Not All Fluorescent Nanodiamonds Are Created Equal: A Comparative Study

Fluorescent nanodiamonds (FNDs) are vital to many emerging nanotechnological applications, from bioimaging and sensing to quantum nanophotonics. Yet, understanding and engineering the properties of fluorescent defects in nanodiamonds remain challenging. The most comprehensive study to date is presented, of the optical and physical properties of five different nanodiamond samples, in which fluorescent nitrogen‐vacancy (NV) centers are created using different fabrication techniques. The FNDs' fluorescence spectra, lifetime, and spin relaxation time (T₁) are investigated via single‐particle confocal fluorescence microscopy and in ensemble measurements in solution (T₁ excepted). Particle sizes and shapes are determined using scanning electron microscopy and correlated with the optical results. Statistical tests are used to explore correlations between the properties of individual particles and also analyze average results to directly compare different fabrication techniques. Spectral unmixing is used to quantify the relative NV charge‐state (NV^? and NV⁰) contributions to the overall fluorescence. A strong variation is found and quantified in the properties of individual particles within all analyzed samples and significant differences between the different particle types. This study is an important contribution toward understanding the properties of NV centers in nanodiamonds. It motivates new approaches to the improved engineering of NV‐containing nanodiamonds for future applications.     

Synthesis,characterisation, luminescence and defect centres in solution combustion synthesised CaZrO3:Tb3+ phosphor

Tb³⁺ doped CaZrO₃ has been prepared by an easy solution combustion synthesis method. The combustion derived powder was investigated by X-ray diffraction, Fourier-transform infrared spectrometry and scanning electron microscopy techniques. A room temperature photoluminescence study showed that the phosphors can be efficiently excited by 251 nm light with a weak emission in the blue and orange region and a strong emission in green light region. CaZrO₃:Tb³⁺ exhibits three thermoluminescence (TL) glow peaks at 126 °C, 200 °C and 480 °C. Electron Spin Resonance (ESR) studies were carried out to study the defect centres induced in the phosphor by gamma irradiation and also to identify the centres responsible for the TL peaks. The room temperature ESR spectrum of irradiated phosphor appears to be a superposition of two distinct centres. One of the centres (centre I) with principal g-value 2.0233 is identified as an O^? ion. Centre II with an axial symmetric g-tensor with principal values g_=1.9986 and g_?=2.0023 is assigned to an F⁺ centre (singly ionised oxygen vacancy). An additional defect centre is observed during thermal annealing experiments and this centre (assigned to F⁺ centre) seems to originate from an F centre (oxygen vacancy with two electrons). The F centre and also the F⁺ centre appear to correlate with the observed high temperature TL peak in CaZrO₃:Tb³⁺ phosphor.     

The form of different charge states of the vacancy in diamond

There are many different centres formed in diamond that have similar optical characteristics such as a sharp zero phonon line and phonon sidebands. Electronic structure and behaviour of some of these defects is analysed in terms of an intrinsic lattice vacancy model. Using such a model (which sometimes provides a clearer interpretation of experimental results) a comparison of stress matrix elements for the GR1, ND1, H3, N3 and 1,945 eV centres is made. It is concluded that the main behaviour of these defects can simply be associated with the different charged states V⁰, V^?, V^2? and V^3?—the role of nitrogen (apart from providing electrons) being to lower the symmetry about the vacancy.     

Hole centres in proton irradiated CaO and SrO

Isolated V^? centres with g⊥ values of 2.0672 and 2.0667 are observed after proton irradiation of CaO and SrO. The behaviour of these centres after thermal and γ-irradiation treatment of the sample indicates that a vacancy aggregate or complex is proton irradiation. The V_OH centre is formed at a higher temperature consistent with the hypothesis that the original proton reacts to form a hydroxyl group.     

Photoluminescence,thermoluminescence and electron spin resonance studies on Eu<Superscript>3+</Superscript> doped Ca<Subscript>3</Subscript>Al<Subscript>2</Subscript>O<Subscript>6</Subscript> red phosphors

Tricalcium aluminate doped with Eu³⁺ was prepared at furnace temperatures as low as 500°C by using the convenient combustion route and examined using powder X-ray diffraction, scanning electron microscope and photoluminescence techniques. A room-temperature photoluminescence study showed that the phosphors can be efficiently excited by UV/Visible region, emitting a red light with a peak wavelength of 616 nm corresponding to the ⁵D₀–⁷F₂ transition of Eu³⁺ ions. The phosphor exhibits three thermoluminescence (TL) peaks at 195°C, 325°C and 390°C. Electron Spin Resonance (ESR) studies were carried out to study the defect centres induced in the phosphor by gamma irradiation and also to identify the defect centres responsible for the TL process. Room-temperature ESR spectrum of irradiated phosphor appears to be a superposition of three distinct centres. One of the centres (centre I) with principal g-value 2.0130 is identified as O⁻ ion while centre II with an axially symmetric principal values g _∥=2.0030 and g _⊥=2.0072 is assigned to an F⁺ centre (singly ionized oxygen vacancy). O⁻ ion (hole centre) correlates with the TL peak at 195°C and the F⁺ centre (electron centre), which acts as a recombination centre, is also correlated to the 195°C TL peak. F⁺ centre further appears to be related to the high temperature peak at 390°C. Centre III is also assigned to an F⁺ centre and seems to be the recombination centre for the TL peak at 325°C.     

Synthesis, characterization, optical absorption, luminescence and defect centres in Er3+ and Yb3+ co-doped MgAl2O4 phosphors

The Er³⁺–Yb³⁺ co-doped MgAl₂O₄ phosphor powders have been prepared by the combustion method. The phosphor powders are well characterized by X-ray diffraction (XRD) and energy dispersive (EDX) techniques. The absorption spectrum of Er³⁺/Er³⁺–Yb³⁺ doped/co-doped phosphor powder has been recorded in the UV–Vis–NIR region of the electro-magnetic spectrum. The evidence for indirect pumping under 980 nm excitation of Er³⁺ from Yb³⁺ was observed in the MgAl₂O₄ matrix material. Electron spin resonance (ESR) studies were carried out to identify the defect centres responsible for the thermally stimulated luminescence (TSL) process in MgAl₂O₄:Er³⁺ phosphor. Three defect centres were identified in irradiated phosphor by ESR measurements which were carried out at room temperature and these were assigned to an O^? ion and F⁺ centres. O^? ion (hole centre) appears to correlate with the low temperature TSL peak at 210 °C and one of the F⁺ centres (electron centre) is related to the high temperature peak at 460 °C.     

Peculiarities of absorption and luminescence spectra of nitrogen-vacancy color centers in diamond crystals

We have studied photoluminescence spectra of nitrogen-vacancy (NV) centers upon their pumping by short-wavelength visible and near-UV radiation. We have shown that NV⁰ centers, as distinct from NV^? centers have an absorption line in the UV range. Inversion of the zero-phonon line of NV^? centers has been shown and interpreted.     

Energy gap,transition temperature and electron-electron interaction of the weak coupling superconductors tin and indium as a function of the mean free path of the electrons

The energy gap and the transition temperature of the weak coupling superconductors tin and indium are measured as a function of the mean free path of the electrons. Both of them increase with decreasing mean free path. The ratioα=2Δ ₀/kT_c increases proportionally to the reciprocal mean free path from 3.6 to 4.0 for tin and from 3.6 to 3.95 for indium. A theoretical consideration is presented showing that the electron-electron interaction is increased in dirty superconductors. Without any fit we find from the theoretical calculation an increase inNV for indium ofδNV=5 · 10^?10 m/l _tr and for tinδNV=2 · 10^?10 m/l _tr (l_tr is the transport mean free path of the electrons). The experiments give an increase inNV for indium ofδNV=4.2 · 10^?10 m/l_tr and for tinδNV=1.7 · 10^?10 m/l _tr. The agreement is surprisingly good.     

Infrared emission and defect centres in Er and Yb codoped Y3Al5O12 phosphors

YAG phosphor powders doped/codoped with Er³⁺/(Er³⁺ + Yb³⁺) have been synthesised by using the solution combustion method. The effect of direct pumping into the ⁴I_11/2 level under 980 nm excitation of doped/codoped Er³⁺/Yb³⁺−Er³⁺ in Y₃Al₅O₁₂ (YAG) phosphor responsible for an infrared (IR) emission peaking at ∼1.53 μm corresponding to the ⁴I_13/2→⁴I_15/2 transition has been studied. YAG exhibits three thermally-stimulated luminescence (TSL) peaks at around 140°C, 210°C and 445°C. Electron spin resonance (ESR) studies were carried out to identify the centres responsible for the TSL peaks. The room temperature ESR spectrum of irradiated phosphor appears to be a superposition of two distinct centres. One of the centres (centre I) with principal g-value 2.0176 is identified as O⁻ ion, while centre II with an isotropic g-factor 2.0020 is assigned to an F⁺ centre (singly ionised oxygen vacancy). An additional defect centre is observed during thermal-annealing experiments and this centre (assigned to F⁺ centre) seems to originate from an F-centre (oxygen vacancy with two electrons) and these two centres appear to correlate with the observed high-temperature TSL peak in YAG phosphor.     

Optically and thermally driven bistability of FH(OH−) centres in KBr: Structure of the bistable configurations

Abstract

Pairs of F centres and OH^? molecules (F_H(OH^?) centres) exhibit a very interesting optical and thermal bistability in KBr. The analysis of our Electron Nuclear Double Resonance (ENDOR) investigation yields structure models. In both bistable configurations the OH^? molecules reside on a next nearest anion site (4^th shell) relative to the F centre. The difference lies in the orientation of the OH^? dipole with respect to the defect pair axis. The thermal bistability will be explained tentatively as being entropy-driven.     

Triple resonance investigation of F centres in BaFCl

Abstract

In BaFCl two types of F centres can be produced, where the electron occupies a Cl^? vacancy [F(Cl^?)-centres] or an F^? vacancy [F(F^?)-centres]. When producing F(F^?)-centres the simultaneous production of F(Cl^?)-centres cannot be avoided. Both ESR spectra overlap strongly. The ENDOR spectra contain very many lines of both centres and have a very complicated angular dependence. It is shown that the angular dependence of the ENDOR spectra of both F centres can be measured separately by applying a double ENDOR technique (called also Triple Resonance). The ratio of the signal heights of the ENDOR lines compared to the double ENDOR lines is characteristic for the neighbour shell to which the nuclei involved belong. The use of this effect for the analysis of the spectra and its explanation are discussed.     

TL, OSL and ESR studies on beryllium oxide

Electron spin resonance (ESR) studies were carried out to identify the defect centres responsible for the thermoluminescence (TL) and optically stimulated luminescence (OSL) processes in BeO phosphor. Two defect centres were identified in irradiated BeO phosphor by ESR measurements, which were carried out at room temperature and these were assigned to an O⁻ ion and Al²⁺ centre. The O⁻ ion (hole centre) correlates with the main 190 °C TL peak. The Al²⁺ centre (electron centre), which acts as a recombination centre, also correlates to the 190 °C TL peak. A third centre, observed during thermal annealing studies, is assigned to an O⁻ ion and is related to the high temperature TL at 317 °C. This centre also appears to be responsible for the observed OSL process in BeO phosphor.     

A rapid combustion process for the preparation of MgSrAl10O17:Eu phosphor and related luminescence and defect investigations

Blue-emitting europium-ion-doped MgSrAl₁₀O₁₇ phosphor, prepared using the combustion method, is described. An efficient phosphor can be prepared by this method in a muffle furnace maintained at 500 °C in a very short time of few minutes. The phosphor is characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy and BET surface area measurements. Photoluminescence (PL) spectra revealed that europium ions were present in divalent oxidation state. The thermoluminescence (TL) glow curve shows two peaks at around 178 and at 354 °C. The defect centres formed in the phosphor are studied using electron spin resonance (ESR). The ESR spectrum indicates the presence of Fe³⁺ ions in the non-irradiated system. Irradiated MgSrAl₁₀O₁₇:Eu exhibits lines due to radiation-sensitive Fe³⁺ ion and a defect centre. The centre is characterized by an isotropic g-value of 2.0012 and is assigned to a F⁺ centre. The radiation-sensitive Fe³⁺ ion appears to correlate with the main TL peak at 178 °C. During irradiation an electron is released from Fe²⁺ and is trapped at an anion vacancy to form F⁺ centre. During heating, an electron is liberated from the defect centre and recombines with Fe³⁺ emitting light.     

Pairs of F centres and CN− molecules in KCl: An ENDOR analysis

Abstract

Pair defects consisting of F centres and CN^? molecules as substitutional impurity anions (F_H(CN^?) centres) exhibit a strong coupling between the F centres and the CN^? molecules. This leads to an optical pumping of the CN^? vibrations via the F centre absorption band. We performed an Electron Nuclear Double Resonance (ENDOR) investigation in order to analyse the microscopic structure of these aggregate centres. The CN^? molecule occupies a [110] nearest anion position with respect to the F centre in two dynamic dipole orientations even at low temperature.     

Thermoluminescence and defect study of MgSiO3 ceramics

Enstatite (MgSiO₃) ceramic powders were synthesised by a low-temperature initiated self-propagating, gas-producing solution combustion process. The prepared powders were characterised by powder X-ray diffraction, scanning electron microscopy and Brunauer–Emmer–Teller specific surface area measurements. Defect centres induced by radiation were studied using the techniques of thermoluminescence (TL) and electron spin resonance (ESR). A well-resolved glow with peak at 178°C and a shouldered peak at 120°C were observed. Two defect centres were identified by ESR measurements, which were carried out at room temperature, and these were assigned to an O^? ion and F⁺ centre. The O^? ion (hole centre) appears to correlate with the main TL peak at 178°C.     

Paramagnetic Centres in Be-Doped MgO Single Crystals

Using a variation of arc-fusion technique, Be-doped MgO single crystals were grown, in which about 0.01% of the cation sites are occupied by Be^2?. This gives rise to a variety of Be-containing paramagnetic centres, easily detectable by EPR. The models of the centres are proposed and the values of their spin-Hamiltonian parameters are determined and discussed. Two of them—V_OH-Be and H-Be centres—stem from well-known paramagnetic centres such as V_OH, and interstitial H atom. In addition, because of the non-central position of the ion, an isolated Be^2? can trap a hole forming a Be^2?O^? centre. The symmetry of the Be^2?O^? centre at T<30K is rhombic, at a higher temperature a motional averaging of the spectrum takes place. It is shown that unusually for V centres physical properties of the V_OH-Be centre (a relatively small g-factor anisotropy and high thermal stability, optical absorption energy and spin-lattice relaxation time) arc caused by the non-central position of the Be^2? ion.     

Magnetic field dependence of triplet-state ONP: theoretical analysis in terms of level anti-crossings

ABSTRACT

A theoretical approach to Optical Nuclear Polarisation (ONP) is described, which is based on the analysis of Level Anti-Crossings (LACs) in triplet states. Here we consider ONP formed in molecular crystals doped with suitable guest molecules and ONP generated in diamond crystals containing negatively charged nitrogen-vacancy (NV^–) centres. In both cases, electron spin polarisation of triplet states generated by light excitation is transferred to nuclei giving rise to ONP. Polarisation transfer is most efficient at LACs; for this reason, we consider in detail crossings of electron–nuclear energy levels and the role of different perturbation terms (coming from isotropic and anisotropic hyperfine coupling, zero-field splitting and sample orientation), which turn these crossings into LACs and give rise to ONP. Analytical results are supported by numerical calculations of the ONP field dependences. Thus, the outlined LAC analysis is a useful approach for interpreting the ONP magnetic field dependence.     

Effect of lattice relaxation on spin density of nitrogen-vacancy centers in diamond and oscillator strength calculations

Using a generalized Hubbard Hamiltonian, many-electron wavefunctions of negatively charged (NV⁻) and neutral nitrogen-vacancy (NV⁰) centers in diamond were calculated. We report the effect of symmetric relaxation of surrounding atoms on the spin density, calculated from the many electron wavefunctions in the ground and excited states. We evaluated the error, that, arises in estimation of spin density when lattice relaxation effect is neglected in Electron Paramagnetic Resonance experiment and showed that the ground state spin density distribution is accessible in outward relaxations. The computed oscillator strengths give a higher efficiency for the 1.945 eV photoluminescence (PL) line of NV⁻ with respect to 2.156 eV PL line of NV⁰ which agrees well with experiment. This result is explained based on the largest the ground state spin among available values for the NV⁻ with respect to NV⁰. The transition probability between degenerate ground and excited states slightly depends on the S _z value. Finally, we report on the electronic configurations which contribute to the ground and excited states and discuss the population variation of electronic configurations with relaxation.     

Electron paramagnetic resonance and electron nuclear double resonance of NaCl:Rh2+: A comparison between similarly doped Bridgman- and solution-grown single crystals

In a recent electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) study several new rhodium related centres in a Bridgman-grown crystal were reported. For two centres a detailed microscopic model was established. The primary Rh²⁺ centre was identified as a [RhCl₆]^4? complex with a single charge compensating Na⁺ vacancy in a next nearest neighbour position. A second centre was identified as a rhodium dimer. Two other Rh²⁺ centres, with apparent axial symmetry, remained unidentified. In the present study a detailed comparison between Rh²⁺ defects in Bridgman- and solution-grown NaCl crystals is presented. One of the previously unidentified Rh²⁺ centres is shown not to be axial but to have orthorhombic-I symmetry and is identified as a cis-[RhCl₆]^4? complex with two next nearest neighbour Na⁺ vacancies. For the primary centre additional ENDOR interactions confirm the model mentioned above. Finally a comparison is made between the temperature stability of the primary and orthorhombic-I centre in Bridgman-grown and similarly doped solution-grown NaCl crystals.     

ODMR Spectroscopy of NV<Superscript>−</Superscript> Centers in Diamond Under High MW Power

We have studied negatively charged nitrogen vacancy centers (NV\(^-\) centers) in diamond using the technique of optically detected magnetic resonance (ODMR). Due to the use of the MW power amplifier we have easily observed in the ODMR spectrum all the lines known to date, including the lines of the excited state of the NV\(^-\) centers. The g values and the hyperfine interaction constants of the ground and the excited triplet states of the NV\(^-\) center are given. Hamiltonian parameters of the ground and the excited triplet states of the NV\(^-\) center at ambient temperature have been confirmed and refined. In addition, an evidence of a slight g value asymmetry of the excited state of the NV\(^-\) center has been obtained.