Upconverting Nanoparticles E-Book

• Herausgeber: Wiley-VCH GmbH
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 1133

Veröffentlichungsjahr: 2022

Beschreibung

Photon upconversion technology using nanoparticles is explained from first principles right up to novel and future applications in imaging, sensing, catalysis, energy technology, biomedicine, and many other areas.

Leseprobe

Table of Contents

Cover

Title Page

Copyright

Preface

1 Introduction to Upconversion and Upconverting Nanoparticles

1.1 Introduction

1.2 Frequency Conversion and Its Various Processes

1.3 Transition Metals and Their Properties

1.4 Rare Earths and Their Properties

1.5 Excitation and De‐excitation Processes of Rare Earths in Solid Materials

1.6 Rate Equations Relevant to UC Mechanism

1.7 Theoretical Description of Optical Characteristics of Rare‐Earth Ions

1.8 An Introduction to Upconverting Nanoparticles

Acknowledgments

References

2 Synthesis Protocol of Upconversion Nanoparticles

2.1 Introduction

2.2 Host Matrix

2.3 Synthetic Strategy of UC Nanomaterials

2.4 Synthesis Techniques for Fabricating Core@shell Architectures

2.5 Other Synthesis Strategies to Develop Lanthanide‐Doped UCNPs

2.6 Conclusion

References

3 Characterization Techniques and Analysis

3.1 Introduction

3.2 X‐Ray Diffraction (XRD)

3.3 X‐ray Photoelectron Spectroscopy (XPS)

3.4 Field Emission Scanning Electron Microscopy (FESEM)

3.5 Transmission Electron Microscopy (TEM)

3.6 Energy‐Dispersive X‐ray Spectroscopy (EDS)

3.7 Thermogravimetric Analysis (TGA)

3.8 Ultraviolet–Visible–Near‐Infrared (UV–Vis–NIR) Absorption Spectroscopy

3.9 Dynamic Light Scattering (DLS)

3.10 Photoluminescence (PL) Study

3.11 Pump Power‐Dependent UC

3.12 Recognition of Emission Color and Colorimetric Theory

Acknowledgment

References

4 Raman and FTIR Spectroscopic Techniques and Their Applications

4.1 Raman Spectroscopy

4.2 Fourier Transform Infrared (FTIR) Spectroscopy

4.3 Applications of Raman Spectroscopy

4.4 Applications of FTIR Spectroscopy

4.5 Raman and FTIR Spectroscopy of Upconverting Nanoparticles

References

5 Fundamental Aspects of Upconverting Nanoparticles (UCNPs) Based on Their Properties

5.1 Introduction

5.2 Elucidation of Dynamics of UCNPs on the Basis of Fluorescence Decay Times

5.3 Measurement of Quantum Yield of UCNPs

References

6 Frequency Upconversion in UCNPs Containing Transition Metal Ions

6.1 Introduction

6.2 Synthesis of Transition Metal Ion‐Activated Luminescent Nanomaterials

6.3 Structural and Optical Characterizations

6.4 Frequency Upconversion and Its Various Mechanisms

6.5 Applications

6.6 Mechanism of Transition Metal Ions in Crystal Field

References

7 Frequency Upconversion in UCNPs Containing Rare‐Earth Ions

7.1 Introduction

7.2 Familiarization with the Spectroscopic Behavior of RE

3

+

Ion‐Doped UCNPs

7.3 Routes to Enhance Upconversion Luminescence in Nanoparticles

7.4 Technological Applications

References

8 Smart Upconverting Nanoparticles and New Types of Upconverting Nanoparticles

8.1 Introduction

8.2 Upconverting Core–Shell Nanostructures

8.3 Hybrid Upconverting Nanoparticles

8.4 Magnetic Upconverting Nanoparticles

8.5 UC‐Based Metal–Organic Frameworks

8.6 Smart UCNPs for Security Applications

8.7 Smart Upconverting Nanoparticles for Biological Applications

8.8 Smart Upconverting Nanoparticles for Sensing

8.9 Conclusion

References

9 Surface Modification and (Bio)Functionalization of Upconverting Nanoparticles

9.1 Introduction

9.2 Upconverting Nanomaterials

9.3 Surface Modification

9.4 Biofunctionalization of Upconverting Materials and Applications

References

10 Frequency Upconversion in Core@shell Nanoparticles

10.1 Introduction

10.2 Synthesis of Core@shell and Core@shell@shell UCNPs

10.3 Frequency Upconversion and Its Various Mechanisms

10.4 Applications

10.5 Conclusion

Acknowledgment

References

11 UCNPs in Solar, Forensic, Security Ink, and Anti‐counterfeiting Applications

11.1 Introduction

11.2 UCNPs for Solar Cells

11.3 Forensic, Security Printing, and Anti‐counterfeiting Applications

11.4 Biomedicals

11.5 Display and Lighting Purposes

References

12 Application of Upconversion in Photocatalysis and Photodetectors

12.1 Introduction

12.2 Photocatalysis

12.3 Photodetector

12.4 Conclusion

References

13 UCNPs in Lighting and Displays

13.1 Introduction

13.2 Major Factors that Affect the UC Emission Efficiency

13.3 UC Mechanisms with Rate Equations

13.4 UCNPs in Solid‐State Laser

13.5 UCNPs in Solid‐State Lighting and Displays

References

14 Upconversion Nanoparticles in pH Sensing Applications

14.1 Introduction

14.2 Basic Properties of UCNPs

14.3 Working Principle of UCNP‐Based pH Sensor

14.4 Photon Upconversion‐Based pH Sensing Systems

14.5 Conclusion

References

Note

15 Upconversion Nanoparticles in Temperature Sensing and Optical Heating Applications

15.1 Introduction

15.2 Classification of Temperature Sensors: Primary and Secondary Thermometers

15.3 Performance of Temperature Sensors

15.4 Temperature Sensing with Luminescence

15.5 Upconversion (UC) and UC‐Based Thermal Sensor of Ln

3+

Ions

15.6 Optical Heating

References

16 Upconverting Nanoparticles in Pollutant Degradation and Hydrogen Generation

16.1 Introduction

16.2 Degradation of Organic Pollutants

16.3 Degradation of Inorganic Pollutants

16.4 Photocatalytic Hydrogen Production

16.5 Conclusion

References

17 Upconverting Nanoparticles in the Detection of Fungicides and Plant Viruses

17.1 Introduction

17.2 Visual Detection of Fungicides

17.3 Detection of Plant Viruses

17.4 Future Challenges Regarding NP‐Based Fungicide and Plant Virus Detection

References

18 Upconversion Nanoparticles in Biological Applications

18.1 Introduction

18.2 Upconversion Nanoparticles in Bioimaging

18.3 Upconversion Nanoparticles in Drug Delivery

18.4 Upconversion in Photodynamic Therapy

18.5 Photothermal Therapy

References

Index

End User License Agreement

List of Tables

Chapter 1

Table 1.1 Electronic configuration of trivalent ionic states of RE elements ...

Chapter 3

Table 3.1 List of upconversion nanomaterial's properties and name of the tec...

Table 3.2 List of upconversion nanoparticles (UCNPs) and their phase structu...

Chapter 5

Table 5.1 Summary of the constant parameters used in the simulations of stud...

Chapter 6

Table 6.1 Electronic configuration of transition elements (d‐block elements)...

Chapter 13

Table 13.1 Upconversion quantum yield in different upconverting nanoparticle...

Chapter 14

Table 14.1 Emission bands of various upconversion materials.

Table 14.2 Upconversion‐based pH sensors with their response time, range, an...

Chapter 15

Table 15.1 Comparison between the calculated value and measured value of tem...

Table 15.2 The temperature generated in different phosphor samples prepared ...

Chapter 16

Table 16.1 Degradation of RhB by typical UC photocatalysts.

Table 16.2 Degradation of MB by typical UC photocatalysts.

Table 16.3 Degradation of MO by typical UC photocatalysts.

Table 16.4 Degradation of various organic pollutants by typical UC photocata...

Table 16.5 Degradation of other pollutants by typical UC photocatalysts.

Table 16.6 Degradation of inorganic pollutants by typical UC photocatalysts....

Table 16.7 Summarized application of UC photocatalytic materials in hydrogen...

Chapter 17

Table 17.1 Summary of some works performed for plant virus biosensing.

List of Illustrations

Chapter 1

Figure 1.1 Basic energy‐level diagrams depicting typical anti‐Stokes process...

Figure 1.2 Schematic representation of possible UC mechanisms: (a) GSA/ESA, ...

Figure 1.3 Tanabe–Sugano diagram for the d

3

electron configuration in the oc...

Figure 1.4 Splitting of 4f energy levels under different perturbations (draw...

Figure 1.5 Energy levels of the trivalent RE ions. Reprinted with permission...

Figure 1.6 Schematic representation of a three‐level energy system. The upco...

Figure 1.7 General strategies to achieve the high efficiency of UCNPs. Sourc...

Chapter 2

Figure 2.1 Flow chart representation of solid‐state reaction method. Source:...

Figure 2.2 Flow chart representation of coprecipitation technique. Source: P...

Figure 2.3 Schematic representation of sol–gel technique.

Figure 2.4 Schematic of hydrothermal synthesis and EDTA functionalization of...

Figure 2.5 Schematic description of the three main steps in solution combust...

Figure 2.6 Flow chart of Stöber technique. Source: PhD thesis of Lakshmi Muk...

Figure 2.7 Schematic illustration of three emulsion evaporation‐based prepar...

Chapter 3

Figure 3.1 Representation of diffraction from atomic plane: (a) Bragg's law ...

Figure 3.2 XRD patterns for cubic α‐NaYF

4

phase (JCPDS no. 77‐2042), mixed α...

Figure 3.3 Schematic representation of ejection of an electron from an atom ...

Figure 3.4 Instrumentation diagram of X‐ray photoelectron spectroscopy. Sour...

Figure 3.5 XPS spectra of NaYF

4

@SnO

2

CSNPs (red line) and NaYF

4

@SnO

2

@Ag NPs ...

Figure 3.6 Schematic of the interactions of an electron beam with a sample a...

Figure 3.7 SEM images of the NaYF

4

:Yb

3+

/Er

3+

UCNPs synthesized using...

Figure 3.8 Comparative illustration of different components of a simple micr...

Figure 3.9 (a) TEM image of the SrSnO

3

:1 at.% Ho

3+

and 3 at.% Yb

3+

; ...

Figure 3.10 Instrumentation of energy‐dispersive X‐ray spectroscopy. Source:...

Figure 3.11 Energy‐dispersive spectra (EDS) of the NaYF

4

:Yb,Tm UCNPs. Source...

Figure 3.12 Schematic diagram of a thermobalance and a control unit. Source:...

Figure 3.13 TGA of the NaYF

4

:Yb

3+

/Er

3+

UCNPs after coating with

poly

...

Figure 3.14 Schematic diagram of a UV–visible spectrophotometer. Source: Fah...

Figure 3.15 UV–vis spectra of UCNPs SrYbF:1% Tm dispersed in chloroform with...

Figure 3.16 Basic setup of a DLS measurement system. The sample is contained...

Figure 3.17 DLS size distribution plot of oleic acid capped hydrophobic NaYF

Figure 3.18 Schematic representation of a fluorescence spectrophotometer. So...

Figure 3.19 Upconversion luminescence spectrum of the NaYF

4

:Yb/Er UCNPs exci...

Figure 3.20 Experimental setup used to observe the UC emission by using the ...

Figure 3.21 Pump power‐dependent upconversion luminescence spectra of the Na...

Figure 3.22 Instrumentation of colorimetry. Source: Ly et al. (2020). Reprin...

Chapter 4

Figure 4.1 A schematic presentation of Stokes Raman, anti‐Stokes Raman, and ...

Figure 4.2 Information obtained by the analysis of Raman peak.

Figure 4.3 Schematic presentation of relaxation processes in molecules.

Figure 4.4 Raman spectra of pure CH

3

C≡N and (CH

3

C≡N + CH

3

OH) at different mo...

Figure 4.5 A schematic energy level diagram of upconversion nanoparticles. S...

Chapter 5

Figure 5.1 (a) Hypothetical decay of fluorescence from an electronically exc...

Figure 5.2 The four main energy transfer processes that are important for up...

Figure 5.3 Schematic representation of the GSA/ESA (a) and GSA/ETU (c) proce...

Figure 5.4 (a) Energy transfer upconversion (ETU) mechanism for two‐photon u...

Figure 5.5 (a) Schematic diagram of Yb/Tm upconversion emission pathways. In...

Figure 5.6 (a–c) Temporal evolution of upconversion emission at ∼480 nm with...

Figure 5.7 (a) Dependence of the frequency UC signal on the Nd

3+

concent...

Figure 5.8 (a) Variation of the lifetime values of different emitting levels...

Figure 5.9 Comparative values of quantum yield (

Ф

) and quantum efficie...

Figure 5.10 Experimental setup of the absolute method of quantum yield measu...

Chapter 6

Figure 6.1 T–S diagram for (a) d

3

electron. Source: Grinberg et al. (2017). ...

Figure 6.2 (a) The UC emission spectra of KZnF

3

:1% Yb

3+

,

x

%Tm

3+

, 2.5...

Figure 6.3 Room temperature UC emission spectra of the optimized Ho

3+

, H...

Figure 6.4 (a)

Upconversion luminescence

(

UCL

) spectra of

LiScSi

2

O

6

(

LSS

):0....

Figure 6.5 (a) UC emission spectra of the NaGdF

4

:Er

3+

/Yb

3+

/

x

Fe

3+

Figure 6.6 (a) Energy‐level diagram for Ni

2+

in octahedral Cl

2

coordinat...

Figure 6.7 (a) Upconversion emission spectra of Y

2

O

3

:Ho

3+

–Yb

3+

and Y

Figure 6.8 (a) Comparative UC emission spectra for different concentrations ...

Figure 6.9 (a) Comparative study of upconversion emission spectra of Bi

4

x

Er

x

Figure 6.10 Schematic energy‐level diagrams of OsX

6

2−

(X = F, Cl, and ...

Figure 6.11 (a) Schematic illustration of synthesis procedures. (b) Photolum...

Chapter 7

Figure 7.1 (a) Comparative study of upconversion emission spectra of 1.0 mol...

Figure 7.2 Upconversion luminescence from the Ho

3+

‐, Yb

3+

‐co‐doped (...

Figure 7.3 (a) UC emission spectra of the Y

2

WO

6

:Tm

3+

and Y

2

WO

6

:Tm

3+

–...

Figure 7.4 (a) The UC luminescence intensities of YPO

4

:20% Yb

3+

/1% Tm

3+

...

Figure 7.5 Room temperature UC emission spectra of (a) NaYF

4

:Yb/Er (18/2 mol...

Figure 7.6 UC fluorescence spectra of hexagonal‐phase (a) NaScF

4

:Yb/Er and (...

Figure 7.7

Ratio of green to red

emissions (

RGR

), and photographs of NaYF

4

:2...

Figure 7.8 (a) Schematic drawing of FRET‐based multicolor silica/NaYF

4

NIR‐t...

Figure 7.9 Schematic illustrations of (a) energy transfer pathway from ICG o...

Figure 7.10 (a) Schematic design of a NaYF

4

@NaYF

4

:Tm@NaYF

4

core–shell–shell ...

Figure 7.11 (a) Color coordinate diagram of 0.4 mol% Ho

3+

–8.0 mol% Yb

3+

...

Figure 7.12 (a) Anticounterfeiting and information concealment schematic dia...

Chapter 8

Figure 8.1 Schematic scheme showing the synthesis method and bimodal lumines...

Figure 8.2 Demonstration of “direct” photoreactions of the DTE derivative tr...

Figure 8.3 Schematic diagram of the hybrid material showing down‐shifting an...

Figure 8.4 Scheme of the luminescence resonance energy transfer NaGdF

4

Er

3+

...

Figure 8.5 Synthesis and characterization of the UC‐IO@Polymer nanohybrid. (...

Figure 8.6 In vivo cell tracking. (a) Fluorescence/upconversion images of mi...

Figure 8.7 (a) Schematic diagram depicting the structure of UCMOFs obtained ...

Figure 8.8 (a) Optical setup for excitation and imaging of patterns. (b) Por...

Figure 8.9 Scheme for producing anticounterfeiting hybrids. (a) Photograph o...

Figure 8.10 Scheme of multimodal imaging: a magnetic nanoparticle, a radionu...

Figure 8.11 (a) Crystal structure of cubic α‐NaYF

4

and hexagonal β‐NaYF

4

. Mn

Chapter 9

Figure 9.1 Pictorial representation of sum frequency generation emission pro...

Figure 9.2 Upconversion emission intensities of upconversion nanoparticles (...

Figure 9.3 The schematic of possible biofunctional molecules and core@shell ...

Figure 9.4 Penetration of various optical wavelength penetration in the skin...

Figure 9.5 Live tracking images at (a) 0, (b) 3, and (c) 6 hours time interv...

Figure 9.6 In vivo imaging of rat:

quantum dot

s (

QD

s) injected into the tran...

Figure 9.7 (a)

Time‐resolved fluorescence‐Förster resonance energy transfer

...

Figure 9.8 In vivo upconversion imaging after injection with NaYF

4

:Yb:Tm@Fe

x

Figure 9.9 In vitro and in vivo images of HEK 293T cells after 5 minutes irr...

Chapter 10

Figure 10.1 Schematic diagram showing frequency downconversion and upconvers...

Figure 10.2 Absorption spectra (black lines) and

photoluminescence

(

PL

) spec...

Figure 10.3 (a) Bandgap tuning after core@shell formation. Middle: core QDs,...

Figure 10.4 Photoluminescence spectra of (a) YVO

4

:5Eu

3+

and (b) YVO

4

:5Eu

Figure 10.5 Schematic diagram showing the core@shell formation: (a) the form...

Figure 10.6 (a) XRD patterns of core: NaYF

4

(i) and core@shell (NaYF

4

@NaGdF

4

Figure 10.7 XPS spectra of core@shell (NaYF

4

@NaGdF

4

) at different excitation...

Figure 10.8 TEM images of (a) hexagonal β‐NaYF

4

:Yb

3+

/Er

3+

(15/2%) co...

Figure 10.9 TEM images of (a) hexagonal β‐NaYF

4

:Yb

3+

/Er

3+

(15/2%) co...

Figure 10.10 TEM images and particle size distribution of (a) hexagonal β‐Na...

Figure 10.11 (a) HAADF TEM images of NaYF

4

core@NaGdF

4

shell NCs at low magn...

Figure 10.12 (a, b) HAADF images of single NaYF

4

core@NaGdF

4

shell NCs and c...

Figure 10.13 ETU and EM processes in core and core@shell particles. The core...

Figure 10.14 UC emission spectra of core@shell nanoparticles: (a) NaErF

4

:0.5...

Figure 10.15 UC emission spectra of (a) core@shell NaYF

4

:Yb‐Tm@NaGdF

4

and (b...

Figure 10.16 (a) Looping mechanism or process in NaYF

4

:1 at.% Tm

3+

at 10...

Figure 10.17 The ground‐state electronic configurations of Yb

3+

(a) and ...

Figure 10.18 (a) Energy level diagram showing sequential energy transfer fro...

Figure 10.19 Possible energy transfer mechanism in highly doped core: NaErF

4

Figure 10.20 (A) Possible core@shell nanostructures in which Er

3+

and Yb

Figure 10.21 NIR III emission spectra of (a) NaErF

4

:0.5 at.% Tm@NaYF

4

and (b...

Figure 10.22 Imaging of MCF 7 after interaction with 250 μg/ml of NPs shows ...

Figure 10.23 Internalization of NPs in HeLa cells. (a) Bright‐field image, (...

Figure 10.24 Four‐modal imaging of the focused tumor from the tumor‐bearing ...

Figure 10.25 FRET mechanism for heat generation. (a) Spectral overlap of UC ...

Figure 10.26 Schematic diagram of killing of cancer cells using FA‐coated na...

Figure 10.27 Schematic diagram showing Boltzmann distribution low for popula...

Figure 10.28 YPO4:Er‐Yb UCNPs: (a) Rise of FIR and temperature with the incr...

Figure 10.29 (a) Schematic diagram showing dual excitation at 800 or 980 nm ...

Figure 10.30 (a) Emission spectra of core@shell NaGdF

4

:Yb‐Er@NaGdF

4

:

x

Nd UCNP...

Figure 10.31 Digital photos illustrating the application of core@shell UCNPs...

Chapter 11

Figure 11.1 (a) A schematic illustration of core–shell–shell nanoparticle's ...

Figure 11.2 (a) Upconversion emission spectra for Er

3+

/Ho

3+

‐codoped ...

Figure 11.3 A schematic of DSSCs attached with the upconverter layer. Source...

Figure 11.4 Number of upconversion material‐based paper publications per yea...

Figure 11.5 Schematic representation of steps of latent fingermark recording...

Figure 11.6 Identification of latent fingerprints using hexagonal NaYF

4

:Ce

3+

...

Figure 11.7 Fingerprint demonstration on (a) paper surface and (b) transpare...

Figure 11.8 Schematic representation of (a) methods of formation of UCNPs co...

Figure 11.9 (a) Schematic diagram of constructing anti‐counterfeiting hybrid...

Figure 11.10 (a) Stamp of Chinese words used to write on plain paper; (b) hi...

Figure 11.11 A schematic illustration of the (a, b) working principle and de...

Figure 11.12 In vivo and in vitro imaging of rats: quantum dots and PEI‐coat...

Figure 11.13 SSOCT B‐scan images of chicken breast tissue; (a) control sampl...

Figure 11.14 Averaged A‐scan profile plot of the SSOCT image over the region...

Figure 11.15 (a) Synthesis of streptavidin‐functionalized NaYF

4

:Yb

3+

,Er

3

...

Figure 11.16 (a) Upconversion luminesce spectra of composition of streptavid...

Figure 11.17 (a) Electroluminescence emission spectra of Yb

3+

/Er

3+

‐c...

Chapter 12

Figure 12.1 Energy‐level scheme showing the UC process in (a) Yb

3+

/Er

3+

...

Figure 12.2 UC emission spectra of (a) La

2

O

3

:Yb

3+

/Er

3+

, (b) La

2

O

3

:Yb

Figure 12.3 Mechanism of photocatalytic process based on a TiO

2

semiconducto...

Figure 12.4 (a) The proposed energy transfer mechanism of NaYF

4

:Yb,Tm@TiO

2

u...

Figure 12.5 (a) Schematic illustration of the photocatalysis mechanism of β‐...

Figure 12.6 (a) FE‐SEM image of BiOBr:Yb

3+

/Er

3+

nano/microcrystals f...

Figure 12.7 Photocatalytic mechanism of Au‐UCNPs/g‐C3N4 under (a) UV and (b)...

Figure 12.8 (a) TEM images of UCNPs‐CdSe nanostructure, (b) schematic repres...

Figure 12.9 (a) Schematic illustration, (b) SEM image of the MAPbI

3

/UCNPs bi...

Figure 12.10 (a) Core–shell structure efficiently converted the incident SWI...

Figure 12.11 (a) Schematic diagram of the UCNPs/graphene hybrid micropyramid...

Chapter 13

Figure 13.1 The energy‐level scheme of a three‐level system and possible upc...

Figure 13.2 (a) Lasing spectra of the microcavity with a bottle‐like geometr...

Figure 13.3 (a) Numerical simulations of resonance spectrum and experimental...

Figure 13.4 Variation of UC emission intensity of green emission with temper...

Figure 13.5 Cathodoluminescence spectra of BaTiO

3

:Er

3+

/Yb

3+

and BaTi...

Figure 13.6 (a) The CIE chromaticity diagram associated with the emission co...

Chapter 14

Figure 14.1 (a) excited state absorption, (b) energy transfer upconversion, ...

Figure 14.2 Schematic diagram of an upconversion‐sensitized pH sensor. The p...

Figure 14.3 (a) Schematic description of the developed nanoprobe and its wor...

Figure 14.4 (a) Upconversion luminescence spectrum of dye‐conjugated UCNPs i...

Figure 14.5 Upconversion response of (a) NaYF

4

:Er

3+

/Yb

3+

@NaYF

4

@Ni an...

Figure 14.6 (a) The upconversion spectrum of Y

2

O

2

S:Er

3+

/Yb

3+

UCNPs (...

Figure 14.7 (a) The upconversion spectra of the graphene oxide–PEI‐NaYF

4

;Er

3

...

Figure 14.8 (a) Imaging calibration row (RGB) of the membrane. The graph at ...

Chapter 15

Figure 15.1 Schematic diagram of the effect of temperature on the emission s...

Figure 15.2 Emission spectra of NaYF

4

:Yb

3+

/Er

3+

UCNPs upon excitatio...

Figure 15.3 (a) UC luminescence spectra of OA‐capped C@S1@S2@S3 NPs in cyclo...

Figure 15.4 (a) UC luminescence spectra of Er

3+

from

2

H

11/2

and

4

S

3/2

le...

Figure 15.5 A variation in the UC emission intensity of

2

H

11/2

 → 

4

I

15/2

(524...

Figure 15.6 Variation of blue UC emission (at 474 nm) intensity with time (i...

Figure 15.7 Variation of blue UC emission (at 475 nm) intensity with time (i...

Figure 15.8 Variation of blue UC emission (at 476 nm) intensity with time (i...

Chapter 16

Figure 16.1 The luminescence decays of the excited‐state levels of Er

3+

...

Figure 16.2 (a and b) SEM images of the UCNPs/CdS/PVP/TBT nanofibers develop...

Figure 16.3 Diagrams of the energy levels of Yb

3+

–Tm

3+

and the upcon...

Figure 16.4 (a) Schematic illustrations for the preparation of the BVO/CF co...

Figure 16.5 (a) Schematic illustration of the NYF/AC composites synthesized ...

Figure 16.6 The proposed photocatalytic mechanism of the A/NYYE/WW photocata...

Figure 16.7 (a) Schematic illustration of the synthesis of UCNPs@Zn

x

Cd

1−x

...

Figure 16.8 (a) The light absorption of each component in the MOF composites...

Figure 16.9 Possible mechanisms for photocatalytic hydrogen evolution of NYF...

Figure 16.10 Schematic of the interaction between the LSPR effect of W

18

O

49

...

Chapter 17

Figure 17.1 Different families of plant virus species. Source: Lefeuvre et a...

Figure 17.2 Jablonski diagram of FRET. Source: Hochreiter et al. (2015).

Figure 17.3 The distance between donor and acceptor can be changed by the am...

Figure 17.4 Structures as well as the absorption spectra of acceptors are ch...

Figure 17.5 Energy‐level diagram of the inner filter effect. Source: Zhang e...

Figure 17.6 Conditions for IFE‐based sensing, absorption spectrum of the abs...

Figure 17.7 Energy‐level diagram of (a) photoinduced electron transfer. Sour...

Figure 17.8 (a) Image of the fabricated sensing system. (b) Graph of quenchi...

Figure 17.9 (a) Schematic diagram of calorimetric detection of thiram via WL...

Figure 17.10 (a) Schematic representation of fluorescence‐based aptasensor f...

Figure 17.11 Scheme for IFE‐based Ag

+

detection. Source: Long et al. (20...

Figure 17.12 IFE‐based detection of the Fe

3+

ions. Source: Chen et al. (...

Figure 17.13 Interaction mechanisms between NPs and plant virus vector syste...

Figure 17.14 Schematic diagram of plant virus detection using antibody‐conju...

Chapter 18

Figure 18.1 (a) Whole‐body images of a BALB/c mouse injected via tail vein w...

Figure 18.2 (a) The UC PL, (b) bright‐field, and (c) merged images of a KB t...

Figure 18.3 Schematic representation of UCNP‐based drug delivery system: (A)...

Figure 18.4 Schematic representation of a polymer‐coated UCNP‐based drug del...

Figure 18.5 Schematic representation of ZnPC PS‐loaded mesoporous silica she...

Figure 18.6 Photothermal therapy and NIR‐controlled drug release of nanocomp...

Guide

Cover Page

Title Page

Copyright

Preface

Table of Contents

Begin Reading

Index

End User License Agreement

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Upconverting Nanoparticles

From Fundamentals to Applications

Edited by Vineet K. Rai

Editor

Prof. Dr. Vineet K. Rai

IIT (ISM) Dhanbad

Department of Physics

Police Line Road

Hirapur, Sardar Patel Nagar

826004 Dhanbad

India

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© BAIVECTOR/Shutterstock

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Preface

The conversion of low‐energy photons into high‐energy photons, known as “frequency upconversion,” using advanced optical materials has become an emerging research field with wide consequence and impact in various scientific areas ranging from healthcare to energy and security. The materials showing frequency upconversion properties are known as upconversion (UC) materials. UC materials reveal variety of applications in different fields, viz. color display, two‐photon imaging in confocal microscopy, WLEDs, high‐density optical data storage, upconvertors, under sea communications, solid‐state lighting, sensors, photovoltaics, photocatalysis, food industry, indicators, anti‐counterfeiting, bioimaging, cancer therapy and other biological fields. It is known that in comparison to ultraviolet (UV) and visible light the near‐infrared (NIR) light is abundant and non‐destructive in nature. It has deep penetration in the organisms and less harmful quality. UC luminescent materials in nanosize range are known as UC nanomaterials or UC nanoparticles (UCNPs). UCNPs excited with non‐destructive NIR light are a better choice than the conventional downconversion nanoparticles because they are free from autofluorescence, have low light penetration, and cause less severe photo‐damage to living organisms. It is notable to mention that the low efficiency of UC materials definitely becomes a major barrier for their application in a wide range. For researchers, it is a top priority to overcome this problem. Several engineered UCNPs, e.g. organic, inorganic, hybrids, and thin films, have been explored widely to obtain highly efficient UC luminescent materials. Usually, organic luminescent materials suffer poor stability under harsh conditions and have poor long‐term reliability, but have a greater ductility than inorganic materials. The inorganic luminescent materials are more durable and possess high thermal stability. So, the hybrid materials consisting of both inorganic and organic components, namely, metal organic frameworks (MOFs), have attracted researchers with enhanced luminescence properties as compared to the bare organic and inorganic materials. To enhance the upconversion efficiency, spherical metal nanoparticles showing plasmon resonance in close proximity of the UCNPs are utilized. The plasmonic nanostructures are widely used to evolve the UCNPs with improved electronic, metallic, and optical properties. When the surface plasmon resonance wavelength of the metallic nanostructure matches with the excitation wavelength of upconversion mechanism, signal enhancement occurs. Usually, the coating of gold (Au) and silver (Ag) nanoparticles is used to tune the luminescence properties of UCNPs, though the nanoparticles exhibit plasmon absorption in 400–600 nm range.

The upconversion emission efficiency can be enhanced by several ways, including doping with sensitizer, non‐lanthanides, and coating with inorganic shell. The non‐lanthanide co‐doping in UCNPs has also been used frequently in order to get enhanced luminescence intensity along with the use of sensitizer ion. The co‐doping of activator and sensitizer ions with proper concentration in an appropriate host matrix is essential to achieve highly efficient UC emission as the concentration quenching has a prejudicial effect on the luminescence intensity. The phonon frequency, stability, cost effectiveness, non‐hygroscopic, and non‐toxic nature of the UC materials are of utmost importance. The security of any important data, currency, etc. has become very crucial to prevent counterfeiting. UCNPs with high luminescence intensity can be validated in anti‐counterfeiting applications. These materials are also utilized for visual exposure of fungicides, thiram, etc., which can be broadly applied in soybeans, apples, wine farming, etc., to avoid crop diseases and excessive use of pesticides. Rare‐earth‐ions‐based UC emission has tremendous advantages in terms of long excited lifetime, sharp emission bandwidth, low autofluorescence, high photostability, high resolution, low toxicity, etc. Rare‐earth ions are found to be very sensitive to even small changes in chemical surroundings. Therefore, it becomes essential to get information about the symmetry, bonding of the probe ion, and how they change their optical properties with chemical composition of the host materials. For getting the high quantum efficiency, concentration of the dopants should be high, but it may cause concentration quenching due to the interaction between the excited and unexcited neighbors. Therefore, the nano‐structured materials containing metallic nanoparticles are of particular interest because the large local field around the rare‐earth ions positioned near the nanoparticles may increase the luminescence efficiency. Among several strategies, the coating of upconversion nanoparticles with inorganic materials shell is an effective method to get enhanced UC luminescence. The core@shell approach offers shielding to the surface particles and thus reduces the surface defects and possibility of quenching. This core@shell architecture is very much beneficial in biomolecule conjugation and thus suitable for many biological applications. Different coating strategies have been employed according to the required application purposes. UCNPs probes can function as multiple contrast agents for concurrent use in altered medicinal imaging modalities by providing corresponding diagnostic information (i.e. MRI and CT). Bio‐conjugation on the surface of the UCNPs shows a much enhanced imaging performance in comparison to the clinically used fluorescent dyes. Innovative bio‐imaging methods are being established by combining the conventional medical imaging modalities using core‐shell structured UCNPs.

The book entitled Upconverting Nanoparticles: From Fundamentals to Applications is completely different from the previously published books in all respects, including the basics, scientific and technological demands. It is divided into eighteen chapters. Chapter 1, authored by Mondal and Rai, introduces the basic concepts of upconversion, and upconversion of nano‐particles. The introduction to frequency upconversion and its various mechanisms, excitation and de‐excitation processes in hosts containing rare‐earth ions along with the spectroscopic properties of rare‐earth ions/transition metals are described in this chapter. The rate equations relevant to excited‐state absorption and energy transfer processes with an overview of the UCNPs have been introduced. Chapter 2, authored by Mukhopadhyay and Rai, describes the synthesis protocol of upconversion nanoparticles. In this chapter introduction to host materials and synthesis strategies of UC nanomaterials like solid‐state reaction, co‐precipitation, sol–gel, hydrothermal, combustion, thermolysis, microwave‐assisted synthesis, core@shell synthesis techniques, etc. have been described. Chapters 3 and 4, authored by Jain et al.; Ojha and Ojha, refer to characterization techniques and analysis; Raman and FTIR spectroscopic techniques and their applications, respectively. Various structural and optical techniques for the characterization of UCNPs, viz. X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), energy‐dispersive X‐ray spectroscopy (EDS), thermogravimetric analysis (TGA), ultraviolet–visible–near infrared (UV–Vis–NIR) absorption spectroscopy, dynamic light scattering (DLS), photoluminescence, Fourier transform infrared (FTIR), have been reported. Chapters 5, 6 and 7, authored by Ranjan et al.; Prasad and Rai; and Pattnaik and Rai, summarize the fundamental aspects of UCNPs based on their properties, frequency upconversion in UCNPs containing transition metal ions, and frequency upconversion in UCNPs containing rare‐earth ions, respectively . Along with introduction the dynamics of UCNPs on the basis of fluorescence decay times, quantum yield measurement of UCNPs, frequency upconversion and its various mechanisms have also been interpreted. The various routes to enhance the upconversion luminescence along with the technological applications of UCNPs have been described.

Chapters 8, 9, and 10, authored by Singh; Dwivedi; and Ningthoujam et al., are devoted to the smart and new type of upconverting nanoparticles; surface modification and (bio) functionalization of upconverting nanoparticles, and frequency upconversion in core@shell nanoparticles, respectively. These chapters outline the upconverting core@shell nanostructures, hybrid upconverting nanoparticles, magnetic‐upconverting nanoparticles, UC‐based metal–organic frameworks, surface modification, bio‐functionalization of upconverting materials, synthesis of core@shell and core@shell@shell UCNPs, and use of UCNPs for security, biological, and sensing applications. Chapters 11, 12, 13, 14, and 15, authored by Kumar, Mishra and Shwetabh; Singh et al.; Dey; Mahata, De and Lee; and Shahi and Rai, deal with the UCNPs in solar, forensic, security ink, and anti‐counterfeiting applications; application of upconversion in photocatalysis and photodetectors; UCNPs in lighting and displays; upconversion nanoparticles in pH‐sensing applications and upconversion nanoparticles in temperature‐sensing and optical heating applications, respectively. Chapters 16, authored by Wang et al., throws the light on UCNPs applications in degradation of organic and inorganic pollutants along with the photocatalytic hydrogen generation. The visual detection of fungicides and plant viruses along with the future challenges have been explained by Kesarwani and Rai in Chapter 17. Chapter 18, authored by Mukherjee and Sahu, involves the application of UCNPs in bio‐imaging, drug delivery, photodynamic therapy, and photothermal therapy.

The present book is outcome of the untiring efforts of all the contributing authors. It will be very much helpful to the researchers as well as the undergraduate and post‐graduate students studying physics, chemistry, materials science, biology, engineering, etc. in gaining a proper understanding about the upconversion luminescence. It was possible to complete this book only due to the great affection and blessings of Gurudev Pt. Shri Ram Sharma Acharya and Gurumataji Mata Bhagawati Devi Sharma. Special thanks to all my family members and research scholars for their motivation and kind support. I would also like to thank the Wiley team involved from the beginning till the completion of the book proposal. As a large number of topics related to the UCNPs and their applications have been covered in this book, there could be the possibility that some of the minute glitches have been missed out. Therefore, genuine suggestions and comments from the readers are welcome. Overall, the research developments on UCNPs and their uses in different fields starting from very basics to advanced level make the present book unique.

Professor (Dr.) Vineet K. RaiDepartment of PhysicsIndian Institute of Technology (Indian School of Mines), Dhanbad, India

1Introduction to Upconversion and Upconverting Nanoparticles

Manisha Mondal1,2 and Vineet Kumar Rai1

1 Indian Institute of Technology (Indian School of Mines), Department of Physics, Laser and Spectroscopy Laboratory, Dhanbad 826004, Jharkhand, India

2 Tezpur University (Central University), Department of Physics, Napaam, Tezpur, Sonitpur 784028, Assam, India

1.1 Introduction

Spectroscopy almost deals with the interaction of light and matter. It provides information about splitting of electromagnetic radiation into its constituent wavelengths. The beginning of spectroscopy lies since the observation of light dispersion through prism by Sir Isaac Newton. Among different spectroscopy techniques, optical spectroscopy delivers an exceptional tool by which one can find detailed information regarding the absorbing and emitting atoms, ions, molecules, defects, their local surroundings, etc. In a term, optical spectroscopy allows light to penetrate inside materials. Optical spectroscopy can be characterized into four parts: absorption, luminescence, reflection, and scattering. A marvelous dimension of research carried out in finding novel luminescent materials plays an important role in optical communication, lighting, medical diagnosis, etc. (Berthou and Jörgensen 1990; Cheng et al. 2013; Jiang et al. 2016; Lin et al. 2016; You et al. 2016; Dey and Rai 2017; Mehra et al. 2020). When an atomic system after absorbing the photons of appropriate frequency transits upward to a higher state and then by the spontaneous emission process, it may return to the ground state. This de‐excitation route is familiar as the luminescence process. The occurrence of luminescence due to excitation of light is known as photoluminescence. On the other hand, luminescence due to excitation of an electron beam is termed as cathodoluminescence, which helps to identify impurities, lattice defects, and crystal distortions. Radioluminescence occurs due to excitation through the highly energetic electromagnetic radiations (i.e. α rays, β rays, and γ rays). The thermoluminescence phenomena are used in radiation dosimetry, dating of minerals and old ceramics, materials characterization, biology, forensic, etc. It occurs when a material radiates light as a consequence of release of energy kept in traps by thermal heating. Electroluminescence occurs due to the passage of electric current over a material. The emission of light due to mechanical disturbance originates triboluminescence. Conferring to the diverse positions of the excitation and emission bands, the luminescent materials can be categorized into Stokes‐ and anti‐Stokes‐type luminescent materials. These processes are typically exemplified by the Jablonski diagram (Jablonski 1935; Jablonski 1993). The luminescent materials are commonly known as phosphors, which means “light bearer,” that consist of host and dopants. In these constituents, lanthanide materials are mainly introduced into the host matrix. Lanthanides have the most complicated electronic structures because of their large number of incomplete 4f energy levels. The present chapter presents a brief outlook on understanding the frequency conversion mechanisms, electronic energy levels of rare‐earth (RE) ions, transition metal ions, theoretical description of the optical characteristics of RE ions, and Upconverting nanoparticles (UCNPs).

1.2 Frequency Conversion and Its Various Processes

The photoluminescent materials are able to display visible emissions via suitable ultraviolet (UV) or near‐infrared (NIR) excitations. In the majority of cases, excitation energy is greater than emitted photon energy; this emission is called as Stokes emission, and the corresponding energy loss is known as Stokes shift. In certain circumstances, emitted energy is higher than absorbed energy; this is known as anti‐Stokes emission.

1.2.1 Stokes Emission

The Stokes‐type emission process possesses two types of features such as downconversion and quantum cutting (Huang et al. 2013; Loo et al. 2019). In quantum cutting process, two or more lower energy photons are emitted for each incident high‐energy photon absorption. In this process, two, three or four low‐energy photons are emitted because of the absorption of one NIR, visible, or ultraviolet photon. In this process, the conversion efficiency is more than 100%. In current years, quantum cutting has acknowledged considerable devotion as a budding method to improve the photovoltaic conversion efficiency of solar cells. On the other hand, in the downconversion process, emission of one lower energy photon takes place because of the absorption of one higher energy photon; thus, the conversion efficiency will not go beyond 100%.

1.2.2 Anti‐Stokes Emission

The anti‐Stokes emission process occur via three processes: two‐photon absorption (TPA), second harmonic generation (SHG), and upconversion (UC) (Figure 1.1) (Pollnau et al. 2000; Gamelin and Gudel 2000; Suijver 2008; Grzybowski and Pietrzak 2013; Chen et al. 2015; Nadort et al. 2016). TPA is a type of nonlinear absorption process that can be defined as the simultaneous absorption of two photons of same or different frequencies by an atom, ion, or molecule. In this process, the electron is promoted from low energy level (i.e. ground state) to excited level, and the energy of the emission transition is equal to the sum of two‐photon energies. As this is a third‐order nonlinear process, it is effective at precise high intensities. TPA was initially anticipated by Maria Goeppert‐Mayar in the year 1931. This was experimentally verified by the laser after its discovery. A number of techniques are used to measure TPA, such as two‐photon excited fluorescence, z‐ scan, nonlinear transmission, etc. On the other hand, SHG, “an optical nonlinear process,” occurs from a virtual state in a medium having second‐order nonlinear susceptibility. This was revealed and experimentally verified by Franken et al. (1961). They detected the second harmonic light when an intense beam of 6943 Å from the ruby laser was passed through the quartz crystal. In this process, two photons of the same frequency interact with a nonlinear material (i.e. medium) and give rise to a new photon of double the frequency or energy of the incident photons. Furthermore, UC is also an anti‐Stokes process that converts the lower energy photons into high‐energy photons, e.g. infrared to visible or UV light (Figure 1.1). It is a stepwise absorption process involving intermediate states (Auzel 1966; Ovsyakin and Feofilov 1966). Basically, among these three processes of converting lower energy photons into higher energy photons, TPA and SHG need a coherent beam as well as a very high excitation beam intensity. In the UC process, coherent pumping and high intensity of the excitation beam are not necessarily required. It occurs even at low intensity of the excitation beam because of the presence of real intermediate states (generally, of metastable nature).

Figure 1.1 Basic energy‐level diagrams depicting typical anti‐Stokes processes.

The materials that exhibit the UC properties are known as upconverting materials. In recent years, these upconverting materials are extensively used in sensing, infrared counters, solid‐state lasers, solar cells, fingerprint detection, security ink, upconverters, biological fields, etc. (Digonnet 1993; Wade et al. 2003; Rai 2007; Wang and Liu 2009; Gu et al. 2013; Li et al. 2013; Wang and Zhang 2014; Chen et al. 2014; Mondal and Rai 2020). Generally, the UC phenomenon observed in these materials is not as simple as depicted in Figure 1.1. Several processes accountable for UC mechanisms are as follows.

1.2.2.1 Ground/Excited‐State Absorption (GSA/ESA)

Ground‐state absorption (GSA) is one of the simplest routes for UC mechanism (Auzel 1973, 2004; Garlick 1976; Rai et al. 2013; Reddy et al. 2018). The process in which the ground‐state ions (i.e. electrons) after absorbing the requisite energy from the pump photons are promoted to the first intermediate level is known as the GSA process. Conversely, sequential absorption of two light quanta by a particular ion is known as ESA process (Auzel 1973, 2004; Garlick 1976; Rai et al. 2013). In the case of ESA process, the ion present in the intermediate state absorbs the second photon and transits upward to the next higher state. For example, the energy‐level diagrams for GSA and ESA mechanisms are presented in Figure 1.2a. Here at first, an ion absorbs the pump photon of energy (=hν, where “h” is Planck's constant and “ν” is the frequency of the incident photon) and reaches to the intermediate state E1 (exhibit long lifetime) from the ground state G via the GSA process and then a second pump photon (of the same energy) excites the ion from E1 state to the next higher state E2. A radiative decay of the ion from the excited state (E2) to the ground state (G) results in UC emission. Thus, a single ion is involved in the whole ESA process. For getting proficient UC emission through the ESA process, a ladder‐like energy‐level arrangement in ions is essential.

Figure 1.2 Schematic representation of possible UC mechanisms: (a) GSA/ESA, (b and c) ETU, (d) cooperative luminescence, (e) cooperative sensitization, (f and g) CR, and (h) PA processes.

1.2.2.2 Energy Transfer Upconversion (ETU)

Like the ESA process, the energy transfer upconversion (ETU) process also involves successive absorption of two energy quanta by the ions to occupy the intermediate (i.e. metastable) state (Figure 1.2). As in the ESA process there is an involvement of single ion, however, ETU operates within two (similar or different) ions. In this mechanism, the involved two dopant ions are termed as sensitizer and activator (Heer et al. 2003; Boyer et al. 2007; Shan et al. 2007; Soni et al. 2015; Mukhopadhyay and Rai 2020; Pattnaik and Rai 2020). At first, both the (different) ions absorb the pump photons from the ground state and then moves to their respective metastable states (E1′ and E1, where E1′ ≅ E1) through the GSA process (Figure 1.2b). After that, the sensitizer ion (present in E1′ state) handovers its excitation energy to the neighboring activator ion (present in E1 state) and relaxes back to the ground state. The activator ion after gaining this excitation energy from the sensitizer reaches to the next higher energy state (E2).

When the two involved dopant ions are similar, these two ions are initially excited to the intermediate state (E1) after receiving the energy from pump photons (Figure 1.2c). The two ions present in the E1 state exchange their energy in such a way that one ion (i.e. donor), after transferring its excitation energy to the other excited ion (i.e. acceptor), decays nonradiatively to the lower energy level (G). The other ion (i.e. acceptor) after getting excitation energy from the first one (i.e. donor) is promoted to the next higher energy state (E2). A radiative transition from state E2 to the ground state (G) generates a photon of energy (=hν1), which is higher than the incident photon energy (=hν) (Figure 1.2). This ETU process is the most efficient UC emission process (Auzel 2004; Rai et al. 2007, 2008). In this process, the dopant ion concentration (which regulates the average distance concerning adjacent dopant ions) plays a key role in the UC emission intensity.

1.2.2.3 Cooperative Luminescence and Cooperative Sensitization Upconversion (CSU)

UC emission by a cooperative energy transfer process involves two ions (one acts as a donor and the other ion as an acceptor). In the cooperative luminescence process, two ions absorb the pump photons successively and reach the higher (intermediate) state E1 (Figure 1.2d). In this intermediate level, these two ions transfer their energy in such a way that one ion (donor) transfers its excitation energy to the other one (acceptor) and the donor returns to the ground state (G). The acceptor, after gaining the excitation energy from the donor, transits upward to a higher energy state, “which is a virtual state.” This virtual state is also known as the cooperative energy state (Lee et al. 1984; Maciel et al. 2000; Diaz‐Torres et al. 2005). From this virtual state, it relaxes radiatively to the ground state (G) via emitting a photon of energy larger than the incident photon energy (Figure 1.2d). On the other hand, in the cooperative sensitization process, when the energy of the two excited ions are transferred to a third ion (ion 2), then it goes from the ground state to an excited state having energy equal to the sum of the energies of the two individual ions (Martín et al. 2001; Salley et al. 2001, 2003). In Figure 1.2e, the excitation energy of the two excited ions (ion 1) present in the state E1 is transferred to a third ion (ion 2). The third ion (ion 2) present in the ground state (G), after absorbing the excitation energy corresponding to the two excited ions (ion 1), moves to its higher state (E2). After that, the third ion from the excited state (E2) relaxes radiatively to the lower levels (say ground state) via emitting the photons of energy higher than that of the incident photon. This process is known as cooperative sensitization, and the emitting state (E2) in this process is a real state (Figure 1.2e). Thus, the cooperative sensitization is more effective than cooperative luminescence because it may compensate the low UC emission efficiency (Dwivedi et al. 2007; Liang et al. 2009).

1.2.2.4 Cross‐relaxation (CR) and Photon Avalanche (PA)

The cross‐relaxation (CR) process occurs due to ion–ion interaction (ions may be similar or different) (Chen et al. 2014; Pattnaik and Rai 2020) (Figure 1.2f,g. The cross‐relaxation between two identical ions/molecules is responsible for self‐quenching (Figure 1.2f). In the self‐quenching process, the intermediate states of both the ions (ion 1) have the same energy (E1). When the cross‐relaxation occurs between two different ions (Figure 1.2g), the first ion shares a part of its excitation energy to the second ion by the process E2 (ion 1) + G (ion 2) → E1 (ion 1) + E1′ (ion 2) (Figure 1.2g). In this process, the first ion (ion 1) initially present in the excited state (E2) interchanges a part of its excitation energy to the second ion (ion 2) that is initially available in the ground state (G). By this way, the decrease in the energy of the first ion (ion 1) is equal to the increase in the energy of the second ion. This results in both the ions/molecules changing simultaneously to the excited state (E1 and E1′). Among the other UC processes, the most exciting process is photon avalanche (PA), which was first experimentally observed in Pr3+‐doped infrared quantum counters (Chivian et al. 1979). Generally, this PA process occurs when the excitation energy exceeds its threshold limit. When the excitation energy is lower than the threshold energy, the emitted intensity is very poor, but as it exceeds the limit, the emitted intensity becomes enormously greater (Joubert 1999; Singh et al. 2011; Zhu et al. 2012; Mondal et al. 2016). For occurrence of PA process, at first, the intermediate level and the upper excited level are populated by the GSA, ESA, and ETU processes. By the CR process between these upper excited level and the ground state of a neighboring ion, two ions are generated in the intermediate level E1 (Figure 1.2h). Now, two ions are available in the intermediate state for the ESA process. Thus, with the feedback looping of ESA and CR processes simultaneously, the number of ions in the intermediate level increases, which give rise to strong UC emission.

The PA process is an unusual pumping process because it may lead to strong UC emission from the upper excited state E2 without any resonant GSA from the ground state (G) to the intermediate state (E1) of ion 2 (Figure 1.2h). The frequency of incident photon is in resonant with state E1′ of ion 1 and the upper excited state E2 of ion 2. An efficient CR process, i.e. E2 (ion 2) + G (ion 1) → E1 (ion 2) + E1 (ion 1), occurs between ion 1 and ion 2. This results in both the ions to occupy the intermediate state E1. These two ions readily populate the level E2 through ESA to further initiate the cross‐relaxation. With the feedback looping of these efficient cross‐relaxation and ESA processes, the number of ions in the intermediate state E1 increases rapidly, which results further an enormous increase in the population of level E2. Thus, in the PA process, a strong UC emission from state E2 to the ground state G (of ion 2) has been observed.

1.3 Transition Metals and Their Properties

The optical centers are necessary for the perfect crystals to exhibit the optical spectra. Depending on the absorption and emission bands of the optical centers present in the pure crystals, they are pertinent for diverse applications, such as optical amplifiers, solid‐state lasers, color displays, absorbers, improving in luminescence brightness, fibers, optical switches, etc. Any element in the periodic table may act as a foreign element in the crystal. However, essentially, a few number of elements can be ionized, which can generate energy levels and thus yield optical features. For industrial applications, the two extremely important elements are transition metals and REs in the periodic table. Transition metal ions are especially used as optically active dopants in tunable solid‐state lasers (Solé et al. 2005). These ions belong to the fourth period of the periodic table with electronic configuration 1s22s22p63s23p63dn, where “n (varies from 1 to 10)” is the number of 3d electrons present in the transition metal ions. Generally, valence electrons are responsible for optical transitions; hence, in the case of transition metals, 3d electrons are accountable. Because of the large radius of transition metal ions as compared to lanthanides and no shielding of valence electrons, strong field effect occurs; hence, they exhibit the broad bands.

The Sugano–Tanabe diagram explains the energy‐level diagram for the transition metal ions (Figure 1.3) (Tanabe and Sugano 1954a,b). The spectroscopic terms for the free ion states of the transition metal ions due to the L‐S interaction are described as 2S+1LJ, where, L, S, and J denote the total orbital angular momentum, total spin angular momentum, and total angular momentum, respectively. The energy separation among the 2S+1L states, i.e. the strength of the electron–electron interaction, can be calculated with the help of Racah parameters (A, B, and C) (Solé et al. 2005). On the basis of octahedral crystal lattice, Sugano and Tanabe explained the occurrence of energy levels in the case of transition metal ions, but by using this diagram, one can also interpret the optical spectra arising from the transition metal ions in different types of host lattices.

Figure 1.3 Tanabe–Sugano diagram for the d3 electron configuration in the octahedral crystal field. Source: Brik et al. (2016). Reprinted with permission of The Electrochemical Society.

This diagram explains the splitting of 2S+1L free ion energy states with the ratio between the strength of the crystal field and the electron–electron interaction strength (symbolized as Dq/B) versus the free ion energy levels (E/B units). In this diagram, the y‐axis is in terms of energy “E” scaled by B (one of the Racah parameters). The splitted terms for 2S+1L