Fabrication and Growth Mechanism of Selenium and Tellurium

High-quality ultrawide Se and Te nanobelts have been prepared through thermal evaporation and deposition of Se and Te bulk powder in a vacuum system. ...
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J. Phys. Chem. C 2007, 111, 12926-12932

Fabrication and Growth Mechanism of Selenium and Tellurium Nanobelts through a Vacuum Vapor Deposition Route Qun Wang, Guo-Dong Li, Yun-Ling Liu, Shuang Xu, Ke-Ji Wang, and Jie-Sheng Chen* State Key Laboratory of Inorganic Synthesis and PreparatiVe Chemistry, College of Chemistry, Jilin UniVersity, Changchun 130012, People’s Republic of China ReceiVed: May 20, 2007; In Final Form: June 26, 2007

High-quality ultrawide Se and Te nanobelts have been prepared through thermal evaporation and deposition of Se and Te bulk powder in a vacuum system. The products were characterized by powder X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and high-resolution transmission electron microscopy (HRTEM). Different Se (Te) nanostructure intermediates have been observed in the preparation process through time-dependent experiments. On the basis of the experimental data, the formation of Se and Te nanobelts is believed to follow a vapor-solid mechanism mediated by agglomeration. Furthermore, it is demonstrated that, when subject to electron beams, the Te nanobelts evolve to helical ones.

1. Introduction One-dimensional (1D) nanostructure materials have drawn much attention due to their importance in understanding the fundamental roles of dimensionality and quantum size effect, and their potential applications as building blocks for electronic/ optical nanodevices.1 Among the various 1D nanostructures, nanobelts or nanoribbons are of particular interest, and since the first discovery of metal oxide nanobelts,2 a variety of inorganic beltlike species including B, C, Bi, Au, GaN, Ga2O3, CdS, and ternary bismuth oxide bromide have been successfully prepared.3 Trigonal selenium (t-Se) and tellurium (t-Te) nanostructure materials show intriguing properties such as unique photoconductivity, nonlinear optical response, and high thermoelectric or piezoelectric responses. They may also serve as ideal precursors for generating other functional materials such as CdSe,4 Ag2Se,5 Pt,6 CuSe,7 Bi2Te3, ZnTe, and CdTe.8 Recently, Se nanobelts have been fabricated through the so-called solution reaction route with the assistance of capping reagents (surfactant or polymers). For instance, Lu et al.9 reported the formation of Se nanobelts using cellulose as both reducing and morphologydirecting agents under simple hydrothermal conditions. Singlecrystal t-Se nanobelts were also prepared by reducing SeO2 with glucose under the functional capping of poly(vinylpyrrolidone) (PVP) via a hydrothermal route.10 Qi and co-workers fabricated single-crystalline nanobelts of trigonal selenium (t-Se) in micellar solutions of nonionic surfactants.11 Bioorganic molecules have been utilized to shape the growth of one-dimensional single-crystalline Se nanoribbons as well.12 In addition, the solution route has also been applied to synthesize tellurium nanobelts. For example, Gautam and Rao presented a controlled synthesis of crystalline tellurium nanobelts through a selfseeding solution process.13 Qian et al. reported a controlled hydrothermal synthesis of thin tellurium nanobelts and nanotubes based on a disproportionation reaction,14 whereas Yu and coworkers have fabricated single-crystalline tellurium nanobelts * To whom correspondence should be addressed. Telephone: (+86)431-85168662. Fax: (+86)-431-85168624. E-mail: [email protected].

Figure 1. Schematic layout of the VVD system for growth of t-Se and t-Te nanobelts on a Si substrate.

from hydrothermal15 and solution processes, respectively.16 Also, trigonal Te 1D nanostructures (including nanobelts) have been prepared under the irradiation of visible light from a commercial lawn lamp in the presence of PVP (K-30) or poly(viny alcohol) (PVA).17 However, generally the solution preparation techniques for Se and Te nanobelts involve complex chemical processes that are not environmentally benign. The vapor-phase approach may be a more economical and greener technique for the formation of Se and Te nanostructures, but so far reports on Se and Te nanobelt (nanoribbon) formation through vapor-phase methods remain scarce. Se nanoribbons with zigzag edges have been prepared via a physical vapor deposition of Se powder at a temperature higher than the melting point of t-Se.18 Trigonal selenium nanowires and nanoribbons have also been formed through a carbothermal chemical vapor deposition route.19 Recently, Geng et al. synthesized tellurium nanobelts with widths ranging from 20 to 300 nm by the reaction of Al2Te3 powder and H2O in a horizontal tube furnace at a relatively high temperature of 500 °C.20 These vapor-phase-derived Se and Te nanobelts (nanoribbons) are usually formed at high evaporation temperatures (>400 °C), which may pose a limitation to the use of deposition substrates and to nanodevice fabrication and integration.21 Fabrication of Se and Te nanobelts through a general, facile, low-temperature vapor-phase route is still rather challenging. The vacuum vapor deposition (VVD) method has been used for the growth of copper nanowires and nanorods, and it is a one-step procedure that only involves copper vapor generation and redeposition on a substrate under a very low pressure or

10.1021/jp073902w CCC: $37.00 © 2007 American Chemical Society Published on Web 08/14/2007

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Figure 3. Series of FE-SEM images of Se nanobelts deposited on Si substrates at different magnifications (a-d), TEM image of a single Se nanobelt (e), and HRTEM image of the Se nanobelt (f) with the corresponding fast Fourier transformation (FFT) pattern in the inset.

Figure 2. Powder X-ray diffraction pattern of as-prepared Se nanobelts on a Si substrate (a) and Raman spectrum of the obtained Se nanobelts (b).

2. Experimental Section

vacuum condition in the absence of carrier gas.22 The VVD method is simple and environmentally friendly because it does not involve carrier gases and harmful chemical processes, and the purity of the as-prepared product is easily guaranteed. In our previous work, highly oriented CuCl nanorod arrays on Si substrate were prepared via evaporating CuCl powder.23 It was found that, if the vapor pressure of the source material reaches 10-3 Torr at an elevated temperature, the vapor species diffuse to the low-temperature zone and grow into 1D nanostructures due to the pressure and temperature differences between the source and the deposition zone. In this paper, we report the preparation of high-quality ultrawide Se nanobelts using the vacuum vapor deposition (VVD) method at an evaporation temperature of 200 °C and a deposition temperature of 100 °C, both temperatures being lower than the melting point of Se (217 °C), in contrast to the case for physical vapor deposition reported previously.18 Similarly, high-quality ultrawide Te nanobelts were prepared at an evaporation temperature of 350 °C and a deposition temperature of 200 °C (Te melting point ) 449 °C). In the preparation processes, the obtained morphologies of Se and Te were strongly dependent on the reaction time and temperature. It is of interest that helical nanobelts of Te were formed upon exposure of the as-deposited Te belts to an electron beam inside the chamber of a scanning electron microscope (SEM). Moreover, the formation mechanism of the Se and Te nanobelts has been elucidated on the basis of the experimental observations.

The Se nanobelt preparation was conducted in a horizontal quartz tube with an inner diameter of 6.50 cm and 30 cm in length. Se powder was directly evaporated onto a Si substrate under controlled vacuum conditions. The substrate was cleaned in an ultrasonic bath of ethanol for 20 min, deionized water for 10 min, and dried in air. The cleaned substrate was then placed at one end of a Pyrex glass tube with the Se powder a certain distance from the substrate (14-16 cm), and the glass tube was subsequently evacuated at room temperature. For deposition of Se nanobelts, the glass tube was heated rapidly (at a rate of 14 °C/min) in a tube furnace from room temperature to 150 °C and then elevated to 200 °C at a rate of 5 °C/min. This temperature was maintained for 30 min. Afterward, the furnace was cooled to room temperature naturally and a product containing black needles was deposited over the Si substrate. We also applied this method to the deposition of Te via the simple replacement of Se by Te powder. The temperature program was changed from room temperature to 300 °C at a rate of 14 °C/min and then elevated to 350 °C at a rate of 5 °C/min, and this temperature was maintained for 30 min. At last, silver-gray needlelike crystals were obtained on the Si substrate when the furnace was cooled to room temperature. The experimental layout for the Se and Te nanobelt preparation is shown in Figure 1. The morphologies and dimensions of the samples were revealed with a JEOL JSM-6700F field-emission scanning electron microscope (FE-SEM). The crystal structure of the asprepared product was confirmed through powder X-ray diffraction (XRD) on a Rigaku D/max 2550 X-ray diffractometer. The Raman spectrum was recorded on a Renishaw Model 1000 Raman spectrometer, with 514.5 nm radiation from a 20 mW

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Figure 6. Schematic representation showing the hexagonal crystal structure of t-Se (Te). Figure 4. Powder X-ray diffraction pattern of as-prepared Te nanobelts on Si substrate. The standard diffraction peak positions and relative intensities of bulk trigonal Te are indicated as bars.

Figure 5. Series of FE-SEM images of Te nanobelts deposited on Si substrates at different magnifications (a-d), TEM image of a single Te nanobelt (e), and HRTEM image of the Te nanobelt (f) with the corresponding fast Fourier transformation (FFT) pattern in the inset.

air-cooled argon ion laser being used as the exciting source. Transmission electron microscopy (TEM) was performed on a JEOL JEM-3010 transmission electron microscope operated at 300 kV. For TEM imaging, the Se and Te products were removed from the corresponding Si substrate by careful scratching. 3. Results and Discussion 3.1. Morphology and Structure Analysis of Se Nanobelts. The simple thermal evaporation of Se powder and condensation of the vapor at a suitable temperature onto a substrate leads to highly pure nanostructural species in high yields. Figure 2a shows the typical XRD pattern of the Se product deposited on the silicon 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 (Se, JCPDS, No. 06-362). Figure 2b shows

the Raman spectrum of the Se nanobelts. In the spectrum there appears an intense resonance peak at 234 cm-1 which is assigned to the vibration of helical selenium chains that only exist in the t-Se phase, whereas the resonance peaks at 256 and 264 cm-1 associated with monoclinic selenium and amorphous selenium, respectively,9 are not observed. Additionally, in the spectrum there appears a peak around 145 cm-1 which is attributable to the transverse optical photon mode, and two peaks at around 437 and 457 cm-1 assignable to the second-order modes of t-Se24 are also observed. Combination of the XRD and the Raman spectroscopic results lead us to conclude that the obtained Se product is well-crystallized trigonal selenium. Figure 3a-d shows a series of typical SEM images of the as-fabricated material at low and high magnifications. The images demonstrate that the material has a beltlike morphology with a width of 500-5000 nm. Such ultrawide Se nanobelts have not been reported previously. The nanobelts appear to be uniform, with an average thickness of 90 nm and a length up to several hundreds of micrometers. Each nanobelt has a very smooth surface. Figure 3e shows the TEM image of a single Se nanobelt with a width of ∼500 nm. A high-resolution TEM (HRTEM) image taken from the belt edge is displayed in Figure 3f, which gives rise to a d-spacing of about 0.5 nm corresponding to the (001) planes of trigonal Se. It is obvious that the nanobelts are grown anisotropically. This image also reveals that the belt is single crystalline. Moreover, the corresponding fast Fourier transformation (FFT) pattern (inset in Figure 3f) can be indexed to the [11h0] zone axis of t-Se, further substantiating that the whole nanobelt is a single crystal grown along the [001] direction. The growth direction is consistent with the inherent helical chain of t-Se. 3.2. Morphology and Structure Analysis of Te Nanobelts. The XRD pattern of the obtained sample in Figure 4 demonstrates that all the peaks can be easily indexed on the basis of the hexagonal crystal structure of tellurium (JCPDS File No. 36-1452) with lattice parameters a ) 0.445 79 nm and c ) 0.592 70 nm of space group P3121 (152). Compared with the standard pattern of trigonal tellurium, an abnormally intense (100) reflection peak was observed in the XRD pattern, indicating that the particles have preferential orientation along a specific direction. A series of representative SEM images (Figure 5a-d) show that the nanobelts with an average width of 5 µm, a thickness of about 100 nm, and a length of several tens of micrometers were obtained. These nanobelts also have very smooth surfaces. The TEM image in Figure 5e displays a clear individual nanobelt. A high-resolution TEM image (Figure 5f) obtained from the edge of the nanobelt shows the regular crystal lattice, indicative of crystal perfection of the structure. The interplanar spacing of 0.59 nm is consistent with the (001) crystal plane which is parallel to the nanobelt axis. The relevant

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Figure 8. SEM image at high and low magnification for Te nanorods obtained when the evaporation temperature is raised from room temperature to 300 °C (a, b), SEM image at high and low magnification for the open Te nanorods obtained when the evaporation temperature is further raised to 350 °C (c, d), TEM image of a single Te open nanorod (e) with the corresponding ED pattern in the inset, and HRTEM image obtained from the tip of an individual open nanorod (f).

Figure 7. SEM images of Se nanospecies obtained when the evaporation temperature is raised from room temperature to 150 °C (a), followed by further raising to 200 °C (b), SEM image of the Se urchinlike nanostructures (c, d), powder X-ray diffraction pattern of as-prepared urchinlike Se nanostructures (e), TEM image of the urchinlike Se nanostructures consisting of nanorods (f), TEM image of a single Se nanorod (g), and HRTEM image of the Se nanorod (h) with the corresponding fast Fourier transformation (FFT) pattern in the inset.

fast Fourier transformation (FFT) pattern is shown in the inset of Figure 5f. These observations confirm that the nanobelts are grown in the preferred direction along the c-axis. 3.3. Nanobelt Growth Mechanism. The shape of a singlecrystalline nanostructure is usually a reflection of the symmetry of a basic crystal lattice. The trigonal crystal structure of t-Se (Te), as shown in Figure 6, is highly anisotropic with a long c-axis. In the structure, there exist helical chains which are formed by covalently bonded atoms, and these chains are bound together through weak van der Waals interactions, resulting in a hexagonal lattice. The anisotropic crystal structure of t-Se (Te) has a strong tendency toward 1D growth along the c-axis;25,26 thereby the formation of Se (Te) nanobelts running along the [001] direction is easily achieved, as observed through TEM. So far, several mechanisms, such as vapor-liquid-solid (VLS),27,3e oxide-assisted growth (OAG),28 and nanobranch coalescence,29 have been proposed to explain the growth of

nanobelts. However, none of these mechanisms seems to be suitable for explanation of the growth of Se (Te) nanobelts produced in the present work. First, no metals were employed as a catalyst in our VVD process, and both SEM and TEM images showed that no catalyst heads were found at the tips of the nanobelts. Therefore, the catalyst-induced VLS mechanism can be ruled out. Second, the evaporation process was carried out under a vacuum condition; hence, the absence of oxygen excludes the possibility of the OAG mechanism. Third, because no nanobranch intermediates were observed in the formation process of the nanobelts, the nanobranch coalescence mechanism should not be involved, either. Time-dependent experiments were carried out to reveal the growth process of the Se nanobelts. The samples obtained at different holding times were first analyzed using field-emission scanning electron microscopy (FE-SEM). Figure 7a shows the typical morphology of the product obtained when the evaporation temperature is raised from room temperature to 150 °C. The main product obtained at this stage consists of nanospheres with a diameter of 100-200 nm. The XRD pattern (not shown) shows that these nanospheres are amorphous. With increasing the evaporation temperature from 150 to 200 °C at a rate of 5 °C/min, large microspheres (Figure 7b) form and short nanorods grow out from the surface of the amorphous microspheres. When the evaporation temperature is maintained at 200 °C for 5 min, the microspheres are completely converted to urchinlike Se structures consisting of nanorods (Figure 7c). Agglomeration can be found (as indicated by the arrow in Figure 7d), and this agglomeration is believed to play an important role in the formation of Se nanobelts. Figure 7e shows the XRD pattern of the urchinlike Se nanostructures, which confirms that the composition of the as-obtained product is t-Se. A TEM

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Figure 9. Representative SEM images showing the roots of the Se (a-c) and Te (d-f) nanobelts.

image of the urchinlike Se structures is presented in Figure 7f, which shows that the Se nanorods in the structures have a diameter of ∼100 nm and a length of ∼500 nm. Figure 7g displays the TEM image of an individual nanorod and Figure 7h its HRTEM image which reveals the single-crystalline nature of the nanorod. The measured spacing of the lattice fringes is 0.5 nm corresponding to the (001) planes of trigonal Se. This means that these nanorods also have a [001] preferred growth direction. The fast Fourier transformation (FFT) of the HRTEM image shown in the inset of Figure 7h confirms the growth direction of the nanorods as well. It is known that Te and Se have similar structures and properties, so the nanobelt formation processes for these two elements may be analogous if not identical. A series of SEM images of the stepwise intermediate samples were also captured to reveal the whole evolution process for the Te nanobelts. Figure 8a,b shows the typical morphology of the product obtained when the evaporation temperature is raised from room temperature to 300 °C. The main product at this stage consists of open nanorods with diameters of 100-200 nm. It is of interest to note that the Te nanorods at this stage appear as hollow boats which have not been observed previously for Te nanostructures synthesized through vapor-phase routes.30-34 When the temperature is elevated from 300 to 350 °C at a rate of 5 °C/min, the open nanorods seem to be larger in size and the boat-shaped morphology is clearer (Figure 8c,d). Figure 8e presents the typical TEM image that demonstrates the morphology of the as-obtained open nanorod, with the selected area electron diffraction (SAED) pattern shown in the inset. The HRTEM image (Figure 8f) obtained from the tip of an individual open nanorod shows a spacing value of 0.59 nm for the lattice fringes, which are perpendicular to the [001] direction. In contrast to the case for Se nanobelt formation, no amorphous tellurium (RTe) spheres appear in the process of Te nanobelt formation, arguably because of the high evaporation temperature employed for the Te deposition. On the basis of the experimental observations, the Se (Te) nanobelt formation process is clearly envisioned. As the temperature increases from room temperature to 150 °C, Se vapor is produced under vacuum and diffuses to the lower temperature zone (where the Si substrate is located), followed by condensation to form Se nanoclusters. These Se nanoclusters are energetically favorable to absorb additional vapor species to form R-Se nanospheres with diameters of 100-200 nm. The diameters of the nanospheres increase further to micrometer scale through continuously accepting the upcoming Se vapor

species. When the temperature reaches the point at which the R-Se transforms to t-Se, t-Se nanorods begin to grow out from the surface of the R-Se microspheres due to their strong tendency toward 1D growth along the c-axis. It has previously been demonstrated that substrate temperature plays an important role in the conversion of R-Se to t-Se.35 Electronic quality R-Se film was prepared at low substrate temperature, and increasing the substrate temperature led to the formation of R-Se film containing crystalline inclusions and eventually to trigonal polycrystalline Se film. A similar sphere-wire transformation phenomenon (R-Se to 1D t-Se nanostructures) has also been observed by other research groups.19,25 In our case, when the temperature of the Si substrate is further increased, more t-Se nanorods grow out to form urchinlike Se nanostructures, and as the upcoming vapor species are continuously attached to the nanorods, agglomeration occurs. The formed agglomerates at this stage serve as roots (Figure 9a-c) for the subsequent nanobelt growth through absorbing further Se vapor species. Similarly, during the formation process of Te nanobelts, agglomeration of open boatlike nanorods occurs. This agglomeration leads to Te roots (Figure 9d-f) from which the final Te nanobelts grow gradually. Interestingly, a similar growth mechanism has also been reported previously for the formation of ZnSe nanobelts and nanowires via a vapor-solid mechanism mediated by a polycrystalline ZnSe film.36 It is concluded that the formation of Se (Te) nanobelts in the VVD system follows the vapor-solid (VS) growth mechanism.2,37 3.4. Effects of Electron Beam Exposure. It is well documented that the melting point of a solid material will be greatly reduced if it exists as a nanostructure.38 Yang and co-workers demonstrated the possibility of manipulating individual Ge nanowires encapsulated in carbon sheaths utilizing techniques such as cutting, interconnecting, and welding due to the low melting points of the nanowires.39,40 Li et al. have observed that electron beam irradiation is a powerful tool in tailoring In2O3 nanotubes and their indium fillings, as a result of the low melting point of In.41 It has also been reported that individual Ag2S nanowires can be thinned out and cut by using a highenergy electron beam and a laser beam.42 In our experiment, we discovered that some Te nanobelts are curved to form helical belts upon exposure to an electron beam at a voltage of 5 kV. Nevertheless, the helical nanobelt formation was typically observed only for ultrathin nanobelts, possibly because the thinner Te possesses a distinctly lower melting point which renders the nanobelt curving easier. In Figure 10 the representative SEM images illustrate the curving process of an

Fabrication of Selenium and Tellurium Nanobelts

J. Phys. Chem. C, Vol. 111, No. 35, 2007 12931 4. Conclusions High-quality ultrawide Se and Te nanobelts have been prepared through a simple VVD route at a relatively low temperature. It is found that the morphology of the obtained products is strongly dependent on the evaporation time and temperature. Se urchinlike nanostructures consisting of nanorods and Te open nanorods are formed as intermediates during the nanobelt preparation process. As the evaporation proceeds, these intermediates undergo agglomeration through particle-nanorod collision, and finally Se and Te nanobelts are grown from the roots that are formed by the agglomeration. The growth of t-Se and t-Te nanobelts is believed to follow a vapor-solid mechanism mediated by agglomeration. In addition, it is also demonstrated that thin Te nanobelts transform to helices upon exposure to an electron beam. The successful preparation of Se and Te nanobelts indicates that the simple VVD technique is effective for morphological transformation of materials that have a relatively high vapor pressure at low temperatures. Acknowledgment. This work was supported by the National Natural Science Foundation of China and the National Basic Research Program of China (2007CB613303). References and Notes

Figure 10. Representative SEM images demonstrating the formation of Te helices upon electron beam exposure at intervals of 100 s. Scale bar: 100 nm.

individual Te nanobelt upon electron beam irradiation at intervals of 100 s. It is seen that the tip of the nanobelt is elongated and the belt starts to curve as the electron beam irradiation proceeds. Finally, the nanobelt coils to form a regular helix. The helix radius and pitch depend on the starting geometry of the Te nanobelt and the time of exposure to the electron beam. Recently, detailed theoretical descriptions of the formation of ZnO nanohelices as a result of competition between elastic energy, surface-polarization-induced energy, volume energy, and defect-induced energy are available.43 On the basis of the experimental data, it was proposed that the formation of ZnO nanohelices is attributed to a consequence of minimizing the total energy consisting of elastic and spontaneous polarization contributions.44 The total energy was found to depend on the nanostructure dimensions, and the critical thickness of the ZnO belts was found to be 15 nm. For thicknesses less than this value, a coiled structure was more energetically favorable, whereas for thicker belts a straight geometry was the stable shape.44 In principle, the behavior of the Te nanobelts should be the same as that of the ZnO nanobelts. In the crystal structure of Te, the covalent bonds within the Te helical chains are stronger than the van der Waals interactions between the chains. If a Te nanobelt is thin enough, the electron beam cannot dissociate the Te covalent bonds but is sufficient to drive the Te nanobelt to curve and subsequently to coil. The Te nanobelt curving upon electron beam irradiation is attributed to its special crystal structure and beltlike morphology, and this phenomenon is reminiscent of the fact that graphite nanoparticles are spontaneously transformed to onion and related nanostructures on electron beam irradiation.45

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