Dissertations / Theses on the topic 'Nanophosphors'

Biosensors are instrumental in the detection of analytes in a wide range of areas including enzyme kinetics and disease diagnosis. A proof-of-principle upconversion nanophosphor (UCNP) based biosensor based on luminescence energy transfer between UCNPs, acting as the energy transfer donor, and enzymes and biologically relevant proteins, the energy transfer acceptor is reported here. Analyte detection has been performed by ratiometric sensing by monitoring the change in the multiple emission bands of the UCNPs. Chapter 1 is an introduction into the emerging field of UCNPs as biosensing agents. These nanoparticles offer numerous advantages over current biosensing agents (namely organic dyes and quantum dots) including resistance to photobleaching and photoblinking, long emissive lifetimes, a large anti-Stokes' shift and near infrared (nIR) excitation to eliminate autofluoresence, and multiple characteristic emission bands for sensing multiple analytes. Chapter 2 describes the synthesis and characterisation of Yb3+/Tm3+ and Yb3+/Er3+ co-doped UCNPs via a range of different preparative methods; thermal decomposition, microwave irradiation and a one-step solvothermal process to produce hydrophilic UNCPs. In addition, commercial UCNPs, kindly donated by Phosphor Technology, were also characterised and used as a benchmark for characterisation of the newly synthesised UCNPs. Chapter 3 describes the detection of the enzyme pentaerythritol tetranitrate reductase (PETNR), through energy transfer between the commercial Yb3+/Tm3+ doped UCNPs and the enzyme using ratiometric sensing. These proof-of-principle results were published in Dalton Transactions. In addition, ratiometric change of the UCNP emission bands was able to monitor the enzyme-substrate turnover in a two electron redox reaction. Chapter 4 describes techniques for increasing the scope and sensitivity of the proof-of-principle UCNP-enzyme biosensing system. Small, hydrophilic Yb3+/Tm3+ and Yb3+/Er3+ doped UCNPs, synthesised in chapter 2, were able to detect glucose oxidase and cytochrome c, in addition to PETNR. Covalent attachment of PETNR to Yb3+/Tm3+ doped UCNPs was additionally achieved. Chapter 5 describes the incorporation of UCNPs into optical ring resonators (ORRs) in order to develop a lost cost, label-free, rapid response biosensor. Drop casting and inkjet printing methods for the deposition of UCNPs onto these devices were investigated and emission of UCNPs was achieved, for the first time, by ORR excitation.

I materiali luminescenti nanostrutturati sono largamente studiati per applicazioni in lampade e display, come scintillatori e nell’imaging biomedico. Pertanto, la ricerca nei nanomateriali prevede lo sviluppo di metodi di sintesi all’avanguardia per il controllo della loro struttura, morfologia e drogaggio. L’utilizzo di polveri nanocristalline per la fabbricazione di nanocompositi permette di ridurre l’incorrere di diverse problematiche come la diffusione della luce emessa; inoltre la dimensione nanometrica dei materiali è un requisito fondamentale per le applicazioni in biotecnologia, per la loro veicolazione attraverso il sangue e la penetrazione nelle cellule. Infine, la realizzazione di nanoparticelle (NP) aventi fase cristallina cubica permetterebbe la progettazione di ceramiche ottiche ad alta densità e quindi di una nuova classe di materiali luminescenti. L’ossido di afnio (HfO2) è stato considerato come fosforo di grande interesse grazie alle sue eccellenti proprietà chimiche e fisiche. In questo lavoro si sono investigate le proprietà di luminescenza e scintillazione di NP di HfO2 di diametro < 5 nm. Le NP pure e drogate con ioni di terre rare (TR) sono state fabbricate attraverso un processo di sintesi appositamente elaborato e ottimizzato. Il lavoro condotto ha permesso di controllare simultaneamente le proprietà strutturali e di luminescenza nelle NP. Particolare attenzione è stata rivolta al ruolo del drogaggio con ioni europio e lutezio tramite sintesi sol-gel non acquosa. L’analisi elementale, la caratterizzazione strutturale e morfologica con XRD, TEM/SEM, insieme alla spettroscopia vibrazionale Raman/IR, hanno confermato la trasformazione della fase cristallina da monoclina a cubica per concentrazioni > 5% mol di ioni Lu3+e Eu3+. Le proprietà ottiche sono state studiate attraverso tecniche di radio- e foto-luminescenza. I risultati ottenuti rappresentano un importante traguardo sia per una migliore comprensione della relazione struttura-proprietà di materiali di dimensione nanometrica, che per l’analisi della applicabilità di questi ultimi in campo tecnologico. In questo lavoro è stata dimostrata la possibilità di modificare lo spettro di emissione delle NP drogando simultaneamente con diverse TR e stabilizzando la fase cubica con l’incorporazione di ioni di Lu3+ otticamente inerte. L’HfO2 è un promettente materiale sia come matrice ospite per le TR che per la sua luminescenza intrinseca. NP non drogate sono state studiate considerando l’effetto della dimensione e della fase cristallina sulla luminescenza. Si è individuata la presenza di una banda di emissione composita nell’intervallo di lunghezze d’onda visibili, possibilmente correlata a difetti di superficie intrinseci o a impurezze del materiale. La sua intensità varia in funzione di trattamenti termici che portano alla modifica della superficie e del diametro delle NP, ed è confrontabile all’efficienza di materiali luminescenti commerciali usati come standard. In parallelo, sono state studiate le proprietà di NP luminescenti per applicazioni biologiche. Le nuove tecniche diagnostiche per immagini in vivo a fluorescenza con alta risoluzione e profondità di penetrazione si basano sulla luminescenza di NP nella finestra di trasparenza del tessuto biologico (1000-1400 nm). Inoltre, l’eccitazione a basse energie porta alla riduzione dell’autofluorescenza generata dai tessuti, componenti intra corporee e molecole organiche della dieta degli animali trattati con le NP. In questa ricerca, è stato dimostrato che l’utilizzo della banda a 1.3 m di ioni di Nd3+ in SrF2 permette di effettuare analisi di biodistribuzione e ottenere immagini in assenza di autofluorescenza e ad alto contrasto. La luminosità, la stabilità chimica e fisica così come l’elevata biocompatibilità rendono le NP di SrF2 promettenti per applicazioni biotecniche, bioimmagini a fluorescenza e future strategie diagnostiche.
Luminescent materials have found a wide variety of applications as phosphors for fluorescent lighting, display devices, X-ray monitoring and imaging, scintillators, and in biomedical imaging. The research on nanostructured materials resulted in the development of novel synthetic methods to control their structure, morphology, and doping. When the size of crystalline powders is tailored down to the nanoscale, several advantages are achieved, like the reduction of the emitted light scattering when fabricating optical nanocomposites. Nanoscale dimensions are also necessary in biotech applications where the material is required to travel in blood vessels and penetrate into cells. Finally, the realization of high density optical ceramics by nanoparticles (NPs) compaction can be pursued, especially with materials that possess cubic crystalline structure, leading to the bottom-up fabrication of a new class of luminescent materials. Hafnium oxide (HfO2) has gained interest in the last years as an attractive nanophosphor because of its excellent physical and chemical properties. In this work, the luminescence and scintillation properties of pure and rare-earth (RE) doped HfO2 NPs with a diameter < 5 nm have been investigated, obtained through a purposely designed synthetic strategy. This work was aimed at controlling the structural properties of NPs while optimizing their optical features. A particular attention has been paid to the role of doping with europium and lutetium ions through the non-aqueous sol-gel method. Structure and morphology characterization by XRD, TEM/SEM, elemental analyses, and Raman/IR vibrational spectroscopies have confirmed the occurrence of the HfO2 cubic polymorph for dopant concentrations larger than 5% mol for trivalent Lu3+ and Eu3+ ions. Optical properties have been investigated by radio- and photo-luminescence spectroscopy. Besides the relevance in application related issues, the results here reported represent an important dataset for a better comprehension of the structure-property relationship in materials confined into nanoscale dimensions. We also demonstrated the possibility of tuning the emission spectrum by multiple RE doping, while deputing the NP cubic structural stabilization to optically inert Lu3+ ions. Given the importance of HfO2 as host material for RE, its intrinsic optical response is also worth of investigation. Undoped HfO2 NPs were studied considering the effect of the size and of the crystal phase. A broad composite emission was observed in the visible range, potentially correlated both to intrinsic surface defects and to impurities. Its intensity can be varied by thermal treatments leading to surface modifications as well as to variations of particle dimensions. Its efficiency has been found to be comparable to that of standard commercial materials, evidencing the potential of pure HfO2 NPs as efficient phosphors. In parallel, we also investigated the use of emitting NPs for biological applications. Novel approaches for high contrast, deep tissue, in vivo fluorescence biomedical imaging are based on infrared-emitting NPs working in the so-called second biological window (1000 -1400 nm), where the partial transparency of tissues allows for the acquisition of high resolution, deep tissue images. In addition, the infrared excitation also leads to a reduction of auto-fluorescence generated by tissues, intra-body components, and specimen's diet. In my work, I exploited how the 1.3m emission band of Nd3+ ions embedded in SrF2 nanoparticles can be used to produce auto-fluorescence free, high contrast fluorescence images and bio-distribution studies. The strong brightness, the chemical and physical stability as well as high biocompatibility make Nd:SrF2 nanocrystals very promising infrared nanoprobes for in vivo imaging experiments in the second biological window.

Nanotechnology has been increasingly employed in forensic science for the detection of latent fingerprints, using multiple techniques from new aluminium nanomaterials for dusting to quantum dot dispersions, to try to increase and enhance areas where prints are likely to be found at scenes of crime. Different substrates use a diverse range of methods to develop prints when they are found and each method has its own drawbacks. It is not viable to use many of these techniques in conditions other than in a laboratory due to the harmful environmental effects they can cause over long term use. With this in mind a new easier to use technique that can be used on any substrate from wood to glass to paper was looked into. A range of nano-sized rare earth phosphor precursors were synthesised using homogeneous precipitation and solid state methods which were then converted to phosphors by firing at 980oC. Eu3+ and Tb3+ doped Y2O3, YVO4 and Y2O2S were chosen for their luminescent intensity. Analysis of each of the phosphors was carried out using multiple techniques and a single host lattice chosen for continuation. Y2O3:Eu3+ and Y2O3:Tb3+ were coated using a modified Stöber process to try and decrease the agglomeration of particles as well as allowing for surface modification to take place. Modifications of the surface were prepared and analysed, and these particles were then used in multiple fingerprint examinations to examine the adherence on fingerprints of different ages. The surface modifications manifested great adherence to the fingerprint residue even after two weeks elapsed and showed great promise after a two year period.

A range of inorganic nanoparticles/nanophosphors that act as ultraviolet radiation absorbers were characterised and assessed in this thesis. Iron doped lithium aluminate phosphor was synthesised using a solid state reaction and also by flame spray pyrolysis. The phosphors prepared by different synthesis methods were characterised to identify their crystal structures and morphologies. Downconverting photoluminescent properties of the phosphors both as pure powders and embedded in polypropylene by co-rotating twin screw extrusion are reported. Zinc oxide nanoparticles made by flame spray pyrolysis were also investigated. They were incorporated into polymers by means of three different approaches including co-rotating twin screw extrusion, spin coating and solvent casting. The resulted composite films were explored to understand the distribution of the zinc oxide nanoparticles. The transmittance and ultraviolet absorption of the nanocomposites were studied and are reported herein. Another set of nanophosphors studied were zinc rich luminescent zinc oxides. They were prepared from the zinc oxide nanoparticles by firing them in a reducing atmosphere. The as-prepared nanophosphors manifested good downconverting photoluminescent properties and maintained their functions when embedded into polystyrene by solvent casting. In this thesis a new route of synthesising aluminium doped zinc oxide nanoparticles was also established. This new approach was based on a series of unexpected results within some trials that were attempting to coat a layer of alumina on the zinc oxide nanoparticles. The concentration of the Al3+ in the final product could be adjusted by tailoring the amount of the Al3+ in the reactants during the synthesis procedures. It was also possible to coat various zinc oxide nanostructures with the aluminium doped zinc oxide.

Ce travail est consacré à l’élaboration de nanoluminophores de formulations β-NaBiF4 et α-NaYF4 monodopés Eu3+, Tb3+, Pr3+, codopés avec Yb3+ et tridopés avec Ce3+. Ces matériaux ont été préparés par coprécipitation, et les paramètres de synthèse ont été optimisés afin de produire des nanocubes (NCs) avec des longueurs d’arête entre 35 et 65 nm, de façon reproductible et avec une distribution de taille étroite. Les NCs de fluorures obtenus ont été caractérisés d’un point de vue structural (DRX, IR), morphologique (MEB et TEM) et optique (photoluminescence) permettant de confirmer leurs puretés cristallines d’une part et d’enregistrer des répartitions spectrales de luminescence conformes à celles attendues d’autre part. L’analyse des spectres d’émission et des déclins de la fluorescence, sous excitations UV et/ou proche infrarouge, a permis de démontrer que des processus de conversion Stokes et anti-Stokes efficaces se produisent dans ces NCs. Les résultats sont discutés en considérant divers chemins de désexcitations radiatives et de transfert d’énergie, permettant de conclure que l’on peut générer sur un même NC les deux processus, même si ces derniers sont en concurrence dans certains cas
This work is devoted to the development of nanophosphors of formulations β-NaBiF4 and α-NaYF4 monodoped Eu3+, Tb3+, Pr3+, codoped with Yb3+ and tridoped with Ce3+. These materials were prepared by coprecipitation, and the synthesis parameters were optimized to produce nanocubes (NCs) with edge lengths between 35 and 65 nm, reproducibly and with a narrow size distribution. The obtained NCs were characterized from a structural (XRD, IR), morphological (SEM and TEM) and optical (photoluminescence) point of view, making it possible to confirm their crystalline purities on the one hand and to record spectral distributions of luminescence in accordance with those expected on the other hand. Analysis of emission spectra and fluorescence decays, under UV and / or near infrared excitations, has demonstrated that efficient Stokes and anti-Stokes conversion processes occur in these NCs. The results are discussed by considering various paths of radiative de-excitations and energy transfer, making it possible to conclude that the two processes can be generated on the same NC, even if the latter are in competition in certain cases

The project is focus on the synthesis and characterization of lanthanide bismuth oxyfluoride particles. The samples are synthesized through homogeneous precipitation using a microwave reactor to heat. Furthermore, by doping with different lanthanides (Tb, Eu, Tb-Eu; Pr; Nd; Yb-Ln, Ln=Er,Yb,Tm) it is tested how the optical response of the systems can be in terms of luminescence spectroscopy. The synthesized Ln3+ NPs are tested for the biological applications as nanothermometer. In recent times Luminescent nanothermometers have been widely investigated because they relate the local temperature of a biological system with their emission, as a result of an external radiation. Through these systems there is the possibility of excitation and / or emission in the so-called first and second biological window. The studies of these project are focused on NPs doped Ln3+ as thermal and emitting probes.

    Luminescentni nanokristali (nanofosfori) na bazi fluorapatita (FAP-a) dopirani elementima retkih zemalja idealni su kontrastni agenti za bio-medicinske primene, kao što su detekcije, snimanja, praćenja i terapije ćelija kancera. Kancer je jedna od najčešćih bolesti modernog doba čiji uspeh lečenja zavisi od rane dijagnostike i neinvazivnog tretmana. Luminescentne nanočestice mogu uneti inovativnu paradigmu u lečenje kancera kombinovanjem biosnimanja, dijagnostike i tretmana. Za studije fluorescentnih biosnimanja nanokristali fluorapatita dopirani retkim zemljama kao kontrastni agenti pružaju značajne prednosti u vidu velikih kontrasta i dugotrajnosti luminescencije, i što je još važnije visoke biokompatibilnosti, netoksičnosti i bioaktivnosti. Glavni ciljevi ove doktorske disertacije su sinteza novih luminescentnih multifotonskih bionanomaterijala na bazi fluorapatita dopiranih jonima prazeodimijuma (Pr³⁺), njihova karakterizacija i evaluacija  primene za fluorescentna biosnimanja kancera. Sintezom nanoprahova u umerenim uslovima metodom ko-precipitacije, a potom sušenjem na 110 ^oC i kalcinacijom na temperaturama od 700 i 1000 ^oC očekuje se pronalaženje najboljih uslova za dobijanje novih nanofosfora koji bi našli i različite bio-medicinske primene u oblasti fluorescentnih biosnimanja. Proučavane su tri vrste PrFAP nanokristala, sa 0,1%, 0,5% i 1% atomskih procenta Pr³⁺, zajedno sa nedopiranim FAP kontrolnim uzorkom. Nivoi energije aktivator jona Pr³⁺ sadrže metastabilna multipletna stanja koja nude mogućnosti efikasnih emisionih linija u više boja u FAP nanokristalima, kao i u infracrvenoj i ultravioletnoj oblasti spektra. Metodom ko-precipitacije na sobnoj temperaturi (25 ^oC), a potom sušenjem na 110 ^oC, sintetisani su monofazni heksagonalni nanokristali PrFAPs nepravilnog sfernog oblika. Termičkom analizom sintetisanih uzoraka, na osnovu detektovanih temperaturnih opsega procesa dekarbonacije i dehidroksilacije, utvrđene su temperature kalcinacije od 700 i 1000 oC. Termička analiza i karakterizacija uzoraka su pokazale da Pr³⁺ joni dovode do stabilizacije FAP strukture na višim temperaturama, što je pripisano unosu lantanoidnih jona sa specifičnim magnetnim osobinama u sistem i stvaranju jačih privlačnih sila sa O^2- anjonima. Nanokristali sušeni na 100 ^oC i kalcinisani na 1000 ^oC, zbog prisustva defekata kristalne rešetke koji zadržavaju emisiju Pr³⁺ jona, nisu pokazali luminescentne karakteristike od značaja za primene u medicinskim fluorescentnim biosnimanjima. Kalcinacijom uzoraka na 700 ^oC izrađen je novi tip aktiviranih fluorapatitnih nanokristala dopiranih prazeodimijumom (PrFAPa) sa ekscitaciono-emisionim profilima u vidljivom delu spektra. Fizičko-hemijska karakterizacija potvrdila je sferne kristale heksagonalne strukture do nanometrske veličine od oko 20 nm. Kvantno-hemijske kalkulacije predvidele su da se joni Pr³⁺ ugrađuju u kristalnu rešetku FAP nanokristala na položaju Ca2 (6h), što je praćeno deformacijama pozicije F^- jona. Pretpostavljeni mehanizam supstitucije je jedan jon Pr³⁺ za jedan Ca²⁺, s delimičnom supstitucijom anjona F^– sa O^2– i OH^– i stvaranjem vakansi usled postizanja neutralnosti sistema. Rezultati in vitro biokompatibilnosti i hemokompatibilnosti pokazali su da nanokristali PrFAPa nisu toksični za žive ćelije. Pored toga, internalizacija PrFAPa nanokristala od strane ćelija kancera kože (A431) i pluća (A549) je proučavana korišćenjem konfokalne mikroskopije i mikroskopije širokog polja zasnovane na fluorescenciji. Nanokristali pokazuju karakterističnu zelenu emisiju na 545 nm (³P₀→³H₅ tranzicija Pr³⁺ jona) i narandžastu emisiju na 600 nm (¹D₂→³H₄), koje su korišćene za razlikovanje od pozadinske autofluorescencije ćelije. Studije dobijenih slika konfokalnom mikroskopijom u plavom, zelenom i crvenom kanalu su otkrile da nanokristali mogu da prepoznaju ćelijsku površinu i da se lepe za nju, ali nisu potvrdile ulazak nanokristala u ćelije. Mikroskopija širokog polja je detektovala emisione prelaze u zelenoj i narandžastoj boji i potvrdila da luminescentni signal dolazi iz unutrašnjosti ćelija. Korišćenjem rezonantne ekscitacije od 488 nm i emisije od 600 nm PrFAPa nanokristala, konfokalnom mikroskopijom ekstrahovan je signal fluorescencije iz unutrašnjosti ćelija kancera. Ortogonalne projekcije u 3D konfokalnim ravnima pokazuju da su nanokristali u stanju da uđu u ćelije kancera i da se raspoređuju po citoplazmi. Sveukupno, ovako dobijeni nanokristali PrFAPa su biokompatibilni i od testiranih uzoraka, aktivirani nanokristali dopirani sa 0,5% Pr³⁺ pokazuju najviše potencijala za primenu u medicinskim fluorescentnim biosnimanjima kao kontrastni agenti.  
Luminescent nanocrystals (nanophosphorus) based on fluorapatite (FAP) doped with rare earth elements are ideal contrast agents for biomedical applications such as cancer cell detection, imaging, tracking and therapy. Cancer is one of the most common diseases of the modern times whose success of the cure depends on early diagnosis and non-invasive treatment. Luminescent nanoparticles can bring an innovative paradigm into the treatment of cancer by combining bioimaging, diagnostics and treatment. Rare earth doped fluorapatite nanocrystals as contrast agents for studies of fluorescence bioimaging, offer significant advantages in terms of high contrasts and long-term luminescence, and more importantly high biocompatibility, non-toxicity and bioactivity. The main objectives of this doctoral dissertation are the synthesis of novel luminescent multiphoton bionanomaterials based on fluorapatites doped with praseodymium ions (Pr³⁺), their characterization and evaluation of their application for cancer fluorescence bioimaging. Synthesis of nanopowders under moderate conditions by the co-precipitation method, followed by dried at 110 °C and calcination at 700 and 1000 °C, is expected to find the best conditions for obtaining new nanophosphors that would find different bio-medical applications in the field of fluorescence bioimaging. Three types of PrFAP nanocrystals were studied, with 0,1%, 0,5%, and 1% atomic percentages of Pr³⁺, together with an undoped FAP control sample. Energy levels of the Pr³⁺ ion activator contain metastable multiplet states that offer the possibility of efficient multi-color emission lines in FAP nanocrystals as well as in the infrared and ultraviolet regions of the spectrum. Single-phase hexagonal nanocrystals PrFAPs of irregular spherical shape were synthesized by the method of co-precipitation at room temperature (25 ^oC) and then drying at 110 ^oC. Thermal analysis of the synthesized samples, based on the detected temperature ranges of the decarbonation and dehydroxylation processes, determined calcination temperatures of 700 and 1000 ^oC. Thermal analysis with characterization showed that Pr³⁺ ions lead to stabilization of the FAP structure at higher temperatures, which was attributed to the entry of lanthanoid ions with specific magnetic properties into the system and the creation of stronger attractive forces with O^2- anions. Nanocrystals dried at 100 ^oC and calcined at 1000 ^oC, due to the presence of crystal lattice defects that quench the emission of Pr³⁺ ions, did not show luminescent characteristics of significance for applications in medical fluorescence imaging. Calcination of the samples at 700 ^oC produced a new type of activated praseodymium doped fluorapatite nanocrystals (PrFAPa) with excitation-emission profiles in the visible part of the spectrum. Physicochemical characterization confirmed spherical crystals of hexagonal structure up to a nanometer size of about 20 nm. Quantum-chemical calculations predicted that Pr³⁺ ions would be embedded in the crystal lattice of FAP nanocrystals at the Ca2 position (6h), which was followed by deformations of the F^- ion position. The assumed substitution mechanism is one Pr3+ ion for one Ca²⁺, with partial substitution of F^– anions with O^2– and OH^– and creation of vacancies due to achieving system neutrality. The results of in vitro biocompatibility and hemocompatibility showed that PrFAP nanocrystals were not toxic to living cells. In addition, the internalization of PrFAPa nanocrystals by skin (A431) and lung (A549) cancer cells was studied using fluorescence-based confocal microscopy and wide-field microscopy. The nanocrystals show characteristic green emission at 545 nm (³P₀→³H₅ transition of Pr³⁺ ion) and orange emission at 600 nm (¹D₂→³H₄), which we use to discriminate from cell autofluorescence. Studies of the images obtained by confocal microscopy in the blue, green, and red channels revealed that nanocrystals could recognize the cell surface and adhere to it, but they did not confirm the entry of nanocrystals into the cells. The wide-field microscopy detected emission transitions in green and orange color, and confirmed that the luminescent signal was coming from inside the cells. Using resonant excitation of PrFAP nanocrystals at 488 nm and emission of 600 nm, confocal microscopy extracted the fluorescence signal from inside the cancer cells. Orthogonal projections across 3D confocal stacks show that the nanocrystals are able to enter the cells positioning themselves within the cytoplasm. Overall, the obtained PrFAPa nanocrystals are biocompatible and of the tested types, the 0,5% Pr³⁺ doped nanocrystals show the highest promise as a tracking nanoparticle probe for bioimaging applications.

碩士
國立成功大學
電機工程學系碩博士班
94
Abstract Zinc sulfide (ZnS), as II-VI semiconductors with a wide band gap energy of 3.68eV, have received much attention due to their excellent luminescence properties and are commercially used in electroluminescence devices. They are candidate materials for phosphors that emit visible light. The major and important applications of phosphors are used as light sources, display devices, radiation detectors and so on. In this study, we prepare the nano-scaled ZnS based phosphors using solid state method and chemical precipitation method. Different dopants (Mn, Cu, Mg, Eu) have been introduced in the system. X-ray diffraction pattern, SEM, TEM, PL and CIE measurements have been used to investigate the characteristics of ZnS-based nano-phosphors for UV-white light LED applications. Firstly , we synthesize and characterize the luminescence properties of ZnS：Mn nanophosphors by solid state method with different S/Zn ratio and under different temperature. When S/Zn ratio is 0.65 and under 300℃, a near white light phosphors are obtained and C.I.E. is (0.309,0.311). Secondly, ZnS:Mn+2,ZnS:Cu+2 and ZnS:Eu+3 phosphors are prepared by chemical-precipitation method. From the emission spectra data, orange light with the emission peak at 593nm for ZnS:Mn+2 phosphors are detected, blue light at 470nm and green light at 520nm for ZnS:Cu+2 phosphors are detected, red light for ZnS:Eu+3 phosphors are also detected. It is possible to obtain white light phosphors by co-doping Cu+2,Eu+3 and Cu+2,Mn+2 in this system Thirdly, ZnS co-doped Mg+2 and Mn+2 phosphors are synthesized by chemical precipitation method. From the emission spectra data, near white light is observed for Zn0.49Mg0.49S: Mn+2 (2mol%) and C.I.E is(0.322,0.292).

碩士
國立交通大學
應用化學系所
92
In this research we have successfully synthesized three series of Gd2O2S:R ( R = Tb3+, Pr3+, Eu3+) nanophosphors via a two-step process by utilizing simple hydrothermal apparatus at 140-200℃, followed by a annealing under H2S atmosphere at 500-1000℃. The correlation between phase purity, photoluminescence and microstructure of Gd2O2S:R nanophosphors were then characterized by X-ray diffraction, spectrofluorimeter, scanning microscope (SEM) and transmission microscope (TEM) techniques. Our research indicates that the pH values and temperature adopted in the hydrothermal synthesis to form nanocrystalline Gd(OH)3 precursor are the most important processing parameters in determining the grain morphology and sizes of Gd2O2S:R nanophosphors. The morphology of nanocrystalline Gd(OH)3 precursor was observed to change from granular to rod-shaped when pH was allowed to vary from 8 to 10. The average diameter of granular Gd2O2S:R was found to be ca. 80 nm, whereas the aspect ratio (c/a) for Gd2O2S:R nano-rods was found to be 10 with length and diameter of 200 nm and 20 nm, respectively, as indicated by TEM investigations. On the other hand, the luminescence and microstructure for bulk and nano-crystalline Gd2O2S:R phosphors prepared from solid-state and two-step hydrothermal routes, respectively, were also compared.

碩士
國立交通大學
應用化學系
91
This research demonstrated the successful synthesis of two series of rare-earth (i.e., Eu3+, Tb3+, Pr3+, Sm3+, Dy3+) -activated R2O2S (R = Y, Gd) via a solvothermal route at 150℃, followed by H2S-annealing at elevated temperatures (i.e., 300-700℃). To establish the appropriate processing conditions, we have investigated the microstructure and luminescent properties of R2O2S nanophosphors, as a function of source and concentration of starting materials, synthetic temperature, solvents and their filling rate, activator concentration and surface modification. As indicated by TEM image analyses, spherical (60 nm in diameter) and rod-shaped (10 nm in diameter and 1.5 m in length) nanophosphors of Y2O2S:Tb have been prepared by solvothermal reactions using methanol (MeOH) and ethylenediamine (en) as a solvent, respectively, followed by H2S annealing.. Furthermore, a rod-shaped nanophosphor of Y2O2S:Tb with 30 nm in diameter and 1.5 m in length has been synthesized by using MeOH/en mixture (50:50, v/v) as a solvent. On the other hand, rod-shaped nanophosphors of Gd2O2S:Tb with 20 nm and 4 m in diameter and length, respectively, have been obtained by using en as a solvent in a solvothermal reaction.

博士
國立交通大學
應用化學系碩博士班
101
In this study, a series of water-soluble YVO4:Bi3+,Eu3+ nanophosphors (NPs), with surfaces functionalized by a branch polyethylenimine (BPEI) polymer, has been synthesized via a facile one-pot hydrothermal method. The crystal morphology can be well controlled by tuning the reaction temperature, pH value and molecular weight of capping agent BPEI. The BPEI-coated YVO4:Bi3+,Eu3+ NPs with high crystallinity show broad band excitation in the 250 to 400 nm near ultraviolet (NUV) region and exhibit a sharp-line emission band centered at 619 nm under the excitation of 350 nm. The folic acid (FA) and epidermal growth factor (EGF) were attached on the BPEI-coated YVO4:Bi3+,Eu3+ NPs and exhibited effective positioning of fluorescent nanophosphors toward the targeted folate-receptor over-expressed HeLa cells or EGFR over-expressed A431 cells with low cytoxicity, respectively. These results demonstrate that the ligand-functionalized BPEI-coated YVO4:Bi3+,Eu3+ NPs show great potential as a new generation biological luminescent probe for bio-imaging applications. For solar cell application, the c-Si solar cells showed an enhancement of 4 % in short-circuit current density and approximately 0.7 % in power conversion efficiency when coated with BPEI-coated YVO4:Bi3+,Eu3+ NPs on the textured cell surface. The current experiments conclude that the BPEI-coated YVO4:Bi3+,Eu3+ NPs can not only act as luminescent down-shifting centers in the UV region but also serve as an antireflection coating for improving the power conversion efficiency of the c-Si solar cell.

碩士
中原大學
物理研究所
105
Nowadays, there usually exists a trade-off between the device efficiency and environmental protection in photonic devices. For example, to harvest all excitons in organic light-emitting materials, expensive heavy metals need to be incorporated into the emitters. Semiconductor nanocrystals with tunable wavelength and pure emission colors have also been commercialized for light-emitting materials in light-conversion nano-phosphors and display backlight. Unfortunately, the most mature semiconductor nanocrystals contain toxic elements (Cd or Pb) and are synthesized in hazardous organic solvent. As a result, it is necessary to develop eco-friendly, non-toxic nanomaterials that can be directly fabricated in an aqueous solution based on cost-effective, element-abundant precursors, while still exhibiting unique photophysical properties. Metal nanoclusters (NCs) with tiny sizes, including AuNCs, AgNCs, and CuNCs can be simply synthesized in an aqueous solution and exhibit some unique photophysical properties that are promising candidates for promising applications in “green photonics”. In the first part, a facile, matrix-free method based on surface modification was used to prepare solid-state non-toxic AuNC nano-phosphors with solid-state enhanced photoluminescence quantum yields (PL-QYs) and lengthened PL lifetime. Those AuNC-nanophosphors also exhibit large Stokes shift due to the emission from intramolecular charge transfer (ICT) state, thus reducing conventional concentration-induced PL quenching and reabsorption losses. In light of our spectroscopic studies and materials characterization, the improved photophysical properties are attributed to surface-modification-induced aggregation, thus restricting molecular surface-ligand motions. Despite aforementioned unique photophysical properties possessed by AuNCs, the issues regarding expensive precursors and long reaction time still need to be addressed. In the second part, we investigated the solid-state photophysical properties of cost-effective, element-abundant CuNCs, which can be simply synthesized in an aqueous solution at room temperature within one hour. The CuNCs exhibit unique solid-state dual-mode emissions of thermally-activated delayed fluorescence and phosphorescence with a short emissive lifetime at room temperature and at ambient environment. Such dual-mode emissions can be attributed to small singlet-triplet energy splitting due to the ICT emission and large spin-orbit coupling arising from heavy-atom effect. To fabricate solid-state nano-phosphors with green PL emission, the carbon nano-dots have also prepared using a simple hydrothermal method. To stabilize the excited states and avoid the formation of solid aggregates, inorganic ionic-crystal matrices were used to protect and disperse the carbon nano-dots. We also investigated the photophysical properties of carbon nano-dots in the solid state. We found some interesting spectral modification behavior when aqueous carbon nano-dots were transferred to solid states and the behind mechanism is still under investigation. Compared with conventional heavy-metal containing semiconductor nanocrystals synthesized in a hazardous solvent, those non-toxic nanomaterials can be directly prepared in an aqueous solution using cost-effective precursors and exhibit some unique photophysical properties, thus would be promising for future applications in “green photonics”. However, the solid-state PL-QY is still poor and need to be further enhanced.

碩士
國立交通大學
應用化學系碩博士班
101
Upconversion phosphors are characterized by the conversion of long- wavelength radiation to short-wavelength radiation. Due to unique properties, these phosphors have been investigated as candidates for biological imaging and spectral converter to enhance their conversion efficiency in solar cells. By using hydrothermal synthesis, the research has successfully prepared YVO4:Yb3+,R3+ (R: Ho3+,Er3+) nanophosphors, which emit red (650 nm; R = Ho) or green (552 nm; R = Er) light, respectively, when excited with 980 nm laser. We have attempted to enhance the luminescence efficiency by employing poly acrylic acid (PAA) and poly vinyl pyrrolidone (PVP) as capping agents and optimizing reaction conditions such as temperature, concentration of dopants and pH values. The nanophosphors were further characterized by XRD, PL, FT-IR, DLS, SEM, and TEM techniques. We have demonstrated that YVO4:Yb3+,R3+ upconversion nanophosphors exhibit great potential for efficiency enhancement of both amorphous silicon and dye-sensitized solar cells.

Semiconductor nanocrystals with sizes comparable to the corresponding bulk excitonic diameter exhibit unique size-dependent electronic and optical properties resulting from quantum confinement effect. Such nanocrystals not only allow the study of evolution of bulk properties from the molecular limit providing important fundamental understandings, but also have great technological implications, leading to intense research over the past several years. Besides tuning the crystal size in the nm regime to obtain novel properties, an additional route to derive new functionalities has been to dope transition metal ions into a semiconductor host. Thus, transition metal doped nanocrystals are of great interest since it allows two independent ways to functionalize semiconductor materials, one via the tunability of properties by size variation and other due to properties of such dopants. Chapter 1 of the thesis provide a general introduction to the subject matters dealt in with this thesis, while the necessary methodologies have been discussed in chapter 2. Chapters 3 and 4 of this thesis deal with nanocrystal doping. Following suggestions in previous literatures that the doping of nanocrystal depends strongly upon the crystal structure of the synthesized host nanocrystal, we have studied the phase-transformation between the somewhat zinc-blende and the usual wurtzite structures for CdS and CdSe nanocrystals in chapter 5. In chapter 6 we have pointed out that a gradient structure is essential to achieve nearly ideal photoluminescence efficiency using heterostructured nanocrystals and also achieved strong two-photon absorptions, adding optical bifunctionality to these nanocrystals. Finally, in chapter 7, we establish different approaches to generate white-light using nanocrystals and their unique advantages, as a first step to realizing white light emitting devices. Chapter 1 provides a brief introduction to various interesting properties and concepts relevant for the studies carried out in the subsequent chapters of this thesis. The present status of the research in the field of semiconductor nanocrystals with an emphasis on synthesizing high quality nanocrystals, doping of nanocrystals and exciting optical properties exhibited by these nanocrystals has been discussed. We have discussed the existing theories and practices of colloidal synthesis that allow us to prepare high quality semiconductor nanocrystals with required size and very narrow size distribution. Optical properties, covering excitonic fine structure, photoluminescence, auger recombination and two-photon absorption have been discussed. We have described heterostructured nanocrystals of different types, particularly in the light of enhancing photoluminescence quantum yield. The difficulty in doping Mn2+ ion in semiconductor nanocrystals and the recent developments in this field have been addressed. Chapter 2 describes experimental and theoretical methodologies that have been employed to study different nanocrystal systems reported in this thesis. The topics covered in this chapter include UV-visible absorption spectroscopy, steady-state and time-resolved luminescence spectroscopy, X-ray diffraction, transmission electron microscopy, electron spin resonance spectroscopy, photoemission spectroscopy, two-photon absorption and least-squared-error fitting. Chapter 3 presents a detailed study of water soluble Mn2+-doped CdS nanocrystals synthesized using colloidal routes. Earlier efforts to dope Mn2+ ion into CdS nanocrystals and therefore, obtain the characteristic orange emission, have been largely impeded by the strong overlap of surface state emission of the host and Mn2+ d-emission. We are the first ones to obtain a distinct Mn2+ d-related emission at around 620 nm, well-separated from the surface state emission with its maximum near 508 nm. In spite of using very high (~30%) concentration of Mn2+ precursor, only ~1% Mn2+ was found in the final product, which is consistent with previous literatures, where Mn2+ doping in such nanocrystals was found to be extremely difficult. Most interestingly, present results establish that Mn2+ ion is found to be incorporated preferentially in the relatively larger sized nanocrystals compared to the smaller sized ones even within the narrow size distribution achieved for a specific reaction condition. We found that 55 oC is the optimum reaction temperature to synthesize Mn2+-doped CdS nanocrystals, at higher reaction temperatures, Mn2+ ions get annealed out of the substitutional sites, leading to a lower level of doping in spite of the formation of larger sized particles. Additionally, we could tune the color of the Mn2+ d- emission from red (620 nm) to yellow (580 nm) by increasing the reaction temperature from 55 oC to 130 oC. Another important aspect is that the synthesized nanocrystals readily dissolve in water without any perceptible effect on the Mn2+ d emission intensity. Chapter 4 discusses the outstanding problem that a semiconductor host in the bulk form can be doped to a large extent, while the same host in the nanocrystal form resist any appreciable level of doping. We first describe two independent models available in literatures to explain this baffling phenomenon. In one, it was suggested that the doping of Mn2+ ion in such nanoclusters is invariably an energetically unfavorable state, thus, Mn2+ ions get annealed out from the host nanocrystal and an increase in reaction temperature facilitate such annealing, a phenomenon known as self-purification. In the second model, it was suggested that the ease of initial adsorption of Mn2+ ions on specific surfaces of a growing nanocrystal, kinetically controls the extent of impurity doping. Specifically, it is easier to dope zinc-blende nanocrystals compared to their wurtzite counterpart. In contrast, the main claim of this chapter is neither crystal structure nor self-purification is as important in nanocrystal doping as lattice mismatch between the dopant and host lattice. To support this claim, we have doped Mn2+ ions into alloyed ZnxCd1-xS nanocrystals. Ionic radius of Mn2+ ion being in between those of Zn2+ and Cd2+ ions, the lattice mismatch between the host ZnxCd1-xS nanocrystal and MnS could be tuned in either side by tuning the composition “x”. It was gratifying to observe that there is an evident maximum of manganese content for Zn0.49Cd0.51S host nanocrystals that has no lattice mismatch with MnS, and the manganese content decreases systematically with increasing compressive as well as tensile lattice mismatches. Based on lattice parameter tuning, we could dope an extraordinarily higher amount of ~7.5% manganese for x = 0.49, at a reaction temperature as high as 310 oC and in a nanocrystal that exhibit wurtzite structure, which was previously suggested unfavorable for doping. These results prove our hypothesis that the strain fields generated because of the lattice mismatch between the dopant and host, are necessarily long range, much longer than typical nanocrystal dimensions and it tends to relieve itself by ejecting the dopant to the surface of nanocrystals, thus, resisting doping in such nanocrystals. High temperature synthesis, on the other hand, leads to a very high photoluminescence efficiency of ~25%. Chapter 5 deals with the phase-control of CdS and CdSe nanocrystals synthesized employing colloidal routes. CdS nanocrystals exhibit a very sensitive phase transformation from zinc-blende to wurtzite structure by increasing the reaction temperature from 280 to 310 oC, which is also accompanied by an increase in particle size from 6 to 6.8 nm, respectively. More importantly, just by changing the S precursor, it has been possible to change the crystal structure of the CdS nanocrystals at a given synthesis temperature of 310 oC. En route, we have synthesized >12 nm zinc-blende CdS nanocrystal, which is the largest one known in literature and that too employing the highest (310 oC) reaction temperature. Thus, our results contradict with the suggestions already in literatures that low reaction temperature and small crystal size favors zinc-blende structure. Also, we could tune crystal structure between zincblende and wurtzite at a given pressure of the reaction vessel and for a given solvent, just by changing the S-precursor, which is again in contradiction to previously made suggestions in literatures that high pressure or noncoordinating solvents favors the formation of zinc-blende nanocrystals. Instead, we believe that the surface energy might be crucial in stabilizing the usually rare zinc-blende structure for such nanocrystals. Chapter 6 is divided into two sections and deals with optically active heterostructured nanocrystals exhibiting high photoluminescence efficiency and strong two-photon absorption. In section-I, we probe the internal structure of extraordinarily luminescent (quantum yield = 85%) CdSeS nanocrystals making a somewhat unconventional use of Photoelectron spectroscopy, using the tunability of the photon energy from the third generation synchrotron radiation source as well as the traditional Mg Kα and Al Kα photon sources. CdSeS nanocrystals synthesized with Se:S precursor ratios 1:5 and 1:50, emitting red and green light have CdSe/CdSeS/CdS core/gradient-shell/shell and CdSeS/CdS gradient-core/shell structure, respectively. Gradient interface/core tunes the lattice parameters continuously between that of CdSe and CdS minimizing the interface related defects which in turn increases the photoluminescence efficiency even beyond that obtained from traditional core/shell nanocrystals, as evidenced by the nearly single exponential photoluminescence decay dynamics exhibited by these nanocrystals. Quantum mechanical calculations further show that a graded-core/shell structure leads to a remarkable spatial collapse and consequently a stronger overlap of the HOMO and LUMO wavefunctions towards the core region and thereby, making these luminescent beyond the traditional core/shell limit. In section-II, we have synthesized hetero-structured nanocrystals with CdSe rich core and CdS-ZnS hybrid shell using a simple single-step reaction. These nanocrystals exhibit a very rare example of an optically bi-functional material, simultaneously exhibiting high (~65%) photoluminescence efficiency and strong two-photon absorption cross-section of 1923 GM. Open-aperture z-scan technique was used to measure two-photon absorptions. Chapter 7 is divided into two sections and deals with the generation of white-light emitting nanophosphors. Section-I addresses the white-light emission from a blend of blue, green and red emitting CdSeS nanocrystals. Different shades of the emitted white-light were achieved by tailoring the composition of the blende. Chromaticity of the emitted light of a particular blend is independent of excitation wavelength. Section-II discusses a new approach to generate white-light by combining surface-state emission of nanocrystalline host and d-electron transitions from dopant centres, with an example of Mn2+-doped CdS nanocrystals. Relative contributions from both surface-state emission and Mn2+ d-emission can be tuned by controlling the dopant concentration to generate white lights of different shades. Similar to section-I, here again the chromaticity of the emitted light is independent of the excitation wavelength; but this approach offers additional advantages. Since the surface state emission as well as the Mn2+ d-emission are relatively less sensitive to a size variation compared to the band-edge emission, the chromaticity of the emitted light is not critically dependent on the particle size. Most importantly, these nanocrystals exhibit a huge stokes shift between the absorption and emission spectra resulting in a complete absence of the well-known self-absorption problem, thus, chromaticity of the white-light emitted by these nanocrystals remains unchanged both in dilute dispersion form as well as in solid state. Also there are two appendices in the thesis. Appendix A discusses the preparation of InP nanocrystals using a novel solvothermal route. Appendix B contains the equations explaining photoemission intensity ratios between Se and S (ISe/IS) for a model nanocrystal with a given internal structure.

Semiconductor nanocrystals with sizes comparable to the corresponding bulk excitonic diameter exhibit unique size-dependent electronic and optical properties resulting from quantum confinement effect. Such nanocrystals not only allow the study of evolution of bulk properties from the molecular limit providing important fundamental understandings, but also have great technological implications, leading to intense research over the past several years. Besides tuning the crystal size in the nm regime to obtain novel properties, an additional route to derive new functionalities has been to dope transition metal ions into a semiconductor host. Thus, transition metal doped nanocrystals are of great interest since it allows two independent ways to functionalize semiconductor materials, one via the tunability of properties by size variation and other due to properties of such dopants. Chapter 1 of the thesis provide a general introduction to the subject matters dealt in with this thesis, while the necessary methodologies have been discussed in chapter 2. Chapters 3 and 4 of this thesis deal with nanocrystal doping. Following suggestions in previous literatures that the doping of nanocrystal depends strongly upon the crystal structure of the synthesized host nanocrystal, we have studied the phase-transformation between the somewhat zinc-blende and the usual wurtzite structures for CdS and CdSe nanocrystals in chapter 5. In chapter 6 we have pointed out that a gradient structure is essential to achieve nearly ideal photoluminescence efficiency using heterostructured nanocrystals and also achieved strong two-photon absorptions, adding optical bifunctionality to these nanocrystals. Finally, in chapter 7, we establish different approaches to generate white-light using nanocrystals and their unique advantages, as a first step to realizing white light emitting devices. Chapter 1 provides a brief introduction to various interesting properties and concepts relevant for the studies carried out in the subsequent chapters of this thesis. The present status of the research in the field of semiconductor nanocrystals with an emphasis on synthesizing high quality nanocrystals, doping of nanocrystals and exciting optical properties exhibited by these nanocrystals has been discussed. We have discussed the existing theories and practices of colloidal synthesis that allow us to prepare high quality semiconductor nanocrystals with required size and very narrow size distribution. Optical properties, covering excitonic fine structure, photoluminescence, auger recombination and two-photon absorption have been discussed. We have described heterostructured nanocrystals of different types, particularly in the light of enhancing photoluminescence quantum yield. The difficulty in doping Mn2+ ion in semiconductor nanocrystals and the recent developments in this field have been addressed. Chapter 2 describes experimental and theoretical methodologies that have been employed to study different nanocrystal systems reported in this thesis. The topics covered in this chapter include UV-visible absorption spectroscopy, steady-state and time-resolved luminescence spectroscopy, X-ray diffraction, transmission electron microscopy, electron spin resonance spectroscopy, photoemission spectroscopy, two-photon absorption and least-squared-error fitting. Chapter 3 presents a detailed study of water soluble Mn2+-doped CdS nanocrystals synthesized using colloidal routes. Earlier efforts to dope Mn2+ ion into CdS nanocrystals and therefore, obtain the characteristic orange emission, have been largely impeded by the strong overlap of surface state emission of the host and Mn2+ d-emission. We are the first ones to obtain a distinct Mn2+ d-related emission at around 620 nm, well-separated from the surface state emission with its maximum near 508 nm. In spite of using very high (~30%) concentration of Mn2+ precursor, only ~1% Mn2+ was found in the final product, which is consistent with previous literatures, where Mn2+ doping in such nanocrystals was found to be extremely difficult. Most interestingly, present results establish that Mn2+ ion is found to be incorporated preferentially in the relatively larger sized nanocrystals compared to the smaller sized ones even within the narrow size distribution achieved for a specific reaction condition. We found that 55 oC is the optimum reaction temperature to synthesize Mn2+-doped CdS nanocrystals, at higher reaction temperatures, Mn2+ ions get annealed out of the substitutional sites, leading to a lower level of doping in spite of the formation of larger sized particles. Additionally, we could tune the color of the Mn2+ d- emission from red (620 nm) to yellow (580 nm) by increasing the reaction temperature from 55 oC to 130 oC. Another important aspect is that the synthesized nanocrystals readily dissolve in water without any perceptible effect on the Mn2+ d emission intensity. Chapter 4 discusses the outstanding problem that a semiconductor host in the bulk form can be doped to a large extent, while the same host in the nanocrystal form resist any appreciable level of doping. We first describe two independent models available in literatures to explain this baffling phenomenon. In one, it was suggested that the doping of Mn2+ ion in such nanoclusters is invariably an energetically unfavorable state, thus, Mn2+ ions get annealed out from the host nanocrystal and an increase in reaction temperature facilitate such annealing, a phenomenon known as self-purification. In the second model, it was suggested that the ease of initial adsorption of Mn2+ ions on specific surfaces of a growing nanocrystal, kinetically controls the extent of impurity doping. Specifically, it is easier to dope zinc-blende nanocrystals compared to their wurtzite counterpart. In contrast, the main claim of this chapter is neither crystal structure nor self-purification is as important in nanocrystal doping as lattice mismatch between the dopant and host lattice. To support this claim, we have doped Mn2+ ions into alloyed ZnxCd1-xS nanocrystals. Ionic radius of Mn2+ ion being in between those of Zn2+ and Cd2+ ions, the lattice mismatch between the host ZnxCd1-xS nanocrystal and MnS could be tuned in either side by tuning the composition “x”. It was gratifying to observe that there is an evident maximum of manganese content for Zn0.49Cd0.51S host nanocrystals that has no lattice mismatch with MnS, and the manganese content decreases systematically with increasing compressive as well as tensile lattice mismatches. Based on lattice parameter tuning, we could dope an extraordinarily higher amount of ~7.5% manganese for x = 0.49, at a reaction temperature as high as 310 oC and in a nanocrystal that exhibit wurtzite structure, which was previously suggested unfavorable for doping. These results prove our hypothesis that the strain fields generated because of the lattice mismatch between the dopant and host, are necessarily long range, much longer than typical nanocrystal dimensions and it tends to relieve itself by ejecting the dopant to the surface of nanocrystals, thus, resisting doping in such nanocrystals. High temperature synthesis, on the other hand, leads to a very high photoluminescence efficiency of ~25%. Chapter 5 deals with the phase-control of CdS and CdSe nanocrystals synthesized employing colloidal routes. CdS nanocrystals exhibit a very sensitive phase transformation from zinc-blende to wurtzite structure by increasing the reaction temperature from 280 to 310 oC, which is also accompanied by an increase in particle size from 6 to 6.8 nm, respectively. More importantly, just by changing the S precursor, it has been possible to change the crystal structure of the CdS nanocrystals at a given synthesis temperature of 310 oC. En route, we have synthesized >12 nm zinc-blende CdS nanocrystal, which is the largest one known in literature and that too employing the highest (310 oC) reaction temperature. Thus, our results contradict with the suggestions already in literatures that low reaction temperature and small crystal size favors zinc-blende structure. Also, we could tune crystal structure between zincblende and wurtzite at a given pressure of the reaction vessel and for a given solvent, just by changing the S-precursor, which is again in contradiction to previously made suggestions in literatures that high pressure or noncoordinating solvents favors the formation of zinc-blende nanocrystals. Instead, we believe that the surface energy might be crucial in stabilizing the usually rare zinc-blende structure for such nanocrystals. Chapter 6 is divided into two sections and deals with optically active heterostructured nanocrystals exhibiting high photoluminescence efficiency and strong two-photon absorption. In section-I, we probe the internal structure of extraordinarily luminescent (quantum yield = 85%) CdSeS nanocrystals making a somewhat unconventional use of Photoelectron spectroscopy, using the tunability of the photon energy from the third generation synchrotron radiation source as well as the traditional Mg Kα and Al Kα photon sources. CdSeS nanocrystals synthesized with Se:S precursor ratios 1:5 and 1:50, emitting red and green light have CdSe/CdSeS/CdS core/gradient-shell/shell and CdSeS/CdS gradient-core/shell structure, respectively. Gradient interface/core tunes the lattice parameters continuously between that of CdSe and CdS minimizing the interface related defects which in turn increases the photoluminescence efficiency even beyond that obtained from traditional core/shell nanocrystals, as evidenced by the nearly single exponential photoluminescence decay dynamics exhibited by these nanocrystals. Quantum mechanical calculations further show that a graded-core/shell structure leads to a remarkable spatial collapse and consequently a stronger overlap of the HOMO and LUMO wavefunctions towards the core region and thereby, making these luminescent beyond the traditional core/shell limit. In section-II, we have synthesized hetero-structured nanocrystals with CdSe rich core and CdS-ZnS hybrid shell using a simple single-step reaction. These nanocrystals exhibit a very rare example of an optically bi-functional material, simultaneously exhibiting high (~65%) photoluminescence efficiency and strong two-photon absorption cross-section of 1923 GM. Open-aperture z-scan technique was used to measure two-photon absorptions. Chapter 7 is divided into two sections and deals with the generation of white-light emitting nanophosphors. Section-I addresses the white-light emission from a blend of blue, green and red emitting CdSeS nanocrystals. Different shades of the emitted white-light were achieved by tailoring the composition of the blende. Chromaticity of the emitted light of a particular blend is independent of excitation wavelength. Section-II discusses a new approach to generate white-light by combining surface-state emission of nanocrystalline host and d-electron transitions from dopant centres, with an example of Mn2+-doped CdS nanocrystals. Relative contributions from both surface-state emission and Mn2+ d-emission can be tuned by controlling the dopant concentration to generate white lights of different shades. Similar to section-I, here again the chromaticity of the emitted light is independent of the excitation wavelength; but this approach offers additional advantages. Since the surface state emission as well as the Mn2+ d-emission are relatively less sensitive to a size variation compared to the band-edge emission, the chromaticity of the emitted light is not critically dependent on the particle size. Most importantly, these nanocrystals exhibit a huge stokes shift between the absorption and emission spectra resulting in a complete absence of the well-known self-absorption problem, thus, chromaticity of the white-light emitted by these nanocrystals remains unchanged both in dilute dispersion form as well as in solid state. Also there are two appendices in the thesis. Appendix A discusses the preparation of InP nanocrystals using a novel solvothermal route. Appendix B contains the equations explaining photoemission intensity ratios between Se and S (ISe/IS) for a model nanocrystal with a given internal structure.

碩士
國立臺北科技大學
光電工程系研究所
105
In this study, the electrical and optical characteristics of the textured crystalline silicon (C-Si) solar cells coated with luminescent down-shifting (LDS) of MAPbBr3 perovskite nanophosphor by spin-on film technique are demonstrated. Due to high reflectance and low spectral response at the ultraviolet (UV) and blue wavelengths (300–450 nm) bands, a higher recombination loss would be exhibited on the surface of photovoltaic devices because the incident photons of higher energy were absorbed within a short distance from the surface. The LDS phosphors materials can absorb high-energy photons and re-emitted lower-energy photons for the applications of solar cells to improve low spectral response at short wavelength band. Otherwise, the large diameter of phosphor had a larger shading and reflecting area to incident lights. The effects will more obvious be presented on the textured solar cell. In this study, the MAPbBr3 nanophosphor layer was appositely deposited by spin-on film technique on the textured C-Si solar cells. The spin methods and the concentration and layer of nanophosphor to achieve high efficiency are also discussed. The samples with nanophosphor concentration of 10 mg/ml and with 1-3 layers of nanophosphor deposited by two-step spinning rate on the textured silicon solar cell with a SiNx anti-reflection coating are prepared for comparing. The SEM analysis, optical reflectance, external quantum efficiency, dark current-voltage and photovoltaic current-voltage measurements of the solar cells with MAPbBr3 nanophosphor layer are measured and compared. The short circuit current density enhancement (ΔJsc) of 3.13% (from 36.48 mA/cm2 to 37.62 mA/cm2) and 4.35% (from 35.83 mA/cm2 to 37.39 mA/cm2), and the conversion efficiency enhancement (Δη) of 3.38% (from 15.08% to 15.59%) and 4.56% (from 15.13% to 15.82%) were obtained for the cells with 1-layer and 2-layer nanophosphor, respectively. However, the performance of current density and conversion efficiency of the textured cells with 3 layers nanophosphor are degraded that ΔJsc of -1.77% (from 37.85 mA/cm2 to 37.18 mA/cm2) and Δη of -1.72% (from 15.13% to 14.87%). The experimental results show that the textured silicon solar cell with 1-2 layer of MAPbBr3 nanophosphor presented a good LDS characteristics. Especially, the efficiency of the cell coated with 2 layers of MAPbBr3 nanophosphor with the concentration of 10 mg/ml is superior to that of the other ones.