Chap 7. Optical properties of Nanomaterials

- Photo-Luminescence (PL)

- Surface Plasmon Effect

Photo-Luminescence (PL)

- A metastable state is necessary.- hh’- It should have direct band gap.

Si, TiO2, Fe2O3……GaAs, CdS, CdSeCdTe

h h’

absorption

emission

Valence Band

Conduction Band

Measurement of PL: by spectrophotometer

1. Emission mode- Excitation wavelength is fixed.- Obtain emission spectra as a function of wavelength.

2. Excitation mode- Emission wavelength is fixed. (The wavelength of emission maxima).- Obtain emission spectra as a function of the excitation wavelength.

Light source

Sample

Monochromator& Slit

Emission & excitation spectra

Excitationlight

Wavelength (nm)

300 400 500 600 700

1. Obtain UV-Vis. absorption spectrum to find out the absorption maxima of the sample.This absorption maxima will be chosen as the excitation wavelengthfor the PL measurement.

2. Monitor the emission spectrum by spectrophotometer. (1st measurement) We can determine emission maxima.

3. Change the measurement mode of spectrophotometer to “emission mode”.Monitor the emission intensity at this emission maxima by varying the excitation wavelength. excitation spectrum

4. From the obtained excitation spectra, determine the optimal excitation wavelength inducing the emission maxima.

5. Monitor emission spectrum again at this excitation wavelength. (2nd measurement)

How to obtain emission and excitation spectra<Experimental procedure>

300 350 400 450 500 550 600

0.0

0.2

0.4

0.6

0.8

1.0

Wavelength (nm)

UV-Vis. absorption spectrum PL from spectrophotometerExcitationspectrum

Emissionspectrum

1st

measurement

2nd

measurement

Absorption maxima

Quantum efficiency in solar cells• External Quantum Efficiency (EQE)The ratio of the number of charge carriers collected by the solar cell to the number of photons irradiated to the solar cell.• Internal Quantum Efficiency (IQE)The ratio of the number of charge carriers collected by the solar cell to the number of photons that are absorbed by the solar cell.

photons irradiated of numberelectrons generated of numberEQE

photons absorbed of numberelectrons generated of numberIQE

• APCE (Absorbed photon to current efficiency) IQE of the generated electrons as a function of the incident light wavelength

• IPCE (Incident photon to current efficiency) EQE of the generated electrons as a function of the incident light wavelength

LHEIPCEAPCE LHE: light harvesting efficiency

LHE = 1 – 10 - absorbance

Quantum efficiency of photoluminescence (PL)• External Quantum Efficiency (EQE)The ratio of number of generated photons to number of irradiated photons

• Internal Quantum Efficiency (IQE)The ratio of number of generated photons to number of absorbed photons

400 500 600 700 800 9000

10

20

30

40

50

60

70

Qua

ntum

Effi

cian

cy(%

)Wavelength (nm)

NCS

NCS

NNHOOC

NN

HOOC

Ru

COO-TBA+

COO-TBA+

N719

solar simulator

IPCE

IPCE spectra

Quantum Efficiency (IQE

=

I: emission intensity A: absorbanceQuinine: a standard fluorescent dye

quinine = 57.7%

UV-Vis. absorption spectrum Emission spectrum by spectrophotometer

Wavelength (nm)300 400 500 600 700

Emissionspectrum

300 350 400 450 500 550 600

0.0

0.2

0.4

0.6

0.8

1.0

Wavelength (nm)

Absorption peak375 nm

photons absorbed of numberphotons emitted of number

sample

quinine

quinine

samplequininesample

AA

IIΦΦ

- UV absorption spectra and emission spectra for the sample and quinine are necessary.

Quinine

Photoluminescence (PL) of quantum-sized nanoparticles (NP)

- R: radius of nanoparticle - m*e and m*h : effective mass of the

electron and hole- : bulk optical dielectric coefficient

Size dependence of band gap; effective mass approximation

E

molecule

HOMO

LUMO

Dots: experimentally observed band gapsSolid line: theoretically calculated band gaps

with the effective mass approximation

Absorption spectra of CdSe NPas a function of size

Band gaps of CdSe NPas a function of size

Electronic spectra of samples consisting of CdS NPs with different mean diameters (Å): (a) 6.4, (b) 7.2, (c) 8.0, (d) 9.3, (e) 11.6, (f) 19.4, (g) 28, and (h) 48. The excitonic transition shifts to higher energy values along with an increase in the molar absorption coefficient, as the particle size decrease.

UV-Vis absorption and PL spectra of CdSe0.34Te0.66 quantum dots in the size range of 2.7-8.6 nm

Comparison of the emission spectra among CdSe, CdTe, and CdSe0.34Te0.66 quantum dots.

- Binary NP emits longer wavelength, compared with CdSe or CdTe NP.

PL of binary NP

All dots were synthesized to have a mean diameter of 5.9 nm (core plus shell) and an overall composition of CdSe0.6Te0.4

- High Eg core / low Eg surface binary NP does not emit PL.

- Low Eg core / high Eg surface binary NP emits shorter wavelength PL, compared with homogeneous binary NP.

- Gradient binary NP emits relatively shorterwavelength PL than the homogeneous one.

◆ Monitoring of the delivered nanoparticles in the organ - Attach fluorecent functional group on the surface of nanoparticles

1. -OH terminated nanoparticlesex) SiO2, zeolite, TiO2, Si, Fe3O4, most of metal oxides

(3-amino propyl)-triethoxysilane)AP-TES

S i O HO H

O H

N H 2

-OH + -O-Si NH2

O

OToluene110oC, 1hr

1-pyrenebutric acid

Formation of amide between –COOH and -NH2

Stirring 2 hrat 25oC

-O-Si NH

O

O

c

+

2. Metal nanoparticlesex) Au, Ag, Pt, Pd, etc.

Fluorescein isothiocyanate (FITC)

SH

Surface Plasmon Effect

▪ Plasmon: a quantum of plasma oscillationa quasiparticle resulting from the quantization of plasma oscillations (just as photons are quantizations of light and phonons are those of lattice vibrations.)

▪ Surface plasmon resonance (SPR) - Resonant, collective oscillation of valence electrons in a metal stimulated by incident light.

- The resonance condition is established when the frequency of light photons matches the natural frequency of surface electrons oscillating against the restoring force of positive nuclei.

▪ Surface plasmon (SP)Surface plasmons (SPs) are coherent oscillations of conduction electrons on a metal surface excited by electromagnetic radiation at a metal-dielectric interface.

<Schematic diagram for a localized surface plasmon of a metal sphere showing the displacement of the electron charge cloud relative to the nuclei>

1) Absorption of visible-light (Surface plasmon absorption) 2) Strong light scattering3) Enhancement of the near-field in the vicinity of particle surface

LSPR is observed in the noble-metal NPs when the incident photon frequency is resonant with the collective oscillation of the conduction electrons confined in the volume of the NPs.

▪ As a result, following phenomena can take place.

◆ LSPR effect

- The spectral position and magnitude of the LSPR absorption band depends on the size, shape, composition, and local dielectric environment.

▪ Spherical metallic NP: Single LSPR absorption band is monitored.

The surface plasmons are unevenly distributed around NP shape dependence of the LSPR absorption spectra

eg.) nanorod: red-shift of LSPR absorption spectra by long axis of NRblue-shift by short axis of NR

1) Surface plasmon absorption

▪ Non-spherical metallic NP

Mie theory

: extinction cross sectionV: NP volume: wavelength of lightm: dielectric constant of medium1 + i2: dielectric constant of NP

- Resonance condition is dependent on NP sizeFor 20 nm-sized Au NP, LSPR: 520 nm, for 100 nm-sized Au, LSPR: 600 nm

[Chem. Rev., 108, 494 (2008)]

♦ Size-dependent absorption peak shift for spherical Au NP

a: 13 nmb: 27 nmc: 36 nmd: 40 nm Size increase of Au NP

red-shift of absorption peak

♦ Au nano-rod

2) Strong light scattering phenomena

Light scattering depending on shape

By the particle dipole plasmon frequency, a resonantly enhanced electric field build up inside the NP. Induces strong light scattering

Au nanocrystals in different shapes

[Chem. Rev., 108, 494 (2008)]

3) Enhancement of the near-field in the vicinity of particle surface- Increase of electric field near the metal NP

e.g.) Enhancement of PL in Si nanocrystal by embedding Ag NP

e.g.) Photocatalytic activity enhancement by embedding Ag NP

(a) Bare TiO2(b) TiO2 with Ag/SiO2 (SiO2 shell thickness: 20 nm)(c) TiO2 with Ag/SiO2 (SiO2 shell thickness: 5 nm)

Absorbance of TiO2 was increased by presence of Ag/SiO2. [JACS, 130, 157 (2008)]

[Solar Energy Materials & Solar Cells, 94, 1481 (2010)]

e.g.) Enhancement of fluorescence from organic dyes- Fluorescence intensity of dye molecules is critically depending on the distance

between dye and Au NPs.

Au NP SiO2Dye

[Langmuir, 2013, 29, 1584-1591.]

- If the distance between Au NP and fluorescent dye is too short, fluorescence of dye is quenched by FRET (fluorescence resonance energy transfer).

Au NP

Dye

Fluorescence resonance energy transfer

fluorescenceh’

Dye excitation is stimulated by LSPR

Fluorescence enhancement