Optical Characterization methodsRayleigh scattering

Raman scattering

transmission

photoluminescence

excitation photons

At a glance Transmission: “untouched” photons Photoluminescence includes fluorescence

(emission within 10-5 s) and phosphorescence (emission after 10-5 s). Emission wavelength usually longer than excitation wavelength (Stokes shift)

Raman scattering: inelastic scattering, in semiconductors, it can be photon-phonon scattering

Rayleigh scattering: elastic scattering, no change in wavelength

Technique Synopsis Solid State Physics

Absorption spectrometry

Scan λin ,

measure intensity of transmission.

Optical absorption, band gap, energy level spacing

Photoluminescence (PL)

Fixed λin (laser),

scan λout

Optical recombination transitions.

Photoluminescence Excitation (PLE)

Fix λout , scan λin

(tunable laser or monochromator)

Sensitive to transitions that “pump” optical emissions.

Raman scattering Laser in, scan λout very close to

λin

Stokes/anti-Stokes peaks provide information about phonon energies.

Energy levels in molecules and semiconductors

Molecular energy level

bulk semiconductor

absorptionemission

PL of bulk semiconductor usually have peak at band gap, while absorption and PLE is broad and can determine density of state.

In molecules absorption and PLE peaks are couple of S1 and S2 with vibrational energy, while PL peaks are couple of S0 and vibrational energy.

CdSe quantum dotPL and PLE peaks in CdSe quantum dots can be used to compute energy spacing and relaxation characteristics for electrons and holes

Energy levels in quantum dots

Left: The evolution of the UV-Vis and PL spectra of the core/shell nanocrystals upon the growth of the CdS shell in a typical reaction.Right: Asymmetric PL of core/shell nanocrystals with five monolayers of CdS shell.

Absorption Spectrometry

SetupBoth setups have a filter or monochromator

for wavelength selection, a transducer and a readout device for data collection.

Double-beam instrument splits the excitation source for faster acquisition and greater accuracy

Our UV-VIS system is a single beam instrument with a monochromator

(a) single-beam instrument, (b) double-beam instrument

Measurement Principle

For single beam instrument, data is acquired twice, once with a reference cell, once with a sample cell in place. Signal ration is taken to give absorbance.

A double-beam instrument adjust zero with the shutter closed; when the shutter opens the absorbance is read directly from the difference amplifier.

Application Characterize optical absorption

Advantage Relatively simple instrument

Disadvantage Limited sensitivity especially when

the change in absorption is small compared to transmission.

Photoluminescence and PLE

Setup A combined PL and PLE system has 2

monochromators for wavelength selection of excitation and emission. A single PL system can have a laser as an excitation source. A tunable laser can also be used instead of the excitation monochromator.

A beam splitter and a reference detector is used to compensate for the variation in excitation intensity

Measurement Principle PL: excitation wavelength is fixed, emission

intensity vs. wavelength is obtained by scanning a monochromator of spectrometer.

PLE: emission is detected at a fixed wavelength while excitation wavelength is scan (by a monochromator or tunable laser) to obtain emission intensity vs. excitation wavelength.

Application Provide both optical absorption and emission

informationAdvantage PLE is similar to absorption in some sense,

with much better sensitivity. Detection limits can be three orders of magnitude smaller than those encountered in absorption spectroscopy.

PL&PLE spectra for quinine solution

Diagram of a PL&PLE system

PLE PL

Photoluminescence Setup: Princeton/ Acton

Excitation laser

fiber optics, f/2.5

CCD1024x256

f = 127 mm f/2.4

f = 63.5 mmf/1.2

xyz stageentranceslit, f/4

cryostat

collimatingmirror

focusingmirror

SP-150 Spectrometerf = 150 mm; f/4dual grating turrets