figure 7 the slight difference in current change for nanowire and nanochain device is mainly ascribe to the geometric factors such as channel lengths and diameters. Meanwhile, because of the high surface-to-volume ratio, the surface states of GaP nanowire play a significant effect on the response of UV, especially the response time and recovery time. In general, the response speed depends on excess carrier lifetime τ and the electron transit time tt at a given condition. In nanowire case, after turning off the light, the excited electrons and holes recombine quickly (short τ and tt) and oxygen adsorption almost does not affect the conductance due to smooth surface. This probably accounts for the fast recovery process in nanowire device. In contrast, the nanochain device not only includes GaP core and shell GaOx layer but also has two amorphous nanospheres on the surface of nanowire as shown in Figure 4b. The amorphous nanospheres may affect the electron transit time tt, further slow the response to UV. In additional, the slow response could be related to the slow adsorption of oxygen on the surface of nanochain device. Further studies are necessary to fully understand the underlying mechanisms that govern the response and recovery time of our devices. 4. Conclusions In summary, GaP/GaOx core-shell nanowires and nanochains have been largely synthesized by thermal evaporation. Microstructure characterization indicates the main products are GaP nanowires formed with amorphous GaOx oxidation layer as shell layers. The droplets generated from capillary force form nanochains as another product with spherical GaOx droplets aligned on the GaP nanowires. Characteristic I-V curves indicate the nanowires and nanochains have good conductivity, and the nanowire devices show obvious field-effect property. A distinct response to UV illumination was observed for the nanowires and nanochains, which are strongly attributed to the nature of nanowires and nanochains. The field effect transistors based on the core-shell GaP/GaOx nanowires has a potential application for chemical sensors. Acknowledgment. This work was supported by the DARPA Grant No. HR0011-07-1-0032 and a research grant from Louisiana Board of Regents Contract Nos. LEQSF(2007-12)-

J. Phys. Chem. C, Vol. 112, No. 47, 2008 18591 ENH-PKSFI-PRS-04 and LEQSF (2008-11)-RD-B-01. W.L.Z. acknowledges partial support from the Research Fund of Key Laboratory for Nanomaterials, Ministry of Education (No. 20071). References and Notes (1) Wong, E. W.; Sheehan, P. E.; Lieber, C. M. Science 1997, 277, 1971. (2) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208. (3) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 92, 1897. (4) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (5) Kim, P.; Lieber, C. M. Science 1999, 286, 2148. (6) Zhou, X. T.; Wang, N.; Lai, H. L.; Peng, H. Y.; Bello, I.; Wong, N. B.; Lee, C. S.; Lee, S. T. Appl. Phys. Lett. 1999, 74, 3942. (7) Kim, J. R.; So, H. M.; Park, J. W.; Kim, J. J.; Kim, J.; Lee, C. J.; Lyu, S. C. Appl. Phys. Lett. 2002, 80, 3548. (8) Berger, L. I. Semiconductor Materials; CRC Press: Boca Raton, FL, 1997. (9) Kang, D.; Ko, J.; Bae, E.; Hyun, J.; Park, W.; Kim, B.; Kim, J.; Lee, C. J. Appl. Phys. 2004, 96, 7574. (10) Kim, B.; Kim, J.; Lee, J.; Kong, K.; Seo, H. J.; Lee, C. J. Phys. ReV. B 2005, 71, 153313. (11) Xu, L.; Su, Y.; Chen, Y. Q.; Xiao, H. H.; Zhu, H. H.; Zhou, Q. T.; Li, S. J. Phys. Chem. B 2006, 110, 6637. (12) Duan, X. F.; Lieber, C. M. AdV. Mater. 2000, 12, 298. (13) Tang, C. C.; Fan, S. S.; de la Chapelle, M. L.; Dang, H. Y.; Li, P. AdV. Mater. 2000, 12, 1346. (14) Shi, W. S.; Zheng, Y. F.; Wang, N.; Lee, C. S.; Lee, S. T. J. Vac. Sci. Technol. B 2001, 19, 1115. (15) Lyu, S. C.; Zhang, Y.; Ruh, H.; Lee, H. J.; Lee, C. J. Chem. Phys. Lett. 2003, 367, 717. (a) Huang, Y.; Yue, S.; Wang, Z.; Wang, Q.; Shi, C.; Xu, Z.; Bai, X. D.; Tang, C.; Gu, C. J. Phys. Chem. B 2006, 110, 796. (16) Han, D. S.; Bae, S. Y.; Seo, H. W.; Kang, Y. J.; Park, J.; Lee, G.; Ahn, J. P.; Kim, S.; Chang, J. J. Phys. Chem. B 2005, 109, 9311. (17) Wang, X. D.; Gao, P. X.; Li, J.; Summers, C. J.; Wang, Z. L. AdV. Mater. 2002, 14, 1732. (18) Jiang, X. C.; Mayers, B.; Herricks, T.; Xia, Y. N. AdV. Mater. 2003, 15, 1470. (19) Ding, Y.; Kong, X. Y.; Wang, Z. L. J. Appl. Phys. 2004, 95, 306. (20) Meng, X. M.; Hu, J. Q.; Jiang, Y.; Lee, C. S.; Lee, S. T. Appl. Phys. Lett. 2003, 83, 2241. (21) Zhang, X. T.; Zhang, J.; Wang, R. M.; Liu, Z. F. Carbon 2004, 42, 1455. (22) Heer, W. A.; Poncharal, P.; Berger, C.; Gezo, J.; Song, Z.; Bettini, J.; Ugarte, D. Science 2005, 307, 907. (23) Rayleigh, L. Proc. R. Soc. A 1879, 8, 425. (24) Goren, S. L. J. Fluid Mech. 1962, 137, 363. (25) Anedda, A.; Serpi, A.; Karavanskii, V. A.; Tiginyanu, I. M.; Ichizli, V. M. J. Appl. Phys. 1995, 67, 3316. (26) Kuriyama, K.; Ushiyama, K.; Ohbora, K.; Miyamoto, Y.; Takeda, S. Phys. ReV. B 1998, 58, 1103. (27) Rose, A. Concepts in PhotoconductiVity and Allied Problems; Krieger: New York, 1978.

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