New insights on the mechanism of semiconductor nanostructures formed during vapor transport at atmospheric pressure - Tran Trung

Journal of Science and Technology 54 (5A) (2016) 107-117 NEW INSIGHTS ON THE MECHANISM OF SEMICONDUCTOR NANOSTRUCTURES FORMED DURING VAPOR TRANSPORT AT ATMOSPHERIC PRESSURE Tran Trung1, *, Hoang Van Han1, Nguyen Van Hieu2,** 1Hung-Yen University of Technology and Education, Khoai Chau, Hung-Yen, Vietnam 2International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST), 1-Dai Co Viet road, Hanoi, Vietnam. *Email: tr_trunghut@yahoo.com Received: 15 July 2016; Accepted for publication: 2 December 2016 ABSTRACT Co-deposition of two types of high aspect ratio nanostructured ZnO involving nanowires and nanotetrapods with a uniform structure were carried out through thermal evaporation and vapor transportation of a mixture of highly pure ZnO and graphite powders in 1:1 weight ratio. The mixture was heated at 1100 oC under various flow rates of N2 and air mixture. The surface morphology and structural characteristics of synthesized ZnO nanostructured materials revealed a highly crystalline structure with an average diameter of about 30 nm and length of several micrometers. The mechanism of co-deposition of ZnO nanowires and nanotetrapods during vapor transport at atmospheric pressure was proposed. Keywords: ZnO nanowires, nanotetrapods, high aspect ratio, co-deposition, vapor phase. 1. INTRODUCTION Among the various classes of one-dimensional nanostructures, semiconductor nanowires are a result of anisotropic nanostructure having the diameter in the order of a nanometer, usually constrained to several tens of nanometers or less and an unconstrained length. These one- dimensions that can be advantageous are the small diameters, large surface area and smooth surfaces of the nanowire materials, then they offer unique opportunities to control the density of states of semiconductors, and in turn their electronic and optical properties. Then semiconductor nanowires possess several unique characteristics. Their ability to be integrated into electronic devices, novel sub-wavelength optical phenomena, their large tolerance for mechanical deformations, their ability to interface with other microscopic and nanoscopic systems in nature, the decoupling of length scales associated different physical phenomena in the radial and axial directions, and their high surface-to-volume ratio, have led to an explosion of applications utilizing these structures. Indeed, since the first report of Radushkevich and Lukianovich, in 1952, on the formation of carbon fibrils by thermal decomposition of carbon oxide during Tran Trung, Hoang Van Han, Nguyen Van Hieu 108 contact with iron, to date a concept “Semiconductor Nanowires” is became a platform for nanoscience and nanotechnology where numerous studies have been carried out to explore nanowires as new building blocks in electronics and photonics [1 – 5], solar cells [6 – 9] sensors/biosensors [10 – 14], and energy applications [15, 16]. In all the mentioned literatures, the method either physical vapor deposition or chemical vapor deposition that were used to synthesize the nanowire materials was obeyed to the vapor– liquid–solid (VLS) mechanism. As to vapor transport depositions, there exist two different processes which either involve metal catalyst for the nanowires (NWs) growth or not. For both just mentioned above, the process consists of mass transfer of the material to the reaction zone in the form of the gas flux of components, their diffusion or molecular beam, adsorption on the surface and surface diffusion of the components to more favorable sites. In case if the mass transfer is sufficient and there is supersaturation of the vapors, the limiting factor of NWs growth is the nucleation rate at the surface. The metal catalyst facilitates growth by introducing a catalytic liquid alloy phase which can rapidly adsorb components to supersaturation levels, and from which crystal growth can subsequently occur from nucleated seeds at the liquid–solid interface by the vapor–liquid–solid (VLS) mechanism. In case of the absence of metal catalyst, the growth of a crystal takes place through direct adsorption of gaseous components onto crystal defects of a solid surface, then aggregated into seeds that act as catalyst. In this situation, the formation and growth of nanowires from these seeds are considered as self-catalyzed process and similar to the one of the vapor-solid mechanism [17]. However the nucleation conditions at the interface are often not so favorable as compared with liquid catalyst. Beside that almost vapor transport depositions to form semiconductor nanowires were carried out in high vacuum of about 10 Torr or in ultimate vacuum of several 10-3 Torr, the formation of semiconductor nanowires by the vapor transport deposition have been carried out at atmospheric pressure [10, 18]. Different from almost the mentioned literatures, in our work the nanowires growth was fully carried out in vapor phase and at atmospheric pressure, as represented in figure 1. Now we focus on the comparing the nanowires growth with VLS mechanism and the one of the nanowires growth in our work. 2. EXPERIMENTAL As the detailed description of the experiment set up was represented in a previous work [10], the ZnO nanowires were synthesized through thermal evaporation and vapor transportation of a mixture of extrapure ZnO powder (Merck, 99.99 %) and graphite powder (Merck) in 1:1 weight ratio that were carefully grounded and mixed. The mixed material was kept in a porcelain boat (with 1 cm diameter and 8 cm long) and loaded into the centre of a horizontal quartz tube furnace (with 100 cm long and 4 cm inner diameter). The tube furnace was heated to 1100 oC at a heating rate of 10 oC/min. One end of the tube was connected to the gas supply and flow control system. A mixture of highly pure nitrogen gas (99.99 %) and air in 1.7:1 volume ratio was flowed at a constant rate of 2880 sccm into the tube furnace. The flow rate was modulated with a digital mass-flow-control system (Aalborg, Model: GFC175-VALD2-A0200, USA). The reactions began within 2–3 min and continued for approximately 13–15 min, depending on the amount of initial materials. A striking point here is to use the ambient atmosphere as the oxygen source for the reaction. The tube furnace was preheated up to and kept at 1100 oC before loading the source material. The product was deposited onto a borosilicate glass cup positioned out of the tube furnace (Fig. 1). The as-synthesized product was observed in cotton-white color and very uniform in diameter and length. The collected ZnO NWs were ultrasonically dispersed in New insights on the mechanism of semiconductor nanostructures formed during vapor transport 109 ethanol for 48 h until a stable dispersion was obtained. The morphology, size distribution, crystallinity, and composition of as-synthesized products were characterized using field emission scanning electron microscope (FESEM 4800, Hitachi, Japan) operating at 10 kV. Transmission electron microscopy (TEM), selected area electron diffraction (SEAD), and high resolution transmission electron microscopy (HRTEM) examinations were conducted using a JEOL JEM- 3010 system at an accelerating voltage of 300 kV. The X-ray diffraction (XRD) patterns were obtained using a Siemens diffractometer with Cu Kα1 radiation. Figure 1: Diagram generation fabrication of ZnO nanomaterials consists of gas suppliers (1), a digital mass-flow-control system (2), a gas mixer (3), a gas adjusted valve (4), a horizontal quartz tube reactor (5), PID controlled program oven (6), the outlet with a borosilicate glass cup. 3. RESULTS AND DISCUSSION Morphology and structure of as-synthesized ZnO The as-synthesized product that was obtained in a borosilicate glass cup positioned at the outlet of the reactor (Fig. 1) was observed in cotton-white color. FE-SEM and TEM studies revealed that the as-synthesized ZnO product can be distinguished into two regions with two different morphologies, nanowires and nanotetrapods, as shown in Figures 3 and 4. In more detailed, it also revealed that structure of the part of the product positioned at the central of the borosilicate glass cup is consisting of ZnO nanowires. Meanwhile structure of the one positioned near by the wall of the cup is ZnO nanotetrapods. Figure 2 presents the XRD pattern of as-grown ZnO NWs, which are identified as hexagonal wurtzite ZnO phase (JCPDS 36-1451) from the XRD analysis results. It is interesting to note that no diffraction peaks corresponding to other phases or impurities are observed. Thus, the XRD pattern clearly reveals the formation of ZnO NWs with wurtzite structure under the conducted experimental conditions. The presence of well-defined diffraction peaks of (100), (002), (101), (102), (110), (103), (200), (112), and (004) clearly reveals that the aggregates of vapor ZnOx molecules are converted into ZnOx cluster form that are then oxidized into ZnO through absorption of oxygen atoms by these surfaces. The strong and sharp ZnO peaks with a narrow spectral width indicate that the synthesized nanowires are highly crystalline. Tran Trung, Hoang Van Han, Nguyen Van Hieu 110 Figure 2. Representation of XRD pattern of ZnO as-synthesized. The overall morphologies of the as-synthesized ZnO nanomaterials were studied with FE- SEM technique. Figure 3(a) shows the low magnification image of the as-synthesized ZnO NWs in high aspect-ratio structure and heap up together. High magnification image of ZnO NWs (Fig. 3(b)) revealed very uniform ZnO NWs with a diameter of approximately 30 nm and a length of several micrometers. Additional structural characterizations were conducted using HRTEM technique. Figure 3(c) shows a low magnification image of a single ZnO nanowire. No voids or tubular structures were observed, which reveals that the growth of ZnO NWs may occur immediately in vapor transportation. For the ZnO product positioned near by the wall of the cup, FE-SEM studies (Fig. 4a) revealed that as-synthesized ZnO materials have structural morphology of nanotetrapods in high aspect-ratio structure and heap up together. A higher magnification image (Fig. 4b) the surface morphology and structural characteristics of synthesized nanotetrapods revealed a high uniform and highly crystalline structure with an average diameter of about 30 nm and length of several micrometers, 3÷5 µm. High-resolution TEM studies of ZnO nanowires (Figs. 3(d)–(f)) and ZnO nanotetrapods (Figs. 4(d)–(f)) also reveal that ZnO nanostructures are clean, atomically sharp and without any sheathed secondary phases or stacking faults. In the lattice-resolved scale, the HRTEM images of ZnO nanowires (Figs. 3(e), (f)) and ZnO nanotetrapods (Figs. 4(f), (h)) show that the length of the lattice fringe of ZnO nanowire is approximately 0.52 nm, corresponding to the (0001) fringes perpendicular to the growth direction, which is consistent with that of the bulk wurtzite ZnO crystal. In addition, SEAD patterns of ZnO nanowires (Figs. 3(d), (f)) and ZnO nanotetrapods (Figs. 4(d) (f)) reveal clearly visible bright spots corresponding to the crystal planes of the hexagonal for 1-D wurtzite ZnO nanostructure. 25.03 31.03 37.03 43.03 49.03 55.03 61.03 67.03 0 100 200 300 400 500 600 In te ns ity (C P S) 2 tetha (2 01 ) (1 12 ) (2 00 ) (1 03 ) (1 10 ) (1 02 ) (1 01 ) (0 02 ) (1 00 ) In te ns ity (a .u ) 2θ (degree) Ne w insights on Figure 3. T the mechan ypical SEM ( ism of semic a), (b), TEM onductor nan (c), and HRTE nanowires Z ostructures M (d)–(f) m nO. formed during icrographs of vapor trans the as-synthe port 111 sized Tran Trung, Hoang Van Han, Nguyen Van Hieu 112 Figure 4. A typical SEM (a), TEM (b) and HRTEM (d)-(h) micrographs of the as-synthesized ZnO nanotetrapods. 10 nm 1 μm 100 nm a) 50 nm 38 picks ~ 10 nm 0.263 nm b) e) d) f) h) New insights on the mechanism of semiconductor nanostructures formed during vapor transport 113 4. MECHANISM OF CO-DEPOSITION OF ZNO NANOWIRES AND NANOTETRAPODS Because of using catalyst-free synthesis method in our work, the growth mechanism of ZnO NWs could not be based on VLS model, involving in a catalyzed-tip or base growth mode, depending on the metal-support interaction to be strong or weak. In our synthesis method, the tube furnace was preheated to 1100 oC. The obtained product was with a white cotton color and very uniform in diameter and length, suggesting that the nucleation and growth of ZnO nanostructures (nanowires and nanotetrapods) may begin during vapor transportation. Indeed, the melting temperature of ZnOx is approximately 419 oC (when x < 1), which is much lower than that of ZnO (1975 oC) [17,19] ZnOx is produced through the reactions (1) and (2) [17]: ଷ ଶZnO(s) + (1−x)C(s) → (1−x)CO(g) + ZnOx(g) (1) ZnO(s) + ሺଵି௫ሻଶ C(s) → ሺଵି௫ሻ ଶ CO2(g) + ZnOx(g) (2) For more detailed reactions, it is not treated in this work and we refer the reader to our previous work [12]. The ZnOx molecules aggregates are converted into ZnO clusters as revealed in reaction (3): ZnOx (clusters, x <1) + ሺଵି௫ሻ ଶ O2 → ZnO (clusters) (3) The ZnO clusters are ideal nuclei centers to form ZnO nanostructures, as discussed below. In this situation, the formation and growth of ZnO nanostructures are considered as self- catalyzed and similar to that of the vapor-solid mechanism [17]. Figure 5. Diagram explaining the formation mechanism of ZnO nanowires under lamina flow conditions. Tran Trung, Hoang Van Han, Nguyen Van Hieu 114 A possible growth mechanism of ZnO nanostructures in our synthesis method can be explained using a heating curve. As shown in Figures 4 and 5, for the formation of ZnO nanowires and nanotetrapods respectively, it takes 110 min for the tube furnace to reach the process temperature of 1100 oC. During the reaction time, the ZnOx vapor, gases CO and CO2 are formed via reactions (1, 2), and after vaporization into separated flows, they are transported in side by side by the supporting gas mixture of nitrogen and air (Fig. 5). A mixture containing a large quantity of graphite with ZnO powder ensures the availability of sufficient concentration of ZnOx vapor and high super saturation condition in the central region of the furnace. However the hydrodynamic regime is under different flow conditions, lamina flow for the supporting gas flows running along the axis of the tube reactor and turbulent flow for the ones running close to the wall (see Figs. 5 and 6). Within every supporting gas flow running under lamina flow conditions, the ZnOx molecules vapor are transported to the low temperature region. During that time they are oxidized further and aggregated into nanoclusters of ZnO via reaction (3) due to the reduction of the transported vapors rate and absorption of ZnOx molecules and aggregates together (Fig. 5). Meanwhile, a slight flow of CO2 prevents aggregation of the vapor ZnOx; therefore, uniform and high aspect ratio ZnO NWs are formed. Figure 6: Diagram explaining the formation mechanism of ZnO nanotetrapods under turbulent flow conditions. In parallel to the lamina supporting gas flows running along the central axis of the tube reactor, the vapors were transported by the supporting gas flows that running along and close to the wall (see Fig. 6) are under turbulent flow conditions. In general, turbulent flow is time- dependent, rotational, and three dimensional. Therefore there usually exist small perturbations imposed on the flow originated from the roughness of a quartz tube reactor, from small variations in the supporting gas flows caused by presence of CO, CO2, ZnOx, ZnO vapors, ZnO clusters (suspended solids) having different mass, etc. These make advantages for oriented connects of ZnO clusters to form some aspects of nanotetrapods in the way to achieve minimum free energy of the system (tetrapod nuclei and ZnO vapor clusters). Figure 5 represents diagram explanation of the formation mechanism of nanotetrapods. As seen the vapor ZnOx molecules that were transported in the supporting gas flow were oxidized further into ZnO vapors and aggregated into ZnO nanoclusters. Under turbulent flow conditions, in the initial stages of the New insights on the mechanism of semiconductor nanostructures formed during vapor transport 115 growth of ZnO nanotetrapods, the ZnO molecules were connected together to form ZnO nanotetrapod nuclei having eight tetrahedral crystals named the octa-twin model as proposed by Takeuchi et al [20]. As FE-SEM and HR-TEM studies revealed that the ZnO nanotetrapods that were synthesized successfully are characteristic of structure of very uniform in diameter and length. It revealed that the adsorption of ZnO molecular vapors and connections of ZnO clusters with a ZnO nanotetrapod nucleus in the way to achieve minimum free energy of the system. This growth is different the growth by VLS or/and VS mechanism that occurred under static conditions with high or ultimate high vacuum where some growth facets of a ZnO nucleus can achieve faster growth rates, while inhibiting the growth rates in other directions [1]. 5. CONCLUSION Using thermal evaporation and vapor transport method, the produced ZnO: nanowires and nanotetrapods were co-deposited under atmospheric pressure. FE-SEM and HRTEM studies revealed that both structure are very uniform and high aspect ratio structure and a diameter of approximately 30 nm and a length of several micrometers. The XRD examinations revealed that the produced ZnO are identified as hexagonal wurtzite ZnO phase and have highly crystallized structure with strong and sharp ZnO peaks of narrow spectral widths but without characteristic peaks of the impurities. The new insights on the mechanism of co-deposition of ZnO nanowires and nanotetrapods during vapor transport were proposed. Acknowledgement. The authors acknowledge the support from the Vietnam Ministry of Education and Training under Scientific Research program for a basic research project B2016-SKH-01 to work at Hung- Yen University of Education and Technology. REFERENCES 1. Z. L. - ZnO nanowire and nanobelt platform for nanotechnology, Mater. Sci. Eng. R Reports. 64 (2009) 33–71. 2. Yang C. - Encoding Electronic Properties by Synthesis of Axial Modulation-Doped Silicon Nanowires, Science (80-. ). 310 (2005) 1304–1307. 3. Minot E. D., Kelkensberg F., M. van Kouwen, J. A. van Dam, Kouwenhoven L. P., Zwiller V., Borgström M. T., Wunnicke O., Verheijen M. A., Bakkers E. P. A. M. - Single Quantum Dot Nanowire LEDs, Nano Lett. 7 (2007) 367–371. 4. Park W. Il, Zheng G., X., Jiang B. Tian, Lieber C. M. - Controlled Synthesis of Millimeter-Long Silicon Nanowires with Uniform Electronic Properties, Nano Lett. 8 (2008) 3004–3009. 5. Qian F., Li Y., Gradecak S., Park H. G., Dong Y., Ding Y., Wang Z. L., Lieber C. M. - Multi-quantum-well nanowire heterostructures for wavelength-controlled lasers., Nat. Mater. 7 (2008) 701–706. 6. Dong Y., Tian B., Kempa T. J., Lieber C. M. - Coaxial Group III−Nitride Nanowire Photovoltaics, Nano Lett. 9 (2009) 2183–2187. 7. Tian B., Zheng X., Kempa T. J., Fang Y., Yu N., Yu G., Huang J., Lieber C. M. - Coaxial silicon nanowires as solar cells and nanoelectronic power sources, Nature. 449 (2007) 885–889. Tran Trung, Hoang Van Han, Nguyen Van Hieu 116 8. Czaban J. A., Thompson D. A., LaPierre R. R. - GaAs Core−Shell Nanowires for Photovoltaic Applications, Nano Lett. 9 (2009) 148–154. 9. Prechtel L., Padilla M., Erhard N., Karl H., Abstreiter G., A. Fontcuberta I Morral, Holleitner A. W. - Time-Resolved Photoinduced Thermoelectric and Transport Currents in GaAs Nanowires, Nano Lett. 12 (2012) 2337–2341. 10. Van Han H., Van Hieu N., Trung T. - A simple method for production of high aspect ratio ZnO Nanowires with uniform structure for NO2 gas sensors, Sci. Adv. Mater. 6 (2014) 1659–1667. 11. Trung D. D., Toan L. D., Hong H. S., Lam T. D., Trung T., Van Hieu N. - Selective detection of carbon dioxide using LaOCl-functionalized SnO2 nanowires for air-quality monitoring, Talanta. 88 (2012) 152–159. 12. Van Hieu N., Duc N. A. P., Trung T., Tuan M. A., Chien N. D. - Gas-sensing properties of tin oxide doped with metal oxides and carbon nanotubes: A competitive sensor for ethanol and liquid petroleum gas, Sensors Actuators B Chem. 144 (2010) 450–456. 13. Liu H., Duan C., Yang C., Chen X., Shen W., Zhu Z. - A novel nitrite biosensor based on the direct electron transfer hemoglobin immobilized in the WO3 nanowires with high length–diameter ratio, Mater. Sci. Eng. C. 53 (2015) 43–49. 14. Ahmad M., Sun H., Hussain M., Karim S., Nisar A., Khan M. - Development of Silver Nanowires Based Highly Sensitive Amperometric Glucose Biosensor, Electroanalysis. 27 (2015) 1498–1506. 15. Marconcini P., Macucci M., Logoteta D., Totaro M. - Is the regime with shot noise suppression by a factor 1/3 achievable in semiconductor devices with mesoscopic dimensions?, (2013) 1–7. 16. Li Z., Zhou Y., Mao W., Zou Z. - Nanowire-based hierarchical tin oxide/zinc stannate hollow microspheres: Enhanced solar energy utilization efficiency for dye-sensitized solar cells and photocatalytic degradation of dyes, J. Power Sources. 274 (2015) 575–581. 17. Yao B. D., Chan Y. F., Wang N. - Formation of ZnO nanostructures by a simple way of thermal evaporation, Appl. Phys. Lett. 81 (2002) 757–759. 18. van Deelen J., Illiberi A., Kniknie B., Steijvers H., Lankhorst A., . Simons P - APCVD of ZnO:Al, insight and control by modeling, Surf. Coat. Technol. 230 (2013) 239–244. 19. A D. Pelton, Binary Alloy Phase Diagrams, 3 (1990). 20. Taylor P., Takeuchi S., Iwanaga H., Fujii M. - Octahedral multiple-twin model of tetrapod ZnO crystals, (n.d.) 37–41. New insights on the mechanism of semiconductor nanostructures formed during vapor transport 117 TÓM TẮT GIẢI THÍCH MỚI VỀ CƠ CHẾ HÍNH THÀNH CẤU TRÚC NANO CỦA VẬT LIỆU BÁN DẪN TRONG QUÁ TRÌNH VẬN CHUYỂN PHA HƠI Ở ẤP SUẤT KHÍ QUYỂN Trần Trung1*, Hoàng Văn Hán1, Nguyễn Văn Hiếu2** 1 Trường Đại học Sư phạm Kỹ thuật Hưng Yên, Khoái Châu, Hưng Yên, Việt Nam 2 Viện ITIMS, Trường Đại học Bách Khoa Hà Nội, Số 1 Đại Cồ Việt, Hà Nội, Việt Nam *Email: tr_trunghut@yahoo.com Trong quá trình bốc bay nhiệt hình thành đồng thời hai dạng nano ZnO là dây nano và nano tetrapod có tỉ số chiều dài/đường kính lớn với cấu trúc đồng nhất. Trong quá trình bốc bay là quá rình vận chuyển pha hơi của hỗn hợp bột ZnO và Các bon với tỉ lệ khối lượng 1:1 tại nhiệt đô 1100 oC bằng dòng hỗn hợp không khí và N2 có lưu lượng khác nhau. Đặc trưng cấu trúc và hình thái bề mặt của vật liệu cấu trúc nano ZnO được tổng hợp có cấu trúc tinh thể cao với đường kính trung binh cỡ 30 nm, chiều dài vài micromet. Báo cáo này chúng tôi đưa ra cơ chế hình thành đồng thời hai dạng vật liệu trong quá trình vận chuyển pha hơi ở ấp suất khí quyển. Từ khóa: dây nano ZnO, nanotetrapod, tỉ số dọc-ngang cao, đồng ngưng tụ, hơi.