Synthesis of GeSe 2 Nanobelts Using Thermal Evaporation and Their Photoelectrical Properties

GeSe 2 nanobelts were synthesized via a simple thermal-evaporation process by using gold particles as catalyst and GeSe 2 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 GeSe 2 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 GeSe 2 nanobelt have been fabricated and their photoelectrical property has been investigated. Results indicate that these nanobelt devices are potential building blocks for optoelectronic applications.


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][4][5], photodetectors [6][7][8], solar cells [9][10][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][16][17].As an important member of IV-VI semiconductors, GeSe 2 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 GeSe 2 nanostructures (nanowires, nanobelts, etc.).For instance, three-dimensional GeSe 2 nanostructures composed of nanobelts were synthesized with chemical vapor deposition (CVD) and have demonstrated very impressive performance in supercapacitor applications [15,18].GeSe 2 nanowires also could be synthesized through decomposing organic ammonium precursor [19].Stepped-surfaced GeSe 2 nanobelts were obtained via CVD process and showed high photoresponsivity and gain [20].Single GeSe 2 nanobelt twoterminal device exhibited high electronic transport properties, photoconductive characteristics, and temperaturedependent electronic characteristics [21].
In most cases, one-dimensional GeSe 2 nanostructures were synthesized via a wet-chemical routine or CVD process by using Ge and Se powders as starting materials [15,[18][19][20].It is also known that both GeSe and GeSe 2 have high stability [17,18].As a result, both GeSe and GeSe 2 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 GeSe 2 nanobelts via a simple thermal evaporation method by using GeSe 2 as starting materials and gold films as catalysts.XRD and TEM results demonstrate that the products are pure phase GeSe 2 .Besides, vapor-solid (VS) mechanism might contribute to the formation of GeSe 2 nanobelts.Devices made of a single GeSe 2 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.

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 GeSe 2 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 GeSe 2 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 K radiation with  = 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.

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, GeSe 2 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 GeSe 2 .Figure 4(a) shows a typical XRD pattern of the assynthesized nanobelts.All the diffraction peaks can be indexed to single-crystalline monoclinic GeSe 2 (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 defectfree.Two d-spacings of 1.68 and 0.70 nm match well with the (010) and (100) planes of monoclinic GeSe 2 , respectively.The reciprocal lattice peaks (Figure 4(d)) which were given by fast Fourier transform (FFT) can be indexed to a monoclinic structure of GeSe 2 .The indexed FFT pattern demonstrates that the growth direction of nanobelt is along [010] direction, which is inconsistent with the [112] growth direction of stepped-surfaced GeSe 2 nanobelts synthesized via VLS mode [20].The top and bottom surfaces of the nanobelts are {001} facets, and the side surfaces are {100} facets.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 GeSe 2 vapor which enters the Au particles to produce Au-Ge-Se alloy particles.When GeSe 2 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 GeSe 2 nanobelt grows along [010] direction.The VS growth direction of section B is along [100] and [−100] directions.The layered structure of the GeSe 2 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 [001] 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 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 [010] direction via VLS mode 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 (O 2 + e − → O 2 − ).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 GeSe 2 nanobelt.

Conclusion
In summary, we demonstrated that a simple thermal evaporation method could be used to synthesize GeSe 2 nanobelts through using gold particles as catalyst and GeSe 2 flakes as starting materials.Most of nanobelts have wide belt, zigzag belt, or narrow belt structures which are mostly single

Figure 2 :
Figure 2: SEM images of the segmented nanobelts with A-B-C structure (A = wide belt, B = zizag belt, and C = narrow belt).(a) A low magnification image of the segmented nanobelts.(b) An enlarged view.(c-e) SEM images taken from section A, section B, and side of section A, respectively.(f-h) SEM images taken from section B.

Figure 4 (Figure 3 :
Figure 3: XPS analysis of the nanobelts.(a) XPS survey spectrum of the as-synthesized nanobelts.(b) High-resolution XPS spectrum of Se 3d3/2 and 3d5/2 core level.(c) High-resolution XPS spectrum of Ge 3d and G-O bond state.

Figure 4 :
Figure 4: (a) XRD pattern of the nanobelts.(b) A TEM image of a nanobelt.(c, d) A HRTEM taken from the edge of the nanobelt and corresponding FFT pattern.(e) A STEM image taken from the tip of a nanobelt.(f-h) EDS maps for Au, Gen and Se, respectively.Scale bar: 100 nm.(i) A STEM image of the nanobelt.Scale bar: 1 m.(j, k) EDS profiles taken along the red and green arrows in (i), respectively.

Figure 7 (
Figure 7(a)  shows the schematic illustration of experimental setup for photoelectrical measurements.A white light with power density of ∼0.7 mW/mm 2 was used as the incident light.Figure7(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.

Figure 5 :Figure 6 :
Figure 5: (a) A TEM image of the zigzag structure.(b-f) The corresponding HRTEM taken from the rectangular area 1-5, respectively.The inset in (b) shows its corresponding FFT pattern.(g, h) Enlarged views of the rectangular area in (f) and corresponding FFT pattern.