Optical barcodes have demonstrated a great potential in multiplexed bioassays and cell tracking for their distinctive spectral fingerprints. The vast majority of optical barcodes were designed to identify a specific target by fluorescence emission spectra, without being able to characterize dynamic changes in response to analytes through time. To overcome these limitations, the concept of the bioresponsive dynamic photonic barcode was proposed by exploiting interfacial energy transfer between a microdroplet cavity and binding molecules. Whispering-gallery modes resulting from cavity-enhanced energy transfer were therefore converted into photonic barcodes to identify binding activities, in which more than trillions of distinctive barcodes could be generated by a single droplet. Dynamic spectral barcoding was achieved by a significant improvement in terms of signal-to-noise ratio upon binding to target molecules. Theoretical studies and experiments were conducted to elucidate the effect of different cavity sizes and analyte concentrations. Time-resolved fluorescence lifetime was implemented to investigate the role of radiative and non-radiative energy transfer. Finally, microdroplet photonic barcodes were employed in biodetection to exhibit great potential in fulfilling biomedical applications.
Here we report X- and gamma-ray scintillation properties from solution-processable perovskite (SPP) halide single crystals and quantum dots. We tune the properties of single crystals by replacing the cation, changing the organic ammonium cation spacer, and varying the halide anion. For quantum dots, we tune the properties by changing the halide ion composition while we also try to replace the lead with the bismuth. Finally, we summarize the advantages and disadvantages of SPP single crystals and quantum dots to pave the way the research for the new high light yield scintillators.
Due to the large exciton binding energy, two-dimensional perovskite has demonstrated the potential as high-performance while low-cost scintillator. In our experiment, first we systemically investigate the effect of Li-ion dopant in phenethylammonium lead bromide, (PEA)2PbBr4 perovskite crystals under soft X-ray radiation of 15 keV. Successful inclusion of Li at four doping concentrations was confirmed by X-ray photoelectron spectroscopy. Li doping causes no substantial change in the crystal structure judging from the X-ray diffraction pattern but induces stronger emission tail as observed in the temperature-dependent X-ray luminescence (XL). Upon higher Li concentration, the emissions become broader due to possible Li trap emission as indicated by increasing traps induced by more Li in the X-ray thermoluminescence spectra. The behavior of negative thermal quenching is found in the XL and it can yield a benefit such as the possible light yield improvement in the X-ray imaging application. After the soft X-ray characterizations, we further explore our crystals in gamma-ray detection. In the gamma-ray pulse height measurement, relatively broad peaks can be resolved with the light yield of about 10,000 photons/MeV at 662 keV. The result from alpha particle pulse height measurement also indicates that we could even utilize our crystals in alpha particle detection at 5.8 MeV. Based on this feature and Li-ion capability as dopants, our crystals promise a good performance in thermal neutron detection. Finally, we can realize a versatile radiation detector that works in broad range of energy from soft to high energy radiation.
We present our recent research results on luminescence of halide perovskites under various excitation sources: photoexcitation, electrical current and high-energy radiation. Our photonics crystal can reduce the emission rate of perovskite film by preventing the photoluminescence coupling to the film, but enhance the out-coupling efficiency by 23.5 times. Our solution-grown single-crystal perovskite hetero-structure was successful with different halide compositions. A pristine single-crystal light emitting device was demonstrated with excellent protection and encapsulation from material synthesis to device characterization. Lastly, with engineered perovskite materials we demonstrate a multifunctional scintillator for high-energy radiation from X-rays, gamma rays to thermal neutrons.
Here we present two techniques, which have advantages in the perovskite single crystal devices. First, we demonstrate modulation-doped layer growth and double heterostructure using a millimeter-sized hybrid halide perovskite crystal as a substrate. We show that previously known limiting factor of halide ion inter-diffusion can be constrained to few microns by (1) using low halide composition gradient and (2) adjusting solution concentrations just above the critical super saturation. In the solvo-thermal growth process, our layer growth time could be conveniently extended as necessary to grow a uniform layer, with only ~5 µm inter-diffusion region. This is a significant improvement compared to few seconds dipping time previously reported for a rapid ion exchange process without any layer growth. The growth of CH3NH3PbBr3 layer on top of CH3NH3Pb(Br0.85Cl0.15)3 bulk substrate is studied for different growth times to obtain up to 30µm layer thickness. Ion diffusion profile, layer thickness and crystallographic orientation have been characterized by cross sectional characterization using Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), and Electron Back-Scattering Diffraction (EBSD) . With this advancement, we are able to grow two consecutive perovskite layers to create a double heterostructure for the first time. Second, we demonstrate an as-grown milliliter-sized perovskite bulk crystal light emitting device. This device can be easily lighten up at low voltage (6-20 V) and at slightly low temperatures than room temperature (160-230 K). We are aiming to integrate both technologies with further optimization to produce efficient, pure-color perovskite light emitting devices for entire visible spectrum with low-cost and simple infrastructure.
Systematic investigation of temperature-dependence on thermal quenching of X-ray luminescence (XL) from various single perovskite crystals was carried out. In the family of methylammonium lead halide perovskites (MAPbX3, MA = methylammonium, X= Cl, Br or I), the quenching temperature of XL decreases from Cl to I. According to our analysis, such behavior is strongly affected by their corresponding decrease of thermal activation energy ▵Eq from 53 ± 3 to 6 ± 1 meV. Different concentrations of Bi3+-doped MAPbBr3 are also prepared and both four-point probe measurement and X-ray thermoluminescence (TL) confirms the successful doping. When we dope MAPbBr3 with Bi3+, Γ0/Γv increases to 78 ± 18 for crystal with Bi/Pb ratio of 1/10 in precursor solution.
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