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Preparation and photoluminescence properties of crystalline GeO2 nanowires
30 November 2001
Chemical Physics Letters 349 (2001) 210±214 www.elsevier.com/locate/cplett
Preparation and photoluminescence properties of crystalline GeO2 nanowires X.C. Wu b
a,*
, W.H. Song b, B. Zhao b, Y.P. Sun b, J.J. Du
b
a Department of Chemistry, Nanjing University, Nanjing 210093, China Laboratory of Internal Friction and Defects in Solids, Institute of Solid State Physics, Academia Sinica, Hefei 230031, China
Received 23 May 2001; in ®nal form 1 October 2001
Abstract Bulk-quantity GeO2 nanowires (GeONWs) have been synthesized by carbothermal reduction reaction between germanium dioxide and active carbons. Transmission electron microscopy (TEM) image shows the formation of the nanowires at a diameter about 50±120 nm and a length up to hundreds of micrometers. The nanowires can emit stable and high brightness blue light at 485 nm (2.56 eV) under excitation at 221 nm (5.61 eV). The intensity of the emission is one order of magnitude higher than that of GeO2 powders. The photoluminescence (PL) may originate from radiative recombination between an electron on VO and a hole on VGe ; VO in the GeONWs. The nanowires may have potential applications in one-dimensional optoelectronic nanodevices. Ó 2001 Published by Elsevier Science B.V.
1. Introduction One-dimensional quantum wires are expected to play a vital role as both interconnects and functional components in future mesoscopic electronic and optical devices, and also to provide an opportunity to test fundamental quantum mechanics concepts [1,2]. The electronic and optical properties of the nanowires strongly depend on size and dimensionality. For instance, as the wire diameter approaches the carrier de Broglie wavelength, quantum con®nement eects shift band gap energies and induce visible photoluminescence (PL) for Si [3]. Optical nanowires may have found
*
Corresponding author. E-mail addresses: [email protected], wuxingcai@chi na.com (X.C. Wu), .
applications in the ®elds of localization of light lower dimensional waveguide, and scanning near®eld optical microscopy (SNOM). Therefore, it is important to synthesize nanowires with optical properties and speci®c sizes that can meet the demands of further applications. So far, several important nanowires (nanobelts) of semiconducting oxides such as ZnO [4], SiO2 [5], SnO2 [6] and so on, have been synthesized. They may be used in designing nanoscale optoelectronic devices. GeO2 nanocrystal is a blue PL material, with peak energies around 3.1 eV (400 nm) and 2.2 eV (563.6 nm) [7,8], and germanium oxide-based glass is thought to be more refractive than the corresponding silicate glass so that the GeO2 nanowires (GeONWs) may be used for nanoconnections in future optoelectronic communication. The fabrication and structure of crystalline GeONWs were once reported, but the nanowires were synthesized
0009-2614/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 1 2 1 3 - 1
X.C. Wu et al. / Chemical Physics Letters 349 (2001) 210±214
211
by the physical evaporation [9] and by the carbon nanotube-con®ned reaction [10], and the PL was not dealt with. Both the above methods are complex relatively, as compared with carbothermal reduction method. In this Letter, we report the carbothermal reduction synthesis and characterization and PL properties of the nanowires. The growth process and PL mechanism of the nanowires have been discussed, respectively. 2. Experimental The preparation processes of the GeONWs were as follows. The mixtures of GeO2 (1.5 g) with active carbon (1.5 g) were ground for 15 min, and put into alumina boat. Then the boat was covered with ceramic plates, placed at the center of conventional horizontal furnace with a quartz tube £ 2:5 cm 100 cm and remained at 840 °C for 3.5 h in ¯owing nitrogen atmosphere (20 ml/min). A white product was found to deposit on the surface of the plate and the inner wall of the boat. Transmission electron microscopy (TEM) images of the products were taken with a JEM-200CX transmission electron microscope. Powder X-ray diraction (XRD) pattern of the products was obtained with Philips PW1700 X-ray diractometer with Cu-Ka radiation. The composition of GeONWs was determined by the X-ray photoemission spectra (XPS), which were recorded on a VGESCALAB MKII X-ray photoemission electron spectrometer. XPS data were collected in the constant analyzer energy (CAE) mode at 20 eV. Mg-Ka (hm 1253:6 eV) radiation was employed as excitation source with an anode voltage of 12 KV and an emission current of 29 mA. PL spectra of the samples were measured in a Hitachi 850 ¯uorescence spectrophotometer with a Xe lamp at room temperature. The excitation wavelength was 221 nm, and the ®lter wavelength was 290 nm. 3. Results and discussion TEM micrograph in Fig. 1a shows the general morphology and dimension of GeONWs. The nanowires have a diameter ranging from 50 to 120
Fig. 1. TEM images of GeONWs: (a) general morphology of GeONWs; (b) selected electronic diraction pattern of GeONWs along [2 2 2] axis.
nm and a length up to hundreds of micrometers by scanning throughout the sample. Fig. 1b is selected area electron diraction (SAED) pattern of the GeONWs along [2 2 2] axis. The SAED micrograph shows that the nanowires are crystalline. The structure of the products was also characterized by XRD. All peaks (Fig. 2) can be determined as hexagonal GeO2 crystal (ASTM card No. 4-497) according to the peak positions and their relative intensities. No carbide such as GeC was observed. The survey spectrum in Fig. 3a also displays C (1s) peaks, which can be attributed to a small amount of the residual. Figs. 3b and c corresponded to the binding energies of Ge (2p) and O (1s) for GeO2 , respectively. The quanti®cation of the peaks reveals that atomic ratio of Ge to O is 1:1.75. The chemical formula of the nanowires is GeO1:75 . Although, the points on the nanowires (con®rmation of the vapor±solid (VS) growth
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X.C. Wu et al. / Chemical Physics Letters 349 (2001) 210±214
Fig. 2. X-ray diraction pattern of the sample.
mechanism) could not be observed, considering that no catalyst was added in the process of the nanowires' growth, and that no eutectic droplets could be seen at the tops of the nanowires, we think that the nanowires grow in terms of the VS mechanism [11,12]. The viewpoint is consistent with that of [8]. The growth process of the nanowires could be divided into three steps. The ®rst step is that GeO2 is reduced by active carbon to Ge and GeO into vapor phase. The second step is that the vapor of Ge and GeO is driven by ¯owing nitrogen atmosphere and deposits on the surface of alumina boat and plate to form de®cient-oxygen germanium oxide crystalline nuclei. The nuclei further grow into nanowires according to helical dislocation mechanism with the proceeding of the carbothermal reduction reaction. Oxygen may be attributed to the incoming of air. The growth process is similar to that of b-Ga2 O3 nanowires too [13]. The most striking properties of the GeONWs are that they emit stable and highly bright blue light. As is shown in Fig. 4a, a broad PL peak was clearly observed at 485 nm under the excitation at 221 nm at room temperature. The intensity of the more intensive peak was found more than one order of magnitude higher than that of the GeO2 powders (spectral purity) as shown in Fig. 4b. The PL peak positions are dierent from either that (400, 563.6 nm) of SiOx ®lm containing Ge and GeO2 nanocrystals [4] or that of the GeO2 powders as shown in Fig. 4c. The GeONWs are prepared
Fig. 3. XPS of the sample.
under reducing condition, and GeO is volatilized at high temperature, so that oxygen vacancies and oxygen±germanium vacancy pairs are easily produced. XPS of the sample has con®rmed that the oxygen defects of the nanowire exist. After the nanowires were annealed in oxygen at 600 °C for 2
X.C. Wu et al. / Chemical Physics Letters 349 (2001) 210±214
213
cies and germanium±oxygen vacancies centers. The acceptors would be formed by VGe ; VO , and the donors would be formed by VO . After excitation of the acceptor, a hole on the acceptor and an electron on a donor are created according to the following formal equation VGe ; VO 00 VO 2hm ! VGe ; VO VO The blue emission occurs via the reverse reaction. Luminescence process would be divided into two steps. First, an electron in donor band is captured by a hole on an acceptor to form a trapped exciton. Secondly, the trapped exciton recombines radiatively emitting a blue photon. An important stokes shift of about 3.05 eV can be noticed, which indicates a strong electron±photon coupling and means that the recombing charges are strongly localized. Certainly, the PL mechanism of the nanowires needs to remain for further investigation. In conclusion, crystalline GeONWs have been synthesized in large scale by using carbothermal reduction approach at 840 °C in ¯owing nitrogen atmosphere. The growth of GeONWs is most likely controlled by the VS mechanism. The intensive blue PL band at 485 nm has been observed, which is attributed into radiative recombination between an electron on VO and a hole on VGe ; VO in the GeONWs. The GeONWs may have potential application in future integrated optical devices.
Acknowledgements
Fig. 4. (a) PL spectrum of the GeONWs at room temperature under excitation at 221 nm. (b) PL spectra of the samples under excitation at 221 nm: (1) PL spectrum of as-grown GeONWs; (2) PL spectrum of GeO2 powders (spectral purity). (c) PL spectra of GeO2 powders under dierent excitation wavelength: (1) at 234 nm; (2) at 221 nm; (3) at 325 nm.
h, PL intensity increased obviously. It is therefore reasonable for us to believe that the intensive blue light emission can be attributed to oxygen vacan-
This work was supported by the Ministry of Science and Technology of China (NKBRSFG19990646), National Science Foundation under contract NSF 59872043, the Fundamental Science Bureau, Academia Sinica. References [1] S.M. Prokes, K.L. Wang, Mater. Res. Sci. Bull. 24 (1999) 13. [2] J. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435.
214
X.C. Wu et al. / Chemical Physics Letters 349 (2001) 210±214
[3] F. Buda, J. Kohano, M. Parrinello, Phys. Rev. Lett. 69 (1992) 1272. [4] M.H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Adv. Mater. 13 (2001) 113. [5] X.C. Wu, W.H. Song, K.Y. Wang, T. Hu, B. Zhao, Y.P. Sun, J.J. Du, Chem. Phys. Lett. 336 (2001) 53. [6] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [7] M. Zacharias, P.M. Fauchet, J. Non-Cryst. Solids 227 (1998) 1058. [8] Y. Maeda, Phys. Rev. B 51 (1995) 1658.
[9] Z.G. Bai, D.P. Yu, H.Z. Zhang, Y. Ding, Y.P. Wang, X.Z. Gai, Q.L. Hang, G.C. Xiong, S.Q. Feng, Chem. Phys. Lett. 303 (1999) 311. [10] Y. Zhang, J. Zhu, Q. Zhang, Y. Yan, N. Wang, X. Zhang, Chem. Phys. Lett. 317 (2000) 504. [11] Y. Mizuhara, M. Noguchi, T. Ishihara, Y. Takita, J. Am. Ceram. Soc. 78 (1995) 109. [12] H.M. Jennings, J. Mater. Sci. 18 (1983) 951. [13] X.C. Wu, W.H. Song, W.D. Huang, M.H. Pu, B. Zhao, Y.P. Sun, J.J. Du, Chem. Phys. Lett. 328 (2000) 5.
Chemical Physics Letters 349 (2001) 210±214 www.elsevier.com/locate/cplett
Preparation and photoluminescence properties of crystalline GeO2 nanowires X.C. Wu b
a,*
, W.H. Song b, B. Zhao b, Y.P. Sun b, J.J. Du
b
a Department of Chemistry, Nanjing University, Nanjing 210093, China Laboratory of Internal Friction and Defects in Solids, Institute of Solid State Physics, Academia Sinica, Hefei 230031, China
Received 23 May 2001; in ®nal form 1 October 2001
Abstract Bulk-quantity GeO2 nanowires (GeONWs) have been synthesized by carbothermal reduction reaction between germanium dioxide and active carbons. Transmission electron microscopy (TEM) image shows the formation of the nanowires at a diameter about 50±120 nm and a length up to hundreds of micrometers. The nanowires can emit stable and high brightness blue light at 485 nm (2.56 eV) under excitation at 221 nm (5.61 eV). The intensity of the emission is one order of magnitude higher than that of GeO2 powders. The photoluminescence (PL) may originate from radiative recombination between an electron on VO and a hole on VGe ; VO in the GeONWs. The nanowires may have potential applications in one-dimensional optoelectronic nanodevices. Ó 2001 Published by Elsevier Science B.V.
1. Introduction One-dimensional quantum wires are expected to play a vital role as both interconnects and functional components in future mesoscopic electronic and optical devices, and also to provide an opportunity to test fundamental quantum mechanics concepts [1,2]. The electronic and optical properties of the nanowires strongly depend on size and dimensionality. For instance, as the wire diameter approaches the carrier de Broglie wavelength, quantum con®nement eects shift band gap energies and induce visible photoluminescence (PL) for Si [3]. Optical nanowires may have found
*
Corresponding author. E-mail addresses: [email protected], wuxingcai@chi na.com (X.C. Wu), .
applications in the ®elds of localization of light lower dimensional waveguide, and scanning near®eld optical microscopy (SNOM). Therefore, it is important to synthesize nanowires with optical properties and speci®c sizes that can meet the demands of further applications. So far, several important nanowires (nanobelts) of semiconducting oxides such as ZnO [4], SiO2 [5], SnO2 [6] and so on, have been synthesized. They may be used in designing nanoscale optoelectronic devices. GeO2 nanocrystal is a blue PL material, with peak energies around 3.1 eV (400 nm) and 2.2 eV (563.6 nm) [7,8], and germanium oxide-based glass is thought to be more refractive than the corresponding silicate glass so that the GeO2 nanowires (GeONWs) may be used for nanoconnections in future optoelectronic communication. The fabrication and structure of crystalline GeONWs were once reported, but the nanowires were synthesized
0009-2614/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 1 2 1 3 - 1
X.C. Wu et al. / Chemical Physics Letters 349 (2001) 210±214
211
by the physical evaporation [9] and by the carbon nanotube-con®ned reaction [10], and the PL was not dealt with. Both the above methods are complex relatively, as compared with carbothermal reduction method. In this Letter, we report the carbothermal reduction synthesis and characterization and PL properties of the nanowires. The growth process and PL mechanism of the nanowires have been discussed, respectively. 2. Experimental The preparation processes of the GeONWs were as follows. The mixtures of GeO2 (1.5 g) with active carbon (1.5 g) were ground for 15 min, and put into alumina boat. Then the boat was covered with ceramic plates, placed at the center of conventional horizontal furnace with a quartz tube £ 2:5 cm 100 cm and remained at 840 °C for 3.5 h in ¯owing nitrogen atmosphere (20 ml/min). A white product was found to deposit on the surface of the plate and the inner wall of the boat. Transmission electron microscopy (TEM) images of the products were taken with a JEM-200CX transmission electron microscope. Powder X-ray diraction (XRD) pattern of the products was obtained with Philips PW1700 X-ray diractometer with Cu-Ka radiation. The composition of GeONWs was determined by the X-ray photoemission spectra (XPS), which were recorded on a VGESCALAB MKII X-ray photoemission electron spectrometer. XPS data were collected in the constant analyzer energy (CAE) mode at 20 eV. Mg-Ka (hm 1253:6 eV) radiation was employed as excitation source with an anode voltage of 12 KV and an emission current of 29 mA. PL spectra of the samples were measured in a Hitachi 850 ¯uorescence spectrophotometer with a Xe lamp at room temperature. The excitation wavelength was 221 nm, and the ®lter wavelength was 290 nm. 3. Results and discussion TEM micrograph in Fig. 1a shows the general morphology and dimension of GeONWs. The nanowires have a diameter ranging from 50 to 120
Fig. 1. TEM images of GeONWs: (a) general morphology of GeONWs; (b) selected electronic diraction pattern of GeONWs along [2 2 2] axis.
nm and a length up to hundreds of micrometers by scanning throughout the sample. Fig. 1b is selected area electron diraction (SAED) pattern of the GeONWs along [2 2 2] axis. The SAED micrograph shows that the nanowires are crystalline. The structure of the products was also characterized by XRD. All peaks (Fig. 2) can be determined as hexagonal GeO2 crystal (ASTM card No. 4-497) according to the peak positions and their relative intensities. No carbide such as GeC was observed. The survey spectrum in Fig. 3a also displays C (1s) peaks, which can be attributed to a small amount of the residual. Figs. 3b and c corresponded to the binding energies of Ge (2p) and O (1s) for GeO2 , respectively. The quanti®cation of the peaks reveals that atomic ratio of Ge to O is 1:1.75. The chemical formula of the nanowires is GeO1:75 . Although, the points on the nanowires (con®rmation of the vapor±solid (VS) growth
212
X.C. Wu et al. / Chemical Physics Letters 349 (2001) 210±214
Fig. 2. X-ray diraction pattern of the sample.
mechanism) could not be observed, considering that no catalyst was added in the process of the nanowires' growth, and that no eutectic droplets could be seen at the tops of the nanowires, we think that the nanowires grow in terms of the VS mechanism [11,12]. The viewpoint is consistent with that of [8]. The growth process of the nanowires could be divided into three steps. The ®rst step is that GeO2 is reduced by active carbon to Ge and GeO into vapor phase. The second step is that the vapor of Ge and GeO is driven by ¯owing nitrogen atmosphere and deposits on the surface of alumina boat and plate to form de®cient-oxygen germanium oxide crystalline nuclei. The nuclei further grow into nanowires according to helical dislocation mechanism with the proceeding of the carbothermal reduction reaction. Oxygen may be attributed to the incoming of air. The growth process is similar to that of b-Ga2 O3 nanowires too [13]. The most striking properties of the GeONWs are that they emit stable and highly bright blue light. As is shown in Fig. 4a, a broad PL peak was clearly observed at 485 nm under the excitation at 221 nm at room temperature. The intensity of the more intensive peak was found more than one order of magnitude higher than that of the GeO2 powders (spectral purity) as shown in Fig. 4b. The PL peak positions are dierent from either that (400, 563.6 nm) of SiOx ®lm containing Ge and GeO2 nanocrystals [4] or that of the GeO2 powders as shown in Fig. 4c. The GeONWs are prepared
Fig. 3. XPS of the sample.
under reducing condition, and GeO is volatilized at high temperature, so that oxygen vacancies and oxygen±germanium vacancy pairs are easily produced. XPS of the sample has con®rmed that the oxygen defects of the nanowire exist. After the nanowires were annealed in oxygen at 600 °C for 2
X.C. Wu et al. / Chemical Physics Letters 349 (2001) 210±214
213
cies and germanium±oxygen vacancies centers. The acceptors would be formed by VGe ; VO , and the donors would be formed by VO . After excitation of the acceptor, a hole on the acceptor and an electron on a donor are created according to the following formal equation VGe ; VO 00 VO 2hm ! VGe ; VO VO The blue emission occurs via the reverse reaction. Luminescence process would be divided into two steps. First, an electron in donor band is captured by a hole on an acceptor to form a trapped exciton. Secondly, the trapped exciton recombines radiatively emitting a blue photon. An important stokes shift of about 3.05 eV can be noticed, which indicates a strong electron±photon coupling and means that the recombing charges are strongly localized. Certainly, the PL mechanism of the nanowires needs to remain for further investigation. In conclusion, crystalline GeONWs have been synthesized in large scale by using carbothermal reduction approach at 840 °C in ¯owing nitrogen atmosphere. The growth of GeONWs is most likely controlled by the VS mechanism. The intensive blue PL band at 485 nm has been observed, which is attributed into radiative recombination between an electron on VO and a hole on VGe ; VO in the GeONWs. The GeONWs may have potential application in future integrated optical devices.
Acknowledgements
Fig. 4. (a) PL spectrum of the GeONWs at room temperature under excitation at 221 nm. (b) PL spectra of the samples under excitation at 221 nm: (1) PL spectrum of as-grown GeONWs; (2) PL spectrum of GeO2 powders (spectral purity). (c) PL spectra of GeO2 powders under dierent excitation wavelength: (1) at 234 nm; (2) at 221 nm; (3) at 325 nm.
h, PL intensity increased obviously. It is therefore reasonable for us to believe that the intensive blue light emission can be attributed to oxygen vacan-
This work was supported by the Ministry of Science and Technology of China (NKBRSFG19990646), National Science Foundation under contract NSF 59872043, the Fundamental Science Bureau, Academia Sinica. References [1] S.M. Prokes, K.L. Wang, Mater. Res. Sci. Bull. 24 (1999) 13. [2] J. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435.
214
X.C. Wu et al. / Chemical Physics Letters 349 (2001) 210±214
[3] F. Buda, J. Kohano, M. Parrinello, Phys. Rev. Lett. 69 (1992) 1272. [4] M.H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Adv. Mater. 13 (2001) 113. [5] X.C. Wu, W.H. Song, K.Y. Wang, T. Hu, B. Zhao, Y.P. Sun, J.J. Du, Chem. Phys. Lett. 336 (2001) 53. [6] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [7] M. Zacharias, P.M. Fauchet, J. Non-Cryst. Solids 227 (1998) 1058. [8] Y. Maeda, Phys. Rev. B 51 (1995) 1658.
[9] Z.G. Bai, D.P. Yu, H.Z. Zhang, Y. Ding, Y.P. Wang, X.Z. Gai, Q.L. Hang, G.C. Xiong, S.Q. Feng, Chem. Phys. Lett. 303 (1999) 311. [10] Y. Zhang, J. Zhu, Q. Zhang, Y. Yan, N. Wang, X. Zhang, Chem. Phys. Lett. 317 (2000) 504. [11] Y. Mizuhara, M. Noguchi, T. Ishihara, Y. Takita, J. Am. Ceram. Soc. 78 (1995) 109. [12] H.M. Jennings, J. Mater. Sci. 18 (1983) 951. [13] X.C. Wu, W.H. Song, W.D. Huang, M.H. Pu, B. Zhao, Y.P. Sun, J.J. Du, Chem. Phys. Lett. 328 (2000) 5.