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Investigation of the surface properties of gold nanowire arrays
Applied Surface Science 258 (2011) 147–150
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Investigation of the surface properties of gold nanowire arrays Huijun Yao a,b,∗ , Dan Mo a , Jinglai Duan a , Yonghui Chen a , Jie Liu a,∗ , Youmei Sun a , Mingdong Hou a , Thomas Schäpers b a b
Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Peter Grünberg Institute (PGI-9) and JARA-FIT Jülich-Aachen Research Alliance, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany
a r t i c l e
i n f o
Article history: Received 11 October 2010 Received in revised form 3 August 2011 Accepted 3 August 2011 Available online 10 August 2011 Keywords: Gold nanowires Surface plasmon resonance (SPR) XPS
a b s t r a c t Gold nanowire arrays with diameters ranging from 45 to 200 nm were obtained via electrochemical deposition within the ion-track templates. The morphology of gold nanowires was imaged by scanning electron microscopy (SEM). The surface properties were investigated by surface plasmon resonance (SPR) and X-ray photoelectron spectroscopy (XPS). The SPR peaks were observed as the gold nanowire arrays embedded in the templates and their intensity decreased after the sample exposed to the air for a certain time due to the formation of chemisorbed oxygen on nanowire surface. The positive binding energy shifts in Au core level was found when the gold nanowire arrays embodied in template and the initialand finial-state effects were introduced to explain this phenomenon. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Gold is the much noble metal due to the lack of reactivity [1] and its unique properties, such as very good electrical and thermal conductivity, high ductility and chemical inertness, which make it useful for fabricating some electronic components used in semiconductor technology. For example, it is used in first- or second-level metallization and as a plating metal during the backside processing of GaAs devices [2]. When the gold size is in the nanometer regime, it will exhibit different electronic properties [3,4] compared to the bulk. The effects of finite nano-size play an important role in the special electronic properties and are already intensively studied in the past [5,6]. Recent years, the nanotechnology has brought us many novel properties about gold. Catalytic activity for example can be strongly enhanced for nanosized metals that normally do not or do only weakly show such behaviour as bulk material. Optical properties may be changed due to redistribution of valence band electrons [7]. The anti-Hall–Petch effect is found in the polycrystalline gold nanowires with grain size around 10 nm [8] and remarkably failure current density as high as 4.94 × 108 A/cm2 is observed in an individual gold nanowire [9]. Recently, several reports have shown that oxidation occurs when a gold film is exposed to highly reactive chemical environments, such as O2 -plasma discharge or in UV/ozone [10,11], typically altering the electrical characteristics of
∗ Corresponding authors at: Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China. Tel.: +86 931 4969334; fax: +86 931 4969334. E-mail addresses: [email protected] (H. Yao), [email protected] (J. Liu). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.08.021
the material. As to gold nanowires, their surface states not only contribute to the electrical properties [12] but also play an important role in the optical properties. The X-ray photoelectron spectroscopy (XPS) is one of the most widely used experimental methods to identify metal oxide species in metal catalysts [13–15]. It can provide us additional information related to the electrical properties on the surface of the gold crystallites [16]. It is also evidenced that the oxygen element as a kind of hydrocarbon contamination adsorbed on the surface of the gold nanowires through XPS spectrum [17]. In this work, the gold nanowires were prepared in ion-track template with electrodeposition method which had been proved to be a kind of powerful tool to control nanowire’s shape, structure and density in nanowire preparing [7,8,18–20]. The changes of the extinction spectra related to gold nanowire surface properties were analyzed. The oxidation of gold nanowires had been conclusively elucidated and chemisorbed oxygen formed on the wire surface. The changing of surface electrical properties because of the initial- and final-state effects was also discussed. However, the relationship between the nobility of gold and the formation of its oxide and the effect of the oxide on the electrical, chemical and physical properties of gold metal still remained unclear. 2. Experiment The 30 m polycarbonate (PC) membranes (Makrofol N, Bayer Leverkusen) were irradiated at the UNILAC linear accelerator of GSI (Darmstadt, Germany) with Au ions (kinetic energy 11.4 MeV/u, fluence 1 × 108 ions/cm2 ) in normal incidence. After irradiation, each side of the membrane was exposed to UV light for 2 h in
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Fig. 1. FESEM images of gold nanowires with diameter of 80 nm. The nanowires are prepared under the deposition voltage of 1.5 V at room temperature and the growth time for (a) and (b) are 1 min and 30 min, respectively.
order to enhance the ratio of track etching rate over bulk etching rate. This step could guarantee the etched pores to be fine cylindrical shape in next etching process. Then, the membranes were etched in 5 mol/L NaOH solution at 50 ◦ C for 1–5 min to obtain ion-track templates with nanopores’ diameters ranging from 45 to 200 nm. During etching process, an ultrasonic field was employed to achieve homogeneous pores etching. Then the membrances were washed immediately in distilled water for three times to remove the residual NaOH solution absorbed on the membrance surface and ion-track pore walls. The cleaned membrances were dried naturally without any stress to avoid nanopore deforming. A gold film with 50 nm thickness was sputtered onto one side of the membrane and reinforced electrochemically with a copper layer in tens microns. This gold/copper layer served later as a conducting substrate cathode and a platinum wire as the anode during electrodepositing gold nanowires in the pores. The employed electrolyte was Na3 Au (SO3 )2 solution (concentration = 0.1 mol/L) and the applied electrodepositing voltage was chosen as 1.5 V. The growth process of gold nanowires was conducted at room temperature and monitored through recording the depositing current versus time curves [21]. After that, the membrane was dissolved by dichloromethane (CH2 Cl2 ) and the gold nanowires still stayed on the gold/copper substrate. The morphology and the size of deposited gold nanowires were investigated by scanning electron microscopy (SEM, JSM 6701). The optical properties of gold nanowire arrays embedded in ion-track template after removing the gold/copper substrate were studied by UV/Vis/NIR spectrophotometer (Lambda 900, Perkin–Elmer) and the surface electrical properties of gold nanowires were investigated by X-ray photoelectron spectroscopy (XPS, PHI-5702, Perkin–Elmer). 3. Results and discussion The gold nanowires were successfully synthesized with different diameters ranging from 45 to 200 nm. SEM images of the resulting gold nanowires reveal that the wires have excellent cylindrical shape, smooth and homogeneous morphology along the wires’ length as illustrated in Fig. 1. The diameter of the gold nanowires depends on the pore size of the template utilized and the length of the nanowires is decided by electrodepositing time. Fig. 1(a) and (b) shows the nanowires with the diameter of 80 nm but the length of 11 m and 30 m, respectively. The extinction spectrum of gold nanowire arrays was explored after removing the gold/copper substrate from the PC template. The schematic representation of spectral measurement is shown in Fig. 2 and the inject light is paralleled to the gold nanowire arrays. The results of the extinction spectra of gold nanowires are shown in Fig. 3. The first extinction peak at 397 nm could be attributed to the PC template and the second larger one is excited by the
Fig. 2. The schematic description of transmission mode for investigating the optical properties of gold nanowire arrays. The gold/copper substrate is removed and the inject light is paralleled to the gold nanowires arrays in this measurement.
transverse mode of gold nanowire arrays [22]. Fig. 3(a) and (b) shows the extinction spectra of gold nanowire arrays with the diameter 100 and 200 nm after exposing to air for 1 day (asprepared) and 125 days, respectively. The extinction peaks of as-prepared gold nanowires with diameter of 100 and 200 nm are initially located at 580 and 690 nm and both shift towards short wavelengths after the samples exposing to air at room temperature for 125 days (Fig. 3 triangle symbols), whilst the intensity of the extinction peaks become weaker than before. Noble metal materials’ optical absorbance properties originate from localized surface plasmon resonance (SPR) and the SPRs are essentially light waves that are trapped on the surface due to the interaction with the free electrons of the metal. The free electrons will oscillate in a collective fashion when the light wave meets the resonance conditions. For the dielectric constant of matrix (PC template), the size and the number concentration of nanowires being constant in our experiment, the dielectric properties of the material (especially the surface of material) are extremely important and play an important role in the intensity and placement of the plasmon resonances [23]. The shifting of the extinction peaks of the gold nanowire arrays with the exposing time could infer the changing of the surface electron statement [12] or surface electrical properties of nanowires. To shed light on the surface properties of gold nanowire before and after exposing to air, the XPS analysis was conducted on the gold nanowire arrays. The gold nanowires embodied in the PC template with diameter of 200 nm was used (the inset of Fig. 4) and surface charge effect (charge buildup on the surface of PC template) was avoided by the standard technique of flooding the sample with thermal electrons. Fig. 4 shows the XPS spectra from the Au-4f core level region of gold nanowire arrays. The spectrum (i) and (ii) in Fig. 4 were measured from as-prepared gold nanowire arrays and nanowires after six months exposing to air, respectively. These two spectra are extremely similar and two main peaks observed
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Fig. 3. Extinction spectra of gold nanowire arrays for different exposing time. The density of the gold nanowire arrays is 108 cm−2 and the length is 30 m, which equates with the membrance’s thickness. The diameters of the gold nanowire investigated here are (a) 100 nm and (b) 200 nm, respectively. The circle means the spectra from as prepared sample and the triangle represents the sample after exposing 125 days.
Fig. 4. XPS spectra corresponding to the Au-4f core level of gold nanowires embodied in the PC template. Spectra (i) and (ii) were measured from as-prepared gold nanowires and exposing to air six months, respectively.
with maxima at 85.7 ± 0.2 and 89.4 ± 0.2 eV are assigned to the 4f7/2 and 4f5/2 core levels of Au0 or Au3+ . For the as-prepared sample, although there is a positive binding energy (BE) shifts compared to Au-bulk (Au0 -4f7/2 = 84.0 eV) [24,25], the XPS signal should attribute to Au0 , which means there is no Au oxide (Au2 O3 ) on the surface of gold nanowires. As to the gold nanowires exposing to air for six months shown in spectrum (ii), there is no more positive BE shifts or additional satellite peaks can be observed, which demonstrates the same kind of electronic state of Au0 as displayed in spectrum (i). The possible reason for the same positive BE shifts (compare to 84.0 eV) in spectrum (i) and (ii) maybe due to the PC template which wraps the gold nanowire arrays as a matrix. In order to avoid the influence of PC template on gold nanowire surface properties, further XPS analysis was done by removing the PC template, as shown in the inset of Fig. 5. In this case, the copper/copper substrate (used as cathode during electrodepositing gold nanowires) is chosen instead of normal gold/copper substrate to eliminate the disturbance of thin gold film on Au-4f XPS signal. In Fig. 5, spectrum (i) and (ii) are also acquired from asprepared gold nanowires and gold nanowires after exposing to air for six months, respectively. They show the similar spectra and the main peaks are located at 84.0 ± 0.2 and 87.7 ± 0.2 eV which consist of Au-bulk (Au0 -4f7/2 = 84.0 eV and Au0 -4f5/2 = 87.7 eV). The two Au-4f XPS spectra indicate that, after dissolving the PC template with dichloromethane, the BE peaks of Au-nanowire move to their “original” positions, which denote pure gold species without any nonmetallic gold (for example, gold oxide). Comparing Figs. 4 and 5, the positive BE shifts in Fig. 4 is strongly dependent on the matrix
Fig. 5. XPS spectra corresponding to the Au-4f core level of gold nanowires without PC template. Spectra (i) and (ii) were acquired from as-prepared gold nanowires and exposing to air six months, respectively.
[26,27]. The positive BE shifts in the core band can be explained by the following two possible mechanisms: (1) initial- and (2) finalstate effects [28]. The initial-state effect is related to the electronic structure of the gold nanowires, and the final-state effect is a manifestation of the positive charges left on the surface of the wires during the photoemission process. The changes in the nanowires electronic structure are unavoidable as the size changes, and initialstate effect can never be neglected independently of the matrix used [28]. In the photoemission final-state, the positive charges will leave on the nanowires when the matrix has poor conductivity and the charges are not instantly neutralized. Positive charges will produce broadening the full width at half maximum of BE and shifts to high binding energy for both core and valence levels [29], as shown in Fig. 4. Comparing the XPS spectra in Figs. 4 and 5, there is no additional species found on the surface of gold nanowires even after exposing to air six months. However, as shown in Fig. 3, there should be something formed or adsorbed on the surface of gold nanowires which decrease the extinction ability after a six months storing in the air. It seems paradox if take the XPS analysis and extinction spectra together here, but exactly not. According to the work of Min et al. [30], there are three types of oxygen species existed on Au (1 1 1): (a) chemisorbed oxygen (oxygen bound to gold that is not part of an ordered phase), (b) oxygen in surface oxide (well-ordered two-dimensional phase), and (c) subsurface oxygen or bulk oxide (three-dimensional phase). For gold nanoparticles or films, only
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undergoing special treatments, such as exposing to atomic oxygen [31] or molecular oxygen assisted with X-ray irradiation [25], the surface oxide or bulk oxide can formed and the oxide species can be detected by XPS. In present study, the gold nanowire arrays are embodied in the PC template, oxygen in surface oxide or subsurface are not formed even after exposing to air for six months. Along the ion-track channel in PC template, the long chain of the organic polymer is broken and fragment of chains forms after ion irradiation and chemical etching. In the process of electrodepositing gold nanowire to the ion-track template, the inner wall of nanopore confines the gold material and has a good contact with it. The deposited gold nanowires can chemisorb oxygen from broken C O or C O bonds which belong to the fragment of chains. With increasing storage time, more oxygen (mainly from fragment) will be chemisorbed on the gold surface and the free electrons decreased, which caused the losing of surface plasmon resonance. The minor charges on the surface of gold nanowire can be detected by the extinction spectra from surface plasmon resonances, but are still under the detectability of XPS. 4. Conclusions The gold nanowire arrays with different diameter and length were obtained in the etched ion-track templates by electrochemical depositing method. Their SPR properties were analyzed using UV/Vis/NIR spectrophotometer. The results showed that the intensity of the extinction peak decreased and the peak position blue shifted after the gold nanowires exposing to atmosphere environment for a certain time. Comparing the XPS results of the gold nanowires embodied in PC template and without template, BE changes in Au core level structure appear to be dominated by the effect of the positive charges left on the nanowire in the photoemission final-state, which produces a broadening and shift to high BE. As one of the oxidization mechanisms, chemisorbed oxygen happened when gold nanowires embedded in the PC template. Finally, the extinction spectra can be considered as an indirect but useful, convenient and lossless method to monitor the change taking place on the surface of nanomaterials. Acknowledgements One of the authors (H.J. Yao) would like to thank the Chinese Academy of Sciences (CAS) for research scholarships as visiting scientist at Forschungszentrum Jülich, Germany. The authors gratefully acknowledge the finance support from the West Light project of the Chinese Academy of Sciences, National Natural Science Foundation of China (no. 11005134, 10805062 and 10975164) and the Natural Science Foundation of Gansu Province (1007RJYA014). The authors also thank the Materials Research Department of GSI (Darmstadt, Germany) for the performing of ion irradiation experiments on polycarbonate membrane and Dr. Besmehn Astrid (Jülich, Germany) for XPS analysis and helpful discussion. References [1] B. Hammer, J.K. Norskov, Why gold is the noblest of all the metals, Nature 376 (1995) 238–240. [2] T. Ishikawa, K. Okaniwa, M. Komaru, K. Kosaki, Y. Mitsui, A High-power gaAsfet having buried plated heat sink for high-performance mmics, IEEE Trans. Electron Devices 41 (1994) 3–9. [3] W.D. Williams, N. Giordano, Experimental study of localization and electronelectron interaction effects in thin Au wires, Phys. Rev. B 33 (1986) 8146–8154. [4] S. Karim, W. Ensinger, T.W. Cornelius, R. Neumann, Investigation of size effects in the electrical resistivity of single electrochemically fabricated gold nanowires, Phys. E: Low-dimensional Syst. Nanostruct. 40 (2008) 3173–3178.
[5] D. Dalacu, L. Martinu, Optical properties of discontinuous gold films: finite-size effects, J. Opt. Soc. Am. B-Opt. Phys. 18 (2001) 85–92. [6] S. Shukla, S. Seal, Cluster size effect observed for gold nanoparticles synthesized by sol-gel technique as studied by X-ray photoelectron spectroscopy, Nanostruct. Mater. 11 (1999) 1181–1193. [7] J.L. Duan, T.W. Cornelius, J. Liu, S. Karim, H.J. Yao, O. Picht, M. Rauber, S. Muller, R. Neumann, Surface plasmon resonances of Cu nanowire arrays, J. Phys. Chem. C 113 (2009) 13583–13587. [8] J. Liu, J.L. Duan, E. Toimil-Molares, S. Karim, T.W. Cornelius, D. Dobrev, H.J. Yao, Y.M. Sun, M.D. Hou, D. Mo, Z.G. Wang, R. Neumann, Electrochemical fabrication of single-crystalline and polycrystalline Au nanowires: the influence of deposition parameters, Nanotechnology 17 (2006) 1922–1926. [9] Y. Peng, T. Cullis, B. Inkson, Accurate electrical testing of individual gold nanowires by in situ scanning electron microscope nanomanipulators, Appl. Phys. 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Barz, A. Gluhoi, B.E. Nieuwenhuys, Y.N. Xie, F. Aldea, M. Neumann, Crystalline and electronic structure of gold nanoclusters determined by EXAFS, XRD and XPS methods, in: 5th International Romanian Conference on advanced materials, Natl. Inst. Optoelectronics, Bucharest Magurele, Romania, 2006, pp. 1555–1560. [17] P. Forrer, F. Schlottig, H. Siegenthaler, M. Textor, Electrochemical preparation and surface properties of gold nanowire arrays formed by the template technique, J. Appl. Electrochem. 30 (2000) 533–541. [18] Y.F. Chen, J. Liu, H.J. Yao, D. Mo, J.L. Duan, M.D. Hou, Y.M. Sun, L. Zhang, K. Maaz, Electrochemical polymerization and characterization of polypyrrole nanowires and nanotubules, Phys. B: Condens. Matter 405 (2010) 2461– 2465. [19] D. Mo, J. Liu, H.J. Yao, J.L. Duan, M.D. Hou, Y.M. Sun, Y.F. Chen, Z.H. Xue, L. Zhang, Preparation and characterization of CdS nanotubes and nanowires by electrochemical synthesis in ion-track templates, J. Cryst. Growth 310 (2008) 612–616. [20] J.L. Duan, J. Liu, H.J. Yao, D. Mo, M.D. Hou, Y.M. Sun, Y.F. Chen, L. Zhang, Controlled synthesis and diameter-dependent optical properties of Cu nanowire arrays, Mater. Sci. Eng. B 147 (2008) 57–62. [21] M.E. Toimil-Molares, V. Buschmann, D. Dobrev, R. Neumann, R. Scholz, I.U. Schuchert, J. Vetter, Single-crystalline copper nanowires produced by electrochemical deposition in polymeric ion track membranes, Adv. Mater. 13 (2001) 62–65. [22] S.Y. Yim, H.G. Ahn, K.C. Je, M. Choi, C.W. Park, H. Ju, S.H. Park, Observation of redshifted strong surface plasmon scattering in single Cu nanowires, Opt. Express 15 (2007) 10282–10287. [23] E.R.I. Cleveland, S. George, S. Kevin, Optical properties of gold nanospheres, Nanoscape 2 (2005) 17–25. [24] B.R. Cuenya, S.H. Baeck, T.F. Jaramillo, E.W. McFarland, Size- and supportdependent electronic and catalytic properties of Au0 /Au3+ nanoparticles synthesized from block copolymer micelles, J. Am. Chem. Soc. 125 (2003) 12928–12934. [25] P. Jiang, S. Porsgaard, F. Borondics, M. Kober, A. Caballero, H. Bluhm, F. Besenbacher, M. Salmeron, Room-temperature reaction of oxygen with gold: an in situ ambient-pressure X-ray photoelectron spectroscopy investigation, J. Am. Chem. Soc. 132 (2010) 2858–2859. [26] R. Radnik, C. Mohr, P. Claus, On the origin of binding energy shifts of core levels of supported gold nanoparticles and dependence of pretreatment and material synthesis, PCCP 5 (2003) 172–177. [27] D. Dalacu, J.E. Klernberg-Sapieha, L. Martinu, Substrate and morphology effects on photoemission from core-levels in gold clusters, Surf. Sci. 472 (2001) 33–40. [28] M.G. Mason, Electronic-structure of supported small metal-clusters, Phys. Rev. B 27 (1983) 748–762. [29] A. Howard, D.N.S. Clark, C.E.J. Mitchell, R.G. Egdell, V.R. Dhanak, Initial and final state effects in photoemission from Au nanoclusters on TiO2 (1 1 0), Surf. Sci. 518 (2002) 210–224. [30] B.K. Min, A.R. Alemozafar, D. Pinnaduwage, X. Deng, C.M. 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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Investigation of the surface properties of gold nanowire arrays Huijun Yao a,b,∗ , Dan Mo a , Jinglai Duan a , Yonghui Chen a , Jie Liu a,∗ , Youmei Sun a , Mingdong Hou a , Thomas Schäpers b a b
Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Peter Grünberg Institute (PGI-9) and JARA-FIT Jülich-Aachen Research Alliance, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany
a r t i c l e
i n f o
Article history: Received 11 October 2010 Received in revised form 3 August 2011 Accepted 3 August 2011 Available online 10 August 2011 Keywords: Gold nanowires Surface plasmon resonance (SPR) XPS
a b s t r a c t Gold nanowire arrays with diameters ranging from 45 to 200 nm were obtained via electrochemical deposition within the ion-track templates. The morphology of gold nanowires was imaged by scanning electron microscopy (SEM). The surface properties were investigated by surface plasmon resonance (SPR) and X-ray photoelectron spectroscopy (XPS). The SPR peaks were observed as the gold nanowire arrays embedded in the templates and their intensity decreased after the sample exposed to the air for a certain time due to the formation of chemisorbed oxygen on nanowire surface. The positive binding energy shifts in Au core level was found when the gold nanowire arrays embodied in template and the initialand finial-state effects were introduced to explain this phenomenon. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Gold is the much noble metal due to the lack of reactivity [1] and its unique properties, such as very good electrical and thermal conductivity, high ductility and chemical inertness, which make it useful for fabricating some electronic components used in semiconductor technology. For example, it is used in first- or second-level metallization and as a plating metal during the backside processing of GaAs devices [2]. When the gold size is in the nanometer regime, it will exhibit different electronic properties [3,4] compared to the bulk. The effects of finite nano-size play an important role in the special electronic properties and are already intensively studied in the past [5,6]. Recent years, the nanotechnology has brought us many novel properties about gold. Catalytic activity for example can be strongly enhanced for nanosized metals that normally do not or do only weakly show such behaviour as bulk material. Optical properties may be changed due to redistribution of valence band electrons [7]. The anti-Hall–Petch effect is found in the polycrystalline gold nanowires with grain size around 10 nm [8] and remarkably failure current density as high as 4.94 × 108 A/cm2 is observed in an individual gold nanowire [9]. Recently, several reports have shown that oxidation occurs when a gold film is exposed to highly reactive chemical environments, such as O2 -plasma discharge or in UV/ozone [10,11], typically altering the electrical characteristics of
∗ Corresponding authors at: Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China. Tel.: +86 931 4969334; fax: +86 931 4969334. E-mail addresses: [email protected] (H. Yao), [email protected] (J. Liu). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.08.021
the material. As to gold nanowires, their surface states not only contribute to the electrical properties [12] but also play an important role in the optical properties. The X-ray photoelectron spectroscopy (XPS) is one of the most widely used experimental methods to identify metal oxide species in metal catalysts [13–15]. It can provide us additional information related to the electrical properties on the surface of the gold crystallites [16]. It is also evidenced that the oxygen element as a kind of hydrocarbon contamination adsorbed on the surface of the gold nanowires through XPS spectrum [17]. In this work, the gold nanowires were prepared in ion-track template with electrodeposition method which had been proved to be a kind of powerful tool to control nanowire’s shape, structure and density in nanowire preparing [7,8,18–20]. The changes of the extinction spectra related to gold nanowire surface properties were analyzed. The oxidation of gold nanowires had been conclusively elucidated and chemisorbed oxygen formed on the wire surface. The changing of surface electrical properties because of the initial- and final-state effects was also discussed. However, the relationship between the nobility of gold and the formation of its oxide and the effect of the oxide on the electrical, chemical and physical properties of gold metal still remained unclear. 2. Experiment The 30 m polycarbonate (PC) membranes (Makrofol N, Bayer Leverkusen) were irradiated at the UNILAC linear accelerator of GSI (Darmstadt, Germany) with Au ions (kinetic energy 11.4 MeV/u, fluence 1 × 108 ions/cm2 ) in normal incidence. After irradiation, each side of the membrane was exposed to UV light for 2 h in
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H. Yao et al. / Applied Surface Science 258 (2011) 147–150
Fig. 1. FESEM images of gold nanowires with diameter of 80 nm. The nanowires are prepared under the deposition voltage of 1.5 V at room temperature and the growth time for (a) and (b) are 1 min and 30 min, respectively.
order to enhance the ratio of track etching rate over bulk etching rate. This step could guarantee the etched pores to be fine cylindrical shape in next etching process. Then, the membranes were etched in 5 mol/L NaOH solution at 50 ◦ C for 1–5 min to obtain ion-track templates with nanopores’ diameters ranging from 45 to 200 nm. During etching process, an ultrasonic field was employed to achieve homogeneous pores etching. Then the membrances were washed immediately in distilled water for three times to remove the residual NaOH solution absorbed on the membrance surface and ion-track pore walls. The cleaned membrances were dried naturally without any stress to avoid nanopore deforming. A gold film with 50 nm thickness was sputtered onto one side of the membrane and reinforced electrochemically with a copper layer in tens microns. This gold/copper layer served later as a conducting substrate cathode and a platinum wire as the anode during electrodepositing gold nanowires in the pores. The employed electrolyte was Na3 Au (SO3 )2 solution (concentration = 0.1 mol/L) and the applied electrodepositing voltage was chosen as 1.5 V. The growth process of gold nanowires was conducted at room temperature and monitored through recording the depositing current versus time curves [21]. After that, the membrane was dissolved by dichloromethane (CH2 Cl2 ) and the gold nanowires still stayed on the gold/copper substrate. The morphology and the size of deposited gold nanowires were investigated by scanning electron microscopy (SEM, JSM 6701). The optical properties of gold nanowire arrays embedded in ion-track template after removing the gold/copper substrate were studied by UV/Vis/NIR spectrophotometer (Lambda 900, Perkin–Elmer) and the surface electrical properties of gold nanowires were investigated by X-ray photoelectron spectroscopy (XPS, PHI-5702, Perkin–Elmer). 3. Results and discussion The gold nanowires were successfully synthesized with different diameters ranging from 45 to 200 nm. SEM images of the resulting gold nanowires reveal that the wires have excellent cylindrical shape, smooth and homogeneous morphology along the wires’ length as illustrated in Fig. 1. The diameter of the gold nanowires depends on the pore size of the template utilized and the length of the nanowires is decided by electrodepositing time. Fig. 1(a) and (b) shows the nanowires with the diameter of 80 nm but the length of 11 m and 30 m, respectively. The extinction spectrum of gold nanowire arrays was explored after removing the gold/copper substrate from the PC template. The schematic representation of spectral measurement is shown in Fig. 2 and the inject light is paralleled to the gold nanowire arrays. The results of the extinction spectra of gold nanowires are shown in Fig. 3. The first extinction peak at 397 nm could be attributed to the PC template and the second larger one is excited by the
Fig. 2. The schematic description of transmission mode for investigating the optical properties of gold nanowire arrays. The gold/copper substrate is removed and the inject light is paralleled to the gold nanowires arrays in this measurement.
transverse mode of gold nanowire arrays [22]. Fig. 3(a) and (b) shows the extinction spectra of gold nanowire arrays with the diameter 100 and 200 nm after exposing to air for 1 day (asprepared) and 125 days, respectively. The extinction peaks of as-prepared gold nanowires with diameter of 100 and 200 nm are initially located at 580 and 690 nm and both shift towards short wavelengths after the samples exposing to air at room temperature for 125 days (Fig. 3 triangle symbols), whilst the intensity of the extinction peaks become weaker than before. Noble metal materials’ optical absorbance properties originate from localized surface plasmon resonance (SPR) and the SPRs are essentially light waves that are trapped on the surface due to the interaction with the free electrons of the metal. The free electrons will oscillate in a collective fashion when the light wave meets the resonance conditions. For the dielectric constant of matrix (PC template), the size and the number concentration of nanowires being constant in our experiment, the dielectric properties of the material (especially the surface of material) are extremely important and play an important role in the intensity and placement of the plasmon resonances [23]. The shifting of the extinction peaks of the gold nanowire arrays with the exposing time could infer the changing of the surface electron statement [12] or surface electrical properties of nanowires. To shed light on the surface properties of gold nanowire before and after exposing to air, the XPS analysis was conducted on the gold nanowire arrays. The gold nanowires embodied in the PC template with diameter of 200 nm was used (the inset of Fig. 4) and surface charge effect (charge buildup on the surface of PC template) was avoided by the standard technique of flooding the sample with thermal electrons. Fig. 4 shows the XPS spectra from the Au-4f core level region of gold nanowire arrays. The spectrum (i) and (ii) in Fig. 4 were measured from as-prepared gold nanowire arrays and nanowires after six months exposing to air, respectively. These two spectra are extremely similar and two main peaks observed
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Fig. 3. Extinction spectra of gold nanowire arrays for different exposing time. The density of the gold nanowire arrays is 108 cm−2 and the length is 30 m, which equates with the membrance’s thickness. The diameters of the gold nanowire investigated here are (a) 100 nm and (b) 200 nm, respectively. The circle means the spectra from as prepared sample and the triangle represents the sample after exposing 125 days.
Fig. 4. XPS spectra corresponding to the Au-4f core level of gold nanowires embodied in the PC template. Spectra (i) and (ii) were measured from as-prepared gold nanowires and exposing to air six months, respectively.
with maxima at 85.7 ± 0.2 and 89.4 ± 0.2 eV are assigned to the 4f7/2 and 4f5/2 core levels of Au0 or Au3+ . For the as-prepared sample, although there is a positive binding energy (BE) shifts compared to Au-bulk (Au0 -4f7/2 = 84.0 eV) [24,25], the XPS signal should attribute to Au0 , which means there is no Au oxide (Au2 O3 ) on the surface of gold nanowires. As to the gold nanowires exposing to air for six months shown in spectrum (ii), there is no more positive BE shifts or additional satellite peaks can be observed, which demonstrates the same kind of electronic state of Au0 as displayed in spectrum (i). The possible reason for the same positive BE shifts (compare to 84.0 eV) in spectrum (i) and (ii) maybe due to the PC template which wraps the gold nanowire arrays as a matrix. In order to avoid the influence of PC template on gold nanowire surface properties, further XPS analysis was done by removing the PC template, as shown in the inset of Fig. 5. In this case, the copper/copper substrate (used as cathode during electrodepositing gold nanowires) is chosen instead of normal gold/copper substrate to eliminate the disturbance of thin gold film on Au-4f XPS signal. In Fig. 5, spectrum (i) and (ii) are also acquired from asprepared gold nanowires and gold nanowires after exposing to air for six months, respectively. They show the similar spectra and the main peaks are located at 84.0 ± 0.2 and 87.7 ± 0.2 eV which consist of Au-bulk (Au0 -4f7/2 = 84.0 eV and Au0 -4f5/2 = 87.7 eV). The two Au-4f XPS spectra indicate that, after dissolving the PC template with dichloromethane, the BE peaks of Au-nanowire move to their “original” positions, which denote pure gold species without any nonmetallic gold (for example, gold oxide). Comparing Figs. 4 and 5, the positive BE shifts in Fig. 4 is strongly dependent on the matrix
Fig. 5. XPS spectra corresponding to the Au-4f core level of gold nanowires without PC template. Spectra (i) and (ii) were acquired from as-prepared gold nanowires and exposing to air six months, respectively.
[26,27]. The positive BE shifts in the core band can be explained by the following two possible mechanisms: (1) initial- and (2) finalstate effects [28]. The initial-state effect is related to the electronic structure of the gold nanowires, and the final-state effect is a manifestation of the positive charges left on the surface of the wires during the photoemission process. The changes in the nanowires electronic structure are unavoidable as the size changes, and initialstate effect can never be neglected independently of the matrix used [28]. In the photoemission final-state, the positive charges will leave on the nanowires when the matrix has poor conductivity and the charges are not instantly neutralized. Positive charges will produce broadening the full width at half maximum of BE and shifts to high binding energy for both core and valence levels [29], as shown in Fig. 4. Comparing the XPS spectra in Figs. 4 and 5, there is no additional species found on the surface of gold nanowires even after exposing to air six months. However, as shown in Fig. 3, there should be something formed or adsorbed on the surface of gold nanowires which decrease the extinction ability after a six months storing in the air. It seems paradox if take the XPS analysis and extinction spectra together here, but exactly not. According to the work of Min et al. [30], there are three types of oxygen species existed on Au (1 1 1): (a) chemisorbed oxygen (oxygen bound to gold that is not part of an ordered phase), (b) oxygen in surface oxide (well-ordered two-dimensional phase), and (c) subsurface oxygen or bulk oxide (three-dimensional phase). For gold nanoparticles or films, only
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undergoing special treatments, such as exposing to atomic oxygen [31] or molecular oxygen assisted with X-ray irradiation [25], the surface oxide or bulk oxide can formed and the oxide species can be detected by XPS. In present study, the gold nanowire arrays are embodied in the PC template, oxygen in surface oxide or subsurface are not formed even after exposing to air for six months. Along the ion-track channel in PC template, the long chain of the organic polymer is broken and fragment of chains forms after ion irradiation and chemical etching. In the process of electrodepositing gold nanowire to the ion-track template, the inner wall of nanopore confines the gold material and has a good contact with it. The deposited gold nanowires can chemisorb oxygen from broken C O or C O bonds which belong to the fragment of chains. With increasing storage time, more oxygen (mainly from fragment) will be chemisorbed on the gold surface and the free electrons decreased, which caused the losing of surface plasmon resonance. The minor charges on the surface of gold nanowire can be detected by the extinction spectra from surface plasmon resonances, but are still under the detectability of XPS. 4. Conclusions The gold nanowire arrays with different diameter and length were obtained in the etched ion-track templates by electrochemical depositing method. Their SPR properties were analyzed using UV/Vis/NIR spectrophotometer. The results showed that the intensity of the extinction peak decreased and the peak position blue shifted after the gold nanowires exposing to atmosphere environment for a certain time. Comparing the XPS results of the gold nanowires embodied in PC template and without template, BE changes in Au core level structure appear to be dominated by the effect of the positive charges left on the nanowire in the photoemission final-state, which produces a broadening and shift to high BE. As one of the oxidization mechanisms, chemisorbed oxygen happened when gold nanowires embedded in the PC template. Finally, the extinction spectra can be considered as an indirect but useful, convenient and lossless method to monitor the change taking place on the surface of nanomaterials. Acknowledgements One of the authors (H.J. Yao) would like to thank the Chinese Academy of Sciences (CAS) for research scholarships as visiting scientist at Forschungszentrum Jülich, Germany. The authors gratefully acknowledge the finance support from the West Light project of the Chinese Academy of Sciences, National Natural Science Foundation of China (no. 11005134, 10805062 and 10975164) and the Natural Science Foundation of Gansu Province (1007RJYA014). The authors also thank the Materials Research Department of GSI (Darmstadt, Germany) for the performing of ion irradiation experiments on polycarbonate membrane and Dr. Besmehn Astrid (Jülich, Germany) for XPS analysis and helpful discussion. References [1] B. Hammer, J.K. Norskov, Why gold is the noblest of all the metals, Nature 376 (1995) 238–240. [2] T. Ishikawa, K. Okaniwa, M. Komaru, K. Kosaki, Y. Mitsui, A High-power gaAsfet having buried plated heat sink for high-performance mmics, IEEE Trans. Electron Devices 41 (1994) 3–9. [3] W.D. Williams, N. Giordano, Experimental study of localization and electronelectron interaction effects in thin Au wires, Phys. Rev. B 33 (1986) 8146–8154. [4] S. Karim, W. Ensinger, T.W. Cornelius, R. Neumann, Investigation of size effects in the electrical resistivity of single electrochemically fabricated gold nanowires, Phys. E: Low-dimensional Syst. Nanostruct. 40 (2008) 3173–3178.
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