\r\n\tA quark exhibits confinement, which means that the quarks are not observed independently but always in combination with other quarks. This makes determining the properties (mass, spin, and parity) impossible to measure directly; these traits must be inferred from the particles composed of them. There are six flavors of quarks: up, down, strange, charm, bottom, and top. The flavor of the quark determines its properties.
\r\n\tThere are three generations of quarks, based on pairs of weak positive/negative, weak isospin. The first generation quarks are up and down quarks, the second-generation quarks are strange and charm quarks, the third generation quarks are top and bottom quarks. The up and down quarks make up protons and neutrons, seen in the nucleus of ordinary matter. They are the lightest and most stable. The heavier quarks are produced in high-energy collisions and rapidly decay into up and down quarks.
\r\n\tThe baryons and mesons known at the time fell into symmetric families of multiplets (octuplets, decuplets) sharing two identical quantum numbers (spin and parity), but differing in an ordered way in others (mass, charge, baryon number and strangeness). The mathematical group to fit this complex situation-SU3, the symmetric, unitary group of dimension 3-was proposed independently by Gell-Mann and Ne'eman. The validity of SU3 was demonstrated by the experiment. A major prediction was that a particle (the omega-minus), an isotopic singlet with spin = 3/2, positive parity, mass of roughly 1,680 MeV, negative charge, baryon number +1, strangeness = -3, and stable to strong decay, should exist to complete the 3/2+ baryon decuplet. It was therefore a major triumph for the scheme when the omega-minus, a baryon with the precise mass, charge, and strangeness predicted, was discovered in 1964. All these facts introduced a quark idea fully into modern physics.
\r\n\r\n\tThis book will be a self-contained collection of scholarly papers targeting an audience of practicing researchers, academics, PhD students and other scientists. The contents of the book will be written by multiple authors and edited by experts in the field.
",isbn:"978-1-83968-313-8",printIsbn:"978-1-83968-312-1",pdfIsbn:"978-1-83968-314-5",doi:null,price:0,priceEur:0,priceUsd:0,slug:null,numberOfPages:0,isOpenForSubmission:!1,hash:"0d9403b5c874f6e63b0686cd7c432e00",bookSignature:"Prof. Zbigniew Piotr Szadkowski",publishedDate:null,coverURL:"https://cdn.intechopen.com/books/images_new/10205.jpg",keywords:"Chiral Symmetries, Weak Interactions, Neutrinoless Double Beta Decay, Deep Inelastic Scattering, Quantum Chromodynamics (QCD), Color Confinement, Quarks Mixing, Cabibbo Angle, Kobayashi-Maskawa Matrix, Quarks Multiplets, CP-Nonconservation, Neutrino Oscillation",numberOfDownloads:56,numberOfWosCitations:0,numberOfCrossrefCitations:0,numberOfDimensionsCitations:0,numberOfTotalCitations:0,isAvailableForWebshopOrdering:!0,dateEndFirstStepPublish:"July 6th 2020",dateEndSecondStepPublish:"October 8th 2020",dateEndThirdStepPublish:"December 7th 2020",dateEndFourthStepPublish:"February 25th 2021",dateEndFifthStepPublish:"April 26th 2021",remainingDaysToSecondStep:"3 months",secondStepPassed:!0,currentStepOfPublishingProcess:4,editedByType:null,kuFlag:!1,biosketch:"The designer of the 2nd level trigger for the fluorescence detector and the designer of the 1st level trigger and the Front-End Boards for the surface detector of the Pierre Auger Observatory.",coeditorOneBiosketch:null,coeditorTwoBiosketch:null,coeditorThreeBiosketch:null,coeditorFourBiosketch:null,coeditorFiveBiosketch:null,editors:[{id:"67836",title:"Prof.",name:"Zbigniew Piotr",middleName:null,surname:"Szadkowski",slug:"zbigniew-piotr-szadkowski",fullName:"Zbigniew Piotr Szadkowski",profilePictureURL:"https://mts.intechopen.com/storage/users/67836/images/system/67836.jpeg",biography:"Dr. Szadkowski completed his Ph.D. with a thesis 'Quarks mixing in chiral symmetries SU4 x SU4 and SU6 x SU6”. Habilitation: „Triggers in the Pierre Auger Observatory: Designs, Implementation and the Impact on the Experimental Results”.\r\nDevelopment of the FPGA-based 2nd level trigger for 24 fluorescence detectors and 1st level trigger for 1660 surface detectors of the Pierre Auger Observatory, FPGA based filters suppressing radio-frequency interferences (RFI) in radio detector of Auger Engineering Radio Array, FPGA based triggers for the Auger surface detectors dedicated for a recognition of very inclined EAS induced by 'old” proton showers or 'young” neutrino showers.\r\nDr. Szadkowski has worked as a research scientist in Michigan Technological University, Associate Professor in College de France, Senior Wissenschaftler, Bergische Universität Wuppertal, and currently is working as the head of the Department of High-Energy Astrophysics and as an Associate Professor at the University of Łódź.",institutionString:"University of Łódź",position:null,outsideEditionCount:0,totalCites:0,totalAuthoredChapters:"4",totalChapterViews:"0",totalEditedBooks:"1",institution:{name:"University of Łódź",institutionURL:null,country:{name:"Poland"}}}],coeditorOne:null,coeditorTwo:null,coeditorThree:null,coeditorFour:null,coeditorFive:null,topics:[{id:"20",title:"Physics",slug:"physics"}],chapters:[{id:"73971",title:"The Inter-Nucleon Up-to-Down Quark Bond and its Implications for Nuclear Binding",slug:"the-inter-nucleon-up-to-down-quark-bond-and-its-implications-for-nuclear-binding",totalDownloads:56,totalCrossrefCites:0,authors:[null]}],productType:{id:"1",title:"Edited Volume",chapterContentType:"chapter",authoredCaption:"Edited by"},personalPublishingAssistant:{id:"247041",firstName:"Dolores",lastName:"Kuzelj",middleName:null,title:"Ms.",imageUrl:"https://mts.intechopen.com/storage/users/247041/images/7108_n.jpg",email:"dolores@intechopen.com",biography:"As an Author Service Manager my responsibilities include monitoring and facilitating all publishing activities for authors and editors. From chapter submission and review, to approval and revision, copyediting and design, until final publication, I work closely with authors and editors to ensure a simple and easy publishing process. I maintain constant and effective communication with authors, editors and reviewers, which allows for a level of personal support that enables contributors to fully commit and concentrate on the chapters they are writing, editing, or reviewing. I assist authors in the preparation of their full chapter submissions and track important deadlines and ensure they are met. I help to coordinate internal processes such as linguistic review, and monitor the technical aspects of the process. As an ASM I am also involved in the acquisition of editors. Whether that be identifying an exceptional author and proposing an editorship collaboration, or contacting researchers who would like the opportunity to work with IntechOpen, I establish and help manage author and editor acquisition and contact."}},relatedBooks:[{type:"book",id:"6768",title:"Cosmic Rays",subtitle:null,isOpenForSubmission:!1,hash:"1578350f18d0bc3abfbcf62278630739",slug:"cosmic-rays",bookSignature:"Zbigniew Szadkowski",coverURL:"https://cdn.intechopen.com/books/images_new/6768.jpg",editedByType:"Edited by",editors:[{id:"67836",title:"Prof.",name:"Zbigniew Piotr",surname:"Szadkowski",slug:"zbigniew-piotr-szadkowski",fullName:"Zbigniew Piotr Szadkowski"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"8356",title:"Metastable, Spintronics Materials and Mechanics of Deformable Bodies",subtitle:"Recent Progress",isOpenForSubmission:!1,hash:"1550f1986ce9bcc0db87d407a8b47078",slug:"solid-state-physics-metastable-spintronics-materials-and-mechanics-of-deformable-bodies-recent-progress",bookSignature:"Subbarayan Sivasankaran, Pramoda Kumar Nayak and Ezgi Günay",coverURL:"https://cdn.intechopen.com/books/images_new/8356.jpg",editedByType:"Edited by",editors:[{id:"190989",title:"Dr.",name:"Subbarayan",surname:"Sivasankaran",slug:"subbarayan-sivasankaran",fullName:"Subbarayan Sivasankaran"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"1591",title:"Infrared Spectroscopy",subtitle:"Materials Science, Engineering and Technology",isOpenForSubmission:!1,hash:"99b4b7b71a8caeb693ed762b40b017f4",slug:"infrared-spectroscopy-materials-science-engineering-and-technology",bookSignature:"Theophile Theophanides",coverURL:"https://cdn.intechopen.com/books/images_new/1591.jpg",editedByType:"Edited by",editors:[{id:"37194",title:"Dr.",name:"Theophanides",surname:"Theophile",slug:"theophanides-theophile",fullName:"Theophanides Theophile"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"3092",title:"Anopheles mosquitoes",subtitle:"New insights into malaria vectors",isOpenForSubmission:!1,hash:"c9e622485316d5e296288bf24d2b0d64",slug:"anopheles-mosquitoes-new-insights-into-malaria-vectors",bookSignature:"Sylvie Manguin",coverURL:"https://cdn.intechopen.com/books/images_new/3092.jpg",editedByType:"Edited by",editors:[{id:"50017",title:"Prof.",name:"Sylvie",surname:"Manguin",slug:"sylvie-manguin",fullName:"Sylvie Manguin"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"3161",title:"Frontiers in Guided Wave Optics and Optoelectronics",subtitle:null,isOpenForSubmission:!1,hash:"deb44e9c99f82bbce1083abea743146c",slug:"frontiers-in-guided-wave-optics-and-optoelectronics",bookSignature:"Bishnu Pal",coverURL:"https://cdn.intechopen.com/books/images_new/3161.jpg",editedByType:"Edited by",editors:[{id:"4782",title:"Prof.",name:"Bishnu",surname:"Pal",slug:"bishnu-pal",fullName:"Bishnu Pal"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"72",title:"Ionic Liquids",subtitle:"Theory, Properties, New Approaches",isOpenForSubmission:!1,hash:"d94ffa3cfa10505e3b1d676d46fcd3f5",slug:"ionic-liquids-theory-properties-new-approaches",bookSignature:"Alexander Kokorin",coverURL:"https://cdn.intechopen.com/books/images_new/72.jpg",editedByType:"Edited by",editors:[{id:"19816",title:"Prof.",name:"Alexander",surname:"Kokorin",slug:"alexander-kokorin",fullName:"Alexander Kokorin"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"1373",title:"Ionic Liquids",subtitle:"Applications and Perspectives",isOpenForSubmission:!1,hash:"5e9ae5ae9167cde4b344e499a792c41c",slug:"ionic-liquids-applications-and-perspectives",bookSignature:"Alexander Kokorin",coverURL:"https://cdn.intechopen.com/books/images_new/1373.jpg",editedByType:"Edited by",editors:[{id:"19816",title:"Prof.",name:"Alexander",surname:"Kokorin",slug:"alexander-kokorin",fullName:"Alexander Kokorin"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"57",title:"Physics and Applications of Graphene",subtitle:"Experiments",isOpenForSubmission:!1,hash:"0e6622a71cf4f02f45bfdd5691e1189a",slug:"physics-and-applications-of-graphene-experiments",bookSignature:"Sergey Mikhailov",coverURL:"https://cdn.intechopen.com/books/images_new/57.jpg",editedByType:"Edited by",editors:[{id:"16042",title:"Dr.",name:"Sergey",surname:"Mikhailov",slug:"sergey-mikhailov",fullName:"Sergey Mikhailov"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"371",title:"Abiotic Stress in Plants",subtitle:"Mechanisms and Adaptations",isOpenForSubmission:!1,hash:"588466f487e307619849d72389178a74",slug:"abiotic-stress-in-plants-mechanisms-and-adaptations",bookSignature:"Arun Shanker and B. Venkateswarlu",coverURL:"https://cdn.intechopen.com/books/images_new/371.jpg",editedByType:"Edited by",editors:[{id:"58592",title:"Dr.",name:"Arun",surname:"Shanker",slug:"arun-shanker",fullName:"Arun Shanker"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"878",title:"Phytochemicals",subtitle:"A Global Perspective of Their Role in Nutrition and Health",isOpenForSubmission:!1,hash:"ec77671f63975ef2d16192897deb6835",slug:"phytochemicals-a-global-perspective-of-their-role-in-nutrition-and-health",bookSignature:"Venketeshwer Rao",coverURL:"https://cdn.intechopen.com/books/images_new/878.jpg",editedByType:"Edited by",editors:[{id:"82663",title:"Dr.",name:"Venketeshwer",surname:"Rao",slug:"venketeshwer-rao",fullName:"Venketeshwer Rao"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}}]},chapter:{item:{type:"chapter",id:"67648",title:"Lead-Free Hybrid Perovskite Light-Harvesting Material for QD-LED Application",doi:"10.5772/intechopen.86836",slug:"lead-free-hybrid-perovskite-light-harvesting-material-for-qd-led-application",body:'Organic-inorganic perovskite light harvesting materials have attracted passionate interest in past few years and raised as one of the most promising solar cells with power conversion efficiency up to 23% as well as excellent performance in LED display devices with high color gamut and color purity [1]. For the photovoltaic (PV) application, perovskite materials were first used in 2009 and for QDs LED applications, perovskite material was used firstly in 2014 [2]. Perovskite-based devices are solution processable that makes it cost-effective and helps in large scale production. Presently, such kind of perovskite solar cells is now comparable to the commercially available solar cells including, silicon, CdTe, CuInGaSe, and GaAs [3]. Perovskite quantum dots (PQDs) also are the most research centered topic in the field of nano-materials and colloidal QDs due to its excellent optical behavior as explained earlier [4].
After long research and development, semiconductor QDs (CdSe, CdZnSe, InP, etc.) and OLEDs have emerged as the leading technologies in the field of display devices [3]. OLEDs have lit pixels, less energy consuming properties, superior contrast, and wider viewing angles. However, synthesis and fabrication process of OLEDs are very complicated that raise the price of devices. On the other hand, PQDs possess tunable, narrow band emission and high PLQY that significantly improve the performance in both EL and PL devices as explained earlier. Existing CdSe and InP QDs have taken decades to be suitable as the robust candidates for displays. Samsung QLED TVs have been in the forefront serving the premium market along with Chinese brands, TCL and Hisense. Samsung Display is building a pilot production facility for QD OLED, which will start production in 2019. Most research is centered on PQDs due to their high photoluminescence (PLQY), tunable band gaps, and narrow emission wavelength. With quantum confinement effects, the emission of PQDs can be controlled with size and ingredients. Hence, PQDs are useful in photovoltaic cells, laser applications, light-emitting diodes (LEDs), and bio-imaging. Promising color purity and emission tunability of QDs make them promising contenders for next-generation displays as a light converter (white-light LEDs) and active-mode QD-LEDs (QLEDs) [3, 5]. Additionally, PQDs work faster than other QDs due to the absence of a deep state, where the electron-hole pair does not immediately revert to its ground state. In this context, PQDs have become promising candidates for the next-generation of w-LEDs for lighting and other display applications [6, 7, 8, 9]. After the discovery of PQDs, more than 7000–8000 peer-reviewed literatures have been published. The fast rate of scientific research publication reveals the interest of the research community in the PQDs topic. Besides excellent performances of PQD technology, some challenges restrict the lead-containing perovskite materials for commercialization. Lead halide perovskite both bulk as well as nanoparticles/QDs are soft and very sensitive with humidity, moisture, light, air, temperature, etc. [10]. Therefore, the most apparent work necessity is to develop the new synthesis routes and new approaches of their more stabilization concerning ambient conditions, water, light, and temperature. Bromide-based perovskite has excellent performance, but the major shortcoming is the stability and presence of toxic element Pb. In this regards, a more in-depth understanding of the mechanism on the radiative and nonradiative recombination is necessarily required [11, 12].
Thus it is an open question that does the organic-inorganic perovskite-based device industry fabrication comes soon? The answer may be controversial because research community will say yes while some will say no because of the toxicity of Pb and long term stability of the materials. According to the U. S. EPA, the maximum amount of Pb2+ in air and water should not be more than 0.1 μg L−1 and 15 μg L−1 respectively [13]. Hence mass production of pure Pb-based perovskite may pollute the Earth due to its toxicity, long degradation lifetime and easy solubility in water. Therefore, we need to develop a new approach to explore environmental friendly Pb free or limited amount of Pb-based perovskite materials for optoelectronic applications. Thus these two main drawbacks of perovskite material restrict it in commercialization. Hence, the demand for nontoxic element-based perovskite materials continues to grow in the past few years [14]. Among these nontoxic alternatives, divalent tin cation has been considered as a right candidate in replacing Pb2+, and the application of tin-halide perovskites in optoelectronic devices has also been investigated. However, Sn2+ undergoes facile oxidation to its tetravalent state (Sn4+), creating a high defect density in the perovskite lattice. These defects would generate trap states in the middle of the band gap, leading to rapid non-radiative relaxation of the exciton. The highest PLQY value of tin-based perovskite nanomaterials can only reach 6.4%, which is still far inferior to the lead-based perovskite nanomaterials (~84%) [15]. Potential applications of Sn-based perovskite material in optoelectronic applications, significant efforts have been made recently to improve the photoluminescence properties of tin perovskite-based nanomaterials. Besides Sn, in place of Pb2+, Zn2+, Mn2+, Ge2+, Cu2+, and Bi3+ already have been investigated for less lead of lead-free perovskite materials for versatile applications. However, in the case of organic-inorganic halide PQDs, there is very less work reported and some research work based on Pb2+ substitution in only inorganic CsPbX3 perovskite systems are going on, and this will be the main focus for the present chapter.
Perovskites structure derives from the crystal structure of titanium calcium oxide (CaTiO3). This was the first discovered perovskite material in the year 1839 by German mineralogist Gustav Rose and structure of perovskite material was studied by Russian mineralogist Lev A. Perovski [16]. After this discovery, a huge amount of perovskite materials have been found from natural resources and as well as synthesized in the lab also. So perovskite materials can be categorized mainly into two categories, one is based on oxides (oxygen anion) and another based on halides (halogen anions). On the basis of this categorization, perovskite materials adopt ABO3 and ABX3 chemical formula, where A and B are the cations and X is halogens such as I, Br, Cl, or mixed halides. ABO3 structured perovskite is generally used for thermoelectric, superconductive and ferromagnetic applications which were discovered in the 18th century. On the other hand, halide-based perovskite material; CsPbX3 was first reported in 1958, and CH3NH3PbX3-based perovskite was seen in 1978. Thus ABX3 perovskite further can be categorized into two subparts, pure inorganic halide perovskite, and organic-inorganic halide perovskite. Mitzi et al. have first time reported the excitonic property of halide perovskites for light-emitting diodes (LEDs) and thin film transistors in the year 1990 [17]. Nowadays, most extensively studied materials are ABX3 structured perovskite semiconductor due to its excellent properties. In ABX3 chemical formula, A, belongs to monovalent inorganic cation such as K+, Rb+, Cs+ or monovalent organic cation like ethylammonium (CH3CH2NH3+, EA), methylammonium (CH3NH3+, MA+), formamidinium ((CH(NH2)+2), FA), B-site stands for divalent cation Pb2+, Sn2+, Ge2+, Ca2+, Sr2+, Ba2+ and X is anion (I−, Br− and Cl−). The properties of perovskite material can be easily tuned by changing the A, B or X site in ABX3 structure [18]. Generally, perovskite materials have cubic structure consists of close-packed AX3 sub-lattice with divalent B-site cations within the six-fold coordinated cavities. B-X bonding of perovskite governs the electronic behavior of perovskite semiconductors while A cation has no direct role for the electronic properties. But the size of the A cation may cause the distortions in symmetry of the material. Besides the cubic phase, perovskite has tetragonal and orthorhombic phase also. For example, MAPbI3 has a tetragonal structure while MAPbBr3 and MAPbCl3 have a cubic structure at room temperature. The symmetry of MAPbI3 perovskite also depends on the temperature and symmetry increases with increase in temperature such as, at a lower temperature it has octahedral symmetry, and at room temperature to (162.2–327 K) has tetragonal white at a higher temperature above 178.8 K shows cubic phase. FAPbI3 perovskite has higher symmetry than MA-based perovskites hence has been most widely investigated [19]. FAPbI3 perovskite has nearly cubic structure at room temperature and inert atmosphere. At open atmosphere, FA shows its yellow phase that is non-perovskite phase of this material, and it is not useful for optoelectronic devices because of the high band gap. This is the only drawback with FA material otherwise FA-based perovskite would exceed those of MA [20].
Every ABX3 structure cannot be a perovskite phase. There is some fixed criteria for the existence of perovskite structure: (1) charge neutrality: that means the charges of cation and anion should be equal. For example, for ABX3 structure, A and B cations have 1+, and 2+ charges (total = 3+) and X anion has 3− charges. So both cation and anion have equal costs, i.e., N(A) + N(B) = 3 N(X), here N belongs to the valence of the A, B and X ions; (2) the Goldschmidt tolerance factor, t and (3) octahedral factor μ. t and μ are the most potent factor that decides the existence and structure of the perovskite material. Tolerance factor t and octahedral factor μ are defined as the following equation:
where rA, rB, and rX are the ionic radii of A, B, and X, respectively. For the perovskite structure, the value of t and μ must be lies between given limit; 0.89 < t < 1.11 and 0.442 < μ < 0.895 [21]. Thus from the above formula, it is clear that the size of the A, B and X ions play an essential role for the perovskite materials. These days the Goldschmidt tolerance factor concept is beneficial for the finding the lead-free perovskite materials based on the ionic radii of implicated ions. Beside B-site replacement, there are too many possibilities of the other ion positions such as A as well as X-site. On the basis of this concept, Kieslich and co-workers have theoretically found out more than 600 hypothetical perovskites that have not been reported yet [22]. Such kind of study can help develop less lead of lead-free perovskite materials on the practical level for commercialization.
Organic-inorganic perovskite materials demonstrate a strong optical absorption and bandgap tuning, long diffusion length, high charge carrier mobility, ambipolar charge transport and high tolerance of defects. Strong light absorbing properties of the material ideally suited for solar cell devices. On the other hand, band tuning properties and a wide range of light emission in the visible range with narrow emission bandwidth (FWHM) suitable for the light emitting devices. Organic-inorganic perovskite materials have sharp optical band edge that indicates the direct bandgap and minimal disordered materials [23]. The bandgap of perovskite material can be controlled by their chemical compositions and crystallite size. For example, CH3NH3PbI3 (red emission), CH3NH3PbBr3 (green emission) and CH3NH3PbCl3 (blue emission) hybrid perovskites have band gap of 1.55, 2.3 and 3.1 eV, respectively [24].
The band gap energy levels of hybrid perovskite are determined by an anti-bonding hybrid state between the cation B-s and anion X-p orbitals. This state is related to the valance band maxima and conduction band minimum. Thus due to their unique ABX3 structure and compositional flexibility, optical properties can be tuned by varying elements at each site. The change in I/Br ratio in CH3NH3PbX3 perovskite can modify the bandgap from 1.55 to 2.3 eV, i.e., PL emission can be tuned red, orange and yellow. This band gap tuning mechanism is due to the hybridization of p and s orbitals such as Br-4p orbital is overlapped with the I-5p orbital and the Pb-6 s orbital. Similarly, with varying the concentration of Cl/Br, we can tune bandgap from 2.3 to 3.1 eV for hybrid perovskites [25]. Another approach is tuning of the band gap in perovskite structure is substitution of (CH3NH3)+ site (A) by Cs+, Rb+ or mixing of both Cs/other alkyl-ammonium cations. With changing the A cation, the bond length with B and X site is also changed that is responsible for the band gap tuning. CsPbI3, CH3NH3PbI3 and FAPbI3 has band gap of 1.73, 1.55 and 1.48 eV, respectively [26]. So from these values, we can see that the band gap of the material decreases with increasing the cation size. Besides A and X site, the band gap energy can be tuned with changing the Pb2+ (B) from other divalent cations like Sn2+, Mn2+, Cd2+, etc. [26].
Depending on the particle size, dimensions and morphology hybrid perovskite materials show different optical energy gap than their bulk counterparts due to their quantum confinement effect. For traditional nano-materials, quantum confinement effect has been widely studied. In recent times, the development of size-controlled perovskite QDs has enabled exhaustive research and developments of quantum confinement effect in QDs. Size of perovskite QDs can be controlled or tuned by ligand OLA. Friend et al. have studied on the size-dependent photon emission from CH3NH3PbBr3 PQDs, where the particle size and the PL emission peak could be tuned by varying the concentration of the perovskite precursors [27]. Quantum confinement effects have also been reported by Kovalenko group in all inorganic CsPbBr3 PQDs. They found that, with a decrease in the diameter of particle size from 11.8 to 3.8 nm, the PL emission peak gradually blue-shifted from 512 to 460 nm [28]. The thickness of the QDs also responsible for the band gap tuning. Thus trough band gap tuning, multi-colored LEDs and white LED can be formed by simple and easy way.
Perovskite quantum dots are considered as the most capable aspirant to the next generation of optoelectronic devices and solar cells technology. But the presence of toxicity due to Pb restricts the commercialization. Hence, currently, most of the research is going on less lead or Pb-free perovskites. The nontoxic and environmentally free perovskite can be prepared by substituting Pb with nontoxic elements that can be achieved through two approaches, i.e., homovalent and heterovalent substitution. Homovalent elements for lead substitution with an isovalent cation such as Sn2+, Mn2+, Cd2+, Zn2+, etc. while heterovalent are aliovalent cations like Bi3+, Sb3+. In this work, we will only discuss homovalent cation substitution of PQDs.
For the Pb-free perovskite materials, many homovalent elements with +2 stable oxidation states can be used. For example, the group- 14 elements Sn2+ and Ge2+, i.e., the same group of Pb2+ can be the best choice for lead substitution. Beside this, transition metals as Cd2+, Mn2+, Fe2+, Cu2+ and Zn2+; alkaline-earth metals such as Ba2+, Sr2+ and Ca2+; rare-earth elements like Eu2+, Yb2+ can be considered for the Pb-free perovskites [29]. Theoretically, these Pb-substitutes are perfect for perovskite structure according to the tolerance factor calculations. But some elements like Ba, Sr, and Ca have a large band gap, so these are not suitable for semiconducting materials. However, working with Cu and Zn in ambient condition will be difficult. In recent years, Sn2+ and Mn2+ are the promising candidates for homovalent substitution in perovskite quantum dots.
The most suitable Pb-substitute is Sn2+ for lead-free perovskite because of the around same ionic radii, binding energy and the same electronic configuration of s2 valence [29]. MASnI3 is the most studied lead-free perovskite material. Sn-based perovskites are direct bandgap semiconductors, like Pb-based perovskites. This means, the valence band maximum and conduction band minimum recline at the same position in k-space (reciprocal space). Similar to the APbX3 structure, the structural, optical and electrical properties of ASnX3 perovskite is also affected by the size of A-site cation and X-site anion. MASnI3 perovskite also has a tetragonal structure, but the band gap is near unity that is lower than those of the MAPbI3 [30]. Due to the quantum confinement effect, PQDs shows different behavior from their bulk counterpart.
In case of the pure inorganic perovskite CsPbI3, band gap of PQDs is 1.85–1.97 eV. Such kind of higher bandgap highly encouraged to find some lower Pb-containing, low band gap, and more stable perovskite material. CsSnI3 pure inorganic can be the satisfied the all requirements for the good PQDs because of its lower binding energy and broad absorption from visible to near infrared region. Due to this different behavior, such kind of perovskite can exhibit higher light harvesting efficiency than other perovskite materials [31]. However, the stability of the Sn2+-based perovskite material is the biggest issue due to higher sensitivity to air, moisture and sometimes with nonpolar organic solvents also [32]. In recent years, many significant approaches have been tried for the stability of the Sn2+ perovskites. For example, SnBr2, SnCl2, and SnF2 had used as a dopant into the CsSnI3 perovskite for the stability of the materials. In all, depends, SnF2 and SnCl2 were found more suitable for the air resistive property. However, stability of the Sn-based material was not satisfactory due to the degradation of the sample within only 3hs [33, 34, 35].
In the year 2017, Liu et al. reported, the alloyed CsSn1−xPbxI3 PQDs, which was more phase stable than its parent CsPbI3 and CsSnI3 PQDs for months in ambient air [36]. They have prepared CsSn1−xPbxI3 PQDs via the standard Schlenk line method under a nitrogen atmosphere. In this method, SnI2/PbI2 mixture was rapidly dissolved in trioctylphoshine into the cesium olate at 120–170°C and purified by antisolvent methyl acetate (MeOAc) washing process. The 140°C reaction temperature was found the best for homogeneous and proper morphology of CsSn1−xPbxI3 PQDs. With the incorporation of Sn, red shifting occurred that can be observed in absorption and PL spectra as shown in Figure 1a.
(a) Optical absorption spectra of different Sn doped PQDs and the inset fig shows their corresponding normalized steady photoluminescence spectra, (b) time-resolved photoluminescence decay for various stoichiometries of Sn, (c) X-ray diffraction patterns and (d) XPS spectra for the CsSn0.60Pb0.40I3 PQDs (as prepared) and those after storage for 150 days.
Additionally, the increase in Sn concentration drastically enhanced the light absorption of CsSn1−xPbxI3 PQDs; however, PLQY was found up to 3% only. On the other hand, CsPbI3-based PQDs had up to 100% efficiency to date. Low PLQY with Sn-based PQDs indicates the increase in intrinsic defects that were allied with Sn vacancies. Such kind of vacancies creates deep level defects which act as non-radiative recombination centers and hence this reduces the efficiency of PQDs. Figure 1b also suggested the average lifetime of CsSn1−xPbxI3 PQDs dramatically decreases with increasing the concentration of Sn. This behavior of Sn containing materials also confirms the occurrence of a large density of quenching defects that support the lower PLQY of the PQDs. Liu also reported that CsSn0.60Pb0.40I3 PQDs was best for performance and stability because of the less- Pb content and widest range of light absorption among the alloyed PQDs. Phase, as well as chemical Stability of CsSn0.60Pb0.40I3 PQDs, is confirmed via XRD and XPS analysis (see Figure 1c and d). XRD Figure 3b suggested that there is no degradation was absorbed up to 5 months that is the first time reported such long time stability with Sn-based material.
Furthermore, there was not found Sn4+ oxidation stage in the XPS analysis of the sample. In Figure 1d XPS graph showed 2 spectra at 486.2 and 494.5 eV that is associated with Sn2+ 3d5/2 and 3d3/2 state, while deficiency of spectra at 486.9 indicates the absence of Sn4+ in perovskite compound. Thus this less lead-containing and air-stable Sn-based PQDs can be the best choice for solar cell devices but for highly efficient LED there will be a need in the improvement of its PLQY. Jellicoe and co-workers had also tried for the Sn-based perovskite, CsSnX3 (X = Cl, Br, I) for the lead-free perovskite PQDs but due to the instability of Sn it cannot be used for practical applications [35]. To overcome the stability problem, Wang group had replaced Sn2+ with Sn4+ and developed stable CsSnI6 PQDs with cubic structure. However it’s PLQY was just 0.48% that is also not sufficient for optoelectronic applications [36]. From the several studied we absorbed that, CsSnI3-based perovskites have low PLQY but good hole conducting properties. Hence it is mostly used for solar cell applications. For sufficient PLQY and more stable Sn-based material was still needed for display devices. For this purpose, CsSnBr3-based perovskite was found more suitable candidate than iodine-based perovskites.
Yang group have reported the fast and less Pb or Pb- free perovskite nanocrystals by introducing the Sn in Pb site and I at Br site [37]. They have used one pot hot injection method for preparation of SnI2 doped CsPbBr3 perovskite nanocrystals. After cation and anion exchange between the CsPbBr3 and SnI2, the lead-free CsSnI3 NCs were obtained. These Pb-free NCs maintained the high PLQY as well as morphology in an inert atmosphere. Figure 2a indicated that, with increasing the amount of SnI2 into CsPbBr3 colloidal solution, dramatically red shifting observed in PL emission and absorption spectra. There was one fascinating result was reported regarding the stability of Sn2+. XRD result predicted that Sn2+ converted into Sn4+ and formed the small amount of Cs2SnI6 phase that indicated the cation as well as anion exchange in the sample. After cation exchange between Pb2+ and Sn2+ and anion exchange between Br−1 and I−1, the PLQY of CsSnI3 and CsPbBr3 NCs was found 59.1 and 73.47% respectively. From this result, the author concluded that the ion exchange did not cause the structural and surface defects. Figure 4b shows the schematic representation of the cation and anion exchange in CsPbxSn1−x(BryI1−y)3 NCs. Halide anions have high migration and the sufficient vacancy diffusion ability that is why after incorporation of SnI2 into CsPbI3, halide anions reacts very fast [38]. On the other hand, cation takes more time to reaction with parent material due to the obligation of high activation energy. Moreover, in most of the cases, cation substitution is obsessed with the concentration of halide vacancies [39]. Thus with this cation and anion approach creates a possibility of higher PLQY of Pb-free perovskite NCs.
(a) UV-Visible and PL spectra of different Sn doped CsPbBr3 NCs (A1–6); the colored image of PQDs under 365 nm UV radiation and (b) schematic diagram of the ion exchange process in SnI2 doped CsPbBr3 NCs.
The 100% Pb replacement with any other divalent cation is impossible till date because of the lower conductivity of substitute element like Sn. In the case of the Sn lower ionic conductivity of PQDs causes many surface defects that make material unstable. Some of the Pb-free perovskite with Sn4+ support that instability and toxicity problem can be resolve by partial substitution of Pb2+ with Sn4+. In this regards, Liu et al. have reported the less lead-containing CsPb1−xSnxBr3 PQDs with PLQY up to 83% [40]. They have performed cation exchange via replacing Pb2+ cation with Sn4+ cation using hot injection method. Partial replacement with Sn4+ in CsPb1−xSnxBr3 exhibited the enhancement in PL emission and after some time increasing the more concentration of Sn4+ ion, decreases the PL performance due to the increasing the impurity phase of Cs2SnBr6. In this work, they found CsPb0.67Sn0.33Br3 was the best composition, and PQDs of material, displayed the high external quantum efficiency and current efficiencies. Figure 3a Shows the variation of PL spectra of CsPb1–xSnxBr3 PQDs and digital picture in UV irradiation (365 nm). This PL spectrum show the highest PL intensity for x = 33% while increasing the concentration of Sn; PL intensity starts to decrease due to impurity and more surface defects. Figure 3b shows the absorption and PL emission spectra of samples that indicated the absence of blue shift that means there is no big effect on band gap after Sn4+ incorporation, but exciton recombination can affect the lifetime and lower PLQY with a higher concentration of Sn4+. A higher concentration of Sn4+ also affected the size; morphology and structure of the QDs (see Figure 3c–g). After x = 0.33 Sn4+ amount in CsPbBr3, the impure phase can be seen in the XRD spectrum and irregular shape and agglomerated particles in TEM due to Cs2SnBr6 byproduct.
(a) Variation in PL intensity of different Sn4+ doped CsPbBr3 PQDs. Inset of samples under 365 nm UV light (b) UV-Visible absorbance and PL spectra of CsPb1−xSnxBr3 (c) XRD pattern of CsPb1−xSnxBr3 PQDs, impurity peaks are shown by stare sign. (d–g) TEM images with (d) x = 0, (e) x = 0.33, (f) x = 0.50 and (g) x = 0.67.
Mn2+ is a transition metal cation, and doping of Mn2+ in II-VI semiconductors has been widely investigated due to its excellent optical, electrical and magnetic properties. All ready many studied have been done on Mn2+ doped ZnS, CdSe, CdS, and ZnSe inorganic QDs in which Mn2+ incorporation can enhance the long-lifetime and interaction between host d-electron of Mn dopants [41]. In compare to the traditional II-VI group semiconductors, the soaring tolerance in perovskite can be the better candidate for promoting the exciton energy transfer to Mn d-d emission to Mn d-state.
The band gap of Mn2+ doped in perovskite materials depends upon the presence of halide anions in a host as well as a dopant. For example, change from Cl− to Br− to I−, the PL emission of perovskite is tuned from blue region to red. In the year 2018, Zhao et al. have reported the Mn2+ doped CsPbCl3 NCs and after that intensive exploration are carried on this work [42]. After incorporation of Mn2+cation in place of Pn2+ site, the XRD spectra shied towards higher angle due to the smaller radii of Mn2+ while PL emission shifted towards higher wavelength because of the d-d transition. Hence undoped CsPbCl3 NCs shows blue emission, but after doping of Mn2+, it tuned into orange emission as shown in Figure 4a–c. It was reported that Cl-based Mn dopant and perovskite host is the best rather than other manganese (II) salts [43]. Via hot injection method, it is tough to doping of Mn2+ into CsPbBr3 and CsPbI3 perovskite structure but through the anion exchange process Mn doping is possible. Such kind of possibility depends upon the bond strength and dissociation energy between Mn-X and Pb-X bond. Figure 4d shows the band gap energy diagram for Mn-doped different perovskite host [44]. As we know Cl-based perovskite NCs has very low PLQY (<5%) in compared to Br- and I-based perovskites. Besides reduction in toxicity, Mn2+ doping helps to enhance the PLQY and lifetime of the perovskite NCs. In this regards, Liu and coworkers fabricated CsPbxMn1−xCl3 PQDs via hot injection method [45]. Mn2+ incorporation in CsPbCl3 not only increases the PLQY of host material from 5 to 54% but also created an additional intense PL emission peak at 580 nm that is stand for bright orange color (see Figure 5a). With increasing the Mn2+ concentration secondary, PL peak gradually shifted from 569 to 587 nm. The highest PLQY (54%) was found for the 46% Mn-doped CsPbCl3 perovskite. The various Mn2+ compositions with tunable emission are shown in schematic Figure 5b. CsPbxMn1–xCl3 PQDs has two emission peaks with two different lifetime values such as 13.9 ns at 390 nm and 1.6 ms at 580 nm as shown in Figure 5a and b. The long decay time for second emission peak near 580 nm is assigned to emission from d-d transition energy transfer mechanism for Mn2+ doped semiconductors. Hence Mn2+ doping also increases the overall lifetime of PQDs that may be an improvement in the stability of PQDs also. A similar study was carried by Yuan et al. in which they have obtained PLQY of up to 60% for 14.6% Mn2+ doped CsPbCl3 [46]. With the help of ligand assisted room temperature method (LART), same work was reported by Zhu group with maximum 41.6% PLQY for 28% Mn2+ incorporation in place of Pb2+ [47]. Through LART, Mn2+:CsPbCl3 can be prepared in concise time duration and room temperature that reduce the cost of experiment, easy, simple method and high production of products.
(a–d) Digital images of CsPbCl3, CsPbBr3, CsPbI3 and Mn: CsPbCl3 nanocrystals under 365 nm UV radiation and (e) band positions of CsPbX3 and Mn2+ d-state.
(a) UV-Visible and PL spectra of CsPb0.54Mn0.46Cl3 PQDs (b) schematic illustration of Mn incorporation in CsPbCl3 PQDs by different molar ratio and reaction temperature, (c and d) PL lifetime of CsPbCl3 and CsPb0.73Mn0.27Cl3 PQDs.
In some cases, Mn2+ is beneficial for stability of CsPbX3 PQDs. Zou et al. have found that Mn2+ incorporation in place of Pb2+ not only reduced toxicity while also enhance the thermal as well as air stability of the perovskite samples [48]. Similar to previous work, Zou group also have worked with CsPbCl3: Mn2+ PQDs and studied different structural and optical enhanced properties. Besides this work, they have also provided experimental data for CsPbBr3: Mn and CsPbI3: Mn PQDs to prove the absence of energy transfer in Br and I containing perovskites. Figure 6(a–c) show the comparison of PL spectra for CsPbCl3: Mn, CsPbBr3: Mn and CsPbI3:Mn PQDs with various Mn2+ concentrations. In this case, PL emission intensity first increases with increasing the Mn2+ concentration and then decreases with further increasing doping concentration which suggests that the Mn2+ ions have a noteworthy impact on the optical properties of PQDs. Presence of additional peak in CsPbCl3: Mn, is the proof of energy transformation, on the other hand, there is no peak shift in Br and I containing PQDs samples due to the mismatch between their optical bang gap absorption and 4T1 → 6T1 transition of Mn2+ [48]. PLQYs of the Cl-based increases and in Br and I increases up to 20% Mn2+ doping and after that dramatically decreases with increasing the Mn2+ concentration (see Figure 6d–f) while lifetime of CsPbBr3: Mn2+ continuously decreases as shown in Figure 6g. Mn2+ doping in CsPbBr3 and CsPbI3 PQDs also enhance air stability as demonstrated in Figure 6h and i. Figure 6h indicates, the degradation of pure CsPbBr3 PQDs after 30 days while Mn2+ doped PQDs is stable up to 120 days while 50%Mn: CsPbI3 PQDs was found stable up to 4 days. As we know CsPbI3 PQDs are unstable in the air, so it is challenging of use for optoelectronic devices. Enhanced stability of Mn: CsPbI3 materials may open the new door for stable red PQDs. The same methodology was reported by Manna group by theoretical and experimental approach also [49]. They reported that unstable CsPbI3 perovskite could be stabilized by incorporation the 10% of Mn2+ and there were no major changes in structural and optical properties.
Comparison of photoluminescence (PL) emission spectra for (a) CsPbCl3:Mn, (b) CsPbBr3:Mn and (c) CsPbI3:Mn QDs doped with different Mn2+ contents (d) absolute PL quantum yields (QYs) for CsPbCl3:Mn QDs doped with different nominal Mn2+ contents ranging from 0 to 80 mol% (e) absolute PL QYs for CsPbBr3:Mn and CsPbI3:Mn QDs doped with different nominal Mn2+ contents ranging from 0 to 60 Mol% (f and g) PL decay curves (left) and lifetimes (right) for excitonic luminescence of CsPbBr3:Mn QDs upon excitation by a 397-nm pulsed laser (h) PL emission photographs for CsPbBr3:Mn QDs coated on the surface of a glass slide with different Mn2+ contents from 0 to 6.2 mol% taken under UV irradiation at indicated time periods and (i) red PL emission photographs for CsPbI3:Mn QDs with different nominal Mn2+ doping concentrations of 0, 20, 40, 50, 60, 80, 100 mol% from left to right respectively, taken at daylight or under UV irradiation at indicated time period.
Cd and Zn2+ Cation exchange in traditional NCs have been extensively studied, but for perovskite material, there are few reports are present till date. A detailed study of cation exchange has not yet been explained, but halide exchange has been explained well. Halide exchange is easier than cation exchange because of low activation energy and diffusion of anion vacancies in the perovskite materials. Stam et al. have experimentally proved that cation exchange in perovskite takes long time [50]. They reported the cation exchange such as Sn2+, Cd2+, and Zn2+ in place of Pb2+ in CsPbBr3 perovskite host. 10% cation exchange in perovskite system results in the reduction in toxicity due to lead and also maintained the good PLQY as well as high color purity. But with these, cation exchange, the blue shift was observed in PL emission due to smaller ionic radii of Cd and Zn than Pb (as shown in Figure 7k). On the other hand, there is very less shift was obtained in Sn2+ due to similar ionic radii as Pb. The Cd2+ doping in CsPbBr3 PQDs was resulting in PL emission variation between 452 and 512 nm while Zn showed between 462 and 512 nm. Furthermore, over 60% of PLQYs was obtained for cation doped PQDs and good stability in ambient conditions. Cation exchange in PQDs reduced the PL lifetime of the CsPbBr3 PQDs. Figure 7a–j TEM and energy dispersive X-ray spectroscopy mapping of Cd2+ and Zn2+ doped CsPbBr3 PQDs. EDS mapping helped to identify the presence of different doped elements in PQDs. Figure 7a–e shows the Cd and Figure 7f-jshows the Zn doped CsPbBr3 PQDs. These mapping analyses also predicted the uniform distribution and lower concentration of dopant into the host material. Zn2+ incorporation enhanced the stability of CsPbI3 black phase in the air also due to improvement in lattice contraction and alloy form of perovskite.
Energy dispersive X-ray spectroscopy mapping of CsPb1−xCdxBr3 and CsPb1−xZnxBr3 nanocrystals. (a) HAADF-STEM image of CsPb1−xCdxBr3 NCs and corresponding maps of (b) Cs, (c) Pb, (d) Br, and (e) Cd, demonstrating the presence of Cd in the perovskite NCs. The inset in panel shows a photograph of a colloidal suspension of the NCs under UV illumination. (f) HAADF-STEM image of CsPb1−xZnxBr3 NCs and the corresponding maps of (g) Cs, (h) Pb, (i) Br, and (j) Zn, indicating the presence of Zn in the perovskite NCs. The inset in panel f shows a photograph of a colloidal suspension of the NCs under UV illumination (k) parent CsPbBr3 NCs (green lines) and product NCs obtained after reaction with different concentrations of CdBr2 (orange lines) and ZnBr2 (blue lines).
As it is well reported that lower PLQYs of Cl-based perovskite materials due to the large band gap. There are many have published on Mn2+ doped CsPbCl3 PQDs, but with doping of, we get orange color so in this case to obtain pure blue color is impossible. On the other hand, other heterojunction-based PQDs such as CH3NH3Bi2X9, Cs3Bi2Br9 and Cs3Sb2Br9 have been developed for blue emission [50]. But still, these blue color emitted PQDs have 46–52% and very unstable that is not suitable for blue LEDs fabrication. To overcome this issue, Mondal et al. reported the effect of Cd incorporation in CsPbCl3 PQDs by hot injection method to enhance the stability and PLQYs of blue color PQDs [51]. For CdCl2 treatment, CsPbCl3 colloidal solution with CHCl3 was mixed with CdCl2 solution (CdCl2 solution was prepared in ethanol) and sonicated for 2–3 min. The obtained colloidal solution was purified by centrifugation with methyl acetate treatments. The CdCl2 treatment of CsPbCl2 dramatically enhanced the from 3–96% PLQYs and narrow PL emission spectra that also indicates the higher color purity of the treated sample.
There are the best thing is that there was no PL or absorbance spectrum shifting observed with CdCl2 treated samples as shown in Figure 8a. From Figure 8b also indicates the huge improvement in PL emission and bright blue emission under UV light radiation compared to untreated CsPbCl3 PQDs. Generally, with doping of Cd2+ or other lower atomic radii elements than Pb2+, has PL emission shift towards blue region due to lower ionic radii. But in this work, the authors did not get any shifting. CdCl2 incorporation not only enhances the PLQYs while it also improved the lifetime and stability of CsPbCl3 PQDs by the four times of the untreated PQDs (Figure 8c). A similar strategy was applied with CsPbBr3 PQDs and improvement in PLQY and lifetime also absorbed in green PQDs also (Figure 8d–f). Thus this work suggests the Pb2+ cation exchange with Cd2+ not only provide high PLQYs while long term stability without disturbing any peak shifting. So such kind of blue and green PQDs can be beneficial for good quality of blue, green or white LEDs and a backlight system. Thus Cd and Zn doped PQDs may be useful for different optoelectronic applications similar to pure PQDs due to its low toxicity and excellent properties.
UV-Visible absorption and PL spectra (a) of the CsPbCl3 NCs before and after the CdCl2 treatment. PL spectra showing dramatic enhancement of PL upon treatment (b) the inset shows the untreated (i) and treated samples (ii) under a UV lamp. (c) PL decay dynamics (λex = 375 nm, λPL = 406 nm) of CsPbCl3 NCs before and after the treatment with CdCl2. PL spectra (d) and PL dynamics (e) of CsPbBr3 NCs before and after the CdBr2 treatment. (f) Change in PL on addition of CdCl2, starting from treated CsPbBr3 NCs (left) to increased addition of CdCl2 (right).
Lead halide-based perovskite QDs, as a promising light-harvesting material for light absorbing and converting light energy, have attracted research community as well as industrialists due to unique properties of PQDs for solid-state lighting (SSL) and flexible color tuning thin film display application. In recent years, organic LED (OLED) and quantum dot LED (QDLED) join the competition of display market and mean to wrest the dominance from LCD. OLED has the advantage of self-luminous, large area fabrication, fast response time, high contrast and application on flexible substrates [2, 3, 4]. Due to such type of benefits, many manufacturers have invested in the development of OLEDs, and now various products based on OLEDs have been commercially available. However, there are some limitations with OLEDs like wide PL emission wavelength, instability of organic molecules, etc. Due to this issue, it is difficult to achieve high color purity and a high-quality full-color display. So it is still required to develop a new variety of display technology with high color purity and PLQY to meet the higher demand of consumers. To overcome the less color purity problem, quantum dots can be used because of its excellent luminescence and color characteristics as we have already discussed in the previous section. There is a chance to achieve the high-resolution color contrast and a better full-color display device in near future moreover in contrast to another semiconductor (inorganic) employed in solid state lighting device perovskite material have also benefited regarding the synthesis and manifesting process. PQDs can also fabricate at low temp, solution processed approach to precisely control the size, shape, and purity of perovskite material which makes PQDs ideal candidates for the display device. In addition, due to the colloidal solution characteristics, PQDs is cheaper to use, easier to process and easier to fabricate in large area, large scale production thus PQDs are often touted as disruptive material that could completely replace traditional inorganic phosphor LEDs or OLEDs, QLEDs can be mainly divided into types one is photoluminescence QD LEDs (based on photoexcited), and another is electroluminescence (EL) QDLEDs (electron- excited QDs) [3]. PL QDLEDs is the most commonly used a type of LEDs. QDs LEDs usually have applications in the high-quality display, high resolution, and high contrast display device as well as in lighting application. For commercialization, much less lead-containing perovskite QDLEDs have been investigated in recent years. Lead substituted perovskites also used for PL LEDs and electroluminescence LEDs. Zou et al. fabricated the EL-based LEDs using CsPbBr3: Mn perovskite QDs [48]. Active LEDs was fabricated by spin coating of PEDOT: PSS on ITO glass substrate as a hole transporting layer, Mn-doped CsPbCl3 as the active layer and TPBi, LiF/Al were used as electrodes by thermal evaporation technique. The schematic illustration of the device is shown in Figure 9a. This active led produced electroluminescence with green color (512–515 nm) for CsPbBr3 and Mn-doped perovskites with high color purity and narrow emission spectrum FWHM of 20 nm (see Figure 9b). There were no significant shifting observed in EL as well as in current density-voltage properties (Figure 9c) of Mn-doped perovskites. Mn doping enhanced the luminescence of LEDs by 1.3 times of the pure PLEDs and EQE and CE also improved from 0.81% and 3.71 cd/A for the CsPbBr3 PLED to 1.49% and 6.40 cd/A for the 3.8 mol% Mn-doped CsPbBr3 PLED device. Similarly, Liu group also fabricated lead substituted active PLEDs with doping of 33 mol% Sn4+ ion consisting of the same device structure as Zou reported [40]. They reported that the presence of Sn4+ helped in the easy injection of charge carriers that is responsible for small turn off voltage as well as large current density. CsPb0.67Sn0.33Br3 PLEDs showed 4.13% EQE, 12500 cd m−2 luminescence, 11.63 cd A−1 of CE, 6.76 lm W−1 and 3.6 V turn-on voltage. Authors have also claimed that such kind of high performance is the best results found in Sn-based PLEDs. So this lead substituted Sn-based PQDs LED can be suitable for different active LEDs as well for backlight-based display devices. On the other hand, CsPbCl3: Mn PQDs has also been fabricated in PLEDs with 2.2 lm W−1 of luminous intensity (Figure 9d–f) and good stability after continuous applying 3.5 V of voltage for 200 h [45]. 27 mol% Mn: CsPbCl3 was mixed with curable resin and coated on 365 nm commercial UV-LED chip for the PL PLED device. Due to d-d transition, Mn doped CsPbCl3 PLED gives bright orange emission with 54% PLQY in ambient conditions. In most of the optoelectronic devices, photo and thermal stability are the crucial issues. So for improving such kind of stability SiO2 coating and KCl/polystyrene or other inorganic materials has been used for encapsulation of perovskite LEDs [42].
(a) Schematic illustration of a typical multilayer-structured PLED device by using pure CsPbBr3 and CsPbBr3: Mn (2.6 mol%) and CsPbBr3: Mn (3.8 mol%) QDs are used as green light emitters. (b) Comparison of normalized EL spectra at an applied voltage of 6 V and their corresponding PL emission spectra for CsPbBr3 and CsPbBr3: Mn (2.6 mol%) and CsPbBr3: Mn (3.8 mol%) QDs when dispersed in cyclohexane solution. The inset shows a photograph of the EL of a representative PLED device. Current density (c) and luminance characteristics for three types of PLEDs based on the pure CsPbBr3 and CsPbBr3: Mn (2.6 mol%), and CsPbBr3: Mn (3.8 mol%) QDs (d) fluorescent image (e), PL emission spectrum (f) CIE chromaticity coordinate of the LED from CsPbxMn1−xCl3 QDs. Inset: Optical image of the LED.
Lead halide-based perovskite material opens a new opportunity in optoelectric since 2012, because of their excellent optoelectronic properties and high power conversion efficiency within a brief time. The big challenge for the perovskite QDs is the presence of toxicity due to lead. Already many groups have worked on leadless or lead-free perovskite materials. 100% replacement of Pb will be complicated but with replacement up to 60–70%, Pb with non-toxic cation may help to the commercialization of less lead-containing perovskite materials. Shortly, less lead or lead-free PQDs have the potential for next-generation optoelectronic devices like backlights, QD TV, flexible and wearable devices, etc. However, to achieve high PLQY, good performances, and stability of lead-free or less lead containing PQDs also be challenging for the research community. Many homovalent lead substitutions like Sn, Mn, Cd, and Zn have been successfully done in place of Pb in different halide containing perovskite nanomaterials. Such kind of replacement of lead not only reduces the toxicity of perovskite while it also improves the performances and stability of the perovskite light-harvesting material. In the halide-based perovskites, dopant engineering is a compelling strategy to tuning the optical, structural, and electrical properties and day by day new depends arrive that make the PQDs more suitable for lead-free perovskite devices. Mn and Cd incorporation in Cl-based perovskite NCs was increased the PLQYs but for Br- and I-based PNCs it is still challenging to get high PLQYs than pure PQDs. So, we need to find a more suitable strategy for less lead-containing high PLQYs perovskite material.
Three-dimensional (3D) shape measurement techniques are widely used in many different fields such as mechanical engineering, industry monitoring, robotics, biomedicine, dressmaking, among others [1]. These techniques can be classified as passive, like in stereo vision in which two or more cameras are used to obtain the 3D reconstruction of a scene, or as active, like in fringe projection profilometry (FPP) in which a projection device is used to project a pattern onto the object to be reconstructed. When compared with other 3D measurement techniques, FPP has the advantages of high measurement accuracy and high density. There are two types of FPP methods: phase shifting and Fourier-transform profilometry (FTP). Phase-shifting methods offer high-resolution measurement at the expense of projecting several patterns onto the object [2, 3, 4], whereas FTP is popular because only one deformed fringe pattern image is needed [5]. For this reason, FTP has been used in many dynamic applications [6] such as vibration measurement of micromechanical devices [7] and measurement of real-time deformation fields [8].
FTP was proposed by Takeda et al. [5, 9] in 1982 and has since become one of the most used methods [3, 10]. Its main advantages are full-field analysis, high precision, noise-robustness [11], among others. In FTP, a Ronchi grating, or a sinusoidal grating, or a fringe pattern from a digital projector is projected onto an object, and the depth information of the object is encoded into the deformed fringe pattern recorded by an image acquisition device as shown in Figure 1. The surface shape can be decoded by calculating the Fourier transform, filtering in the spatial frequency domain, and calculating the inverse Fourier transform. Compared with other fringe analysis methods, FTP can accomplish a fully automatic distinction between a depression and an elevation of the object shape. It requires no fringe order assignments or fringe center determination, and it needs no interpolation between fringes because it gives height distribution at each pixel over the entire field. Since FTP requires only one or two images of the deformed fringe pattern, it has become one of the most popular methods for real-time 3D reconstruction of dynamic scenes.
Fringe projection system.
Although FTP has been extensively studied and used in many applications, to the best of our knowledge a complete reference in which the implementation details are fully described is nonexistent. In this chapter, we describe the FTP fundamentals and the implementation of an FTP system in LabVIEW one of the most used engineering development platforms for data acquisition and laboratory automation. The chapter is organized as follows. In Section 2 we describe the FTP fundamentals and a general calibration method, in Section 3 we describe how FTP is implemented in LabVIEW, and finally in Section 4 we show three applications of FTP for 3D reconstruction.
There are many implementations of FPP. However, all share the same underlying principle. A typical FPP setup consists of a projection device and a camera as shown in Figure 1. A fringe pattern is projected onto a test object, and the resulting image is acquired by the camera from a different direction. The acquired fringe pattern image is distorted according to the object shape. In terms of information theory, it is said that the object shape is encoded into a deformed fringe pattern acquired by the camera. The object shape is recovered/decoded by comparison to the original (undeformed) fringe pattern image. Therefore, the phase shift between the reference and the deformed image contains the information of the object shape.
By projecting a fringe pattern onto the reference plane, the fringe pattern (with period
Likewise, when the object is placed on the reference plane, the deformed fringe pattern observed through the camera is given by
where
with
where
Principle of the filtering via Fourier transform (FT) method. IFT, inverse FT.
Next, the phase of the fringe patterns is recovered using the Fourier Transform method. Using one-dimensional notation for simplicity, when we compute the Fourier transform of Eqs. (1) and (2) the Fourier spectrum of the fringe signals splits intro three spectrum components separated from each other, which gives
as shown in two dimensions in Figure 2. With an appropriate filter function, for instance, a Hanning filter, the spectra are filtered to let only the fundamental component
where
The variable related to height distribution is the phase change
with
where
The calibration of FPP systems plays an essential role in the accuracy of the 3D reconstructions. Here we describe a simple yet extensively used calibration called the reference-plane-based technique, i.e., to convert the unwrapped phase map
The optical axis geometry of the FTP measurement system is depicted in Figure 3. The optical axis
Fringe projection system.
The triangles
Combining Eqs. (12) and (13) a proportional relation between the phase map and the surface height can be obtained for any point
where
where
We have shown how the object surface height is related to the recovered phase through FTP. The model described by Eq. (15) has many underlying assumptions and is often extended to cover more degrees of freedom. Moreover, a general calibration process in FPP can be carried out employing the methodology shown in Figure 4. First, we propose a model that best describes the system, while also considering metrological requirements such as speed, robustness, accuracy, flexibility and reconstruction scale. Some authors have proposed to use several calibration models based on polynomial or fractional fitting functions [13, 14], bilinear interpolation by look-up table (LUT) [15] and stereo triangulation [16, 17, 18]. These calibration models require different strategies or techniques that allow relating metric coordinates with phase values. In step II, we select or design a strategy that fits the proposed calibration model and characteristics of the elements to a given experimental setup, such as the type of projector (i.e., analog or digital projection) and camera (i.e., monochrome or color). These strategies consist in projecting and capturing fringe patterns onto 3D-objects [19] or 2D-targets [16, 20] with highly accurate known measurements. In some cases, the calibration consists in displacing the targets along the
General calibration methodology.
In this section, we explain the details of the FTP software implementation in LabVIEW. LabVIEW stands for Laboratory Virtual Instrument Engineering Workbench and is a system-design platform and development environment for a visual programming language from National Instruments [21]. It allows integrating hardware, acquiring and analyzing data, and sharing results. Because it is a visual programming language based on function blocks, it is a highly intuitive integrated development environment (IDE) for engineers and scientists familiar with block diagrams and flowcharts. Every LabVIEW block diagram also has an associated front panel, which is the user interface of the application.
The acquisition and processing strategies described in this section require the installation of the following software components:
NI vision acquisition software, which installs NI-IMAQdx. This software driver allows the integration of cameras with different control protocols such as USB3 Vision, GigE Vision devices, IEEE 1394 cameras compatible with IIDC, IP (Ethernet) and DirectShow compatible USB devices (e.g., cameras, webcams, microscopes, scanners). NI vision acquisition software also includes the driver NI-IMAQ for acquiring from analog cameras, digital parallel and Camera Link, as well as NI Smart Cameras. This hardware compatibility is the main advantage of using LabVIEW for vision systems. This compatibility greatly facilitates the development of applications for different types of cameras and busses.
NI vision development module (VDM). This package provides machine vision and image processing functions. It includes IMAQ Vision, a library of powerful functions for vision processing. In this library, there is a group of VIs that analyze and process images in the frequency domain. We will make use of these functions throughout the entire chapter.
NI VDM and Vision Acquisition Software are supported on the following operating systems:
• Windows 10; Windows 8.1; Windows 7 (SP1) 32-bit; Windows 7 (SP1) 64-bit; Windows Embedded Standard 7 (SP1); Windows Server 2012 R2 64-bit; Windows Server 2008 R2 (SP1) 64-bit.
There are two primary ways to obtain images in LabVIEW: loading an image file or acquiring directly from a camera. The wiring diagram in Figure 5(a) illustrates how to perform a continuous (grab) acquisition in LabVIEW using Vision Acquisition Software. A Grab acquisition begins by initializing the camera specified by the Camera Name Control and configuring the driver for acquiring images continuously. Using IMAQ Create, we create a temporary memory location for the acquired image. This function returns an IMAQ image reference to the buffer in memory where the image is stored. The reference is the input to the IMAQ Grab VI for starting the acquisition. The grabbed image is displayed on the LabVIEW front panel using an Image Indicator (see Figure 5(b)), which points to the location in memory referenced by the IMAQ image reference. A while loop statement allows adding each grabbed image to the image indicator as a single frame. Finally, the image acquisition is finished by calling the IMAQ close VI that releases resources associated with the camera and the interface.
Grab acquisition in LabVIEW. (a) Block diagram. (b) Image indicator in front panel.
The acquired image is written to a file in a specified format by using the IMAQ Write File 2 VI. The graphics file formats supported by this function are BMP (windows bitmap), JPEG, PNG (portable network graphics), and TIFF (tagged image file format). However, note that lossy compression formats, such as JPEG, introduce image artifacts and should be avoided to ensure accurate image-based measurements. The saved image can be displayed in a secondary image indicator by enabling the Snapshot option. When enabling the Snapshot Mode, the Image Display control continues to display the image as it was when the image was saved during the Case Structure execution, even when the inspection image has changed. To configure the Image Display control for working in Snapshot Mode, right-click on the control on the front panel and enable the Snapshot option.
Another way to acquire an image using a camera is presented in the Figure 6. This example uses the NI Vision Acquisition Express to perform the acquisition stage. The Vision Acquisition Express VI is located in the Vision Express palette in LabVIEW, and it is commonly used to quickly develop image acquisition applications due to its versatility and intuitive development environment. Double-clicking on the Vision Acquisition Express VI makes a configuration window appear which allows choosing a device from the list of available acquisition sources, selecting an acquisition type, and configuring the acquisition settings. Concerning the acquisition types, there are four main modes: single acquisition with processing, continuous acquisition with inline processing, finite acquisition with inline processing and finite acquisition with post-processing. The last two acquisition types are similar, except that for a finite acquisition with post-processing the images are only available after they are all acquired. The configuration of the acquisition settings is one of the most relevant processes during configuration and allows the simultaneous manipulation of camera attributes like Exposure Time, Trigger Mode, Gain, Gamma Factor, among others. For this example, we configured the acquisition for working in a continuous acquisition with inline processing mode, which continuously acquires images until an event stops the acquisition. Additionally, the Exposure Time attribute can be modified during the acquisition process by using a Numeric Control. As with the example in Figure 5, the captured image is displayed in a secondary image indicator during the Case Structure execution.
Continuous acquisition using IMAQ vision acquisition express. (a) Block diagram. (b) Image indicator in front panel.
In Fringe Projection systems, the manipulation of certain camera attributes (e.g., the Exposure Time attribute) is required to capture high-quality images and to enable to work under different lighting environments with different constraints. In the example above, we introduced the possibility of manipulating camera attributes during acquisition using the Vision Acquisition Express. This manipulation of attributes is also possible by programming a simple snap, grab, or sequence operation based on low-level VIs (as in the example in Figure 5) using IMAQdx property nodes. The attribute manipulation requires providing the property node with the name of the attribute we want to modify and identifying the attribute representation, which can be an integer, float, Boolean, enumeration, string or command. In general, cameras share many attributes; however, they often have specific attributes depending on the manufacturer. These attributes should be known beforehand to ensure good acquisition control. At the development stage, LabVIEW does not know or display the name of the attributes or representations. Furthermore, if the documentation is not available, we suggest using the Measurement and Automation Explorer (MAX). MAX is a tool that allows the configuration of different acquisition parameters and is useful when it is required to manipulate attributes of a device with a specific interface within the LabVIEW programming environment. For example, suppose we want to modify the exposure time of our camera (Basler Aca 1600-60gm), but we do not have information about supported attributes. Here is where MAX becomes a powerful tool for vision system developers. This attribute verification is done by selecting the desired attribute from the Camera Attributes tab in the Measurement and Automation Explorer and identifying its name (i.e., ExposureTimeAbs) and representation (i.e., floating-point format). Therefore, the section of the block diagram inside a red box in Figure 5 can be modified in order to allow setting the ExposureTimeAbs attribute value using a Property Node as shown in Figure 7.
Setting the ExposureTimeAbs attribute value using a property node.
Both acquisition methods have their advantages and disadvantages concerning their implementation in vision systems. On the one hand, the use of the NI Vision Acquisition Express allows to quickly and easily develop acquisition applications, even without having a high knowledge of the tools for image acquisition offered by LabVIEW. However, this could be a disadvantage if our purpose is to have complete control over the acquisition. On the other hand, the low-level VIs provide greater control and versatility over the application development, but the implementation of vision systems based on low-level VIs can be a complicated task for novice users of NI Vision Acquisition Software and LabVIEW.
Once the acquired fringe image file has been written to disk, it is loaded for processing. The block diagram in Figure 8 illustrates how to perform this procedure in LabVIEW. The IMAQ ReadFile VI opens and reads an image from a file stored on the computer into an image reference. The loaded pixels are converted automatically into the image type supplied by IMAQ Create VI. From now on we refer to the Fringe Image to the loaded fringe image.
Reading an image file in LabVIEW.
In the previous section, we described several acquisition methods for capturing images from a camera in LabVIEW. However, in fringe projection systems there are many different fringe pattern projection technologies and choosing the correct one becomes extremely important for an accurate three-dimensional reconstruction. A fringe pattern projector can be considered as an analog device (e.g., LED pattern projector) or as a digital device (e.g., DLP, LCoS, and LCD digital display technologies). LED pattern projectors are ideal for high-resolution three-dimensional reconstruction applications. If equipped with an objective lens and a stripe pattern reticle, these projectors offer great versatility for manipulating the optics of the system and obtaining results according to the metrological requirements. The main disadvantage of this type of projection system is the impossibility of manipulating the projected fringe pattern. Therefore, its use is often restricted to techniques in which only a single fringe image is necessary to obtain the 3D information, such as in the case of FTP.
Fringe Projection systems can also take advantage of a computer to generate sinusoidal fringe patterns that are projected using a digital projector. The key to a successful 3D reconstruction system based on digital fringe projection focuses on generating high-quality fringes to meet the metrological requirements. Ideally, assuming the projector is linear in that it projects grayscale values ranging from 0 to 255 (0 black, and 255 white), the computer-generated fringe patterns can be described as follows,
where
Block diagram for fringe pattern generation.
An alternative to a block diagram implementation of Eq. (16) LabVIEW provides a MathScript RT Module as a scripting language. The module allows the combination of textual and graphical approaches for algorithm development. In Figure 10 we provide an example on how to use the MathScript RT Module for fringe generation in LabVIEW.
Fringe pattern generation example using the LabVIEW MathScript RT module.
Once the fringe images have been generated, they are sent to a digital video projector for projection. A video projector is essentially a second monitor. Therefore the fringe image is displayed by using the External Display VIs provided by the NI Vision Development Module. Here, we use IMAQ WindDraw VI to display the image in an external image window. The image window appears automatically when the VI is executed. Having beforehand the information from all the available displays on the computer, including their resolution and bounding rectangles, we set the position of the image window to be displayed on the desired monitor. This setting is done with IMAQ WindMove VI. Additionally, using IMAQ WindSetup VI the appearance and attributes of the window can be modified to hide the title bar. Note that the default value for this attribute is TRUE which shows the title bar. The block diagram in Figure 11 illustrates a projection stage in LabVIEW. Here, we use a Property Node for obtaining the information about all the monitors on the computer. The Disp.AllMonitors property Returns information about their bounding rectangles and bit depths.
Second monitor configuration in LabVIEW.
Phase retrieval is carried out by Fourier transform profilometry. In LabVIEW, the IMAQ FFT VI computes the discrete Fourier transform of the fringe image. This function creates a complex image in which low frequencies are located at the edges, and high frequencies are grouped at the center of the image. Note that for the IMAQ FFT VI a reference to the destination image must be specified and configured as a Complex(CSG) image. Once the deformed fringe pattern is 2-D Fourier transformed, the resulting spectra are converted into a complex 2D array to perform the filtering procedure, thus obtaining the fundamental frequency spectrum in the frequency domain. The following step is to compute the inverse Fourier transform of the fundamental component. The Inverse FFT VI is for computing the inverse discrete Fourier transform (IDFT) of a complex 2D array. By using this function, we calculate the inverse FFT of the fundamental component which contains the 3D information. Finally, we obtain the phase by applying Eq. (11). Here, we use Complex To Re/Im Function to break the complex 2D array into its rectangular components and Inverse Tangent(2 Input) Function for performing the arctangent operation. With the example in Figure 12(a) we illustrate the phase retrieval process in LabVIEW. In this figure, the Fringe Image and Hanning W refer to the fringe pattern image shown in Figure 12(b) and the Hanning window filter array, respectively. The resultant wrapped phase map is shown in Figure 12(c).
Phase retrieval process in LabVIEW. (a) Block diagram. (b) Fringe pattern image. (c) Wrapped phase map.
In Section 2 we showed that in FTP a filtering procedure is performed to obtain the fundamental frequency spectrum in the frequency domain. Once the Fourier transform is computed, the resultant spectrum is filtered by a 2-D Hanning window defined by Eq. (6). In LabVIEW, the IMAQ Select Rectangle VI is commonly used to specify a rectangular region of interest (ROI) in an image. We use the IMAQ Select Rectangle VI for manually selecting the region in the Fourier spectrum corresponding to the fundamental frequency component. Here, the image is displayed in an external display window and through the use of the rectangle tools, provided by the IMAQ Select Rectangle VI, we estimate the optimal size and location of the filtering window that guarantees the separation between the fundamental frequency component and other unwanted contributions. The block diagram shown in Figure 13(a) indicates the IMAQ Select Rectangle VI to manually select the region corresponding to the first order spectrum. The Fringe Image is the fringe pattern image in Figure 12(b). The IMAQ FFT VI computes the discrete Fourier transform of the Fringe Image. The resultant complex spectrum is displayed using an external display window as shown in Figure 13(b). By using the selection tools located on the right side of the window, we can manually select the rectangular area of interest.
Manual selection of the filtering window. (a) Block diagram. (b) External display window and rectangle tools.
The IMAQ Select Rectangle VI returns the coordinates (i.e., left, top, right and button) of the chosen rectangle as a cluster. Therefore, it is necessary to access each element from the cluster to extract the window information. For this reason, we add the Unbundle By Name function to the block diagram which unbundles a cluster element by name. Based on this information, we calculate the size and location of the Hanning window filter. Finally, using the Hanning Window VI two 1-D Hanning windows are created whose lengths correspond to the size of
Hanning filter design in LabVIEW. (a) Continuation of the block diagram in Figure 13(a). (b) Fourier transform magnitude spectra displayed by the external window in Figure 13(b). dx and dy relate to the size in x and y of the filtering window, respectively. (c) 2D-hanning window.
The phase unwrapping process is carried out comparing the wrapped phase at neighborhoods and adding, or subtracting, an integer number of
Bidimensional phase unwrapping in LabVIEW. (a) Wrapped phase map. (b) Unwrapped phase map.
FPP is often used as a non-contact surface analysis technique in industry inspection. In this section, we show the 3D surface reconstruction of a dented steel pipe. A dent is a permanent plastic deformation of the cross-section of the pipe. In the example shown in Figure 16 the dent was produced penetrating the pipe with a diamond cone indenter. In Figure 16(a) and (b) we show the tested object, and the deformed fringe pattern image, respectively. The goal is to measure the depth of the dent with high accuracy and to obtain the surface shape of the pipe for subsequent deformation analysis. In Figure 16(c) and (d), we show the wrapped, and unwrapped phases obtained by FTP, respectively. The unwrapped phase map is converted to metric coordinates using a calibration model. In Figure 17(a), we show the reconstructed pipe shape with the texture map. A profile across the reconstructed pipe, thought the dent, is shown in Figure 17(b). Analyzing this profile, we can measure the depth of the dent to approximately 4 mm.
FTP analysis of a indented pipe. (a) Texture image. (b) Deformed fringe pattern. (c) Wrapped phase. (d) Unwrapped phase.
(a) 3D reconstructed shape. (b) Cross section of the 3D reconstruction.
Another application of FPP is in facial metrology, where several patterns are projected onto the face to obtain a 3D digital model. 3D shape measurement of faces plays an important role in several fields like in the biomedical sciences, biometrics, security, and entertainment. Human face models are widely used in medical applications for 3D facial expression recognition [24] and measurement of stretch marks [25]. Usually, the main challenge is the movement of the patient. The movement can produce errors or noise in the 3D reconstruction affecting its accuracy. Hence, 3D scanning techniques that require few images in the reconstruction process, like FTP, are commonly used. In Figure 18 we show an experimental result of reconstructing a live human face. The captured image with the deformed fringe pattern is shown in Figure 18(a). In Figure 18(b) and (c) we show the 3D geometry acquired rendered in shaded mode and with texture mapping, respectively. Note that several facial regions with hairs, like the eyebrows, are reconstructed with high detail. While other areas, under shadows, like the right side of the nose, are not correctly reconstructed.
(a) Fringe pattern onto face. (b) 3D rendered model in shaded mode. (c) 3D rendered model with color texture mapping.
Finally, another area where FPP has frequently been used is in cultural heritage preservation. The preservation of cultural heritage works requires accurately scanning sculptures, archeological remains, paintings, etc. In Figure 19 we show the 3D reconstruction of a sculpture replica.
FTP 3D reconstruction of a sculpture replica of “Figura reclinada 92 - Gertrudis” by Fernando Botero [23]. (a) Texture image. (b) 3D reconstruction.
This work has been partly funded by Colciencias (Fondo Nacional de Financiamiento para la Ciencia, la Tecnología y la Innovación Francisco José de Caldas) project (538871552485) and by the Universidad Tecnológica de Bolívar (Dirección de Investigación, Emprendimiento e Innovación). J. Pineda and R. Vargas thank Universidad Tecnológica de Bolívar for a Master’s degree scholarship
Authors are listed below with their open access chapters linked via author name:
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\n\nFrede Blaabjerg 2015-18
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\n\nZhigang Chen 2016, 2018
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USA, CRC Press Taylor & Francis, Asia Pacific, Trans Tech Publications Ltd., Switzerland, and Materials Science Forum, USA. 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Omar obtained\nhis Bachelor degree in electrical and\nelectronics engineering from Universiti\nSains Malaysia in 2002, Master of Science in electronics\nengineering from Open University\nMalaysia in 2008 and PhD in optical physics from Universiti\nSains Malaysia in 2012. His research mainly\nfocuses on the development of optical\nand electronics systems for spectroscopy\napplication in environmental monitoring,\nagriculture and dermatology. He has\nmore than 10 years of teaching\nexperience in subjects related to\nelectronics, mathematics and applied optics for\nuniversity students and industrial engineers.",institutionString:null,institution:{name:"Universiti Sains Malaysia",country:{name:"Malaysia"}}},{id:"191072",title:"Prof.",name:"A. K. M. Aminul",middleName:null,surname:"Islam",slug:"a.-k.-m.-aminul-islam",fullName:"A. K. M. Aminul Islam",position:null,profilePictureURL:"https://mts.intechopen.com/storage/users/191072/images/system/191072.jpg",biography:"Prof. Dr. A. K. M. 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