The rising demand for radiation detection materials in many applications has led to extensive research on scintillators. The ability of a scintillator to absorb high-energy (kiloelectronvolt-scale) X-ray photons and convert the absorbed energy into low-energy visible photons is critical for applications in radiation exposure monitoring, security inspection, X-ray astronomy and medical radiography. However, conventional scintillators are generally synthesized by crystallization at a high temperature and their radioluminescence is difficult to tune across the visible spectrum. Here we describe experimental investigations of a series of all-inorganic perovskite nanocrystals comprising caesium and lead atoms and their response to X-ray irradiation. These nanocrystal scintillators exhibit strong X-ray absorption and intense radioluminescence at visible wavelengths. Unlike bulk inorganic scintillators, these perovskite nanomaterials are solution-processable at a relatively low temperature and can generate X-ray-induced emissions that are easily tunable across the visible spectrum by tailoring the anionic component of colloidal precursors during their synthesis. These features allow the fabrication of flexible and highly sensitive X-ray detectors with a detection limit of 13 nanograys per second, which is about 400 times lower than typical medical imaging doses. We show that these colour-tunable perovskite nanocrystal scintillators can provide a convenient visualization tool for X-ray radiography, as the associated image can be directly recorded by standard digital cameras. We also demonstrate their direct integration with commercial flat-panel imagers and their utility in examining electronic circuit boards under low-dose X-ray illumination.

The synthesis of lanthanide-activated phosphors is pertinent to many emerging applications, ranging from high-resolution luminescence imaging to next-generation volumetric full-color display. In particular, the optical processes governed by the 4f-5d transitions of divalent and trivalent lanthanides have been the key to enabling precisely tuned color emission. The fundamental importance of lanthanide-activated phosphors for the physical and biomedical sciences has led to rapid development of novel synthetic methodologies and relevant tools that allow for probing the dynamics of energy transfer processes. Here, we review recent progress in developing methods for preparing lanthanide-activated phosphors, especially those featuring 4f-5d optical transitions. Particular attention will be devoted to two widely studied dopants, Ce and Eu. The nature of the 4f-5d transition is examined by combining phenomenological theories with quantum mechanical calculations. An emphasis is placed on the correlation of host crystal structures with the 5d-4f luminescence characteristics of lanthanides, including quantum yield, emission color, decay rate, and thermal quenching behavior. Several parameters, namely Debye temperature and dielectric constant of the host crystal, geometrical structure of coordination polyhedron around the luminescent center, and the accurate energies of 4f and 5d levels, as well as the position of 4f and 5d levels relative to the valence and conduction bands of the hosts, are addressed as basic criteria for high-throughput computational design of lanthanide-activated phosphors.

The optical properties of five different nanocrystalline Y2O3:Er3+, Yb3+ samples are presented and discussed. Green and red emission was observed following excitation with 488 nm and attributed to 2H11/2, 4S3/2→4I15/2, and 4F9/2→4I15/2 transitions, respectively. Striking red enhancement was observed in the upconversion spectra when exciting the Y2O3:Er3+, Yb3+ samples with 978 nm, and it became more pronounced with an increase in Yb3+ concentration. A cross relaxation mechanism (4F7/2→4F9/2 and 4F9/2←4I11/2) was responsible for directly populating the 4F9/2 state but did not explain the difference in the magnitude of red enhancement between identically doped bulk and nanocrystalline Y2O3:Er3+, Yb3+ samples. The 4F9/2 level was populated via a nonresonant mechanism that involved the 4F9/2←4I13/2 transition that is more prevalent in the nanocrystals, which is due to the high energy phonons inherent in this type of material. In nanocrystalline Y2O3:Er3+, Yb3+, we observe a change in the upconversion mechanism responsible for populating the 4S3/2 state, from a two-photon to a three-photon process with an increase in Yb3+ concentration. An explanation to account for this behavior is presented.

In this study, we report on the remarkable two-photon excited fluorescence efficiency in the "biological window" of CaF(2):Tm(3+),Yb(3+) nanoparticles. On the basis of the strong Tm(3+) ion emission (at around 800 nm), tissue penetration depths as large as 2 mm have been demonstrated, which are more than 4 times those achievable based on the visible emissions in comparable CaF(2):Er(3+),Yb(3+) nanoparticles. The outstanding penetration depth, together with the fluorescence thermal sensitivity demonstrated here, makes CaF(2):Tm(3+),Yb(3+) nanoparticles ideal candidates as multifunctional nanoprobes for high contrast and highly penetrating in vivo fluorescence imaging applications.

Optical quantum memories are essential elements in quantum networks for long-distance distribution of quantum entanglement. Scalable development of quantum network nodes requires on-chip qubit storage functionality with control of the readout time. We demonstrate a high-fidelity nanophotonic quantum memory based on a mesoscopic neodymium ensemble coupled to a photonic crystal cavity. The nanocavity enables >95% spin polarization for efficient initialization of the atomic frequency comb memory and time bin–selective readout through an enhanced optical Stark shift of the comb frequencies. Our solid-state memory is integrable with other chip-scale photon source and detector devices for multiplexed quantum and classical information processing at the network nodes.

The luminescent properties of 1 mol % Eu3+-doped cubic Lu2O3 nanocrystals prepared by a combustion synthesis route were investigated. The visible emission spectrum of the europium doped Lu2O3 nanocrystals indicate that the structural environment surrounding the dopant Eu3+ ion is distorted when compared to a bulk sample with a micrometer particle size. From the resulting emission spectra, the Ω2 and Ω4 Judd-Ofelt intensity parameters were calculated. The lifetimes of the 5D0 excited state for both the C2 and C3i sites were found to be nearly double that found for a similarly doped sample with larger particle size (bulk sample). This behavior is attributed to a change in the refractive index of the nanocrystalline material that in turn modifies the oscillator strength of the 4f ↔ 4f transitions.

The electronic structure of four ternary-metal oxides containing isolated vanadate ions is studied. Zircon-type YVO 4 , YbVO 4 , LuVO 4 , and NdVO 4 are investigated by high-pressure optical-absorption measurements up to 20 GPa. Firstprinciples calculations based on density-functional theory were also performed to analyze the electronic band structure as a function of pressure. The electronic structure near the Fermi level originates largely from molecular orbitals of the vanadate ion, but cation substitution influence these electronic states. The studied ortovanadates, with the exception of NdVO 4 , undergo a zircon-scheelite structural phase transition that causes a collapse of the band-gap energy. The pressure coefficient dE g /dP show positive values for the zircon phase and negative values for the scheelite phase. NdVO 4 undergoes a zircon-monazite-scheelite structural sequence with two associated band-gap collapses.

We investigated the upconversion properties of nanocrystalline and bulk Y 2 O 3 :Er 3+ as a function of the erbium concentration (1, 2, 5, 10, 25, and 35 mol %). Following excitation with 980 nm, upconverted emission is observed from the 2 H 11/2 , 4 S 3/2 , and 4 F 9/2 excited states to the 4 I 15/2 ground state centered at 525, 550, and 660 nm, respectively. As the dopant concentration is increased, the upconverted luminescence revealed not only an overall increase in intensity but also an enhancement of the red ( 4 F 9/2 f 4 I 15/2 ) emission with respect to the green ( 2 H 11/2 , 4 S 3/2 f 4 I 15/2 ) emission. A cross-relaxation process is involved in populating the 4 F 9/2 state, which bypasses the green-emitting states. Blue upconversion, observed in bulk Y 2 O 3 :Er 3+ only, also showed a concentration dependence. The population of the 2 P 3/2 state was achieved through a three-step phonon-assisted energy-transfer process.