Abstract
GeSe2 nanobelts were synthesized via a simple thermal-evaporation process by using gold particles as catalyst and GeSe2 flakes as starting materials. The morphology, crystal structure, and composition were characterized with scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TEM), X-ray diffraction spectroscopy (XRD), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray spectroscopy (EDS). SEM micrographs show that most of GeSe2 nanobelts have distinct segmented structures (wide belt, zigzag belt, and narrow belt). A possible mechanism was proposed for the growth of segmented nanobelts. It is possible that the growth of the segmented nanobelts is dominated by both vapor-liquid-solid and vapor-solid mechanisms. Devices made of single GeSe2 nanobelt have been fabricated and their photoelectrical property has been investigated. Results indicate that these nanobelt devices are potential building blocks for optoelectronic applications.
1. Introduction
Recently, one-dimensional semiconductor nanostructures, such as nanowires and nanobelts or nanoribbons, have attracted much attention because of their excellent physical properties and their unique structure for device applications [1]. They are considered to be excellent building blocks for devices such as field effect transistors [2], field emitters [3–5], photodetectors [6–8], solar cells [9–11], nanolasers [12], and chemical [13] and biological sensors [14]. These devices demonstrate improved features in comparison to those using bulk materials. For instance, photodetectors fabricated using nanowire or nanobelt always exhibit high sensitivity to light because of their high ratio of surface to volume [8].
IV–VI chalcogenides with layered crystal structure have been widely employed in construction of devices used in optics, thermoelectrics, and optoelectronics [15–17]. As an important member of IV–VI semiconductors, GeSe2 with a wide band gap (~2.7 eV) was found to have promising applications in electronics, optoelectronics, and renewable energy devices [18]. Therefore, much attention has been drawn towards the preparation and the study of property of GeSe2 nanostructures (nanowires, nanobelts, etc.). For instance, three-dimensional GeSe2 nanostructures composed of nanobelts were synthesized with chemical vapor deposition (CVD) and have demonstrated very impressive performance in supercapacitor applications [15, 18]. GeSe2 nanowires also could be synthesized through decomposing organic ammonium precursor [19]. Stepped-surfaced GeSe2 nanobelts were obtained via CVD process and showed high photoresponsivity and gain [20]. Single GeSe2 nanobelt two-terminal device exhibited high electronic transport properties, photoconductive characteristics, and temperature-dependent electronic characteristics [21].
In most cases, one-dimensional GeSe2 nanostructures were synthesized via a wet-chemical routine or CVD process by using Ge and Se powders as starting materials [15, 18–20]. It is also known that both GeSe and GeSe2 have high stability [17, 18]. As a result, both GeSe and GeSe2 phases probably form simultaneously in the CVD process when Se and Ge are used as precursors. Besides, the melting point of Ge (938.25°C) is much higher than that of Se (217°C), resulting in different evaporation rate of the precursors and a complex process of the growth.
We synthesized GeSe2 nanobelts via a simple thermal evaporation method by using GeSe2 as starting materials and gold films as catalysts. XRD and TEM results demonstrate that the products are pure phase GeSe2. Besides, vapor-solid (VS) mechanism might contribute to the formation of GeSe2 nanobelts. Devices made of a single GeSe2 nanobelt were fabricated and their photoelectrical property was investigated. Results indicate that these prepared nanobelts have good photoelectrical properties and potential application in optoelectronic devices. The detailed results and the discussion about the synthesized nanobelts and their photoelectrical properties are given in the following sections.
2. Experimental Section
The experimental setup used for nanobelt synthesis consists of a horizontal tube furnace, a quartz tube, a gas supply, and a control system (Figure 1). Commercial GeSe2 flakes (20 mg, purity 99.99%, J&K Scientific Ltd.) used as source materials were positioned in the center of the furnace. The Au-coated (~20 nm) Si substrates were placed at the downstream zone to collect products. After the furnace was fully flushed with high-purity argon gas for 30 min, the system was heated up to 650°C at a rate of 30°C/min. The Ar flux was kept at 100 standard cubic centimeters per minute (sccm) and the deposition temperature was ~450°C. After 1 h of growth, the system was cooled to room temperature. The yellow products on the substrates were GeSe2 nanobelts.
The morphology, crystal structure, and composition of as-synthesized products were characterized using SEM (FEI Nova NanoSEM200), XRD (Bruker D8 Advanced X-ray diffractometer, Cu Kα radiation with λ = 0.15418 nm), TEM (JEOL 2100F, 200 kV), and EDS. The binding energy of the samples was examined by XPS (Kratos AXIS Ultra DLD).
To evaluate the photoelectrical properties of the nanobelts, two-terminal device made of a single nanobelt was fabricated. The as-synthesized nanobelts were dispersed on p+-Si wafer with marks and then treated with electron beam lithography (30 kV, 110 pA, Nanometer Pattern Generation System installed in the SEM), metallization, and lift-off process to define the Cr (10 nm)/Au (100 nm) contacts with the nanobelt. The electrodes were used for interconnecting and the marks were used for aligning in the following electron beam lithography. The current-voltage (I-V) characteristics of the device were investigated in air and at room temperature with a semiconductor characterization system (4200 SCS, Keithley Instruments Inc., USA). The power density of the incident light was calibrated by using a standard silicon photodetectors.
3. Results and Discussion
Figure 2 shows the SEM images of the as-prepared nanobelts. As can be seen from the SEM images, most of the nanobelts have segmented A-B-C structure (A = wide belt, B = zigzag belt, and C = narrow belt). The length of the A-B-C structured nanobelts varies from hundreds of micrometers to millimeters (Figures 2(a) and 2(b)). Sections A and C are commonly seen belt-like structure (Figures 2(c)–2(e)). The thickness of most segmented nanobelts ranged from 60 to 200 nm (Figure 2(e)). Besides, section C is narrower than section A. In Figure 2(d), particle (catalyst) can be clearly observed due to contrast difference, indicating that VLS mechanism probably dominates the growth of the A-B-C structured nanobelts. Interestingly, section B has zigzag structure. Figure 2(f) clearly reveals that the zigzag structure (indicated by arrows) serves as a transition region connecting sections A and C. Figures 2(g) and 2(h) show the enlarged views of section B and the zigzag structure is clearly observed. It is well known that there is a good correlation between the nanobelt diameter and the catalyst size during VLS growth [22]. Although the size of section C and particle on the tip match well, section B has different shape and size. Therefore, VLS mechanism only partly makes contribution to the growth of nanobelt possibly. As can be seen from Figures 2(g) and 2(h), the growth of flake-like branch on the backbone might be governed by vapor solid (VS) rather than VLS mechanism. Besides, some small branches were also observed possibly due to different growth stages. As a layer-structured chalcogenides, GeSe2 consists of Se-Ge-Se layers stacked together via van der Waals interaction [20]. The branches with different thickness are found (e.g., indicated by arrows 3 and 4 in Figure 2(h)), implying that each layer has different growth speed. The segmented nanobelts have uniform thickness at section A. Besides, the growth direction is from section A to C according to the top growth mechanism. Therefore, it is reasonable to conclude that the zigzag structures are not an ultimate state and they would grow into nanobelt with the same structure as section A if growth continues.
XPS was employed to derive composition information of the as-synthesized nanobelts. The binding energy of all the survey and finely scanned spectrums was calibrated by the standard reference of the C 1s (284.8 eV) signal. Typical XPS survey spectrum (Figure 3(a)) indicates the presence of Ge and Se. Figure 3(b) shows that the peaks of Se 3d3/2 and Se 3d5/2 core level are at 55.1 and 54.4 eV, respectively. The peak position of Ge 3d core level is at 31.1 eV which is consistent with the literature [20]. The Ge 3d core level spectrum is asymmetric and a small shoulder on the high energy side is found (Figure 3(c)). This is caused by the Ge–O states (corresponding to 33 eV) on the surfaces [23]. No signals from Ge–Ge (29.1 eV) and Se–Se (55.4 eV) [20] were detected in the as-synthesized nanobelts, demonstrating that the products are pure phase GeSe2.
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Figure 4(a) shows a typical XRD pattern of the as-synthesized nanobelts. All the diffraction peaks can be indexed to single-crystalline monoclinic GeSe2 (JCPDS file: 71-0117). A representative TEM image (Figure 4(b)) further confirms the belt-like morphology. Figure 4(c) shows the high-resolution TEM (HRTEM) image of the nanobelt edge, confirming that the nanobelt is a single crystal and defect-free. Two d-spacings of 1.68 and 0.70 nm match well with the (010) and (100) planes of monoclinic GeSe2, respectively. The reciprocal lattice peaks (Figure 4(d)) which were given by fast Fourier transform (FFT) can be indexed to a monoclinic structure of GeSe2. The indexed FFT pattern demonstrates that the growth direction of nanobelt is along direction, which is inconsistent with the growth direction of stepped-surfaced GeSe2 nanobelts synthesized via VLS mode [20]. The top and bottom surfaces of the nanobelts are facets, and the side surfaces are facets.
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Figure 4(e) shows the scanning transmission electron microscope (STEM) image of a GeSe2 nanobelt with catalyst droplet on the tip. Figures 4(f)–4(h) show elemental distribution of Au, Ge, and Se, respectively. Au was detected only at the catalyst droplet, whereas Ge and Se were observed in the nanobelt as well as the catalyst droplet. This further confirms the VLS growth of the nanobelts. Under high temperature, Au film changes into liquid droplet. Meanwhile, high temperature causes the formation of GeSe2 vapor which enters the Au particles to produce Au–Ge–Se alloy particles. When GeSe2 is oversaturated, it recrystallizes out of alloy particles, nucleates, and further grows to nanobelt. Figure 4(i) is the STEM image of single nanobelt. Figures 4(j) and 4(k) display EDS profiles along the red and green arrows in Figure 4(i), respectively. The STEM-EDS results confirm the uniform distribution of Ge and Se in the nanobelt. The atomic ratio between Se and Ge is close to stoichiometric ratio (2 : 1).
HRTEM was employed to investigate the structure of section B. Figure 5(a) shows a representative TEM image of a zigzag structure. Figure 5(b) is the HRTEM image of selected region marked by rectangle 1 in Figure 5(a) and the inset shows the corresponding FFT pattern. It can be found that the crystal structure of branch is identical to that of the backbone, further confirming that the growth of branches is from the backbone. Figures 5(c)–5(f), respectively, show the HRTEM images of position marked by rectangles 2–5 in Figure 5(a). Figure 5(g) gives an enlarged view of the region marked by rectangular area in Figure 5(f) and the corresponding FFT pattern is shown in Figure 5(h). These results indicate that the GeSe2 nanobelt grows along direction. The VS growth direction of section B is along and directions. The layered structure of the GeSe2 is clearly observed in Figures 5(b) and 5(f), which is consistent with previous report [15] that Se-Ge-Se layers are assembled along direction. As a result, the VS growth is parallel to (001) plane rather than perpendicular to (001). The HRTEM results also suggest that the VS growth speed of each layer is not equal and this result is in agreement with the SEM observation (Figure 2(h)). With the VS growth continuing, the side surfaces are changed from and facets (Figures 5(f) and 5(g)) to , , and facets (Figures 5(c) and 5(d)).
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Based on the above analysis, a possible growth mechanism is proposed for the formation of the segmented nanobelts. Firstly, Au–Ge–Se alloy particles act as catalyst and induce the growth along direction via VLS mode (Figure 6(a)). The two side surfaces of a nanobelt in this growth stage are parallel to (100) plane. Once the narrow nanobelt is formed, its side surfaces become nucleation sites which direct the VS growth along and directions (Figure 6(b)). However, because the growth speed of the VLS mode is faster than that of VS mode, zigzag structure forms (Figure 6(b)). With the VLS and VS growth proceeding simultaneously, the zigzag product evolutes to nanobelt with the structure of section A and the length of the section C further increase (Figure 6(c)), resulting in the formation of A-B-C structured products.
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Compared with bulk materials, nanostructured semiconductors have large surface-to-volume ratio and dangling bonds on the material surfaces. Therefore, there are numerous oxygen molecules absorbed on their surface which attract the electron of photoexcited carriers (O2 + e−→). This can improve the separation of photoexcited carriers and increase photocurrent [24]. To evaluate the photoelectrical properties of the as-synthesized nanobelts, two-terminal device was fabricated through using a single GeSe2 nanobelt. Figure 7(a) shows the schematic illustration of experimental setup for photoelectrical measurements. A white light with power density of ~0.7 mW/mm2 was used as the incident light. Figure 7(b) gives the current-voltage (I-V) characteristics of the device under incident light illumination or without illumination. The asymmetry and nonlinearity of I-V curves indicate the formation of Schottky contact between the nanobelt and the metal electrodes. Compared with the dark current at a bias of 1 V, the photocurrent increases by ~52 times. This indicates that the as-synthesized nanobelts have good photoelectrical properties.
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4. Conclusion
In summary, we demonstrated that a simple thermal evaporation method could be used to synthesize GeSe2 nanobelts through using gold particles as catalyst and GeSe2 flakes as starting materials. Most of nanobelts have wide belt, zigzag belt, or narrow belt structures which are mostly single crystals. A possible mechanism was proposed for explaining the growth of segmented nanobelts. The growth occurs first along direction via VLS mode and then VS mode starts along and directions. These two growth modes result in the formation of segmented nanobelts. The as-synthesized GeSe2 nanobelts exhibit excellent photoelectrical property.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This work was supported by NSFC (nos. 61001005 and 11375174), National 973 Project of China (nos. 2012CB934300 and 2012CB619200), and NSFZJ (Y1110734).