Universidade de Aveiro Departamento de F´ısica, 2016
Ricardo Liz de
Castilho Fernandes
D´ıodos Emissores de Luz Verde Para Ilumina¸
c˜
ao de
Estado S´
olido
Universidade de Aveiro Departamento de F´ısica, 2016
Ricardo Liz de
Castilho Fernandes
D´ıodos Emissores de Luz Verde Para Ilumina¸
c˜
ao de
Estado S´
olido
Green Emitting Diodes for Solid State Lighting
Disserta¸c˜ao apresentada `a Universidade de Aveiro para cumprimento dos requesitos necess´arios `a obten¸c˜ao do grau de Mestre em Engenharia F´ısica, realizada sob a orienta¸c˜ao cient´ıfica da Doutora Maria Rute de Amorim e S´a Ferreira Andr´e, Professora Associada com Agrega¸c˜ao do Departamento de F´ısica da Universidade de Aveiro e do Doutor Lu´ıs Ant´onio Ferreira Martins Dias Carlos, Professor Catedr´atico do Departamento de F´ısica da Universidade de Aveiro.
o j´uri / the jury
presidente / president Professor Doutor Luis Miguel Rino Cerveira da Silva
Professor Auxiliar
vogais / examiners committee Professora Doutora Maria Rute de Amorim e S´a Ferreira Andr´e
Professora associada com agraga¸c˜ao (orientador)
Doutora Ana Maria de Matos Charas
Investigadora principal no Instituto de Telecomunica¸c˜oes e Instituto Superior T´ecnico
agradecimentos / acknowledgements
Em primeiro lugar queria agradecer aos meus orientadores Professora Doutora Maria Rute de Amorim e S´a Ferreira Andr´e e Professor Doutor Lu´ıs Ant´onio Ferreira Martins Dias Carlos. `A Doutora Raquel Rond˜ao por todo o apoio que me deu durante o decorrer da tese e `a Doutora Mariela Nolasco. Gostaria tamb´em de agradecer `as seguintes pessoas pela sua contribui¸c˜ao directa nesta disserta¸c˜ao: Vˆania Freitas, Sandra Correia, Rita Frias e Ruben dos Santos. Por fim, gostaria tamb´em de agradecer `as seguintes pessoas pela sua amizade, ajuda e apoio prestado durante todo o meu percurso acad´emico: Ao meu irm˜ao Le-andro Fernandes, Ana Quaresma, Joana Rocha, Tiago “Zizu” Azevedo e David Tavares.
This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID /CTM /50011/2013), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement.
Palavras Chave D´ıodos emissores de luz (LEDs), Ln3+, fotoluminescˆencia, lumin´ofero,
h´ıbridos orgˆanicos-inorgˆanicos.
Resumo Nos anos recentes a ilumina¸c˜ao de estado s´olido impulsionou alter-nativas de ilumina¸c˜ao eficientes e ecol´ogicas. Os desafios correntes envolvem o desenvolvimento de materiais emissores de luz que con-vertem radia¸c˜ao de uma determinada energia para radia¸c˜ao de ener-gia mais baixa, na gama do vis´ıvel. Esta tese estuda um complexo novo, Tb(NaI)3(H2O)2 onde NaI ´e o ´acido nalid´ıxico, que emite na
regi˜ao do verde e ´e est´avel sob ilumina¸c˜ao no ultravioleta. Este foi incorporado em materiais h´ıbridos orgˆanico-inorgˆanico tripodais com dois pesos moleculares m´edios (3000 e 5000 g.mol–1, denominados
t-U(3000) e t-U(5000) respetivamente) que permitem o processamento de mon´olitos e filmes com forma e espessura controlada. Estes h´ıbridos tamb´em aumentam o rendimento quˆantico absoluto de emiss˜ao de ∼0.11 medidos para o Tb(NaI)3(H2O)2 isolado para ∼0.82 ap´os
incor-pora¸c˜ao no t-U(5000). Foi tamb´em demonstrado o potencial de usar estes materiais h´ıbridos como emissores na regi˜ao verde para uso em ilumina¸c˜ao de estado s´olido atrav´es do revestimento do d´ıodo emissor na regi˜ao ultravioleta (365 nm). Este LED apresenta uma efic´acia de 1.3 lm.W−1.
Keywords Light emitting diodes (LEDs), Ln3+, photoluminescent, phosphor,
organic-inorganic hybrids.
Abstract In the last few years, solid state light-emitting diodes (LEDs) have been driving the lighting industry towards energy efficient and environ-mental friendly lighting. Current challenges encompass efficient and low-cost downconverting photoluminescent phosphors with emission in the visible region. This thesis will cover a novel UV-photostable green emitting complex, Tb(NaI)3(H2O)2 where NaI is nalidixic acid,
was incorporated into organic-inorganic tripodal hybrid materials with two average molecular weights (3000 and 5000 g.mol–1, termed as
t-U(5000) and t-U(3000), respectively) which enable the easy shaping of monoliths and films with controlled thickness. Moreover, the hy-brid hosts boost the Tb3+ green absolute emission quantum yield from
∼0.11 measured for the isolated Tb(NaI)3(H2O)2 complex to ∼0.82
after incorporation into t-U(5000). The potential use of the hybrid materials as UV-down converting green-emitting phosphors for solid state lighting was demonstrated by means of coating a near-UV LED (365 nm). This LED shows an efficacy of 1.3 lm.W−1.
Contents
Contents i
List of Figures ii
1 Introduction 1
2 Background 6
2.1 Experimental Characterization Techniques . . . 6
2.1.1 Ultra Violet(UV)-Visible Absorption Spectroscopy . . . 6
2.1.2 Infrared Absorption Spectroscopy . . . 7
2.1.3 Raman Spectroscopy . . . 9
2.1.4 Nuclear Magnetic Resonance . . . 10
2.1.5 Luminescence . . . 11
2.2 Lanthanide Ions . . . 14
3 Synthesis and Processing 16 3.1 Experimental Section . . . 16
3.2 Results and discussion . . . 21
3.2.1 29Si magic-angle spinning (MAS) nuclear magnetic resonance (NMR) and 13C cross polarization (CP) MAS NMR spectra . . . 21
3.2.2 Vibrational studies . . . 22
4 Optical Properties 26 4.1 Experimental Details . . . 26
4.2 Results and Discussion . . . 27
4.2.1 UV-visible absorption spectroscopy . . . 27
4.2.2 Photoluminescence Spectroscopy . . . 28
4.2.3 UV Photostability . . . 35
5 UV-down converting green emitting LED 37 5.1 Device Fabrication . . . 37
5.2 Optical Properties . . . 39
6 Conclusions 43
List of Figures
1.1 (a) Scheme showing three common ways to produce white light based on LEDs. (b) Comparison of the spectrum of ideal sunlight with those of two WLED configurations. . . 2 2.1 (a) An illustration of the relationship between the spectrum of an an
ide-alized laser line and its interference pattern. (b) Diagram of a Michelson interferometer. . . 7 2.2 Scheme ilustrating the optical processes that take place when an IR beam in
a crystal of high refractive index, nc, encounters a sample of lower refractive
index ns. θi is the angle of incidence, θR is the angle of refraction, θr is the
angle of reflectance, and θc is the critical angle above which total internal
reflection takes place. . . 8 2.3 Diagram of the Rayleigh and Raman scattering processes. The lowest energy
vibrational state is m and the higher is n. . . 10 2.4 Jablonski diagram that illustrates some of the electronic states of a molecule
and the transitions between them. . . 12 2.5 Schematic diagram showing elements used to measure photoluminescence
spectra. . . 13 2.6 Energy levels of the 4fN configurations of the trivalent lanthanide ions. . . 15
3.1 Schematic for the synthesis process of Tb(NaI)3(H2O)2. . . 17
3.2 Photos of the Tb(NaI)3(H2O)2 complex in powdered form and of the hybrid
grown monolith, tU(5000)Tb-M, under (a) natural daylight and (b) UV illumination (365 nm). . . 17 3.3 Schematic representation of the synthesis of the non-hydrolyzed precursors
t-U(5000) and t-U(3000). . . 18 3.4 Photo of the tU(5000)Tb-F under UV illumination (365 nm). . . 19 3.5 Optical microscopy image of the cross section of the tU(5000)Tb-F under
UV illumination (365 nm). . . 19 3.6 29Si MAS NMR spectra of (a) tU(5000)Tb-M and (b) tU(3000)Tb-M. . . . 21 3.7 13C MAS NMR spectra of (a) tU(5000)Tb-M and (b) tU(3000)Tb-M. . . . 22
3.8 (a) ATR spectra of HNaI ligand (black line) and Tb(NaI)3(H2O)2
com-plex (red line). (b) Raman spectra of HNaI ligand (black line) and Tb(NaI)3(H2O)2 complex (red line). . . 22
3.9 FT-IR (700-2000 cm−1) and Raman (2700-3100 cm−1) spectra of tU(5000) (black line) and tU(5000)Tb-M hybrid samples (red line). . . 23 3.10 ATR (700-2000 cm−1) and Raman (2700-3100 cm−1) spectra of
tU(3000)Tb-M (black line) and tU(3000)Tb-F (red line) hybrid samples. . . 25 4.1 UV-visible absorption spectra of a) Tb(NaI)3(H2O)2 in ethanol (black line)
and NaI in ethanol (red line) b) tU(5000)-M (green line), tU(3000)-M (blue line), tU(5000)Tb-M (cyan line) and tU(3000)Tb-M (magenta line) and c) tU(5000)-F (green line), tU(3000)-F (blue line), tU(5000)Tb-F (cyan line) and tU(3000)Tb-F (magenta line). . . 27 4.2 Excitation spectra (300 K) of Tb(NaI)3(H2O)2 (black line),
tU(5000)Tb-M (red line), tU(3000)Tb-tU(5000)Tb-M (green line), tU(5000)Tb-F (blue line) and tU(3000)Tb-F (cyan line), monitored at 545 nm. The inset depicts the maximization of the 7F6 → 5D4 transition. . . 29
4.3 Emission spectra (300 K) of (a) Tb(NaI)3(H2O)2, (b) tU(5000)Tb-M, (c)
tU(3000)Tb-M (d) tU(5000)Tb-F and (e) tU(3000)Tb-F excited at (lower color) 275 nm and at (upper color) 340 nm. . . 30 4.4 Excitation spectra (a) of tU(5000) (black and red lines for emission at 305
nm and 420 nm respectively) and tU(3000) (green and blue lines for emission at 305 nm and 420 nm respectively) and emission spectra (b) of tU(5000) (black and red lines for excitation at 270 nm and 340 nm respectively) and tU(3000) (green and blue lines for excitation at 270 nm and 340 nm respectively). . . 31 4.5 High-resolution emission spectra (18 K) of Tb(NaI)3(H2O)2 (black line),
tU(5000)Tb-M (red line) and tU(3000)Tb-M (blue line) excited at 275 nm. The inset shows the magnified transitions for a)5D4 →7F6 b)5D4 →7F5and
c)5D
4 →7F4. . . 31
4.6 Emission decay curves of Tb(NaI)3(H2O)2 at 300K (black circle) and at 18
K (red triangle) excited at 356 nm and monitored at 545 nm. The solid lines (cyan and yellow) represent the data best fit (r2 >0.99), using a single
exponential function. The respective residual plots are shown on the right side. . . 32 4.7 Emission decay curves of tU(5000)Tb-M at 300K (black circle) and at 18
K (red triangle) excited at 356 nm and monitored at 545 nm. The solid lines (cyan and yellow) represent the data best fit (r2 >0.99), using a single exponential function. The respective residual plots are shown on the right side. . . 33
4.8 Emission decay curves of tU(3000)Tb-M at 300K (black circle) and at 18 K (red triangle) excited at 356 nm and monitored at 545 nm. The solid lines (cyan and yellow) represent the data best fit (r2 >0.99), using a single
exponential function. The respective residual plots are shown on the right side. . . 33 4.9 Emission decay curves of tU(5000)Tb-F (black circle) and tU(3000)Tb-F
(red triangle) excited at 356 nm and monitored at 545 nm. The solid lines (cyan and yellow) represent the data best fit (r2 >0.99), using a single
ex-ponential function. The respective residual plots are shown on the right side. . . 34 4.10 Spectral radiance (300 K) of (a) Tb(NaI)3(H2O)2 and (b) tU(5000)Tb-M
excited at 365 nm, as function of the irradiation time and (c) integrated radiant flux variation of the5D4 → 7F5 transition with the irradiation time
for Tb(NaI)3(H2O)2 (black dot) and tU(5000)Tb-M (red square). . . 36
5.1 Emission spectrum of the UVLED365-SMD under 20×10−3 A forward current. 38 5.2 Photos of the: (a) 3D rendering of the PCB and (b) printed PCB with
soldered SMDs. . . 38 5.3 Photograph of PCB with soldered SMDs covered with t-U(5000)Tb hybrid. 39 5.4 Photos of the (a) 3D rendering of the single SMD PCB and (b) printed PCB
with soldered SMD and connecting cables. . . 40 5.5 (a) Spectral radiance of the Tb3+hybrid material coupled with the SMD-UV
Chapter 1
Introduction
General lighting is one of society’s fundamental needs and it constitutes a significant portion of any state’s economic budget, job market and carbon footprint [1]. From the last reported values, it is estimated that around 19 % of the energy consumed worldwide was used for lighting [2], with this figure being expected to increase as outdoor lighting has been steadily increasing globally at a rate of 3–6 % per year [1], and also as the standard of living increases around the world [3]. Concerns over the systematic rise of lighting energy consumption and the increasing interest in green and sustainable energies led to a paradigm shift in the lighting technologies as filament-based incandescent technologies gave way to gas-plasma-based fluorescent technologies and more recently solid state lighting (SSL) [4]. Looking at the advances in SSL in the last decades, lighting solution industries turned to the new emerging technology of the light emitting diodes (LEDs) as they present ad-vantages which outweigh the investment cost. SSL performance is characterized by the luminous efficacy (LE | lm.W−1) that accounts for the ratio of the emitted luminous flux (lm) and the consumed electric power (W). Typically, to satisfy general indoor lighting requirements, either incandescent light-bulbs (100 W) are used with low LE of 15 lm.W−1 or/and fluorescent bulbs with larger LE values, around 100 lm.W−1 [5]. For the case of outdoor lighting, high pressure sodium lamps are typically used with LE values arround 140 lm.W−1 [6]. Nowaday’s white light emitting LEDs (WLEDs) have already reached values of upwards 300 lm.W−1 at 350 mA forward current [7]. Such high LE achieved by WLEDs compared with that of conventional lighting yields the need of less energy required to output the same luminous flux (lm) which means that energy costs for general indoor and outdoor lighting can be greatly reduced. Also by taking advantage of WLEDs abil-ity to be frequently powered on and off with less worn-out device damage, much unlike incandescent and fluorescent lamps, “smart” luminaries can be built that only switch on when light is needed, thus reducing even further energy consumption [5]. WLEDs can also be used in displays of scalable sizes from large electrical billboard to small mobile phone displays due to their reduced dimensions and weight. Moreover, LED-based light sources permit a controlled spectral, geometric and radiance output. With these comes the possibility of tuning the colour hue in order to fit different spectral profiles, such as, for example, one to coincide with the Sun’s spectrum for that time of the day. This
applica-tion serves the purpose of controlling the amount of blue light that a person receives in so avoiding overexpose of the human body to excessive amounts of blue light which is known to be disruptive for the human/plants circadian rhythms [8, 9]. Such WLEDs simulate the natural colour hue of the Sun’s light that reaches earth at specific times resulting in an artificial lighting solution that is in tune with the circadian physiology [10].
After the major developments on the blue light emitting LED in the early 90s [11] and with the further development on UV-emitting LEDs in combination with efficient phosphor materials, it became possible to generate white light based on LEDs in a commercially viable way [12, 13]. To produce white light emission from LEDs, distinct solutions are proposed in the literature. There are three main common ways of producing WLEDs as shown in Figure 1.1a [14]. One of the three, and the more distinct one, is the use of individual red, green, and blue (RGB) chips assembled in a single device. Another way is to produce additional colours from a monochromatic emitting LED by coupling it with a material (phosphor) that absorbs radiation from a higher-energy wavelength and converts totally, or partially, to a lower energy emission (down-converting) [13]. From this approach, two viable ways appear with one being a blue LED chip covered with a yellow down-converting phosphor that allows some of the exciting blue radiation to bleed through (partial conversion) and the other the use of a InxGa1−xN based near UV (380–410 nm)
LED chip joined with red, green and blue (RGB) down-converting phosphors.
(a)
(b)
Figure 1.1: (a) Scheme showing three common ways to produce white light based on LEDs. (b) Comparison of the spectrum of ideal sunlight with those of two WLED configurations. [14]
A comparison of how well the emission spectrum of the two configurations (Figure 1.1a) compare to the ideal Sun light can be observed in Figure 1.1b [14]. Here we can see that even though the blue emitting LED with yellow phosphor covers almost the entire visible spectrum, its relative intensity per wavelength is a mismatch compared to the ideal sunlight spectrum resulting in a poor colour rendering index (CRI). The UV emitting LED
coupled with RGB phosphors has the advantage that each phosphor provides a much closer representation for each respective colour in the visible spectral range, however it lacks in relative intensity of blue light when compared to the previous example [15].
Naturally the use of any of these approaches comes with their respective advantages and drawbacks, with these playing the deciding factor on the choice of application depending on the context. The summary of advantages and drawbacks are listed in Table 1.1.
Table 1.1: List of advantages and drawbacks for each of the three popular approaches of generating white light (Figure 1.1a).
Approaches Advantages and/or Drawbacks
Blue LED + Yellow Phosphor
⊕ High theoretical efficacy
⊕ Cheapest configuration to produce Low colour rendering index (CRI) Ultraviolet LED + Phosphor mixture ⊕ Higher CRI
Lower efficacy Red, Green and Blue (RGB) LEDs
⊕ Dynamic control of white light hue ⊕ Good CRI
Requires complex circuitry
The drawback of the RGB approach is that it requires distinct LEDs that must be individually adjusted to balance the emission intensity of each colour for the generation of homogeneous white light, which is a difficult task because each illuminance (lm.m−2) distribution is different from the other, leading to complex circuitry [15]. Furthermore, the differential ageing of the LEDs is also another drawback. In the case of phosphor mixing, this results in efficiency losses due to self-absorption, and the different degradation rates of the individual phosphors lead to changes in emission colour over time [16]. The single-phosphor-converted blue LED device is currently the most common WLED produced today, due to the high LE of blue LEDs and yellow-emitting Y3Al5O12:Ce3+ (YAG:Ce3+)
phosphor, despite the limitation of CRI(<80) due to the lack of green and red components, current dependence of chromaticity and high correlated colour temperatures (CCT>4000 K) [13].
The most suitable reported phosphors for WLEDs emit in yellow, green and red spectral ranges. Examples of yellow emitting phosphors are YAG:Ce3+, under blue excitation (460 nm) [17] and (Sr1.7,Ba0.2)SiO4:0.1Eu2+ excited in a broader region from the UV to the
visible (300-500 nm). Green phosphors include (Ba1.1,Sr0.7)SiO4:0.2Eu2+ has an emission
band with peak at 528 nm and is excited in a continuous band from 300 nm to 500 nm, LuAG:Ce3+ and Lu
3Al5O12:Ce3+ with an emission peak at 528 nm under UV (340 nm)
and blue (460 nm) excitation. Red phosphors are mainly based on (Sr,Ba)2Si5N8:Eu2+
with an emission peak at 632 nm excited in a broad spectral region (300-600 nm) and (Sr,Ca)SiAlN3:Eu2+ emitting around 635 nm under UV-visible excitation (380-450 nm)
Currently, the majority of newly reported phosphor compounds are Ln3+-doped ox-ides and nitrox-ides, Eu2+-doped sulfides and borophosphates and organic electroluminescent
materials [15].
Since its first reported version back in 1997, WLED technology has gone through nu-merous improvements in the performance of its individual components as well as the output colour quality. Current performance for commercial single phosphor WLEDs are typically around 130 lm.W−1 with a CRI of 70, such is the case for a Lumileds model Luxeon T [18], while for multiple phosphor WLEDs efficacy is lower (110 lm.W−1) but with a higher CRI of 85 [19]. Theoretical performance projections point for a target of 310 lm.W−1 to be reached by the year 2020 [6], of which the latest reported values are close for one of the popular approaches at WLEDs [7] but for the others there is still plenty research to be done.
Phosphor materials by themselves are limited to their intrinsic characteristics but, by incorporating them with certain host matrices, their properties can be improved. In what concerns material developed organic-inorganic hybrids, incorporating optically active centres, such as lanthanide based complexes, are good candidates as phosphors for the new generation of WLEDs. These organic-inorganic hybrid materials can be classified according to the interaction established between the organic and inorganic counterparts which form the whole. Divided in two classes, the Class I hybrid’s organic molecules are contained within the inorganic network while Class II materials the organic and inorganic molecules are linked in a covalent bond. This thesis will focus on a particular siloxane-based Class II hybrid. These type of materials possess several characteristics that are advantageous to the concerning topic. From a chemistry point of view, they are relatively facile to produce with controlled purity, as they are synthesized from pure precursors, they can be processed at low temperature and allow the encapsulation of large amounts of emitting centres [20]. From an optics point of view, these materials provide high transmission of light, easy control of the refractive index by altering precursor proportions and protection of the isolated emitting centres by the organic-inorganic host enabling the control of nonradiative decay pathways [20].
The emitting centres used in this thesis are trivalent lanthanide ions (Ln3+) which are
known for emitting light in the visible spectrum. These ions do not have efficient direct photoexcitation however this can be improved by using lanthanide complexes, in which the ligands incorporate organic chromophores linked with a strong bond to the 4f metal centre. These chromophores are also particularly useful as they provide effective absorption over a broad spectral range with the absorbed energy then being transferred to the nearby Ln3+ which then can lead to radiative energy transitions. This process is called lanthanide
luminescence sensitization or it is also referred to as antenna effect [20].
The main objective for this thesis is to characterize the optical properties of the studied materials so as to assess their potential as phosphors in order to make for a possible component in an RGB phosphor mixture approach for WLEDs. The Tb(NaI)3(H2O)2
complex structural and optical characteristics will be fist reported on this thesis as well as its incorporation in a tripodal organic-inorganic hybrid. A green phosphor converted LED (pcLED) is proposed and characterized using the most efficient developed hybrid.
In recent years many studies have been conducted exploring the luminescent capabilities and diverse applications of various complexes based on Ln3+ as well as diverse molecules
that enable sensitizing processes [21–23]. This thesis will focus on one complex and study its optical properties, both before and after incorporation into the hybrid, so as to assess its capabilities for acting as a phosphor. The reasoning for incorporating this complex inside a hybrid matrix, along with the previous mentioned characteristics, is due to this class of materials reported capabilities of improving photostability, increasing the overall absorption regions for the complex either being in relative intensity or in range, protecting the complex from environmental degradation and providing additional processing methods for the underlying materials. The complex, Tb(NaI)3(H2O)2, is a novel one and is composed
of two ligands, the neutral one (H2O) and nalidixic acid (NaI). The later one is mostly
known for being part of the quinolone group, in fact it was the first quilone to be developed, that are well known for generally being used as synthetic antibiotics [24].
This thesis is divided in 6 chapters that describe the synthesis, structural characteri-zation, photoluminescence and a LED prototype. The first chapter describes the context in which these materials are inserted, the state of the art of the underlying technology used in LED lighting and the objectives for this thesis. The second chapter introduces the background techniques and concepts used in the research of the materials in concern. The third chapter describes the synthesis process for the complex and organic-inorganic hybrid materials, the processing methods for each sample and structural characterization. The fourth chapter presents the photoluminescent characteristics of the samples studied namely emission and excitation spectra, emission lifetimes, quantum yield and photostability. The fifth chapter presents a LED application for the Tb3+ based hybrid. Chapter 6 finishes
with the closing conclusions for this thesis and presents some suggestions for future work with the concerning materials.
Chapter 2
Background
2.1
Experimental Characterization Techniques
2.1.1
Ultra Violet(UV)-Visible Absorption Spectroscopy
When a sample is illuminated by radiation with intensity I0, it is attenuated after
traversing the sample, which means the intensity Itof the traversed beam is lower than I0.
One of the reasons for this occurrence is due to absorption of radiation by the sample. This occurs when the beam frequency is resonant with the transition energy from a ground state to an excited state, resulting in the absorption of photons from the trespassing beam [25]. The amount of radiation that is attenuated after passing through a material, or the beam intensity differential, dI, after trespassing a differential thickness of dl is given by:
dI = −α × I × dl (2.1)
where I is the radiation intensity located at the distance l of the material and α is the absorption coefficient. By integrating the equation 2.1, we obtain:
I = I0× e−α×l (2.2)
which is called the Beer-Lambert law which can also be written as: T = 10−A; T = I
I0
(2.3) where T is the transmittance and A is the absorbance. The later one is defined as:
A = − log T = ε × c × l (2.4)
where c is the concentration of the absorbing element and ε is the molar absorption co-efficient [26]. By combining equations 2.2 and 2.4 we can now obtain a more practical deduction for the absorption coefficient:
α = A
Absorption spectra are usually registered by spectrometers in a UV-visible spectral range, typically 240-800 nm, and can be configured in different modes to measure optical density, transmittance or absorbance.
2.1.2
Infrared Absorption Spectroscopy
Fourier Transform Infrared
Fourier Transform Infrared (FTIR) spectroscopy is a technique where the properties of the data collected in an interference pattern are mathematically transformed through Fourier’s function yielding a spectrum. The interference pattern is obtained by using two beams of radiation with different optical paths that when superposed generate an interference pattern or an interferogram, as seen in Figure 2.1a.
(a) (b)
Figure 2.1: (a) An illustration of the relationship between the spectrum of an an idealized laser line and its interference pattern [27]. (b) Diagram of a Michelson interferometer [27].
The raw data from an interference pattern generated by the initial beam of radiation and a beam that traversed the sample only gives the intensity of the combined interference and the spacing between the peaks. After applying a Fourier’s transformation on the raw data, the result can be expressed in the quantity of energy, in cm−1, for each interference pattern observed.
To generate an interference pattern a Michelson interferometer is generally used. With this device configuration radiation from an infrared source is split into two beams by using a beamsplitter, as can be seen in Figure 2.1b, which is a device that is designed to transmit a fraction of the incident light that reached it and the remaining light is reflected. The light transmitted by the beamsplitter travels towards a fixed mirror while the reflected light moves in direction to a moving mirror. After both beams are reflected by each mirror they are superposed in the beamsplitter which then guides the resulting interference beam toward the sample. The interference pattern generated without a sample is recorded and then compared to the patterns generated by including a sample in the beam’s path. The pattern difference indicates a change in intensity that is analysed for a specific frequency
that is then mathematically converted into an intensity versus wavenumber plot that revels the sample’s infrared absorption spectrum [27].
Attenuated Total Reflectance
Attenuated total reflectance (ATR) is a technique that also permits to measure an IR spectra, encompassing the FT-IR variant, based upon internal total reflectance of traversing radiation. ATR spectroscopy is often used in the middle IR region of the electromagnetic spectrum which is usually defined as the wavenumber region from 4000 to 400 cm−1 (from 2.5 to 25 µm). The predominant features in this spectral region are primarily due to molecular vibrations which give information about the functional groups present in the molecule, as well as the structure of the molecule. The structure of molecules can be identified by their IR spectra, by way of comparison with a reference determined in the lab or by using IR libraries [28]. A schematic of the working principles can be found in Figure 2.2.
Figure 2.2: Scheme ilustrating the optical processes that take place when an IR beam in a crystal of high refractive index, nc, encounters a sample of lower refractive index ns. θi is
the angle of incidence, θR is the angle of refraction, θr is the angle of reflectance, and θc is
the critical angle above which total internal reflection takes place [27].
The principle of ATR consists on a beam of radiation (IR in this case) that travels through an optically dense material, such as a crystal of some sort with high refractive index nc, which encounters a boundary with a sample with lower refractive index ns. This
starting condition implies that this technique is best suited for samples with refraction indexes lower than the available testing material. The doted line in Figure 2.2 represents the normal plane to the surface of the sample, the angle that the direction of the incoming beam makes with the normal plane is the angle of incidence, θi, and when the IR beam
reaches the boundary of the two media, it will be reflected with angle θr. For small incident
reflected part, that remains inside de crystal, and a refracted part that traverses into the sample with a refraction angle θR. According to Snell’s law:
sin(θi)
sin(θR)
= ns nc
(2.6) as θi increases, θR also increases to a point which it becomes 90◦resulting in the IR beam
not leaving the crystal due to all of the radiation reflecting off the internal surface of the crystal, hence total internal reflectance. The minimum incident angle required for total reflectance is called the critical angle, θc [27].
At the point of internal reflectance, the incoming and outgoing IR beams occupy the same volume and, under the right conditions, θi > θc, these two beams undergo constructive
interference resulting in an IR beam that at the point of internal reflectance possesses amplitude greater than of the incident or refracted beams [27]. The resulting interference beam created at the boundary of the two mediums forms what is called an evanescent wave. This wave sticks up above the surface of the crystal, usually between less than a µm to upwards of 10 µm depending upon the materials and radiation. Since the evanescent wave trespasses onto the sample, it’s intensity is partially absorbed by the sample and thus attenuated. This beam travels parallel to the interface between the two mediums onto the end of the crystal where it is collected. A background spectrum is obtained of the crystal and is then the difference between the clean beam and the total reflected one that represents the samples characteristics [27].
2.1.3
Raman Spectroscopy
When light interacts with matter photons may be absorbed, scattered or may not interact with the material at all, passing straight through it. The scattered photons can be observed by collecting them at an angle to the incident light beam, provided there is no absorption from any electronic transitions which have similar energies to that of the incident light. In the scattering process, light interacts with the molecule and polarizes it giving a state called a “virtual state” which is not stable resulting in the photon being quickly re-radiated. The photons will be scattered with very small frequency changes, as the electrons are comparatively lightweight. This scattering process is regarded as elastic scattering and is the dominant process, being referred to as Rayleigh scattering for molecules. If nuclear motion is induced during the scattering process, energy will be transferred either from the incident photon to the molecule or from the molecule to the scattered photon. In these cases the process is inelastic and the energy of the scattered photon is different from that of the incident photon by one vibrational unit. This is called Raman scattering. This is an inherently weak process in the sense that only one in every 106-108 photons which scatter is Raman scattered, however with modern lasers and sensors, very high power densities can be delivered to very small samples albeit at the cost of sample degradation and unwanted fluorescence. Since the virtual states are not real states of the molecule but are created when the radiation interacts with the electrons and causes polarization, the energy of these states is determined by the frequency of the radiation source used. The Raman scattering
process involves a energy transition from the ground vibrational state m from absorption of energy by the molecule to the virtual state followed by relaxation to a excited vibrational state n. This is called Stokes scattering. However, due to thermal energy, some molecules may be present in an excited state such as n as seen in Figure 2.3.
m
n Vibrational
states Virtual states
Stokes Rayleigh anti-Stokes
Figure 2.3: Diagram of the Rayleigh and Raman scattering processes. The lowest energy vibrational state is m and the higher is n.
Scattering from these states to the ground state m is called anti-Stokes scattering and involves transfer of energy to the scattered photon. Intense Raman scattering occurs from vibrations which cause a change in the polarizability of the electron cloud around the molecule. Usually, symmetric vibrations cause the largest changes and give the greatest scattering. This contrasts with IR absorption where the most intense absorption is caused by a change in dipole and hence asymmetric vibrations which cause this are the most intense. Not all vibrations of a molecule need, or in some cases can, be both IR and Raman active with the two techniques usually giving quite different intensity patterns and as such they are typically used complement to each other. Moreover in Raman spectroscopy, a single frequency of radiation is used to irradiate a sample unlike IR absorption which requires matching of the incident radiation to the energy difference between the ground and excited states. The Raman scattering spectra is obtained by subtracting the collected scattered energy from the original radiation so that the differences in energy correspond to the ground and excited vibrational states (n and m in Figure 2.3) [29].
2.1.4
Nuclear Magnetic Resonance
Nuclear magnetic resonance (NMR) spectroscopy allows the study of molecular struc-ture through measurement of the interaction of an oscillating radio-frequency electromag-netic filed with a collection of nuclei immersed in a strong external magelectromag-netic field [30]. NMR exploits the magnetic properties of nuclei to provide information on molecular struc-ture. For example an hydrogen nucleus has angular momenum properties coming from its charged nucleus and intrinsic spin which creates a magnetic filed. The spin properties of protons and neutrons in the nuclei of heavier elements combine to define the overall spin
of the nucleus. When the atomic number and atomic mass are even, the nucleus has no magnetic properties and as such those nuclei are considered not to be spining, therefore are not detected in NMR experiments. This means even numbered elements are invisible to NMR, however isotopes of those elements can amount to non zero spin and thus can be observed [31].
NMR spectroscopy measures the precession frequencies of individual nuclear magnetic moments in an external magnetic field B0 and the rate of nuclear spin reorientation after
excitation (NMR relaxation). The external magnetic field B0 induces circulating currents
in the electron cloud, which, in turn, generate an induced magnetic field Bi, which is in
the order of 10−4 × B0. The individual spins sense the sum of the external and locally
induced magnetic fields (B0+Bi), resulting in distinguishable magnetic fields (and NMR
frequencies) for any atomic environment in molecules. NMR characteristics depend on the isotope-specific nuclear properties, the applied B0 and the local chemical environments.
Therefore, any atom within a molecule will show individual NMR-relevant properties, allowing the assembly of molecular structures from NMR spectra in the case of resolvable mixtures and the reconstruction of key structural principles in the case of nonresolvable biogeochemical mixtures. NMR spectra are visualized as plots of line intensity versus frequency ωi, expressed as dimensionless, magnetic field independent units of chemical
shift δi. The chemical shift δi denotes the fractional change in frequency induced by the
change in chemical environments normalized to B0 and is expressed by parts per million
(ppm). The accessible chemical shift range varies from about 15 ppm for the case of 1H
NMR up to 22000 ppm in the case of59Co NMR spectroscopy [32].
2.1.5
Luminescence
Luminescence is any nonthermal processes where the emission of light by any substance occurs from radiative transitions of electronically excited states and can be divided in two categories, fluorescence and phosphorescence. These two categories are differentiated by the nature of their excited state where fluorescence occurs from singlet excited states (S) and phosphorescence from triplet excited states (T) resulting in different emission lifetimes [33]. When fluorescence occurs the emission ceases almost immediately after withdrawing the exciting source, whereas with phosphorescence the emission lasts for some time after removing the excitation [34]. When an electron is excited from its ground state to a higher energy state, it has a natural tendency to return to its lowest energy state and thus, after a certain time passes, τ , it returns back to its original place through processes of recombination.
In order for luminescence to occur the electron has to recombine in a radiative manner that results in a photon emission with frequency ν = Eh where E is the energy between the electron’s ground state and the previous occupied excited state and h is the Plank constant.
The excitation type for luminescence determines the kind of luminescence that takes place, for instance if excitation comes from absorbed photons resulting in radiative recom-bination is called photoluminescence and if it is obtained electronically via p-n junction, it
is a form of electroluminescence. When the photon energy of the incident source is lower than the difference between two electronic states, these photons are not absorbed and the material is transparent to such radiation energy [34].
While in fundamental levels, electrons in the same orbital have opposite spin values (s1 = +12 and s2 = −12), making the sum of spins equal to zero. Therefore the fundamental
state has multiplicity (M=2S+1=1) of one, which means that the ground state is designated as a singlet (S0). In the event of optical absorption, the electrons will be excited and,
excluding situations of spin inversion, the excited state will also be a singlet (S1), meaning
it has the same multiplicity of the ground state. However in the event of spin inversion, the two electrons will have the same spin and so the total spin (S=s1+s2) will be one which
means the excited state will have a multiplicity of 3 and so this state is called a triplet (T1).
Absorption involving a triplet state are generally forbidden by the spin selection rule, as allowed transitions must involve the promotion of electrons without change in their spin (∆S=0). Relaxation of the spin selection rule can be allowed through strong spin-orbit coupling which, as an example, occurs in the case of rare earth ions [34].
S0 S1 T1 Absorption Fluorescence (10 -7 -10 -9 s) Phosphoresce nce (10-10 -3 s)
Inter System Crossing
Figure 2.4: Jablonski diagram that illustrates some of the electronic states of a molecule and the transitions between them.
As it was already stated, the transition processes of electrons that return to their fundamental state can be either radiative or nonradiative. Radiative processes are divided between fluorescence and phosphorescence which differ by the kind of transitions, from S1
to S0 and from T1 to S0 respectively, and from the time-scale of these transitions with
one ranging 10−7 s to even lower durations and the other higher than 10−5 s. A diagram of these type of transitions is present in Figure 2.4. The nonradiative can be explained by processes of internal conversion, where an electron close to a ground state relaxes via transitions between vibrational energy levels while transferring excess energy to nearby molecules as heat, and intersystem crossing where the electron in an upper S1 excited
The occurrence of nonradiative transitions means that in the event of an emission, it will occur at lower energy than that of the absorbed photons. This energy difference between the maximum of the emission and of the absorption spectra ascribed to the same electronic transition is known as Stokes–shift [34].
To measure photoluminescence, a typical configuration such as in figure 2.5 is used where the sample is excited by a lamp, which emits in a wide band comprising of the total visible spectrum in addition to near IR and near UV radiations. A monochromator is present in the light path after the broad band emitting lamp to choose the excitation wavelength and before the detector so as to measure each individual wavelength emitted by the sample.
Figure 2.5: Schematic diagram showing elements used to measure photoluminescence spec-tra [25].
With this kind of configuration two kinds of spectra can be acquired, emission spectra and excitation spectra. To register the emission spectra, the configuration uses a fixed excitation wavelength and the emitted light intensity is measured at different wavelengths by scanning the emission with the monochromator. For the excitation spectra, the emis-sion monochromator measures a fixed emisemis-sion wavelength while the excitation wavelength is scanned in a defined spectral range.
When studying a material’s photoluminescence characteristics it is important to quan-tify the material’s luminescent efficiency through the fluorescence quantum yield defined as: [33]
ΦF =
number of emitted photons
number of absorbed photons (2.7)
In order to measure the total emitted light, from both the excitation source as well as the sample, an integrating sphere is used. This is an optical component consisting of a hollow spherical cavity with its interior covered with a diffuse white reflective coating allowing for a uniform scattering or diffusing effect. Photons incident on any point on the inner surface are distributed equally to all other points. This device also has pinholes
where photons passes through and are directed toward the sample or an exit connected to the sensor. Photons emitted by the sample suffer various subsequent reflections in the inner reflective surface of the sphere eventually reaching the detector [25].
2.2
Lanthanide Ions
Lanthanide elements (Ln) are a series of fifteen metallic elements with atomic numbers ranging from 57 to 71. These elements are characterised by the gradual filing of their outermost sub-shell 4f and 5f while sharing the same basic electronic configuration as Xenon: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6. In solid state these elements are
generally found in a state of triple ionization, their most prevalent oxidation state in which they are found in nature [20], caused by the loss of the last 3 electrons from their outer shell which then results in their valence band to take the form of 4f1 to 4f14, with the
exception of La3+ due to having an empty 4f shell. With these electronic configurations in
place, the resulting observed transitions from the ions emission spectrum now comes from within the 4fN energy states, with N being the number of electrons.
Within the Ln group, Ce3+, Sm3+,Eu3+,Tb3+ and Dy3+are the ions typically associated
with energy transitions corresponding with the visible spectrum. Gd3+ whose atomic number is located between Eu3+ and Tb3+ stands out as it lacks any infrared and visible
absorption or fluorescence due to its energy gap between its fundamental energy sate and its first excited state being higher than these spectral regions (≈ 32000 cm−1) [35].
For Ln3+ ions within a compound, the 4fN energy states, as seen in Figure 2.6, are
split under the influence of an electrostatic field generated by the surrounding compound’s molecules or crystal lattice. This leads to well defined narrow lines, which at low temper-atures (≈10 K) can be compared to the spectral lines of free atoms and molecules. The atom like nature of these spectra reveals that there is week interaction between the ion and its surrounding field, which is consistent with the de-localization of the 4fN shell to the interior of the 5s2 and 5p6 outermost shells. This implies that a shielding effect to the 4fN
orbital is taking place caused by the ion’s exterior shells in such a way that it weakens the lattice’s electrostatic field influence on the ion’s inner shells. This shielding effect allows for interpreting the local field’s interaction as a disturbance to the atom’s energy levels of the 4fN shell.
This thesis focus on Tb3+based materials, so it is pertinent to review its characteristics. In its trivalent state, Tb3+ has a 4f8 sub-shell configuration, meaning its sub-shell is filled over half. According to Hund’s third rule, the lowest fundamental energy levels are organized inversely depending on the state of the outermost sub-shell where for a half filled outer shell or under the lowest total angular momentum number, J, corresponds to the lowest energy, the reverse occurs for the outermost shell more than half-filled , such is the case for Tb3+, where the highest J value corresponds to the lowest energy state. Thus the Tb3+-related emission spectra will be composed of transitions in the green spectral range between the 5D
Chapter 3
Synthesis and Processing
All the material’s synthesis was carried out in straight collaboration with Doctor Mariela Nolasco and Doctor Raquel Rond˜ao from CICECO-Aveiro Institute of Materi-als, Universidade de Aveiro.
3.1
Experimental Section
Materials
The chemicals terbium(III) chloride hexahydrate (TbCl3.6H2O; 99,99 %,
Sigma-Aldrich), Nalidixic acid (1-ethyl-1,4-dihydro-7-methyl-4-oxo-1,8-naftiridine-3-carboxylic acid, HNaI, ≥ 98 %, Sigma-Aldrich), sodium hydroxide (NaOH, Merck), Polyoxypropy-lene (POP) triamine Jeffamine R T5000 (Jeffamine R T5000, 97 %, Huntsman) and Polyoxypropylene (POP) triamine Jeffamine R
T3000 (Jeffamine R
T3000, 97 %, Hunts-man), 3-isocyanatepropyltriethoxysilane (ICPTES, 95 %, Aldrich), tetrahydrofuran (THF, 99.9 %, Sigma-Aldrich), ethanol (EtOH, 99.8 %, Fluka Riedel-de Ha¨en and Fisher Scientific), hydrochloric acid (HCl, 37 %, Sigma-Aldrich) were used as received. High purity distilled water was used in all experiments.
Synthesis
For the synthesis of Tb(NaI)3(H2O)2 complex (Figure 3.1) an amount of 3 mmol of
HNaI was dissolved in 10 mL of CH3CH2OH and the pH of this solution was adjusted to 6
by adding an appropriate amount of an aqueous NaOH solution (5% w/v). Then a solution of 1 mmol of TbCl3.6H2O in 5 mL of water was added dropwise to the ethanolic solution
of NaI. The white solid product was filtrated, washed with water and dried in desiccators at room temperature. Yield: 80%. Anal. Calcd (%) for C36H23TbN6O11: C48.98, H 3.51,
Figure 3.1: Schematic for the synthesis process of Tb(NaI)3(H2O)2.
The obtained complex is a white powder that under UV irradiation shows a green emission (further details are present in Chapter 4), as is shown in the Figures 3.2a and 3.2b.
(a) (b)
Figure 3.2: Photos of the Tb(NaI)3(H2O)2 complex in powdered form and of the hybrid
grown monolith, tU(5000)Tb-M, under (a) natural daylight and (b) UV illumination (365 nm).
Synthesis of the tri-ureasil hybrid precursors
In this work two non-hydrolized precursors were prepared using triamine precursor with different average molar weight , namely Jeffamine R 3000 (3000 g.mol−1) and Jeffamine R 5000 (5000 g.mol−1). Tri-ureasil hybrid precursors (t-UPTES(5000) and tUPTES(3000)) are formed from cross-links between the organic and the inorganic components obtained
by reacting each of the three –NH2 groups of Jeffamines with the –N=C=O group of
ICPTES, in THF, under magnetic stirring at room temperature for 24 h [37–39]. The molar ratio of Jeffamine to ICPTES is 1:3. The non-hydrolyzed precursors tUPTES(5000) and t- UPTES(3000) were obtained as transparent liquids after evaporation of THF under vacuum at room temperature.
(a) Jeffamine R (Y) where Y=3000 and 5000
for x+y+z≈50 and ≈85 respectively.
(b) 3 isocyanatepropyltriethoxysilane (3 -ICPTES)
(c) t-UPTES(Y) where Y=3000 and 5000 for x+y+z≈50 and ≈85 respectively.
Figure 3.3: Schematic representation of the synthesis of the non-hydrolyzed precursor of t-U(5000) and t-U(3000) [38].
Tri-ureasils doped with Tb(NaI)3(H2O)2 complex
tU(5000) and tU(3000) using the following procedure. A solution of Tb(NaI)3(H2O)2
(5.4 mg, 6×10−6 mol) in 1 mL of ethanol was left under magnetic stirring for 30 min. This ethanolic solution plus 20 µL of distilled water was added to 1.5 g of the tri-ureasil non-hydrolysed precursors, tUPTES(5000) and tUPTES(3000), under magnetic stirring at room temperature for 15 min. Then, 40 µL of HCl 0.5 M were added to catalyze the hydrol-ysis reactions. Part of the suspension was casted into a polystyrene mould (1.0×1.0×3.0 cm3), covered with Parafilm R, and kept at 40 ◦C for 2 days inside a oven (Vacucell 22,
MMM Medcenter GmbH). The remaining part of the suspension was used to process thin films deposited on glass substrates (NORMAX, 7.6×2.6×0.1 cm3) using a spin coater
(SPIN 150-NPP, APT) accelerated at 1000 rpm for 60 s. The films, Figure 3.4, were dried at 40◦C for 24 h, for complete solvent removal. The hybrid samples processed as monoliths and films will be hereafter termed as tU(Y)Tb-M and tU(Y)Tb-F, with Y=5000 and 3000, respectively. The film layer’s thickness were measured with a microscope, Figure 3.5, that showed them to be 20 ± 2 µm thick. The microscopic images were recorded using an Olym-pus BX51 Bright field microscope (10× objective), in the reflection mode, equipped with a digital CCD camera (Retiga 4000R, QImaging) used to capture the microphotographs (exposure time of 20 ms) of the films under illumination of an UV-emitting LED.
Figure 3.4: Photo of the tU(5000)Tb-F under UV illumination (365 nm).
Figure 3.5: Optical microscopy image of the cross section of the tU(5000)Tb-F under UV illumination (365 nm).
Table 3.1: List and designation of the prepared samples.
Complex Host Processing Designation
Tb(NaI)3(H2O)2 - Powder Tb(NaI)3(H2O)2
Tb(NaI)3(H2O)2 t-U(3000) Monolith tU(3000)Tb-M
Tb(NaI)3(H2O)2 t-U(3000) Film tU(3000)Tb-F
Tb(NaI)3(H2O)2 t-U(5000) Monolith tU(5000)Tb-M
Tb(NaI)3(H2O)2 t-U(5000) Film tU(5000)Tb-F
Elemental analysis
Elemental analyses for C, H, and N were performed with a CHNS-932 elemental analyzer with standard combustion conditions and handling of the samples in air.
29Si magic-angle spinning (MAS) nuclear magnetic resonance (NMR) and 13C cross polarization (CP) MAS NMR spectra
The 29Si MAS NMR spectra were recorded with a Bruker III Avance 400 and Bruker III Avance 500 (9.4 T) spectrometer at 79.49 and 100.62 MHz, respectively. 29Si MAS
NMR spectra were recorded with 2 µs (∼ 30F◦) rf pulses, a recycle delay of 60 s and at a 5.0 kHz spinning rate. The 13C CP/MAS NMR spectra were recorded with 4 µs 1H 90◦ pulses, 2 ms contact time, a recycle delay of 4 s and at a spinning rate of 8 kHz. Chemical shifts (δ) are quoted in ppm from TMS.
Infrared spectroscopy
IR spectra (FT-IR) were obtained in KBr pellets using a MATTSON 7000 FTIR Spec-trometer. The spectra were collected in the 350-4000 cm−1 range by averaging 256 scans at a spectral resolution of 2 cm−1. Attenuated total reflectance (ATR) FT-IR spectra were measured on the MATTSON 7000 FTIR Spectrometer instrument equipped with a Specac Golden Gate Mk II ATR accessory having a diamond top-plate and KRS-5 focusing lenses.
FT-Raman spectroscopy The FT-Raman spectra were recorded on a FT Bruker RFS-100 spectrometer using a Nd:YAG laser (Coherent Compass-1064/500N) with excita-tion wavelength of 1064 nm. The spectra were collected in the 150-4000 cm−1 range at a spectral resolution of 2 cm−1.
3.2
Results and discussion
3.2.1
29Si magic-angle spinning (MAS) nuclear magnetic
reso-nance (NMR) and
13C cross polarization (CP) MAS NMR
spectra
The 29Si MAS NMR spectra of tU(5000)Tb-M and tU(3000)Tb-M, as seen in Figures
3.6a and 3.6b display broad signals around –46 and –49 ppm, –58 ppm, and –69 ppm, ascribed to R’Si(OSi)(OR)2 (T1), R’Si(OSi)2(OR) (T2) and R’Si(OSi)3 (T3) silicon
envi-ronments [38]. ppm −30 −60 −90 −120 −150 T1 T 2 T3 RSi(OSi)2(OH) R'Si(OSi)3 RSi(OSi)(OH)2 Q4 Si(OSi)4 (a) ppm −30 −60 −90 −120 −150 T1 T2 T3 R'Si(OSi)3 RSi(OSi)(OH)2 RSi(OSi)2(OH) Q4 Si(OSi)4 (b)
Figure 3.6: 29Si MAS NMR spectra of (a) tU(5000)Tb-M and (b) tU(3000)Tb-M.
Contrary to that found for the non-doped tU(5000) host [37, 38, 40], the 29Si MAS NMR spectra also shows a broad signal at –111 ppm assigned to the the (≡SiO)4Si
(Q4, siloxane) local environment [41] indicating the pre-hydrolysis of the tUPTES(Y) precursors. The condensation degree values were estimated using the expression c=1
3(%T
1+2×%T2+3×%T3), yielding values of 0.67 and 0.74 for tU(5000)Tb-M and
tU(3000)Tb-M, respectively.
The 13C MAS NMR spectra of tU(5000)Tb-M, Figure 3.7a, and tU(3000)Tb-M, Figure
3.7b, are dominated by a pair of peaks located at 75.5 ppm and 73.5 ppm, related to the methine and methylene groups of the polyoxypropylene polymer chains respectively [37–39]. The intense peak at 17.7 ppm is due to the methyl groups of the oxypropylene repeat units [37–39].
In te n si ty ( a. u .) ppm 120 100 80 60 40 20 0 CH(CH3)CH2 CH(CH3)CH2 CH(CH3)CH2 NCH2CH2CH2 Si-NCH2CH2CH2 Si-(a) In te n si ty ( a. u .) ppm 120 100 80 60 40 20 0 CH(CH3)CH2 CH(CH3)CH2 CH(CH3)CH2 NCH2CH2CH2 Si-NCH2CH2CH2 Si-(b)
Figure 3.7: 13C MAS NMR spectra of (a) tU(5000)Tb-M and (b) tU(3000)Tb-M.
3.2.2
Vibrational studies
The C, H and N microanalysis percentage experimental/calculated values (see experi-mental section) found for Tb(NaI)3(H2O)2 show that the Tb3+ ion has reacted with HNaI
in a metal-to-ligand mole ratio of 1:3 and that two molecules of water are involved in all complexes. The composition of the first coordination sphere was also confirmed by some coordination-sensitive modes observed in the ATR and Raman spectra, Figure 3.8a and 3.8b, respectively. In te n si ty ( a. u .) Wavenumber (cm-1) 400 800 1200 1600 3000 3500 3350 1708 1654 1615 1603 15 62 15 22 1500 1603 16 72 15 62 1583 15 38 15 19 16 15 (a) In te n si ty ( a. u .) Wavenumber(cm-1) 400 800 1200 1600 3000 3500 3440 1654 1614 1604 15 86 15 58 15 22 14 99 1712 16 50 15 58 15 40 15 15 (b)
Figure 3.8: (a) ATR spectra of HNaI ligand (black line) and Tb(NaI)3(H2O)2 complex (red
line). (b) Raman spectra of HNaI ligand (black line) and Tb(NaI)3(H2O)2 complex (red
In the 1480-1750 cm−1 region, from the free HNaI ligand to Tb(NaI)3(H2O)2, the ATR
bands at 1708/1672/not active/1615/1603/1583/1562/1538/1519 cm−1, Figure 3.8a black line, with correspondence with the Raman bands at 1712/not active/1650/1614/1604/not active/1558/1540/1515 cm−1, Figure 3.8b black line, shift to not active/not ac-tive/1654/1615/1603/not active/1564/1522/1500 cm−1, Figure 3.8a red line, and not active/not active/1654/1614/1604/1586/1558/1522/1499 cm−1, Figure 3.8b red line. The HNaI bands at 1708 cm−1 (with correspondence Raman band at 1712 cm−1) is attributed to the carbonyl C=O stretching band while the bands at 1672 cm−1, Figure 3.8a black line, and 1650 cm−1, Figure 3.8b black line, are attributed to the asymmetric and symmetric carboxylic acid C=O stretching band.
From the observed results we can assume that the NaI is coordinated to Tb3+ ion via
one carboxylate oxygen atom and oxygen atom of the carbonyl group. The broad band around 3350 cm−1, Figure 3.8a red line, and the medium band near 3440 cm−1, Figure 3.8b red line, are associated with the coordinated water molecules. ATR (cm−1): 3350 broad, 1654 sh, 1615 s, 1603 s, 1564 vs, 1522 m, 1500 m, 1442 vs, 1393 m, 1345 m, 1319 m, 1291 m, 1256 s, 1230 m, 1132 m, 1110 w, 1096 w, 1034 vw, 1012 sh, 960 vw, 943 sh, 896 w, 815 s, 785 w, 757 m, 704 vw, 682 sh, 657 m, 635 w. Raman (cm−1):3440 w, 3350 sh, 3070 m, 2983 m, 2932 vs, 1654 m, 1614 m, 1604 sh, 1586 m, 1558 s, 1522 m, 1499 vw, 1440 m, 1387 vs, 1342 m, 1318 s, 1292 m, 1260 m, 1229 w, 1167 w, 1132 w, 1109 m, 1090 m, 1050 m, 962 m, 942 w, 895 m, 816 sh, 788 s, 705 vs, 658 w, 637 w, 559 m, 549 m, 507 sh, 495 m, 435 m, 398 m, 378 sh, 322 m, 300 w, 262 w, 200 m.
Valuable information about the effect of incorporating Tb(NaI)3(H2O)2 complex within
the constrained environment of the tri-ureasils matrices may be gained from the vibrational analysis of the spectral signatures of the polyether backbone and of urea linkages of the matrices. Figure 3.9 displays the FT-IR (700-2000 cm−1 region) and Raman (2700-3100 cm−1 region) spectra recorded for the non-doped tU(5000) and tU(5000)Tb-M, black line and red line respectively. Similar information was obtained for tU(3000) and tU(3000)Tb-M. In te n si ty ( a. u .) Wavenumber (cm-1) 900 1200 1500 1800 2700 3000 1106 927 1014 12511375 15671644 1720 2871 2933 2973
Figure 3.9: FT-IR (700-2000 cm−1) and Raman (2700-3100 cm−1) spectra of tU(5000) (black line) and tU(5000)Tb-M hybrid samples (red line).
The vibrational bands of the tU(5000) are very strong and broad and as the concentra-tion of Tb3+ complex is too low, most of the bands due to the Tb(NaI)
3(H2O)2 complex are
not evident. Analysis of the tU(5000) spectra, Figure 3.9 black line, leads us to divide the vibration bands into two groups, namely vibrations originating from the polyether back-bone and from urea [NHC(=O)NH] linkages and, for which, the assignment proposed below is based on those reported elsewhere for the FT-IR spectra of tri-ureasils [37,38] and analo-gous urea derived organic-inorganic hybrids (so-called di-ureasils) [42–44]. Concerning the polyether backbone FT-IR signatures in the 750-1500 cm−1 region, the medium-to-weak intensity band at 927 cm−1 provides evidence that in tri-ureasil the PEO chains attain complete disorder and the medium intensity peak at 1014 cm−1 is attributed to the strong coupled vibrations of CH2 rocking, and C-O and C-C stretching modes. The most intense
FT-IR band, attributed to the C-O stretching mode, is found at 1106 cm−1 and the broad envelope bands with maxima at 1251 and 1375 cm−1 are related to CH2 twisting and
wag-ging modes, respectively. Concerning the polyether backbone Raman signatures in the 2700-3100 cm−1 region, Figure 3.1, the tU(5000) spectra present three narrower intense bands at 2871, 2933 and 2973 cm−1 assigned to the CH2 symmetric and asymmetric and
CH3 asymmetric stretching ν-C-H modes, respectively.
The spectral signatures of the urea [NHC(=O)NH] linkages are observed in two FT-IR distinct regions, namely 1500-1600 cm−1 and 1600-1800 cm−1, and are related to the so-called “amide II” and “amide I” vibrations, respectively. The “amide II” mode, that is a mixture of C-N and C-C stretching modes and in-plane bending mode of N-H group, is located at 1567 cm−1 and does not change when Tb(NaI)3(H2O)2 complex is incorporated.
In turns, for the “amide I” vibration, a highly complex vibration associated essentially with the C=O stretching mode [37,38,44], although the profile of the band maxima at 1644 cm−1 remains unchanged, the shoulder at 1720 cm−1 slightly changes, being intensified. In this region, the profile of “amide I” band of the tU(5000) matrix produces four individual components at ca. 1724, 1700, 1664 and 1640 cm−1 (obtained by classical curve-fitting procedures [37, 38]). The bands at ca. 1724, 1700, and 1664 cm−1 are ascribed to C=O groups of disordered hydrogen-bonded POP/urea aggregates of increasing strength while the 1640 cm−1 feature is assigned to C=O groups included in significantly more ordered hydrogen-bonded urea/urea aggregates [37, 38, 44].
Despite the low Tb3+concentration, the incorporation of the Tb(NaI)3(H2O)2 complex
into the hybrid matrix disturbs the “amide I” region leading to a band redistribution. This suggests that the coordination ability of the hybrid host is strong enough to replace water molecules from the Tb3+ first coordination sphere by carbonyl groups of the urea moieties, which is in agreement with the PL results (as will be discussed later). Aiming at studying the effect of the materials processing through spin-coating in the local-structure of the hybrid samples, films of tU(3000)/tU(5000) hosts and films incorporating Tb(NaI)3(H2O)2
complex were studied by ATR and Raman spectroscopies. The ATR (700-2000 cm−1 re-gion) and Raman (2700-3100 cm−1region) of tU(3000)Tb-M (black line) and tU(3000)Tb-F (red line) are presented in Figure 3.10, with similar information obtained for tU(5000)Tb-M/tU(5000)Tb-F.
In te n si ty ( a. u .) Wavenumber (cm-1) 900 1200 1500 1800 2700 3000 1565 1640 1722 2871 2933 2973
Figure 3.10: ATR (700-2000 cm−1) and Raman (2700-3100 cm−1) spectra of tU(3000)Tb-M (black line) and tU(3000)Tb-F (red line) hybrid samples.
All spectra show an overall similar profile dominated by the strong and broad bands of the host tri-ureasil material and most of bands due to the Tb(NaI)3(H2O)2 complex
are not evident. Therefore, this observation does not allow us to conclude with certainty about the no change of the Tb3+ local coordination with the materials processing but they
are not inconsistent with that possibility. It should be noted, however, that the materials processing through spin-coating, Figure 3.10 red line, is accompanied by a change in the intensity of the “amide” vibrations at 1565, 1640 and 1722 cm−1 relative to the total spectrum. The possibility of a structure with a lower degree of organization may be the explanation to that (in accordance with the PL results discussed in Chapter 4).
Chapter 4
Optical Properties
4.1
Experimental Details
The photoluminescence spectra were recorded at 18 K and at 300 K with a Horiba Scientific modular double grating excitation spectrofluorimeter and a TRIAX 320 emission monochromator (Fluorolog-3) coupled to a R928 Hamamatsu photomultiplier, using front face acquisition mode. The excitation source was a 450 W Xe arc lamp. The emission spectra were corrected for detection and optical spectral response of the spectrofluorimeter and the excitation spectra were corrected for the spectral distribution of the lamp intensity using a photodiode reference detector.
Emission decay curves were recorded on a Fluorolog R TCSPC spectrofluorometer from Horiba Scientific coupled to a TBX-04 photomultiplier tube module (950 V), 200 ns time-to-amplitude converter and 70 ns delay using a Horiba Scientific pulsed diode (SpectraLED-355, peak at 356 nm) as excitation source. Both photoluminescence spectra and lifetime measurements were performed in solid state.
The room temperature emission quantum yields (φF) were measured in solid state using
the Hamamatsu C9920-02 setup with a 150 W Xe lamp coupled to a monochromator for wavelength discrimination, an integration sphere as sample chamber and a multichannel analyser for signal detection. Three measurements were made for each sample and the average values obtained are reported with accuracy within 10 % according to the manu-facturer.
The photostability under UV irradiation was performed at room temperature, using an integrating sphere (ISP 150L-131) from Instrument Systems and a LED emitting at 365 nm (Multi-Channel LED Light Source from Ocean Optics) as excitation source. The maximum exposure time was 4 days. Two ethanolic solutions were prepared for Tb(NaI)3(H2O)2
and NaI for use in UV-visible absorption spectra with concentrations 10−5 mol.dm−3 and 9.4×10−5 mol.dm−3 respectively.
4.2
Results and Discussion
4.2.1
UV-visible absorption spectroscopy
Figure 4.1 compares the UV-visible absorption spectra of Tb(NaI)3(H2O)2, NaI,
tU(5000), tU(3000), tU(5000)Tb-M, tU(5000)Tb-F, tU(3000)Tb-M and tU(3000)Tb-F.
A b so rb an ce (a .u .) 257 a) α (c m -1) 5 10 15 20 b) α (c m -1) 0 5.0×102 1.0×103 1.5×103 2.0×103 Wavelength (nm) 240 280 320 360 400 330 c)
Figure 4.1: UV-visible absorption spectra of a) Tb(NaI)3(H2O)2in ethanol (black line) and
NaI in ethanol (red line) b) tU(5000)-M (green line), tU(3000)-M (blue line), tU(5000)Tb-M (cyan line) and tU(3000)Tb-M (magenta line) and c) tU(5000)-F (green line), tU(3000)-F (blue line), tU(5000)Tb-F (cyan line) and tU(3000)Tb-F (magenta line).
The electronic absorption spectrum of the Tb(NaI)3(H2O)2 shows two broad bands
with maxima at 330 nm and 257 nm arising from the excited singlet states (S) of the NaI ligands, namely S1 (330 nm) and S2 (257 nm), whose absorption spectrum is also
depicted in Figure 4.1a. After the Tb(NaI)3(H2O)2 incorporation into the tU(5000) host,
the absorption spectrum of the t-U(5000)Tb-M (Figure 4.1b cyan line) became broader revealing bands with maxima at ∼260 nm and ∼335 nm followed by smaller broad band with maximum at ∼355 nm. It is observed that these bands result from the contribution of the S levels and from the tU(5000) excited states (Figure 4.1b, green line). The red-shift from 330 nm to 335 nm (∼450 cm−1) observed for S1 and changes in Full Width Half
Max-imum (FWHM) of the absorption bands observed after the complex incorporation into the tri-ureasil suggests that the electron distribution of the conjugated system involving the ligand chelated ring changed when Tb(NaI)3(H2O)2 was incorporated into the tU(5000)
matrix, in good agreement with the complex coordination to the urea cross-linkages, as the vibrational studies pointed out. These changes where previously observed for other lanthanide-based complexes [45]. This indicates changes to the integrity of the first-sphere ligands in the hybrid material as the structural changes are expected to occur outside the coordination polyhedron. The same conclusion has already been reached for analogous hybrid system based on the Eu(tta)3Phen complex (tta=2-thenoyltrifluoracetonate) and
the poly(ε-caprolactone) siloxane organic–inorganic biohybrid [46] and for t-U(5000) mod-ified by Eu(tta)3ephen (ephen=5,6-epoxy-5,6-dihydro-[1,10]) [45]. For the tU(3000)Tb-M
similar results are found, albeit lacking the maxima at ∼355 nm, pointing out that the average molecules weight did not affect the Tb3+ local coordination.
The UV-visible absorption spectra of the hybrid material processed as thin film were also measured, Figure 4.1c. Comparing these spectra with that of the monolithic samples, a blue-shift is observed, namely S2 from tU(3000)Tb-M deviates from 262 to ∼240 nm
(∼3500 cm−1) and from tU(5000)Tb-M 260 nm to 240 nm (∼3205 cm−1). The S1 level of
both tU(3000)Tb-M and tU(5000)Tb-M deviates from ∼335 nm to ∼285 nm (∼5240 cm−1). This blue shift has already been observed for other sol-gel derived hybrid materials and it was ascribed to differences in the gelification and condensation rates between spin-coating process in the film and the sol-gel reactions in the monoliths [47, 48]. For thin films, the forced solvent extraction is much faster, compared to that of bulk monoliths, resulting in a structure with a lower degree of organization. This distortion on the symmetry could cause the change in the energy levels positions of the hybrid host [47, 48] as it is expected that the Tb local coordination remains unaffected, as was suggested in the FT-IR discussion (Chapter 3).
4.2.2
Photoluminescence Spectroscopy
Figure 4.2 shows the room-temperature excitation spectra of Tb(NaI)3(H2O)2,
tU(5000)Tb-M, tU(3000)Tb-M, tU(5000)Tb-F and tU(3000)Tb-F monitored within the Tb3+ 5D
4 →7F5 transition. The spectrum of tU(5000)Tb-M reveals two main components
at 352 nm and at 270 nm ascribed to the S1 and S2 levels also observed in the absorption
