Functional Metal Oxide Nanobelts: From Materials to Nanodevices

1. Functional Metal Oxide Nanobelts: From Materials to Nanodevices ——————————————————————————————————————— Research Proposal By Jingpeng Wang March 2006

2. Outline ——————————————————————————————————————— • Introduction • Research group of Dr. ZhongLin Wang • Metal oxide nanobelt and its structural derivatives • Application in nano-devices • Proposed Research Objectives • Short-term and long-term plans • Summary • Reference

3. Introduction - Dr. Zhong Lin (ZL) Wang ——————————————————————————————————————— Highlight • Authored and co-authored 4 scientific reference and textbooks and over 350 journal articles, 40 review papers and book chapters, edited and co-edited 10 volumes of books on nanotechnology, and held 8 patents and provisional patents. • Papers have been cited over 9000 times placing him on the list of the top 25 most cited authors in the world from 1992. • Reported the “nanobelt” in 2001, which was considered to be a ground breaking work and was reported by over 20 media including USA Today, Science News, BBC News, and CNN. • Regents’ Professor • COE Distinguished Professor • Director, Center for Nanostructure Characterization (CNC) • Georgia Institute of Technology http://www.nanoscience.gatech.edu/zlwang/wang.html

4. The Family of Nanobelts ——————————————————————————————————————— • Semiconducting oxide nanobelts first discovered in 2001, being considered in the same category as the discovery of nanotubes. • The as-synthesized oxide nanobelts are highly pure, structurally uniform, and single crystalline, and most of them are free from defects and dislocations. • They have a rectangle-like cross section with typical widths of 30 to 300 nm, thickness of 5-20 nm, and lengths of up to a few mm. • The belt-like morphology appears to be a distinctive and common structural characteristic for the family of semiconducting oxides with cations of different valence states. TEM and HRTEM images of ZnO nanobelts showing their geometrical shape and electron diffraction pattern

5. Synthesis and Manipulation Technique ——————————————————————————————————————— • Physical Vapor Deposition (PVD): thermal evaporation of oxide powders under controlled conditions without the presence of catalyst. The desired oxide powders were placed at the center of an alumina tube that was inserted in a horizontal tube furnace, where the temperature, pressure, and evaporation time were controlled. • The as-synthesized oxide nanobelts can be sectioned at specified locations into various lengths using either an AFM probe or a focused electron beam.

6. Growth Mechanism ——————————————————————————————————————— Vapor-Solid self-catalyzed process • Vaporized into molecular species at high temperature • Condensed onto the substrate at a lower temperature region, forming a small nucleus • Newly arrived molecules continue to deposit on the formed nucleus, while the surfaces that have lower energy, such as the side surfaces, start to form. • The high growth temperature(800-1000˚C) ensures the high mobility of the atoms and molecules (more molecules stick on the rough growth front, not accumulating onto the side surfaces) ZL Wang, Annu. Rev. Phys. Chem. 2004. 55:159 • The rough structure of the tip leads to a rapid accumulation of incoming molecules, resulting in the fast formation of a nanobelt. • The newly arrived molecules randomly diffuse on the surface and finally find the lower-energy sites at the growth front. (unlikely to stick to the edge of the nanobelts because of the unbalanced coordination and possibly higher energy.) • The size of the nanobelt cross section is determined by the growth temperature and supersaturation ratio in kinetics of crystal growth.

7. Single-crystal Nanorings ——————————————————————————————————————— • Freestanding single-crystal complete nanorings of ZnO (SnO2) were formed via a spontaneous self-coiling process during the growth of polar nanobelts.(1400˚C-30min) • Rings have typical diameters of 1~4 um and thickness of 10~30 nm. (yield 40%) • The tetrahedral coordination in ZnO results in a noncentral symmetric structure, in which the oppositely charged ions produce positively charged (0001)-Zn and negatively charged (0001¯)-O polar surfaces. The nanoring appeared to be initiated by circular folding of a nanobelt, caused by long-range electrostatic interaction. Coaxial and uniradial loop-by-loop winding of the nanobelt formed a complete ring. Short-range chemical bonding among the loops resulted in a single-crystal structure. • The self-coiling is likely to be driven by minimizing the energy contributed by polar charges, surface area, and elastic deformation. X. Y. Kong, Y. Ding, R. S. Yang, and Z. L. Wang, Science 303, 1348 (2004).

8. Single-crystal Nanohelices/Nanosprings ——————————————————————————————————————— • Growth condition-800 ˚C-20min + 1400˚C-2hr (10% yield) • Polar surfaces result in a normal dipole moment and spontaneous polarization along the basal plane. • If the surface charges are uncompensated during the growth, the spontaneous polarization induces electrostatic energy due to the dipole moment, but rolling up to form a circular ring would neutralize the overall dipole moment, reducing the electrostatic energy. • Bending of the nanobelt produces elastic energy. • The stable shape of the nanobelt is determined by the minimization of the total energy contributed by spontaneous polarization and elasticity. • If the nanobelt is rolled uniradially loop by loop, the repulsive force between the charged surfaces stretches the nanohelix, while the elastic deformation force pulls the loops together; the balance between the two forms the nanohelix/nanospring.

9. Spontaneous Polarization-Induced Structural Conversion ——————————————————————————————————————— • Multilooped ring formed by folding the nanobelt with its polar direction pointing to the axial direction of the ring. • Nanospring, nanospiral and nanohelixes created by folding the polar nanobelt with its polar direction pointing towards the center. • From the energy point of view, the driving force for a polar-surface dominated nanobelt to fold itself into a ring or spring is to reduce electrostatic energy. • Rule of thumb: The nanoring is stable if the ratio between nanobelt thickness (t) and the radius (R) of the nanoring is smaller than ca. 3%. If the t/R is ca. 6% to 13%, which is much larger than the (3%) permitted for forming a nanoring by electrostatic polar charges, nanohelix is likely to be formed. (The only way to control t/R is the growth temperature.)

10. Application of Nanobelt in Functional Nanodevices ——————————————————————————————————————— • Such nanobelt oxides are semiconducting and piezoelectric materials that have been used for fabrication of nanosize functional devices of key importance for nanosystems and biotechnology, such as field-effect transistors, gas sensors, nanoresonators, and nanocantilevers. • Field-effect transistors: ZnO and SnO2 • The principle of this device is that controlling the gate voltage controls the current flowing from the source to the drain. • E-beam lithography fabricated field-effect transistor (FET) using a single ZnO nanobelt is shown below. • Such nanobelt can be doped by annealing in reduced oxygen environments, increasing conductivity and decreasing the gate threshold voltage, indicating the feasibility of tuning device by controlling oxygen vacancies. Arnold MS, Avouris Ph, Pan ZW, Wang ZL. 2003. J. Phys. Chem. B 107:659–63

11. Application of Nanobelt in Functional Nanodevices ——————————————————————————————————————— • Gas sensors: • The fundamental sensing mechanism of metal oxide–based gas sensors relies on a change in electrical conductivity due to the interaction process between the surface complexes, such as O-, O2- , H+, and OH-, reactive chemical species and the gas molecules to be detected. • Nanobelts of semiconducting oxide are very promising for sensors because their surface to volume ratio is very high, the oxide is single crystalline, the faces exposed to the gaseous environment are always the same, and the size is likely to produce a complete depletion of carriers inside the belt. • Gas sensors have been made using SnO2 nanobelts. Response curve of the conductance through nanobelts to the concentration of the surface-adsorbed CO, ethanol, and NO2 gases at two different temperatures. The sensitivity of the sensor is on the level of a few parts per billion. Comini E, Faglia G, Sberveglieri G, Pan ZW, Wang ZL. 2002. Appl. Phys. Lett. 81:1869–71

12. Application of Nanobelt in Functional Nanodevices ——————————————————————————————————————— • Nanocantilevers used as SPM: • The most conventional cantilever is based on Si, Si3N4 or SiC, which is fabricated by an e-beam or optical lithography technique and has typical dimensions of thickness 100 nm, width 5 um, and length 50 um. • Semiconducting nanobelts are ideal candidates for cantilever applications. Structurally they are defect-free single crystals, providing excellent mechanical properties. The reduced dimensions of nanobelt cantilevers offer a significant increase in cantilever sensitivity. • Nanobelts as ultrasmall nanocantilever arrays aligned on a silicon chip. (SEM) • An enlarged SEM image recorded from the nanobelt cantilever. Hughes W, Wang ZL. 2003. Appl. Phys. Lett. 82:2886–88

13. Proposed Research Objectives ——————————————————————————————————————— • Short Term Investigation I: • Controlled growth is required to control nanobelts' size, size distribution, shape, crystal structure, defect distribution, and even surface structure (atomic termination, surface polarization). • A thorough understanding of the growth mechanism is the key. A valid (albeit tedious) way to control the size of nanobelts is to properly control the temperature, pressure, and growth time. • Tentative plan: (expected morphology-nanowire/belt/ring/spring/helix/saw) • Temperature programming: (m.p. dependent) starting temperature(700,800,900˚C); 1st hold-up growth temp(1000,1100,1200˚C); 2nd hold-up growth temp(1300~1400˚C); final deposition temp and annealing temp(1/3 of melting temperature). Growth time (hold-up time): 0.5~2 hr. • Ar or He Carrier gas pressures: (O2 deficient environment; heat distribution) 0, 100,200,300,400,500 mbar. • Interested source materials: ZnO, SnO2, CdSe, CdO, ZnSe • Experimant number: over 500.

14. Short Term Investigation II. ——————————————————————————————————————— • Only binary metal oxides(ZnO, SnO2, CdO,Ga2O3, PbO2) have been investigated in terms of forming nanobelt. • Metal sulfides, carbides and other rare component semiconductors are also worth studying (ZnS, CdSe, and ZnSe, In2O3, Ge3N4, Bi2S3, SiC, GaP, etc.). • In semiconductor production, doping refers to the process of intentionally introducing impurities into an extremely pure (also referred to as intrinsic) semiconductor in order to change its electrical properties. • Adding dopants of group III-V elements into the binary nanobelt oxides is needed to improve their multifunctionality, which will possibly change the crystal structures as well as electromechanical properties.(Al, Ga, In, C, Si, Ge, N, P.)

15. Proposed Research Objectives ——————————————————————————————————————— • Long Term: • For sensor applications, the nanobelts may have the required sensitivity, but the selectivity needs to be improved. This requires the synthesis of composite nanobelts such as heterostructure, junction, and barrier. Surface functionalization of the nanobelts is also an important topic. • Techniques are required to grow nanobelts into aligned arrays, onto patterned substrates, and in self-assembly structures with functionality. This is a key step toward nanosystem integration. • Development of techniques for integration of nanobelts with other nano- and microstructures such as nanoelectromechanical and biosensing systems is needed. (Scott showed a good case…)

16. Summary ——————————————————————————————————————— • Among the group of ZnO, SnO2, In2O3, Ga2O3, CdO, and PbO2, which belong to different crystallographic systems and structures, a generic shape of nanobelt structure has been synthesized. • The vapor-solid growth mechanism is driven by the spontaneous polarization-induced process. • The oxides are intrinsic semiconductors, which have been used for fabrication of nanosize functional devices such as field-effect transistors and gas sensors, nanoresonators,and nanocantilevers. These devices will have important applications in nanosystems and biotechnology. • As for the future of nanotechnology and applications in nanosystems and biotechnology, there are a lot of issues to be investigated.

17. References ——————————————————————————————————————— • Z. W. Pan, Z. R. Dai, and Z. L. Wang, Science 291, 1947 (2001). • Z. L. Wang, X. Y. Kong, and J. M. Zuo, Phys. Rev. Lett. 91, 185502 (2003). • X. Y. Kong and Z. L. Wang, Nano Lett. 3, 1625 (2003). • X. Y. Kong, Y. Ding, R. S. Yang, and Z. L. Wang, Science 303, 1348 (2004). • W. Hughes and Z. L. Wang, J. Am. Chem. Soc. 126, 2709 (2004). • P. X. Gao and Z. L. Wang, J. Phys. Chem. B 106, 12653 (2002). • J. Y. Lao, J. G. Wen, and Z. F. Ren, Nano Lett. 2, 1287 (2002). • P. X. Gao and Z. L. Wang, Appl. Phys. Lett. 84, 2883 (2004). • Z. L. Wang, J. Phys.: Condens. Matter 16, R829 (2004). • Z. L. Wang, X. Y. Kong, Y. Ding, P. X. Gao, W. L. Hughes, R. S. Yang, and Y. Zhang, Adv. Funct. Mater. 14, 943 (2004). • Z. L. Wang and Z. C. Kang, Functional and Smart Materials—Structure Evolution and Structure Analysis sPlenum, New York, (1998). • Y. Dai, Y. Zhang, and Z. L. Wang, Solid State Commun. 126, 629 (2003). • ZL Wang, et al., J. AM. CHEM. SOC. 2006,(128)5, 1467 • http://www.nanoscience.gatech.edu/zlwang/index.html