nanostrukturphysik (nanostructure physics) · β is related to emitter geometry and crystal...
TRANSCRIPT
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Fachgebiet 3D-Nanostrukturierung, Institut für Physik
Contact: [email protected]; [email protected]
Office: Unterpoerlitzer Straße 38 (Heisenbergbau) (tel: 3748) http://www.tu-ilmenau.de/3dnanostrukturierung/
Vorlesung: Thursday 7:00 – 8:30, F 3001 (Faradaybau)
Übung: Friday (G), 11:00 – 12:30, C 110
Prof. Yong Lei & Dr. Yang Xu
(a) (b2) (b1)
UTAM-prepared free-standing one-dimensional surface nanostructures on Si
substrates: Ni nanowire arrays (a) and carbon nanotube arrays (b).
Nanostrukturphysik (Nanostructure Physics)
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]://www.tu-ilmenau.de/3dnanostrukturierung/http://www.tu-ilmenau.de/3dnanostrukturierung/http://www.tu-ilmenau.de/3dnanostrukturierung/
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• Class 1: A general introduction of fundamentals of nano-structured materials
• Class 2: Structures and properties of nanocrystalline materials
• Class 3: Graphene
• Class 4: 2D atomically thin nanosheets
• Class 5: Optical properties of 1D nanostructures and nano-generator
• Class 6: Carbon nanotubes
• Class 7: Solar water splitting I: fundamentals
• Class 8: Solar water splitting II: nanostructures for water splitting
• Class 9: Lithium-ion batteries: Si nanostructures
• Class 10: Sodium-ion batteries and other ion batteries, and Supercapacitors
• Class 11: Solar cells
• Class 12: Other nanostructures
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Large-scale free-standing metallic nanowires for 3D surface patterns: (Left): top view of
nanowire array of an area of about 775 μm2. (Right): high regularity of nanowire arrays.
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Large-scale free-standing metallic nanowires for 3D surface patterns: (Left): top view of
nanowire array of an area of about 775 μm2. (Right): high regularity of nanowire arrays.
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Features of optical properties of 1-D nanostructures
• Sharp and discrete features in absorption spectra &
‘‘band-edge’’ PL (photoluminescence) shift and
enhancement – results of quantum confinement effect.
• Anisotropic PL, high polarized along axial direction – the
large dielectric contrast between nanowire and surrounding
environment.
• Efficient migration of electrons and holes to surface of
nano-structures allows them to participate in chemical
reactions before recombining – enhance the efficiency of
solar cells.
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Features of optical properties of 1-D nanostructures
• Nanowires with optical properties tuned by changing
aspect ratio.
• Single crystalline and well faceted nanowires can function
as effective resonance cavities; lasing properties.
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Si nanowires synthesized at
500 oC at pressures of 200 bar
(A and B) and 270 bar (C and
D). The nanowires are highly
crystalline. Two planes (100) and (110). In both (B) and (D),
the lattice planes are separated
by 3.14 Ǻ.
1st finding (obvious evidence) of the quantum confinement effect of 1-D
nanostructures: Holmes JD, Science, 2000. Defect-free Si nanowires with uniform diameters range from 4 to 5 nm and length of
several micrometers- by a solution-phase approach.
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The absorption edge of Si nanowires
was strongly blue-shifted from the bulk
indirect band gap of 1.1 eV and showed
sharp discrete absorption features and
strong band-edge PL, results from
quantum confinement effects.
The 100> oriented wires have a much
higher exciton energy than that of the
110> oriented wires.
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Band-Gap Variation of Size- and Shape-Controlled CdSe
Quantum Rods (Li LS, Nano Letters, 2001)
TEM images of 4 CdSe nanorod samples.
The scale bar is 50 nm.
PL spectra of 3.7 nm wide CdSe rods with
lengths: 9.2, 11.5, 28.0, 37.2 nm, respectively
(from left to right), excited at 450 nm.
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Band gap of CdSe quantum rods vs length and width viewed from two different
angles. The data are fit in 1/length (1/L), 1/width (1/W), and aspect ratio (L/W).
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10 periods of Zn0.8Mg0.2O/ZnO on ZnO nanorods.
A: 1.1 nm wells and B: 2.5 nm wells.
Quantum confinement observed in ZnO/ZnMgO multiple quantum
well nanorod heterostructures (Park WI, Advanced Materials, 2003)
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Quantum confinement in 1-D nanostructures
Quantum confinement can be approximately described by a
simple particle-in-a-box type mode:
ΔE = 1/dn (d: diameter, 1 ≤ n ≤ 2) size dependence of
bandgap.
The quantum confinement effect in semiconductor nanodots
and nanowires did not exactly follow the particle-in-a-box
prediction.
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A particle-in-a-cylinder mode:
The calculated energy shift ΔE, relative to the bulk band gap as a
function of the nanowire radius R, is given by:
Quantum confinement in 1-D nanostructures
L is the length of the cylinder, m* is the reduced effective exciton mass
(memh /(me + mh)), ħ is Planck’s constant, e is the electron charge.
The first term represents the size-dependent kinetic energy confinement
by the walls of the nanowire cylinder.
The second term is the attractive Coulomb interaction between electron
and hole.
This mode provides excellent fits to experimental results.
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Nanowire Lasing (Yang PD‘s group)
Nanowires with flat facets at both end can be used as optical resonance
cavities to generate coherent light. UV lasing at RT has been
demonstrated for ZnO and GaN nanowires with epitaxial arrays and single
nanowires.
ZnO and GaN are wide bandgap semiconductors (3.37, 3.42 eV) suitable
for UV-blue optoelectronics. The large binding energy for excitons in ZnO
(∼ 60 meV) permits lasing via exciton-exciton recombination at low excitation conditions.
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Well-faceted nanowires with diameters from 100 to 500 nm
support axial Fabry-Perot waveguide mode:
Δλ = λ2/[2Ln(λ)]
where L is the cavity length and n(λ) is the group index of
refraction.
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Nanowire Lasing The transition from spontaneous PL to lasing is achieved by exciting high
density of wires via pulsed UV illumination (pumping).
3 regions: (a) spontaneous emission, (b) stimulated emission (lasing starting)
above a certain threshold, (c) saturation (lasing) at high pump power.
The lasing thresholds vary several orders of magnitude as a consequence of
different nanowire dimensions, the lowest threshold observed for ZnO is ∼70 nJ cm-2 and for GaN ∼500 nJ cm-2.
(a) Spectra of light emission
from GaN/AlGaN core-shell
nanowires below, near and
above lasing threshold
(about 2–3 µJ/cm2).
(b) The power dependence
of output integrated emission
intensity.
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Lasing emission localized at ends of nanowires, suggests strong
waveguide behavior - consistent with axial Fabry-Perot mode.
spontaneous emission
stimulated emission (lasing)
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The ultrafast dynamics of the lasing in ZnO nanowires and nanobelts
Transient PL to detect carrier relaxation dynamics near lasing threshold.
(a) SEM of nanowire array
and (b) single nanowire
dispersed on sapphire
substrate. Inset: far-field
image of nanowire emission.
(c) SEM of nanobelts and (d)
single dispersed belt on
silicon. Inset: far-field image
of belt lasing emission.
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The ultrafast dynamics of the lasing in ZnO nanowires and nanobelts
Above the lasing threshold, a fast decay of PL was observed, with a fast
component (< 10 ps) corresponding to exciton-exciton lasing and a slow
component (∼70 ps) owing to free-exciton spontaneous emission.
(a) PL/lasing spectra of single ZnO nanowire near lasing threshold (excitation
∼1 µJ/cm2) and (b) transient PL response. Long decay component is about 70 ps and short component is about 9 ps (red) and 4 ps (black).
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Nanowire Lasing
The useful applications for nanowire lasers require that they
are integrated in circuits and activated by electron-injection
rather than optical pumping.
Lieber and coworkers have made progress in this direction by
assembling n-type CdS nanowire (Fabry-Perot cavities) on p-
Si wafers to form the required heterojunction for electrical-
driven lasing:
Single-nanowire electrically driven lasers
(Lieber et al., Nature 2003)
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First, optical-pumped
Single-nanowire
lasers
a, A nanowire as an optical waveguide, with facet ends of a Fabry–Perot cavity.
b, A faceted CdS nanowire end.
c, RT PL image of a CdS nanowire excited (pumping power 10 mW) about 15 mm
away from the nanowire end. The white arrow highlight the nanowire end.
d, PL spectra obtained from the body of nanowire (blue) and the end of nanowire (green) at low pump power (10 mW). e, Spectrum from the nanowire end at higher
pump power (80 mW) showing periodic intensity variation.
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Optically pumped nanowire laser:
Emission spectra from a CdS nanowire end with a pumping power of 190, 197 and
200 mW (red, blue and green) recorded at 8 K.
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To investigate nanowire injection lasers, a hybrid structure was used: n-type
CdS nanowire laser cavities are assembled onto p-Si electrodes. An image
of a typical device is shown in b. Images of the RT electroluminescence
produced in forward bias from these hybrid structures (b) exhibit strong
emission from the exposed CdS nanowire ends.
Single-nanowire electrical-driven lasers
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(d) At low injection currents, the end emission shows a broad peak (spontaneous
emission).
Above 200 mA threshold, the spectrum quickly collapsed into a few very sharp peaks
with a dominant emission at 509.6 nm, come from both spontaneous and stimulated
emission.
(e) Low-T measurements on independent CdS injection laser devices show
spontaneous emission spectrum, it can collapse to a sharp peak of lasing, the results
are very similar to the low-T optically pumped results.
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Techniques for device fabrication of semiconductor lasers is costly and
difficult to integrate directly with Si microelectronics.
There are considerable interests in using organic molecules, polymers,
and inorganic nanostructures for lasers, because these materials can be
integrated into devices by chemical processing. And stimulated emission
or lasing have been reported for optically pumped inorganic nanowires
and organic systems.
Electrical-driven nanowire lasers might be assembled in arrays
capable of emitting a wide range of colors, used in flat displays.
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Field-emission Display (FED)
Much attentions to explore using of semiconductor 1-D nanostructures as
field-emitters: low work functions, high aspect ratios, high
mechanical stability, high electrical and thermal conductivity.
Field-emission is one of the main features of nanostructures, and is
of great commercial interest in FEDs and other electronic devices.
Progresses in the synthesis and assembly of nanostructures has resulted
in a considerable increase in the current density and lowering of turn-on
voltage.
Besides CNTs, some other inorganic semiconductor nanostructures used for field-
emitters, such as ZnO, Si, WO3, SiC, ZnS, AlN.
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Field-emission is a quantum tunneling process: electrons pass
from an emitting material (negatively biased) to the anode
through a barrier (vacuum) with a high electric field.
Highly dependent both on properties of material and shape of cathode, materials
with higher aspect ratios and sharper edges (nanowires or nanotubes) produce
higher field-emission currents.
The current density J produced by an electric field E (Fowler–Nordheim equation):
J = (Aβ2E2/ø)exp(-Bø3/2/βE), or ln(J/E2) = ln(Aβ2/ø) – Bø3/2/βE, (1)
I = S × J, E = V/d, (2)
(A: 1.54 × 10-6 A eV V-2, B: 6.83 × 103 eV-3/2 V μm-1, S: emitting area, V: applied potential, I: emission current, β: field enhancement factor, d: distance between sample and anode, ø: work function)
β is related to emitter geometry and crystal structure, and spatial distribution of
emitting centers: β = h/r, (h is the height and r is radius of curvature of emitter).
Materials with elongated geometry and sharp tips or edges can greatly increase an
emission current.
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(a) The emission occurs from tip of an
emitter. (b) The emitter can have different
emission currents depending upon the tip
geometry, such as (i) round tip, (ii) blunt tip
and (iii) conical tip. (W. Z. Wang, et al., Adv. Mater., 2006)
The emission current is strongly
dependent on three factors:
(i) work function of an emitter surface,
(ii) radius of curvature of the emitter end,
(iii) emission area.
A lower work function material can
produce a higher electron emission
current. However, not all low work
function materials are ideal for constructing field-emission cathodes
For a given material, emission current
can be enhanced by increasing its
aspect ratio, assembling it into arrays, or decorating its surface with a lower
work function material.
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Aligned ultra-long ZnO nanobelts. (a) nanobelts with length of several mm. (b) belt-like
structures with a width up to 6 μm. (c) TEM image of a single belt. Its transparency to
electron beam (can see a copper TEM grid beneath belt) clearly reflects much smaller
thickness of belts compared to widths. The HRTEM image of this belt is shown in (d)
shows the perfect crystallinity and defect-free nature of nanobelts. (Wang WZ, Adv Mater, 2006)
ZnO 1-D nanostructures (belts and wires)
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Field-emission performances of aligned ultra-
long ZnO nanobelts. (a) field emission current
density–applied field (J–F) curve. The turn-on
electric field is about 1.3 V μm-1. (b) Fowler–
Nordheim plot of nanobelts, fits well to the linear relationship given by Fowler–Nordheim
equation: ln(J/E2) = ln(Aβ2/ø) – Bø3/2/βE
From the slope of fitted straight line in (b), the
ZnO nanobelts have a very high field-
enhancement factor of 1.4 × 104, which is the result of the extremely high aspect ratio of the
emitter geometry.
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ZnS nanobelt arrays
These nanobelts form in bundles. Within a bundle they are aligned. In many cases
even perfectly parallel ensembles are visible. The electron diffraction patterns show
the similar orientation of nanobelts along the [001] direction. HRTEM images of an
individual belt display the defect-free (001) lattice plane of wurtzite Zn and confirm
the [001] growth direction.
ZnS nanobelt arrays. Length of
belts is about several hundreds
of micrometers; some of them
may even be as long as a
millimeter.
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The orientation-ordered
ZnS nanobelt arrays have
much improved field-
emission properties as
compared to random
nanowires: a low turn-on
field (~ 3.55 V μm-1) and a
high field-enhancement
factor (~ 1850).
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Some effective routes to enhance the field-emission performances: e.g. only ~ 1 V μm-1 turn-on field at a 3 mA cm-2 current density was achieved from
ultra-sharp (~ 1 nm diameter) and ultra-high density (109 – 3 × 1011 cm-2) SiC-capped Si nanotip arrays.
SiC-capped silicon nanotips: (a) nanotips of about 1 μm height with an aspect ratio
of about 1000; (b) the high density nature of nanotip arrays. (c) TEM image of a
SiC-capped Si nanotip. The inset is a magnified lattice image at the interface
between the Si and SiC. (Lo HC, Appl Phys Lett, 2003)
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A typical field emission data obtained from SiC-capped silicon nanotips
demonstrating ultralow turn-on electric fields (only ~ 1 V μm-1 turn-on field
at a 3 mA cm-2 current density).
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WO3 1-D nanostructures (Chen J, Appl Phys Lett, 2007)
FED: WO3 nanowires as cathode, with a cathode plate (consists of nano-emitters on a
substrate). Anode: phosphor screen. Gate plate: a ceramic plate with round apertures.
Metallic strips were prepared on both sides of ceramic plate (perpendicular to each
other while electrically insulated by ceramic). 8 × 8 arrays of WO3 nanowires on a Si wafer (a). Diameter of each cathode is ~ 1 mm, distance between pixels is 2.5 mm. The dark spots on anode correspond to the pixels (b).
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The functioning of the device, where Arabic and Chinese characters appear by
switching of individual spots. Each pixel could be accurately addressed without
interference.
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ZnO 1-D nanostructures and nano-generator
Among the known 1D nanomaterials, ZnO has three key
advantages:
It has both semiconductor and piezoelectric properties;
It is relatively bio-safe and bio-compatible, and can be used for
biomedical applications with little toxicity;
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Mechanism of piezoelectric discharging of
ZnO NW with AFM scanning (Z. L.
Wang et. al., Science 312, 242 (2006)
When a ZnO NW is bent by a Pt-coated
AFM tip, a strain is produced. Stretched
side has positive potential and compressed
side has negative potential. Schottky diode
is formed (Pt/ZnO).
Two processes:
When tip contacts and bends NWs, interface
of tip and stretched side is a reversely
biased Schottky diode (ΔV=Vm–VS+0), external
electrons can flow across interface under
driving of piezoelectric potential, resulting
in a discharging. (current output process).
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Nanogenerator based on piezoelectric behavior of ZnO nanowires
Implantable biomedical electronic device (pacemakers), is fast
increasing in the past two decades.
A major shortcoming: all implantable biomedical devices need battery
replacement. Surgeries to replace battery 15K EUR + possible danger.
Highly desirable for implanted biomedical devices to be self-powered,
harvest electrical energy from natural energies in human body.
Fabrication of nanoscale generators as power supplies for
implantable biomedical devices:
a self-powered implantable biomedical device, avoid the medical
surgery to replace batteries
reduce the size of integrated system of a device and its power source.
The realization of a highly efficient nano-generator with sufficient
energy output to power a biomedical device presents an important issue
to the fields of biomedical technology as well as nano-science.
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Side view (schematic) of assembled structure of proposed nano-generator:
ordered ZnO nanowire arrays with top of nanowires extended into holes of a Pt
nano-porous membrane. Insets show a template-prepared Ni membrane and Ni
nanowire arrays.
Metallic membrane
Nanowire array
Lei Y., Jiao Z., Wu M. H., Wilde G., “Ordered Arrays of Nanostructures and Applications
in High-Efficient Nano-Generators”, Advanced Engineering Materials, 9, 343, 2007.
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No current when NW is not deflected (a). Piezoelectric discharging is generated when NW is
deflected by liquid flow and touch Pt, no matter what direction of liquid flow [(b) and (c)].
(a) (b) (c)
Estimated output piezoelectric power of 1 NW ~ 5×10-13 W. Output power from about 1010 NWs in 1 cm2 area of nano-generator ~ 5×10-3 W. This 5mW nano-generator is sufficient to directly power a low-energy device like a pacemaker.
Arrayed NWs will be deflected at almost same time and in same way → discharges of
different NWs are collected at same time, realizing a stable and large DC output → power a
real device. When nano-generator is implanted into human body, different mechanical
(body movement, muscle stretching, and blood pressure) and hydraulic (flow of body fluid
and contraction of blood vessel) pressures on liquid sac will lead to a continuous wavy
motion of water inside sac, forcing the ZnO NWs to contact Pt pore-walls continuously,
thus resulting in a continuous piezoelectric discharging of each NW.
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Direct-Current Nanogenerator
(from Z. L. Wang et. al., Science 316, 102 (2007))
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Optical applications of the metallic 1-D nanostructures
- Nanometer-sized Metallic Barcodes
Multimetal nanorods encoded with nanometer-sized stripes can be
prepared.
Complex striping patterns are prepared by sequential electrochemical
deposition of metal ions into templates with uniformly sized pores (AAOs).
The different reflectivity of adjacent stripes enables identification of the
striping patterns by conventional light microscopy.
This readout mechanism does not interfere with the use of fluorescence
for detection of analytes bound to rods, as demonstrated by DNA and
protein bioassays → bioanalysis and biodetection
(Nicewarner-Pena, et al., Science, 2001)
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Synthesis of barcoded nanorods
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Barcoded 1-D nanorods. (A) SEM (left)
and optical microscope image (right) of Au-
Ag-Au rods. (C) optical microscopy image
of Ag-Au-Ag barcode rod. Top: High
contrast was observed between Ag (brighter sections) and Au (dark middle
section) with 430-nm excitation. Bottom:
No contrast using 600-nm excitation. (D)
optical images for a rod of Au-Ag-Ni-Pd-Pt
with illumination at 430, 520, and 600 nm.
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Optical (A) and SEM (B) images of an Au-Ag multi-stripe rod with about 550-
nm Au stripes and Ag stripes of 240, 170, 110, and 60 nm (top to bottom).
The same rod is shown in both images.
Thus, it should be possible to distinguish large numbers of barcode patterns.
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Bioassays on barcoded rods
using fluorescence detection.
(A) ‘Sandwich’ DNA
hybridization assay:
(i) fluorescence readout; (ii)
shows the rod ID. (iii) and (iv)
fluorescence readout and the
rod ID, respectively.
(B) A simultaneous sandwich
immuno-assay performed on
barcode rods: (i) reflectance
optical microscopy image,
which gives barcode rod ID;
(ii) and (iii) show the
fluorescence readout with
FITC and Texas Red filter
sets, respectively.
Immuno-assays on 2 different barcoded rods (1: Au-Ag-Au, 2 Au-Ni-Au).
Type 1 with capture antibody to human IgG, Type 2 with capture
antibody to rabbit IgG. Samples were exposed to 2rd antibodies. Each
2rd antibody was labelled with fluorophores of different colors (green
for antibody to human IgG, red for antibody to rabbit IgG). 2 types of
rods are able to selectively bind their target analytes.
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• Class 1: A general introduction of fundamentals of nano-structured materials
• Class 2: Structures and properties of nanocrystalline materials
• Class 3: Graphene
• Class 4: 2D atomically thin nanosheets
• Class 5: Optical properties of 1D nanostructures and nano-generator
• Class 6: Carbon nanotubes
• Class 7: Solar water splitting I: fundamentals
• Class 8: Solar water splitting II: nanostructures for water splitting
• Class 9: Lithium-ion batteries: Si nanostructures
• Class 10: Sodium-ion batteries and other ion batteries, and Supercapacitors
• Class 11: Solar cells
• Class 12: Other nanostructures
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Thank you and have a nice day!