Transcript 幻灯片 1 - University of Guelph

Functional Metal Oxide Nanobelts:
From Materials to Nanodevices
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Research Proposal
By Jingpeng Wang
March 2006
Outline
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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
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Introduction - Dr. Zhong Lin (ZL) Wang
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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
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The Family of Nanobelts
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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.
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TEM and HRTEM images of ZnO nanobelts showing their geometrical shape and electron diffraction pattern
Synthesis and Manipulation Technique
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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.
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Growth Mechanism
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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.
ZL Wang, Annu. Rev. Phys. Chem. 2004. 55:159
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)
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 lowerenergy 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 6
supersaturation ratio in kinetics of crystal growth.
Single-crystal Nanorings
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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).
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Single-crystal Nanohelices/Nanosprings
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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
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nanohelix/nanospring.
Spontaneous Polarization-Induced Structural Conversion
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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.) 9
Application of Nanobelt in Functional Nanodevices
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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.
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Arnold MS, Avouris Ph, Pan ZW, Wang ZL. 2003. J. Phys. Chem. B 107:659–63
Application of Nanobelt in Functional Nanodevices
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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.
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Comini E, Faglia G, Sberveglieri G, Pan ZW, Wang ZL. 2002. Appl. Phys. Lett. 81:1869–71
Application of Nanobelt in Functional Nanodevices
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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.
(a) Nanobelts as ultrasmall
nanocantilever arrays
aligned on a silicon chip.
(SEM)
(b) An enlarged SEM image
recorded from the nanobelt
cantilever.
Hughes W, Wang ZL. 2003. Appl. Phys. Lett. 82:2886–88
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Proposed Research Objectives
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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.
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Short
Term
Investigation
II.
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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,
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Ge, N, P.)
Proposed Research Objectives
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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…)
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Summary
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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.
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References
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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
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