Aligned selenium microtubes array: Synthesis, growth mechanism and photoelectrical properties

Aligned selenium microtubes array: Synthesis, growth mechanism and photoelectrical properties

Chemical Physics Letters 510 (2011) 87–92 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/loca...

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Chemical Physics Letters 510 (2011) 87–92

Contents lists available at ScienceDirect

Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett

Aligned selenium microtubes array: Synthesis, growth mechanism and photoelectrical properties Emanuela Filippo ⇑, Daniela Manno, Antonio Serra Department of Material Science, University of Salento I, 73100 Lecce, Italy

a r t i c l e

i n f o

Article history: Received 4 April 2011 In final form 30 April 2011 Available online 6 May 2011

a b s t r a c t Aligned selenium microtubes array vertically grown on a silicon substrate was synthesized in a tubular furnace under argon flow at an evaporation temperature of 300 °C. The microtubes were characterized by Raman spectroscopy, X-ray diffraction, UV–vis spectroscopy, scanning and transmission electron microscopy. The photoelectrical properties of the microtube array with light were investigated. It was found a stable relative increase of the conductivity by 180% when the sample was taken from the dark and exposed with tungsten light and a sharp on/off switching behavior. These results hold promise for the fabrication of microtubes-detector arrays. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction In the past decade, single-crystal one dimensional (1D) nanostructures such as rods, wires, belts, ribbons and tubes have become the focus of intensive research owing to their unique properties such as space-confined transport phenomena and potential applications such as fluid paths, reservoirs in catalysis, fuel cells, sensors, separation systems and other novel applications in nanodevices [1]. Iijima’s discovery of carbon nanotube in 1991 inspired the interest in tubular inorganic nanostructures, in further, due to their unique physical and chemical properties that exhibit wide and practical applications [2]. Trigonal selenium (t-Se) has a variety of interesting properties, such as high photoconductivity, a relatively low melting point and nonlinear optical response; it has been used in solar cells, rectifiers, photographic exposure meters, medical diagnostics and xerography [3]. Recently, inspired by the unique 1-D nanostructure, much work has been concentrated on the synthesis of selenium nanowires and nanorods [4–9]. It has been reported that t-Se is apt to grow into 1-D nanostructure owing to its unique chains of atom that is favor of anisotropic growth. Compared to nanowires, the reports of selenium nanotubes are much fewer and most of the reported methods are chemical solution-based routes [9] which involved dissolution of selenium powder in various solvents [10], the reduction of different selenium precursors such as H2SeO3 [11], Na2SeSO3 [12], Na2SeO3 [13]. Recently, Zhang and coworkers presented a low-temperature refluxing method to synthesize multiarmed tubular crystalline t-Se using SeO2, PVP and hydrazine hydrate [14]; in a previous work, Zhang and coworkers [15] synthesized nanotubes by a compound ⇑ Corresponding author. Fax: +39 832297100. E-mail address: emanuela.fi[email protected] (E. Filippo). 0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.04.098

solution method composed of hydrothermal process and following sonication, using N2H4H2O as reducing agent. The research of alternative method to growth selenium nanotube let Zhang and coworkers [16] to prepare selenium nanotubes on the surface of Au sheet electrode by cyclic voltammetry, using a soft-template. To our knowledge, up to now only the above reports are about the synthesis of crystalline selenium nanotubes, and the formation mechanism is not very clear. We have concentrated on the exploration of a direct physical deposition process to prepare microstructures of t-Se because this method is convenient and reproducible and does not involve complicated chemical reactions. Recently, via a vapor–solid process (VS), we successfully prepared selenium microtubes and we found that the evaporation rate of selenium source was a crucial factor in the formation of the microtubes (slow evaporation rate let to the formation of microrods) [17]. However, the vapor phase synthesis of the nano and microtubes is not common and no report has demonstrated the vapor phase synthesis of tubular microstructures Se array. Yet, it is amazing that few reports on the photoconductivity of isolated Se 1-D nanomaterials have been proposed and that there have been very limited reports on the physical properties of the nanostructures of Se, especially as an electrical or photonic device [6,18,19]. Moreover, to best of our knowledge, only Deng et al. [20] reported the study of the photoconductivity of selenium nanowires array. Here we report for the first time the large scale, low-cost and rapid synthesis of selenium microtubes array via a vapor deposition method in a horizontal tube furnace under high argon flow gas at an evaporation temperature of 300 °C, without the use of any template. Large-yield t-Se microtubes array was synthesized with high purity using only selenium metal powder for the growth of the microtubes. The as-obtained arrays were characterized and their

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optical and photoelectrical properties were tested. As expected, the photoconductivity of the products illuminated with a tungsten– halogen lamp as light source had remarkable change, suggesting that the microtubes array might be employed as micro-devices or photo-switches. 2. Experimental The synthesis process of Se microtubes array was carried out in a horizontal quartz tube furnace. The selenium powder (P99.9%) was provided from Sigma–Aldrich. It was placed on a silicon substrate at the center of the tube and it was directly evaporated onto a silicon substrates located 20 cm far away, under a constant flow of argon gas (800 sccm, 99.9%). Before experiments, all substrates were washed by ultrasonication in a mixture of Millipore water and non-ionic detergent, followed by thorough rinsing with Millipore water and ethanol for many times to get rid of any remnants of nonionic detergent and dried prior to use. All the employed reactants from commercial sources were analytical grade and were used as received without further purification. The tube furnace was rapidly heated in 15 min from room temperature to 300 °C under Ar flow and maintained for 50 min. Afterwards, the flow of argon was stopped and the furnace was naturally cooled to room temperature. At the end of the deposition experiment, we observed the formation of aligned microtubes arrays, vertically grown from the substrate. Raman spectroscopy, X-ray diffraction (XRD), UV–vis spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the composition and the structure of the obtained microstructures. Raman scattering measurements were obtained by backscattering geometry with a Renishaw spectrometer coupled to a Leica metallographic microscope. An argon-ion laser operated at a wavelength of 514.5 nm and a 10 mW incident power to avoid thermal effects provided excitation. Raman shifts were corrected by using silicon (1 1 1) reference spectra after each measurement. X-ray diffraction measurements were carried out in the reflection mode on a Mini Flex Rigaku model diffractometer with Cu Ka radiation (k = 0.154 056 nm). The X-ray diffraction data were collected at a scanning rate of 0.02 degrees per second in 2h ranging from 15° to 80°. UV–vis spectra were recorded using a Varian Cary 5 spectrophotometer. The morphology of the obtained microstructures have been studied using a scanning electron microscope SEM-JEOL JSM 5010LV and a transmission electron Hitachi H-7100 microscope operated at an accelerating voltage of 100 kV. In order to prepare

the sample for TEM observations, the silicon substrate with aligned selenium microtubes array was immersed in ethanol solution and ultrasonically bathed for 20 min. Then, a little amount of ethanol was dropped onto a copper grid covered by porous carbon and let to dry slowly in air. A Keithley 6517A electrometer was employed as V-source and current meter. Photocurrent measurements were performed on the microtubes array using a tungsten–halogen lamp as light source, equipped with a series of neutral filters to reduce the light power. Photoconductivity measurements were performed at a constant temperature of 200 K, using a helium cryostat Galileo. Current changes were measured with a constantly applied potential of 0.5 V across the microtubes. The photoresponse as a function of photon energy has been detected with a Jobyn-Yvon H10 single grating monochromator. The trend of photocurrent vs. power was linear in the integrated power range 0.5–100.0 mW/mm2. 3. Results and discussion 3.1. Raman and X-ray diffraction measurements The crystallinity and purity of the obtained products were studied by Raman and XRD scattering measurements. Figure 1a shows a typical Raman spectrum of the as-prepared microtubes. The most intense resonance peak was observed at around 236.0 cm1 (no signals of the 256 cm1 peak for monoclinic Se and the 264 cm1 peak for a-Se), which is attributed to the vibration of helical chains of selenium that only exists in the trigonal phase. Additionally, in the spectrum there appeared a peak around 143.9 cm1 which was attributable to the transverse optical photon mode [21]. Figure 1b shows the typical XRD patterns of the Se microtubes grown on Si substrate; as comparison, the standard diffraction peak position and the relative intensities of bulk trigonal Se are also indicated as bars in the same Figure. All of the strong and sharp diffraction peaks of the XRD pattern can be indexed on the basis of the trigonal phase of selenium with lattice constants of a = 0.4365 nm and c = 0.4948 nm [JCPDS 73-0465; space group P3121]. Raman and XRD results indicated that the as-prepared Se has high purity in phase. 3.2. Scanning and transmission electron microscopy The morphology of the Se microtubes was examined by SEM. Figure 2a and b reported, respectively, low and high magnification SEM image of the cross-sectional view of the microtubes grown directly on the surface of the substrate. It is evident that almost vertically aligned microtubes with high density were found on

Figure 1. (a) Typical Raman spectrum and (b) XRD pattern of the as-prepared Se microtubes. The bars indicated the standard diffraction peak positions and relative intensities of bulk trigonal Se.

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Figure 2. (a) Low and (b) high magnification SEM images of the cross-sectional view of aligned microtubes arrays; (c) Single microtube with hexagonal cross-section.

Figure 4 clearly evidenced that the tubular microstructures tended to grow from a microparticle almost normal to the substrate. A careful observation of these growing microstructures revealed that they were not complete tubes but groove-like structures with semicircular cross section, with length in the range 3–350 lm and the diameter in the range 3–10 lm.

3.3. UV–vis absorption measurements

Figure 3. TEM images of (a) a bundle of microtube showing the base-up growth process, (b) microtubes coming out from a tangle. Inset: SAED pattern taken from the tangle.

the entire surface of the Si substrate. The Se microtubes were straight, had smooth surface, exhibited a uniform diameter along their length and had hexagonal cross-section (Figure 2c). They were in the range of 5–12 lm in diameter and their length ranged from 100 to 700 lm. SEM observations suggested that the growth of Se microtubes followed a base-up growth process, as also illustrated in the TEM image of Figure 3a. TEM images of Figure 3b showed that the tubes were hollow and had walls with a thickness comprised between 1.5 and 5.0 lm. Moreover, SAD pattern (inset of (Figure 3b) could be indexed according to the trigonal structure of selenium [JCPDS 73-0465] and confirmed that the microtubes were crystalline in nature. In order to confirm the base-up growth process of the microtubes array, we performed a new deposition route and stopped the evaporation after 25 min. SEM images shown in

UV–vis absorption measurement is one of the most important methods to reveal the energy structures and optical properties of semiconductor nanocrystals [22]. In our experiment, the prepared microtubes were scratched from the substrate by using sharp forceps and dispersed in ethanol at a concentration of about 103 mol/l. The UV–vis absorption spectrum of the Se microtubes solution is shown in Figure 5; it was recorded using a quartz cell (1 cm path length) and pure ethanol as a blank. The spectrum clearly exhibited an absorption band at 2.26 eV, which could be attributed to interchain interactions and was slightly blue-shifted in comparison to the value expected for bulk t-Se (2.1 eV) [6]. Similar observation has been made by Bogomolov et al. [23] in their spectroscopic absorption studies on isolated chains of Se atoms, who confirmed that the strong absorption at 2.2 eV came from the intermolecular transition between the spiraling chains of selenium. This transition was only present in the trigonal phase and not in the solid Se of any other allotropic form (amorphous and monoclinic phases). The optical absorption coefficient a was calculated according to the following equation

a ¼ ð2:303  103 AqÞ=lc:

ð1Þ

where A is the absorbance of the sample, q is the real density of Se (4.8 g cm3), l is the pathlength (1 cm) and c is the concentration of the suspension [24]. The optical band gap was determined from

Figure 4. SEM images of Se microstructures synthesized in the furnace maintaining the temperature of 300 °C for 25 min.

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Figure 5. UV-Vis absorption spectrum of the Se microtubes dispersed in ethanol.

Figure 6. Linear plot of Y/Y’ as a function of photon energy.

the analysis of the spectral dependence of the absorption near the fundamental absorption edge (Figure 5). In this region, the absorption coefficient a is well described by t relation [25]

The formation of the microtubes can be explained as being similar to the growth mechanism proposed by Filippo et al. [17] for the formation of Se microrods in a tubular furnace and to the nucleation–dissolution–recrystallization mechanism proposed by Xi and co-workers for the wet chemical synthesis of t-Se nanotubes [30]. The initial stage in the growth of the microtubes array was the formation of microparticles on the substrate. During the heating, the upcoming selenium vapor were transferred on the surface of the microparticles and due to the anisotropic crystal structure, there was a strong tendency toward 1D grow along the c-axis [6]. Moreover, it is likely that the effect of the collision of Se vapor with Ar atoms at high Ar flow rates could reduce the amount of Se atoms available for the growth. So, there were not enough atoms for the growth of the solid rod-like crystals observed in our previous work [17]. This would lead to the undersaturation in the central part of the growing regions of the surface of the microparticles [31] and to the formation of groove-like microstructures very similar to the ones obtained by Xi et al. after 8 h of hydrothermal reaction [30]. The continuous feeding of selenium atoms on the surface of the microparticles could diffuse into two directions: circumferential diffusion and diffusion parallel to the tube axis [0 0 1], which will induce selenium crystal growth along the circumferential

Y ¼ ahv ¼ Bðhv  Eg Þm

ð2Þ

where hm is the energy of incident photons and Eg is the value of the optical band gap corresponding to transitions indicated by the value of m. The factor B depends on transitions probability and can be assumed to be constant within the optical frequency range. Taking the first derivative Y0 of Eq. (2) with respect to photon energy, we obtain

Y=Y 0 ¼ ðhv  Eg Þ=m

ð3Þ

The value of Y and Y0 can be deduced from experimental data and the linear plot of Eq. (3) as a function of the photon energy can be drawn. Both m and Eg can easily be determined from the slope and the intercept, respectively. This procedure in shown in Figure 6. In this figure, a linear region is evident and m is about 0.5. This dependence is peculiar of direct allowed interband transitions and is in good agreement with the literature data [26]. The optical band gap value determined from Figure 6 is 1.9 eV. It was observed that the band gap value was higher than that of bulk Se (1.7 eV) [27] due to quantum confinement [28]. 3.4. Growth mechanism In a vapor-phase deposition route, the formation of Se microtubes was strongly affected by various synthesis parameters such as the evaporation temperature, deposition temperature, temperature gradient and the gas flow rate. Generally, the vapor phase deposition occurs via vapor–liquid–solid (VLS) or vapor–solid (VS) growth mechanism [29]. The VLS mechanism generally needs the presence of another metallic particle with which the depositing material forms eutectic alloy. Growth occurs from a liquid alloy droplet as more and more material is incorporated into it from vapors and is precipitated out forming crystal due to supersaturation. In the case of the VS growth mechanism, the gaseous species (supersaturated at lower deposition temperature of the substrate) are directly incorporated into the solid phase leading to the growth of the crystal. In the present study, no other metallic particle has been used and the deposition occurs below the melting point of Se, indicating that the growth occurs via a VS process.

Figure 7. Se microtubes array encapsulated in the polymer matrix (PVA). Inset: High magnification SEM image of a microtube encapsulated in the polymer.

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Figure 8. (a) On-off light excitation response of the microtube array; (b) On-off light excitation response under a periodic illumination.

direction and along the tube axis direction [30]. Because of the anisotropic crystal structure of selenium, Xi et al. speculated that the growth rate along the tube axis direction was faster than the growth rate along the circumferential direction. So combining the difference of the growth rate with the undersaturation in the central part of the growing regions, groove-like microtubes could be first formed. As heating proceeded, the groove-like microstructures gradually developed into microtubes which often exhibited unclosed segments at the ends Figures 2c and 4, which further demonstrated that the tube axis direction growth rate is faster than the circumferential direction growth rate. 3.5. Photoconductivity of selenium microtubes array After structural characterization of the microtubes, the voids between them were slowly filled with polyvinyl alcohol (PVA) matrix; then, the encapsulated array of microtubes (2  2 mm2) were carefully removed from the Si substrate. Figure 7 shows the microtubes array encapsulated in the polymer matrix; the inset of Figure 7 revealed that the microtubes extended above the polymer matrix by few microns. To perform photoconductivity measurements, the top surface of the encapsulated array was coated with an optically semitransparent Au film (d = 25 nm). Meanwhile, a nontransparent (d = 100 nm) Au coating was evaporated on the bottom surface and was used as a bottom electrode. The encapsulated microtubes array has been connected to external circuit simply by contacting the top and bottom surfaces with a thin metal wire. The simplicity of electrical contact of the microtubes array by directly connecting them to an external circuit is a result of the fabrication method. The vertically aligned microtubes were encapsulated in a polymer matrix which formed a protective and mechanically tough sheath that enabled simple and macroscopic manipulation to be carried out in a straightforward fashion without expensive nanomanipulation techniques, circumventing a major challenge to the large scale integration. Moreover, the photoconductivity of the microtubes array was unaffected by changes in the local environment due to contamination or humidity, as they were clad in the polymer matrix. The on–off light excitation response of the microtube is shown in Figure 8a and b. In detail, when the tungsten–halogen lamp was switched on, conductivity immediately increased and it was saturated within 10–20 s. After the light was switched off, the current returned to its original value within 10–20 s (Figure 8a). This process was repeated more times under a periodic (50 s) illumination of light at 0.5 V bias voltage and a prompt generation of reproducible and stable photocurrent was observed. It was clearly evident that the photocurrent value of the first cycle has been achieved in the subsequent cycles (Figure 8b). The observed photoresponse corresponds to relative increase in photocurrent which is approximately 180% of the initial (light off).

4. Conclusion In summary, synthesis of aligned single-crystalline Se microtubes array can be easily and rapidly realized by physical vapor deposition method. Raman, XRD and SAED demonstrated that the obtained Se microtubes were highly crystalline and pure; SEM and TEM images established that the microtubes were in the range of 5–12 lm in diameter, that their length ranged from 100 to 700 lm and that the walls had a thickness comprised between 1.5 and 5.0 lm. The microtubes array displayed efficient and stable photoconductivity properties. There was found a relative increase of the photoconductivity by 180 times when the sample was taken from the dark and exposed with tungsten light. Moreover, the microtubes exhibited sharp on/off switching behaviour, suggesting that they are potentially good photosensor material and that can be useful in the fabrication of micro-devices or photo-switches. Acknowledgement The authors are grateful to Miss A.R. De Bartolomeo for valuable technical support. References [1] P. Mohanty, T. Kang, B. Kim, J. Park, J. Phys. Chem. B 110 (2006) 791. [2] S.M. Lee, Y.H. Lee, Y.G. Hwang, J. Elsener, D. Porezag, T. Frauenheim, Phys. Rev. B 60 (1999) 7788. [3] K. Tang, D. Yu, F. Wang, Z. Wang, Cryst. Growth Des. 6 (2006) 2159. [4] B. Mayers, K. Liu, D. Sunderland, Y. Xia, Chem. Mater. 15 (2003) 3852. [5] B. Gates, B. Mayers, A. Grossman, Y. Xia, Adv. Mater. 14 (2002) 1749. [6] B. Gates, B. Mayers, B. Cattle, Y. Xia, Adv. Funct. Mater. 12 (2002) 219. [7] B. Gates, Y. Yin, Y. Xia, J. Am. Chem. Soc. 122 (2000) 12582. [8] U.K. Gautam, M. Nath, C.N.R. Rao, J. Mater. Chem. 13 (2003) 2845. [9] U.K. Gautam, C.N.R. Rao, J. Mater. Chem. 14 (2004) 2530. [10] J. Lu, Y. Xie, F. Xu, L. Zhu, J. Mater. Chem. 12 (2002) 2755. [11] S.Q. Yuantao, H. Li, Chem. Lett. 32 (2003) 448. [12] Y. Ma, L. Qi, J. Ma, H. Cheng, Adv. Mater. 16 (2004) 1023. [13] M. Chen, L. Gao, Chem. Phys. Lett. 417 (2006) 132. [14] B. Zhang et al., Small 3 (2007) 101. [15] H. Zhang, D.R. Yang, Y.J. Ji, X.Y. Ma, J. Xu, D.L. Que, J. Phys. Chem. B 108 (2004) 1179. [16] S.Y. Zhang, J. Zhang, Y. Liu, X. Ma, H.Y. Chen, Electrochim. Acta 50 (2005) 4365. [17] E. Filippo, D. Manno, A. Serra, Cryst. Growth Des. 10 (2010) 4890. [18] P. Liu, Y.R. Ma, W.W. Cai, Z.Z. Wang, J. Wang, L.M. Qi, D.M. Chen, Nanotechnology 18 (2007) 205704. [19] L. Cheng, M. Shao, D. Chen, X. Wei, F. Wang, J. Hua, J. Mater. Sci.: Mater. Electron. 19 (2008) 1209. [20] D.S. Deng, N.D. Orf, S. Danto, A.F. Abouraddy, J.D. Joannopoulos, Y. Fink, Appl. Phys. Lett. 96 (2010) 023102 1. [21] G. Lucovsky, A. Mooradian, W. Taylor, G.B. Wright, R.C. Keejer, Solid State Commun. 5 (1967) 113. [22] H. Gu, Y. Hu, J. You, Z. Hu, Y. Yuan, T. Zhang, J. Appl. Phys. 101 (2007) 024319 1. [23] V.N. Bogomolov, S.V. Kholodkevich, S.G. Romanov, L.S. Agroskin, Soild State Commun. 47 (1983) 181. [24] Y.W. Zhang, R. Si, C.S. Liao, C.H. Yan, J. Phys. Chem. B 107 (2003) 10159. [25] D. Manno, A. Serra, M. Di Giulio, G. Micocci, A. Tepore, Thin Solid Films 324 (1998) 44.

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