Fachgebiet 3D-Nanostrukturierung, Institut für Physik

Contact: yong.lei@tu-ilmenau.de; yang.xu@tu-ilmenau.de

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:yong.lei@tu-ilmenau.demailto:yong.lei@tu-ilmenau.demailto:yong.lei@tu-ilmenau.demailto:yang.xu@tu-ilmenau.demailto:yang.xu@tu-ilmenau.demailto:yang.xu@tu-ilmenau.dehttp://www.tu-ilmenau.de/3dnanostrukturierung/http://www.tu-ilmenau.de/3dnanostrukturierung/http://www.tu-ilmenau.de/3dnanostrukturierung/

• 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

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.

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.

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.

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.

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.

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.

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.

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).

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)

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.

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.

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.

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.

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.

Lasing emission localized at ends of nanowires, suggests strong

waveguide behavior - consistent with axial Fabry-Perot mode.

spontaneous emission

stimulated emission (lasing)

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.

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).

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)

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.

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.

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

(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.

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.

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.

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.

(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.

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)

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.

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.

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).

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)

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).

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).

The functioning of the device, where Arabic and Chinese characters appear by

switching of individual spots. Each pixel could be accurately addressed without

interference.

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;

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).

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.

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.

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.

Direct-Current Nanogenerator

(from Z. L. Wang et. al., Science 316, 102 (2007))

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)

Synthesis of barcoded nanorods

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.

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.

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.

• 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

Thank you and have a nice day!