Characterization and Growth Mechanism of Selenium Microtubes

DOI: 10.1021/cg1012632

Characterization and Growth Mechanism of Selenium Microtubes Synthesized by a Vapor Phase Deposition Route

2010, Vol. 10 4890–4897

Emanuela Filippo,* Daniela Manno, and Antonio Serra Department of Material Science, University of Salento I, 73100 Lecce, Italy Received July 19, 2010

ABSTRACT: A simple and rapid vapor deposition route has been developed for the growth of trigonal phase selenium microtubes in a horizontal tubular furnace under argon flow gas. Selenium powder was evaporated by heating at 300 °C, and the vapors were condensed on different quartz substrates located at 70-140 °C. It was found that the morphologies of the products were strongly affected by small variations of the temperatures of the deposition zones. It was observed that the growth of microtubes was initiated by formation of nearly spherical microparticles with smooth surfaces; the smooth microspheres were first covered by a rough layer and then they slowly became empty. The additional selenium atoms transported from the heated part of the furnace or coming from the consumption of the inner core of the rough microparticles continued to adsorb on the empty microspheres, allowing two possible growth mechanisms. If the additional Se atoms preferentially went to the circumferential edges of the empty microspheres, crystalline microtubes with no defects were formed; however, Se atoms could also follow a spiral growth mechanism starting from the empty shells. This second growth mechanism led to the formation of semiclosed tubular structures with irregular surfaces, which developed into the relatively completed uniform microtubes with smooth surfaces. The morphology, microstructure, and chemical composition of the microtubes were characterized by various means (X-ray diffraction, energy-dispersive X-ray spectroscopy, Raman spectroscopy, UV-vis spectroscopy, scanning electron microscopy, and transmission electron microscopy). The as-grown Se microtubes may find application as rapid response photosensors and photocells.

1. Introduction Tubular micro- and nanocrystals with several different areas of contact (borders, inner and outer surfaces, and structured tube walls) that can be further functionalized in many ways have been the subject of intensive research for many years.1-3 Among them, semiconductor tubes are of particular importance because of their intrinsic properties that exhibit wide practical and potential applications.4 As examples, hexagonal selenium, usually called trigonal selenium5 (t-Se), is an indirect gap semiconductor with excellent photoelectrical and semiconducting properties.6 In addition, the relatively low melting point (217 °C), high piezoelectric, catalytic activity, and indispensability in the human body have made Se an important material.7 Heretofore, a lot of effort has been devoted to the synthesis of t-Se one-dimensional (1D) nanostructures. For example, Gates et al. have fabricated t-Se nanowires by precipitating R-Se, which dispersed in colloids through the reaction between selenious acid and excess hydrazine.8 Cheng et al. have fabricated single-crystal Se nanowires by the direct conversion of polycrystalline selenium powder via a hydrothermal process.9 Gautam and co-workers10 described a solutionbased synthesis of selenium nanorods via the reaction of Se powder with NaBH4 at a temperature of 30 °C. They found out that portion of the dissolved selenium precipitated as the t-Se nanoparticles, which acted as nuclei to form nanorods. The rods grew in length with time while the diameters remained almost the same. Moreover, they also prepared t-Se nanowires in high yield through the thermal decomposition of [(CH3)4N]4Ge4Se10 in a furnace at 600 °C under argon *Corresponding author. Phone: 0039 832297101; fax 0039 832297100; e-mail: [email protected]. pubs.acs.org/crystal

Published on Web 10/15/2010

gas flow. Zhang et al. reported the growth of selenium nanowires and nanotubes by means of a hydrothermal process and a following sonication.11 Most of these reported methods are chemical solution-based routes and involve complex chemical processes that are not environmentally benign. Compared with the above-mentioned methods, the physical evaporation approach remains a valuable technique for fabrication of high crystalline low-dimensional materials because it is a more economical, greener, and simpler technique.12 However, to the best of our knowledge, few works have reported the synthesis of selenium nanostructures via vapor phase growth. Cao et al. prepared uniform nanowires, networks built by “Y” nanowires junctions and mixture of microspheres and hexagonal prisms via thermal evaporation and deposition, but their method required a very long deposition time: 8 h in order to obtain uniform nanowires and 20 h in order to obtain networks or a mixture of prisms and microspheres.13 Ren et al.12 and Zhang et al.7 synthesized nanowires using a vapor-phase growth with the assistance of silicon powder and active carbon, respectively, as catalysts mixed with selenium powder. Wang et al.14 obtained ultrawide Se and Te nanobelts using a vacuum vapor deposition method at an evaporation temperature of 200 °C. Jiang et al.15 synthesized selenium products with different shapes from selenium powder in a quartz tube, followed by laser ablation. They pointed out that without the assistance of laser no rods were produced. It is clearly evident that it is still a challenge to fabricate onedimensional (1D) Se nano/microstructures by vapor phase routes and to study their growth mechanism. In this work, we describe the preparation of selenium microtubes on a large scale using the vapor deposition method at an evaporation temperature of 300 °C. Our preparation r 2010 American Chemical Society

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Crystal Growth & Design, Vol. 10, No. 11, 2010

Figure 1. (a) Schematic diagram of the experimental setup showing the tubular furnace with gas flow arrangement. (b) Different deposition temperature zones located along the flow of the Ar gas.

method required a relatively short deposition time (1 h) and did not involve the use of any catalyst or template. Furthermore, we studied the products obtained at different temperatures of the deposition substrates (70-140 °C) in order to understand the possible growth mechanism of the microtubes. 2. Experimental Section Materials. The synthesis process of Se microtubes was carried out in a horizontal quartz tube furnace. The selenium powder (g99.9%) was provided from Sigma-Aldrich. It was placed on a quartz substrate and it was directly evaporated onto different quartz substrates, under a constant flow of argon gas (99.9%). Before experiments, all quartz substrates were washed by ultrasonication in a mixture of Millipore water and nonionic 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. After they were dried, the quartz substrates were placed downstream (deposition zone) inside the horizontal tube furnace equipped with a temperature control system; the quartz substrate on which Se powder was located was placed at the center of the tube (evaporation zone). The system was evacuated by a mechanic pump, before introducing the flow of argon gas. In Figure 1a, the schematic diagram of the experimental setup showing the horizontal tubular furnace with gas flowing arrangement is presented; Figure 1b gives the temperature distribution curve along the direction of the carrying gas at the setting temperature (300 °C). It can be seen that the depositing temperature of the five samples gradually decreased from 140 to 70 °C. The tube furnace was rapidly heated in 10 min from room temperature to 150 °C and maintained for 2 min; then the temperature was elevated to 300 °C in 10 min and maintained for 60 min. Afterward, the flow of argon was stopped and the furnace was naturally cooled to room temperature. During the present experiment, the temperature of the deposition zone was kept at 70-140 °C, while the temperature the evaporation zone was kept at 300 °C. Gray samples consisting of needles visible to the naked eyes were formed on the outlet of the inner tube as well as on the edge of the quartz substrate located at 70 °C, while the other substrates were covered by a dark brick-red opaque layer. X-ray diffraction (XRD), Raman spectroscopy, UV-vis spectroscopy, scanning electron microscopy (SEM) coupled with energy dispersive X-ray analysis (SEM-EDAX), and transmission electron microscopy (TEM) were used to characterize the composition and the structure of the obtained microstructures. Instrumentation. X-ray diffraction measurements were carried out in the reflection mode on a Mini Flex Rigaku model diffractometer with Cu KR radiation (λ=0.154056 nm). The X-ray diffraction data were collected at a scanning rate of 0.02 deg/s in 2θ ranging from 15° to 80°. Raman scattering measurements were obtained by backscattering geometry with a Renishaw spectrometer coupled to a Leica

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Figure 2. XRD pattern of the (a) as-prepared Se microtubes, (b) Se powder used as the source material. (c) Standard diffraction peak positions and relative intensities of bulk trigonal Se. 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 (111) reference spectra after each measurement. UV-vis spectra were recorded in the range between 300 and 900 nm using a Varian Cary 5 spectrophotometer. The morphology and elemental composition of the obtained microstructures were performed using a scanning electron microscope SEM-JEOL JSM 5010LV equipped with an Oxford X-ray energy dispersive (EDX) microanalysis system. The TEM images were taken using a Hitachi H-7100 instrument operated at an accelerating voltage of 120 kV.

3. Results and Discussion The simple thermal evaporation of Se powder and condensation of the vapors at a substrate temperature at 70-80 °C led to pure microtubes in high yields. Figure 2a,b shows the typical XRD patterns of the Se product deposited on quartz substrates and the diffraction pattern of the Se powder used as the source material, respectively. The XRD data were collected directly from the microtubes as they were deposited on the substrate. 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]. Very sharp peaks in Figure 2a indicated the high quality of deposited Se single crystals. As comparison, the standard diffraction peak position and the relative intensities of bulk trigonal Se are indicated as bars in Figure 2c. The abnormally strong (100) reflection peak relative to the bulk t-Se indicated that the microtubes were grown preferentially along the [001] direction16,17 with infinite, helical chains of selenium atoms packed parallel to each other along the c-axis; moreover, unusually strong (hk0) reflection peaks and weak (hkl ) reflection peaks (l ¼ 6 0) suggested that the assynthesized selenium microtubes might lie on the surface of the quartz substrate.18 Since only selenium peaks were detected in the EDX spectrum (Figure S1, Supporting Information), except the peaks from silicon and oxygen due to the use of quartz substrate for the deposition, it was confirmed that the microtubes contained only pure selenium. The Raman scattering spectrum provided further evidence that the selenium microtubes were in the trigonal phase, due to its characteristic signals in the spectrum. Typically, the resonance peak of t-Se is located at ∼235 cm-1, whereas those of

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Figure 3. Room temperature UV-vis absorption spectrum of (a) Se microtubes, (b) Se used as source material.

amorphous selenium and monoclinic selenium are centered at ∼264 cm-1 and ∼256 cm-1, respectively.19,20 In the Raman scattering spectrum of the as-prepared microtubes (Figure S2, Supporting Information), an intense resonant peak around 236.6 cm-1 was found. It could be attributed to the vibration of helical selenium chain and confirmed that selenium only existed in the trigonal phase. Additionally, in the spectrum there appeared a peak around 145.1 cm-1, which was attributable to the transverse optical photon mode, and two peaks at around 440.2 cm-1 and 457.6 cm-1 assignable to the second order modes of t-Se were also observed. They correspond to the combination mode and overtone of the phonons at the edge point (M) of the Brillouin zone.21 Combination of the XRD, EDX, and the Raman spectroscopy results led us to conclude that the obtained Se product was well-crystallized trigonal selenium. The optical properties of metallic selenium are of interest due to its photoconductive, photovoltaic, and rectifying response on illumination with visible light. We performed optical studies on the obtained microtubes in order to evaluate their potential optical properties. The absorption spectrum of the Se microtubes can be decomposed into five bands with peaks at 385, 445, 540, 665, and 780 nm, as shown in Figure 3a. Maxima at 385, 445, and 540 nm are close to the absorption peaks at 354, 452, and 590 nm observed for the t-Se nanowires obtained by sonication of R-Se in ethanol22 and also to the peaks at 375, 460, and 575 nm observed for Se nanowires grown in the pores of anodic alumina membrane as template.23 These values are also consistent with reflectivity measurements on t-Se single crystals.24 It is worth noting that peaks above 540 nm can be solely attributed to interchain interactions perpendicular to the c-axis within a given t-Se crystal, as confirmed by Bogomolov et al. in their studies of isolated polymer chains of Se and Te atoms formed in zeolite pores.25 Furthermore, the position of the low-energy peaks at high wavelengths may provide instructive information for the interchain interactions as well as the degree of crystal perfection.22 The absorption spectrum acquired on Se powder used as the source material and dispersed in aqueous solution is reported in Figure 3b. We could find that the absorption spectrum of the t-Se powder exhibited features at 325, 371, 525, 662, and 798 nm, in good agreement with the values expected for the trigonal bulk Se.24 The growth of different Se microstructures was observed on the wall of the furnace as well as on the substrates located at deposition zones with slightly different temperatures, along the length of the quartz tube. From SEM observations, it could be possible to deduce that the morphology of the

obtained selenium microstructures dramatically depended on the change of the temperature of the deposition substrates. The SEM micrograph shown in Figure 4a reveals that in the temperature deposition zone of 125-140 °C, the product deposited onto the substrate was composed of regular micrometer-sized spheres with smooth surfaces and a diameter between 3 and 8 μm. A careful observation at higher magnification revealed that on the surface of some microspheres appeared a growing outer covering (Figure 4b). As the substrate temperature slowly decreased (110-120 °C), the microspheres with an average diameter in the range of 411 μm presented a very coarse surface, a hole on each sphere appeared, and so the inner core became visible (Figure 4c). The outer radial structure grew with radial alignment pointing to the inner core center, and the inner core showed nonradial streaks or it was completely smooth. Some spheres with a growing outer covering were still present (Figure 4d). SEM observations of the products deposited on the substrate set at a temperature of about 95-105 °C were shown in Figure 4e; they suggested that the holes gradually grew bigger and bigger until they hollowed the sphere. This consumption process continued until the microsphere was completely emptied, leaving only an empty shell having a size in the range 4-15 μm, as well shown in Figure 4f. Figure 4h shows typical SEM images of the as-fabricated material at low and high magnifications, respectively, deposited on the substrate at a temperature 80-90 °C. The images demonstrate that the material was deposited in large scale, had a tube morphology with an hexagonal/triangular cross-section, an average diameter of 7-15 μm, and a length ranging from 40 to 100 μm, indicating that the growth of the selenium microtubes along the c-axis based on strong covalent bonds was faster than that along the circumferential direction based on weak van der Waals interactions between helical chains.26 The microtubes tended to lie along the substrate surface, and they could be classified into two groups: those with regular smooth surfaces and uniform diameter and those with an irregular outer surface and not so uniform in diameter. SEM image at higher magnification of the irregular microtubes (Figure 4h) clearly shows the presence of semiclosed tubular structures. On the basis of these observations, it was possible to conclude that the regular microtubes were gradually formed from the absorption of selenium atoms transported from the heated part of the furnace on the circumferential edges of the empty microspheres because of the free energy difference. Furthermore, the empty microparticles could also follow a spiral growth mechanism on more than a layer, getting the irregular microtubes shown in Figure 4h.

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Figure 5. SEM image of the edge of the quartz substrate set just at the outlet of the furnace. Inset: cross-section of a microtube.

Figure 6. (a-c) TEM images of different Se microtubes.

Figure 4. Low and high magnification SEM images of the Se microstructures synthesized at the temperature deposition zones of (a, b) 125-140 °C; (c, d) 110-120 °C; (e, f) 95-105 °C; (g, h) 80-90 °C; (i, j) 70-80 °C.

At this deposition temperature, the as-described irregular, not uniform microtubes were much more abundant than the regular, uniform ones. In a little bit lower temperature deposition zone (70-80 °C), uniform microtubes with a diameter of about 7-15 μm and a length ranging from 70 to 150 μm were mainly observed (Figure 4i,j). This fact allowed us to suppose that the regular microtubes just continued their growth while the irregular ones developed into relatively completed uniform microtubes. The observation of the edge of the quartz substrate set just at the outlet of the furnace is presented in Figure 5. It revealed that the microtubes had a length up to several hundreds of micrometers (140-1100 μm) and a mean diameter in the range 7-20 μm. They exhibited hexagonal cross-section

and their surfaces were rather regular and smooth (inset of Figure 5); furthermore, the microtubes exhibited walls with a thickness comprised between 0.6 and 3 μm. The microtubes at different growth stages were analyzed using TEM in order to confirm that a hollow structure was always present along the whole tubes. TEM observations confirmed that the irregular tubes were hollow structures, consisting of wrapped-up layers, as shown in Figure 6a,b. It was also evident that the regular microtubes with smooth surfaces exhibited a considerably uniform thickness and straightness along their longitudinal axis (Figure 6c). The growth process of selenium microtubes is depicted in Figure 7. It was observed that the growth of microtubes was initiated by formation of nearly spherical microparticles with smooth surfaces (Figure 7a); the smooth microspheres were first covered by a rough layer (Figure 7b,c,d) and then they slowly became empty (Figure 7e,f ). The additional selenium atoms transported from the heated part of the furnace or coming from the consumption of the inner core of the rough microparticles continued to adsorb on the empty microspheres, allowing the growth of microtubes along the [001] direction. On the basis of our observations, it was possible to suggest that if the additional Se atoms preferentially went to

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Figure 7. Series of SEM images supporting the possible growth mechanism of the microtubes: (a) smooth microparticles obtained with depositing temperature kept at 125-140 °C; (b-d) microparticles covered by a rough layer obtained with depositing temperature kept at 110-120 °C; (e, f) empty microparticles obtained with depositing temperature kept at 95-105 °C; (g, h) semiclosed tubular structures with irregular surfaces obtained with depositing temperature kept at 80-90 °C; (i) long uniform microtubes obtained with depositing temperature kept at 70-80 °C.

the circumferential edges of the empty microspheres, crystalline microtubes with no defects (Figure 7i) were formed; however, Se atoms could also follow a spiral growth mechanism, starting from the empty shells. This second growth mechanism led to the formation of semiclosed tubular structures with irregular surfaces (Figure 7g,h), which developed into the relatively completed uniform microtubes with smooth surfaces, in the little bit lower temperature deposition zone. The microstructures observed in the different zones depend on the vapor pressure and supersaturation of Se in the zone, which is dependent on the evaporation temperature, deposition temperature, temperature gradient, and gas flow rate.27,28 Generally, the growth behavior of t-Se microstructures in the vapor phase processes is rather complicated due to their sensitive nature to the thermal treatment and there will be numerous cases in keeping with different evaporation/deposition temperatures. However from the view of the melting point of Se, the vapor phase deposition processes can be classified into two types: a vapor-liquid-solid (VLS) process or a vapor solid (VS) process. If the temperature in the deposition zone is higher than the melting point of Se (>217 °C), the t-crystal growth will undergo a VLS process. In other words, selenium in the deposition zone exists in the form of gaseous molecules or clusters first, then gaseous selenium aggregates into droplets on the substrate, and finally droplets will solidify when the tube furnace is cooled to room temperature. If the

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temperature in the deposition zone is set lower than the melting point of selenium while the evaporation temperature is higher than the melting point of Se, gaseous selenium molecules or clusters will directly aggregate into solid state species and, consequently, t-Se crystal growth undergoes a VS process.13 In a VS process, the growth behavior is more complicated relative to those in VLS processes because the dynamic growth rate of Se crystals is not only linked to the evaporation/ deposition temperature but also to the temperature gradient between the evaporation and the deposition temperatures.13 In our experiment, the growth followed a VS process, and we found that the final morphologies of the products were strongly affected by small variations of the temperature of the deposition zone. The temperature of the evaporation zone was set at 300 °C; the control of the temperature gradient in the reaction furnace between the evaporation and the deposition temperatures allowed controlling the degree of supersaturation. In the higher temperature region (95-140 °C), the degree of supersaturation is very slight,29 formation of many seeds on the substrate would greatly reduce the concentration of Se atoms near the substrate. So, the nucleus and the growth of selenium crystals might be kinetically controlled and the products should exist in the forms of spherical crystals due to the too fast evaporation rate of selenium source, getting smooth microspheres. As soon as the nucleation occurred, the additional upcoming Se atoms were absorbed on the Se clusters until microparticles with rough surfaces were formed. The treatment in the furnace somehow formed a crater on the spheres, broke them, and they slowly became empty. The edge of the crater or the empty shell acted as the initiation spot and further directional growth of the selenium crystal along a preferential direction yielding tube products, aided by the argon flux. In the lower temperature region, the degree of supersaturation is higher than in the higher temperature region at the surface of the substrate. It facilitated the nucleation of a large number of seeds and also the number of atoms available on the surface of a seed was larger with a reduced mobility of the atoms. This would favor the formation of a large amount of microtubes which were lying down on the substrate. The preferential growth tended to occur along the unique [001] direction because of the highly anisotropic trigonal lattice. In addition, selenium atoms in infinite spiral chains parallel to the c-axis are bound with covalent bonds while the chains are packed into a hexagonal lattice through van der Waals interactions. To favor the strong covalent bonds within helical chains over the relatively weak van der Waals forces among chains, the growth rate along the [001] direction should be faster relative to that along the [010] and [100] directions.30,31 In the first step of the observed growth process, the formation of spherical microparticles with smooth surface was due to the fast evaporation rate of selenium source. In order to speculate this assessment, we carried out a new experiment in which we allowed Se to evaporate more slowly. The tube furnace was rapidly heated in 10 min from room temperature to 150 °C and maintained for 2 min; then the temperature was elevated to 300 °C in 40 min and maintained for 60 min. Afterward, the flow of argon was stopped and the furnace was naturally cooled to room temperature. SEM observations revealed a drastic change in the morphology and in the growth mechanism of the obtained microstructures, as shown in Figure 8.

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Figure 8. Low and high magnification SEM images of the Se microstructures synthesized lowering the evaporation rate, at the temperature deposition zones of (a, b) 125-140 °C; (c, d) 110-120 °C; (e, f) 95-105 °C; (g, h) 80-90 °C; (i, j) 70-80 °C.

Figure 8a shows that the typical morphology of the products deposited on the substrate located at 125-140 °C mainly consisted of microparticles with a diameter in the range 7-15 μm. The microparticles were not exactly spherical and exhibited rough surfaces with numerous small protuberances (Figure 8b). As the substrate temperature slowly decreased (110-120 °C), the microparticles appeared as irregular urchin-like structures consisting of microrods with a diameter in the range 2-5 μm and a length of 4-21 μm (Figure 8c,d). When the temperature of the deposition substrate decreased to the values 95-105 °C and 80-90 °C, the microrods growing out of the urchin-like structures became longer with a length in the range of 9-50 μm (Figure 8e,f and 8g,h). Figure 8i,j shows that on the substrate located at 70-80 °C


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the main product consisted of microrods with diameters of 2.5-6 μm and lengths of 12-60 μm. On the basis of the experimental observations, the following growth process of Se microrods could be proposed. As the temperature in the furnace increased, Se vapor was produced and diffused to the lower temperature zones, followed by the condensation to form Se nanoclusters. These nanoclusters were energetically favorable to absorb additional vapor species until microparticles were formed.14 At this growth stage, the amount of upcoming Se vapor played an important role in the morphology of the formed microparticles. If the Se evaporated quickly, the large amount of Se atoms available for the growth led to the formation of regular microparticles with smooth surfaces and spherical shape; on the other hand, when the Se evaporated more slowly, the upcoming Se vapor species were slowly accepted by the nanoclusters leading to the formation of microparticles with rough surfaces and a more irregular shape. The protuberances on the microparticles surfaces provided many energy sites for nanocrystalline growth.32 So when the Se atoms were transferred onto the surfaces of the microparticles, they spontaneously nucleated onto the small protuberances. Furthermore, due to the anisotropic crystal structure, there was an intrinsic tendency for the nucleation growth along the 1D direction and microrods began to grow out from the surface of the microparticles.33 As microrods grew, the microparticles were slowly consumed, until they disappeared. A similar transformation phenomenon from microparticles to 1D microstructures has also been observed by other research groups, even in different experimental conditions. Wang et al.14 reported the preparation of ultrawide Se and Te nanobelts through a vacuum vapor deposition route at an evaporation temperature of 200 and 350 °C, respectively. Time-dependent experiments were carried out to reveal that the growth process of the nanobelts consisted of the following main steps: microparticles, urchinlike microstructures made by nanorods, nanobelts. The same growth process was proposed by Zhang et al.7 for the nanowires and nanoribbons formed in a carbothermal chemical vapor deposition route. Xi et al.33 reported a hydrothermal reaction to make t-Se nanotubes and suggested that their growth was governed by a nucleation-dissolution-recrystallization mechanism. In this mechanism, t-Se nanoparticles were initially formed, and then they grew onto large particles until microspheres were formed. The microspheres gradually dissolved in the solution and the atoms were transferred onto the surfaces of the spheres, nucleating spontaneously onto the small protuberances. So, groove-like structures were first formed coming out the microspheres; then, these groove-like structures developed into nanotubes. So we could assess that the formation of spherical crystals with smooth surfaces was essential for the growth of microtubes; moreover, when the evaporation rate was lowered, it was possible to observe different morphologies and a completely different growth mechanism. 4. Photoconductivity of a Single Selenium Microtube Historically, photoconductivity was first observed in selenium more than 100 years ago. Intensive studies have been carried out on this photoconductor due to its commercial application in xerography. Most of these studies were focused on thin films of R-Se or large single crystals of t-Se.34 For the photocurrent investigation, a single selenium microtube was mounted on the top of a multifinger device with

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Figure 9. Dynamic on-off photocurrent response under an irradiance power of 100 mW/mm2 at a pressure of 10-3 mbar of (a) Se microtube and (b) bulk t-Se. Inset: Single Se microtube mounted on a multifinger device.

interdigitated Au electrodes fabricated on Al2O3 substrates using conventional lithography, gold evaporation, and lift-off processes. The electrodes thickness was 90 nm and the distance between the opposite electrodes was about 200 μm. A Keithley 6517A electrometer was employed as V-source and current meter. The on-off photoconductivity was investigated by using a tungsten-halogen lamp illumination of 100 mW/mm2 at a pressure of 10-3 mbar. 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 microtube. The on-off light excitation response of a single Se microtube is shown in Figure 9a; the inset shows the microtube, with a size of about 20800 μm, mounted on the multifinger device above-described. In Figure 9b, the photocurrent response under the same experimental conditions of a 15 μm thick film of t-Se is also presented. Se film was obtained by thermal deposition onto ultrasonically cleaned quartz substrate at a temperature of 300 °C. During film formation, the deposition chamber was evacuated to a pressure of about 510-7 mbar. A Maxtek quartz crystal monitor was employed to control the growing rate of the film and used to stop the growth at a thickness of 15 μm. After the deposition the film thickness was checked by Tencor Alpha Step 200 Stylus profilometer. It was evident that after the excitation a prompt generation of reproducible and stable photocurrent was observed in both samples. In order to compare the two responses, we referred to the relative change of photocurrent, expressed as (IMAX IMIN)/IMIN. It was found that the t-Se film showed a relative change of photocurrent of about 40%, while the microtube showed a relative change of photocurrent of about 180%. 5. Conclusions In summary, synthesis of single-crystalline Se microtubes can be easily and rapidly realized by direct physical vapor deposition approach. The XRD, EDAX, and Raman scattering results all demonstrated that the obtained Se microtubes were highly crystalline and very pure. Slight alteration of temperature of the deposition substrates led to a significant change of morphologies of deposited products. As a result, lower substrate temperatures produced a longer tube, since microspheres could exist at a higher temperature. In order to explain the formation of t-Se nanotubes, nanowires, and/or nanobelts, the growth mechanism usually reported in the literature involved the following main mechanisms: the oriented nanoparticles attachment,11,35,36 the nucleation-dissolution-recrystallization, 36 and the

surfactant-directed growth mechanism.35 Here, we reported and documented for the first time a new growth process involving the formation of smooth microparticles, rough microparticles, empty shells, short irregular microtubes, and longer uniform microtubes. The synthetic approach described here had a number of advantages, such as being simple, practical, low-cost, common, and rapid. It involved environmentally benign conditions with relatively low energy consumption and relatively low temperatures (300 °C). Interestingly, these microtubes displayed good photoconductivity properties. Supporting Information Available: EDX spectrum and Raman spectrum of the prepared Se microtubes. This material is available free of charge via the Internet at http://pubs.acs.org.

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