Cathodoluminescence in electron microscopy:from phosphor evaluation to single particle analysis

Philippe F. Smet, Lisa I.D.J. Martin, Jeroen Wattez, Filip Strubbe, Jonas J. Joos, Jonas Botterman, Katleen Korthout, Sofie Abé, Heleen Sijbom, Dirk Poelman

http://LumiLab.UGent.behttp://tiny.cc/amazingCL

philippe.smet@ugent.be@pfsmet

ICOM 2015 – Budva/Montenegro – September 1 2015

‘Cold light’: generation of light in a non-thermal way

• Photoluminescence (PL)• Cathodoluminescence (CL)• Electroluminescence (EL)• Chemoluminescence• Bioluminescence• Radioluminescence (RL)• Triboluminescence• Sonoluminescence• Thermoluminescence (TL) (?)

CL: a brief introduction

Focused electron beam(1 to 30 keV)

Heat

Light (CL)

White x-rays

Characteristic x-rays

Auger e-

Secondary e-

Backscattered e-

Transmitted e-Diffracted e-

Sample

CL: a brief introduction

CL: a brief introduction

Cathode ray tube (CRT)Field emission display (FED/SED)

CL

CL: a brief introduction

4µm

4µm

25keV

SrS

Casino

CL: Spatial resolution

STEM-CL

Zagonel et al, Nano Letters 2011(11) 568-573Nanometer scale spectral imaging of quantum emitters in nanowires

Halfgeleiders?

20nm

EDX detector

Electron detectors

Spectrograph

CCD

SEM chamber

Optical fiber

Setup - 1. Add light collection and analysis to SEM-EDX

Data collection method51200 spectra

Phosphor (nm)

FWHM (nm)

400 nm

445 nm

(K) (ns)

SrSi2O2N2:Eu2+ 541 74 ++ + 0.62 450 905

Ba3Si6O12N2:Eu2+ 524 75 + - 0.39 435 1270

ZnGa2S4:Eu2+ 542 50 + ++ 0.14 242 135

SrGa2S4:Eu2+ 534 49 ++ ++ 0.71 460 450

Case 1: ZnGa2S4:Eu

Case 1: ZnGa2S4:Eu

ZnGa2S4:Eu (1%)

EuGa2S4

Case 1: ZnGa2S4:Eu

Saturated green emission, but...• Severe thermal quenching• Short lifetime• Low quantum efficiency (<20%)

Origin of green luminescence?

Case 1: ZnGa2S4:Eu

EDX: chemical compositionat the microscopic scale

Zn in green, Eu in red

J.J. Joos et al, Optical Materials Express 3 (2013) 1338

Case 1: ZnGa2S4:Eu

Zn in green, Eu in red

Formation of EuGa2S4 percipitates

J.J. Joos et al, Optical Materials Express 3 (2013) 1338

Case 1: ZnGa2S4:Eu

CL mapping EDX mappingOptical behaviour Chemical composition

Case 2: Sr0.25Ba0.75Si2O2N2:Eu

Case 2: Sr0.25Ba0.75Si2O2N2:Eu

Case 2: Sr0.25Ba0.75Si2O2N2:Eu

Case 2: Sr0.25Ba0.75Si2O2N2:Eu

PL, 10K

PL, 295K

Case 2: Sr0.25Ba0.75Si2O2N2:Eu

3 2

1

Case 2: Sr0.25Ba0.75Si2O2N2:Eu

Temperaturestage

EDX detector

Electron detectors

Spectrograph

CCD

SEM chamber

Optical fiber

Setup - 2. Add a temperature stage

Case 3: Thermal quenching in SrGa2S4:EuSr

Si2O

2N2:E

u2+

Ba3S

i 6O12

N2:E

u2+

ZnGa

2S4:E

u2+

SrGa

2S4:E

u2+

5µm

Total CL intensity

Peak emission wavelength

l(nm)FWHM

nm

Case 3: Thermal quenching in Sr0.9Eu0.1Ga2S4

2.5µm

Total CL intensity (-20°C)

CL (100°C)/CL (-20°C)

60%

100%

80%

EDX Eu-L signal

Total CL intensity (100°C)

Case 3: Thermal quenching in Sr0.9Eu0.1Ga2S4

Recording CL maps at different temperaturesAligning all maps by appropriate shifting/skewing

5µm

Temperature (K)

Local variations in Eu concentrationaffect global thermal quenching

Case 3: Thermal quenching in Sr0.9Eu0.1Ga2S4

Case 4: Lifetimes in SEM-CL

D. den Engelsen et al., Ultramicroscopy 2015 http://dx.doi.org/10.1016/j.ultramic.2015.05.009

Temperaturestage

EDX detector

Electron detectors

Spectrograph

ICCD

SEM chamber

Optical fiber

Beam blanker

Pulse generator

Setup - 3. Add a fast beam blanker

Eu-rich

Ca-rich

t-mapping

Case 4: Luminescence lifetime in (Ca,Eu)2SiS4

D. Poelman and P.F. Smet, Physica B 439 (2014) 35-40

3 points are sufficient to calculate t - saves time!

Case 4: Luminescence lifetime in (Ca,Eu)2SiS4

D. Poelman and P.F. Smet, Physica B 439 (2014) 35-40

Eu-rich

Ca-rich

t-mapping

Case 4: Luminescence lifetime in (Ca,Eu)2SiS4

D. Poelman and P.F. Smet, Physica B 439 (2014) 35-40

Temperaturestage

EDX detector

Electron detectors

Spectrograph

ICCD

SEM chamber

Optical fiber

Setup - 4. Collecting optics. Let’s integrate?

Case 5: Single particle analysis – SrS:Eu

K. Korthout et al. / Optical Materials 33 (2011) 1128–1130

Case 5: Single particle analysis – SrS:Eu

Case 5: Single particle analysis – SrS:Eu

Case 5: Single particle analysis – SrS:Eu

Case 5: Single particle analysis – Ray tracing simulation

Case 5: Single particle analysis - Ray tracing simulation

Case 5: Single particle analysis - Ray tracing simulation

Case 5: Single particle analysis

620nm

622nm

624nm

CL intensity

Emission barycenter

Reabsorption playsa role!

Application notes

Type of materials• Impurity doped phosphors: + to ++• Quantum dots: -• Organic compounds: --

Spatial resolution• Depends on sample morphology• Thickness and composition• Accelerating voltage

Collection optics: integrating setup or fiber?• CL emission intensity• Need for BSE/EDX• Topic of interest

K2SiF6:Mn4+

Case 6: Hybrid conversion layers (phosphor + quantum dots)

CdSe/CdS core-shell quantum dots

SrGa2S4:Eu phosphorOrganic binder

Conclusions

• ‘Standard’ photoluminescence research at microscale

• Discriminates between bulk and local behaviour

• Interesting add-on in phosphor research

• Standalone for single-particle analysis

CL in electron microscopy

Acknowledgments

Thank you for your attention!

http://LumiLab.UGent.behttp://tiny.cc/amazingCL

philippe.smet@ugent.be@pfsmet