Alan G. Joly

Carrier dynamics in α‐Fe2O3 (0001) thin films and single crystals probed by femtosecond transient absorption and reflectivity

Femtosecond transient reflectivity and absorption are used to measure the carrier lifetimes in α‐Fe2O3 thin films and single crystals. The results from the thin films show that initially excited hot electrons relax to the band edge within 300fs and then recombine with holes or trap within 5ps. The trapped electrons have a lifetime of hundreds of picoseconds. Transient reflectivity measurements from hematite (α‐Fe2O3) single crystals show similar but slightly faster dynamics leading to the conclusion that the short carrier lifetimes in these materials are due primarily to trapping to Fe d‐d states in the band gap. In the hematite single crystal, the transient reflectivity displays oscillations due to the formation of longitudinal acoustic phonons generated following absorption of the ultrashort excitation pulse.

Investigation of water-soluble x-ray luminescence nanoparticles for photodynamic activation

In this letter, we report the synthesis of LaF3:Tb3+–meso-tetra(4-carboxyphenyl) porphine (MTCP) nanoparticle conjugates and investigate the energy transfer as well as singlet oxygen generation following x-ray irradiation. Our observations indicate that LaF3:Tb3+-MTCP nanoparticle conjugates are efficient photodynamic agents that can be initiated by x-rays at a reasonably low dose. The addition of folic acid to facilitate targeting to folate receptors on tumor cells has no effect on the quantum yield of singlet oxygen production in the nanoparticle-MTCP conjugates. Our pilot studies indicate that water-soluble scintillation nanoparticles can potentially be used to activate photodynamic therapy as a promising deep cancer treatment modality.

Temperature and pressure dependences of the Mn2+ and donor-acceptor emissions in ZnS : Mn2+ nanoparticles

Temperature and pressure dependent measurements have been performed on 3.5 nm ZnS:Mn2+ nanoparticles. As temperature increases, the donor-acceptor (DA) emission of ZnS:Mn2+ nanoparticles at 440 nm shifts to longer wavelengths while the Mn2+ emission (T-4(1)-(6)A(1)) shifts to shorter wavelengths. Both the DA and Mn2+ emission intensities decrease with temperature with the intensity decrease of the DA emission being much more pronounced. The intensity decreases are fit well with the theory of thermal quenching. As pressure increases, the Mn2+ emission shifts to longer wavelengths while the DA emission wavelength remains almost constant. The pressure coefficient of the DA emission in ZnS:Mn2+ nanoparticles is approximately -3.2 meV/GPa, which is significantly smaller than that measured for bulk materials. The relatively weak pressure dependence of the DA emission is attributed to the increase of the binding energies and the localization of the defect wave functions in nanoparticles. The pressure coefficient of Mn2+ emission in ZnS:Mn2+ nanoparticles is roughly -34.3 meV/GPa, consistent with crystal field theory. The results indicate that the energy transfer from the ZnS host to Mn2+ ions is mainly from the recombination of carriers localized at Mn2+ ions

X-ray luminescence of LaF3:Tb3+ and LaF3:Ce3+,Tb3+ water-soluble nanoparticles

We report x-ray luminescence from LaF3:Ce3+,Tb3+ and LaF3:Tb3+ water-soluble nanoparticles. The x-ray luminescence is dominated by emission from Tb3+ ions, similar to photoluminescence spectra of the nanoparticle aqueous solutions and spectra from nanoparticle powders precipitated from the aqueous samples. Coating the nanoparticles with an insulating inorganic LaF3 or organic H2N–(CH2)10–COOH layer can enhance the x-ray luminescence from the aqueous nanoparticles. This enhancement is most likely due to the decreased energy loss due to the particle-solvent interactions.

Upconversion luminescence from CdSe nanoparticles

Efficient upconversion luminescence has been observed from CdSe nanoparticles ranging in size from 2.5 to 6 nm. The upconversion luminescence exhibits a near-quadratic laser power dependence. Emissions from both excitons and trap states are observed in the upconversion and photoluminescence spectra, and in the upconversion luminescence the emission from the trap states is enhanced relative to the trap-state emission in the photoluminescence. The upconversion decay lifetimes are slightly longer than the photoluminescence decay lifetimes. Time-resolved spectral measurements indicate that this is due to the involvement of long decay components from surface or trap states. Both the photoluminescence and upconversion luminescence decrease in intensity with increasing temperature due mainly to thermal quenching. All the observations indicate that trap states work as emitters rather than as intermediate states for upconversion luminescence and that two-photon absorption is the likely excitation mechanism.

Upconversion luminescence of Eu3+ and Mn2+ in ZnS : Mn2+, Eu3+ codoped nanoparticles

Strong upconversion luminescence of both Mn2+ and Eu3+ is observed in ZnS:Mn2+, Eu3+ codoped nanoparticles. Laser power dependencies and spectroscopic data show that the upconversion emission is due to two-photon excitation of each specific dopant ion. The relative differences in two-photon excitation cross section result in different relative intensities for the Eu3+ and Mn2+ upconversion at different wavelengths. Spectroscopic data and luminescence lifetime data indicate no evidence of energy transfer between the Mn2+ and Eu3+ ions.

Folic acid-CdTe quantum dot conjugates and their applications for cancer cell targeting

In this study, we report the preparation, luminescence, and targeting properties of folic acid-CdTe quantum dot conjugates. Water-soluble CdTe quantum dots were synthesized and conjugated with folic acid using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide-N-hydroxysuccinimide chemistry. The influence of folic acid on the luminescence properties of CdTe quantum dots was investigated, and no energy transfer between them was observed. To investigate the efficiency of folic acid-CdTe nanoconjugates for tumor targeting, pure CdTe quantum dots and folic acid-coated CdTe quantum dots were incubated with human nasopharyngeal epidermal carcinoma cell line with positive expressing folic acid receptors (KB cells) and lung cancer cells without expression of folic acid receptors (A549 cells). For the cancer cells with positive folate receptors (KB cells), the uptake for CdTe quantum dots is very low, but for folic acid-CdTe nanoconjugates, the uptake is very high. For the lung cancer cells without folate receptors (A549 cells), the uptake for folic acid-CdTe nanoconjugates is also very low. The results indicate that folic acid is an effective targeting molecule for tumor cells with overexpressed folate receptors.

Plasmonic field enhancement of individual nanoparticles by correlated scanning and photoemission electron microscopy

We present results of a combined two-photon photoemission and scanning electron microscopy investigation to determine the electromagnetic enhancement factors of silver-coated spherical nanoparticles deposited on an atomically flat mica substrate. Femtosecond laser excitation of the nanoparticles produces intense photoemission, attributed to near-resonant excitation of localized surface plasmons. Enhancement factors are determined by comparing the respective two-photon photoemission yields measured for single nanoparticles and the surrounding flat surface. For p-polarized, 400 nm (∼3.1 eV) femtosecond radiation, a distribution of enhancement factors is found with a large percentage (67%) of the nanoparticles falling within a median range. A correlated scanning electron microscopy analysis demonstrated that the nanoparticles typifying the median of the distribution are characterized by spherical shapes and relatively smooth silver film morphologies. In contrast, the largest enhancement factors were produced by a small percentage (7%) of particles that displayed silver coating defects that altered the overall particle structure. Comparisons are made between the experimentally measured enhancement factors and previously reported calculations of the localized near-field enhancement for isolated silver nanoparticles.

Selective laser desorption of ionic surfaces: Resonant surface excitation of KBr

We demonstrate evidence of selective laser-induced desorption of ground state Br(2P3/2) and spin–orbit excited state Br(2P1/2) atoms from KBr single crystals following 6.4 eV irradiation. Laser excitation tuned selectively to a surface resonance below the first bulk absorption band excites surface states preferentially leading to surface specific reactions while inducing relatively insignificant bulk reaction. The experimental results are supported by embedded cluster ab initio calculations that indicate a reduced surface exciton energy compared to that of the bulk exciton with a slight further reduction for steps and kink sites. Low fluence irradiation of cleaved KBr crystals, near the calculated surface exciton energy of 6.2 eV, produces hyperthermal Br(2P3/2) emission without a significant thermal or Br(2P1/2) component. The hyperthermal emission is shown theoretically to be characteristic of surface induced reaction of exciton decomposition while thermal emission is attributed to bulk photoreaction.

Characterization of Salicylate-Alumina Surface Complexes by Polarized Fluorescence Spectroscopy

Speciation of salicylate anions (o-hydroxybenzoate) adsorbed on aqueous colloidal alumina (δ-Al2O3) was determined by polarized fluorescence excitation spectroscopy. Adsorption isotherms and pH and ionic strength edges suggest the existence of both inner-sphere and outer-sphere salicylate surface complexes. Spectroscopic characteristics of inner-sphere surface-salicylate complexes (one bidentate and two monodentate) were identified through comparison of suspension spectra with remarkably similar fluorescence and excitation spectra of solution phase Al-salicylate complexes. The large fluorescence Stokes gap of the aqueous salicylate anion is highly sensitive to complexation, resulting in spectral shifts characteristic of aluminum binding in the three inner-sphere salicylate complexes. These species appear to be present even at extremely low surface coverage, and the relative distributions are dependent on pH, ionic strength, and the relative concentrations of alumina and salicylate. The bidentate complex, however, is the predominant species at low surface coverages. Fluorescence anisotropy measurements, both steady-state and time-resolved, demonstrate that the bidentate and monodentate surface complexes do not undergo rotational reorientation on the time-scale of the fluorescence (τf = 4.0 ns), consistent with inner-sphere, polar covalent binding of these salicylate complexes to alumina surface sites. At high surface coverage, time-resolved anisotropy measurements suggest the existence of a surface salicylate species that is rotationally hindered (τr = 31 ps ) relative to free solution phase salicylate ions (τ r = 20 ps ). This behavior is consistent with an electrostatically bound outer-sphere complex suggested by the pH and ionic-strength sorption edges.