2009. "Excitation, Ionization, and Desorption: How Sub-band gap Photons Modify the Structure of Oxide Nanoparticles." Journal of Physical Chemistry C 113(4):1274-1279. Abstract Nanoparticles of wide-band-gap materials MgO and CaO, subjected to low-intensity ultraviolet irradiation with 266 nm (4.66 eV) photons, emit hyperthermal oxygen atoms with kinetic energies up to ~ 0.4 eV. We use ab initio embedded cluster methods to study theoretically a variety of elementary photoinduced processes at both ideal and defect-containing surfaces of these nanoparticles and develop a mechanism for the desorption process. The proposed mechanism includes multiple local photoexcitations resulting in sequential formation of localized excitons, their ionization, and further excitations. It is suggested that judicious choice of sub-band-gap photon energies can be used to selectively modify surfaces of nanomaterials.
2009. " Using Thin Films to Screen Possible Scintillator Materials ." IEEE Transactions on Nuclear Science 56(3, PT 3): 1650-1654. Abstract The discovery and optimization of new scintillators has traditionally been a rather slow process due to the difficulties of single crystal growth. This paper discusses the production of polycrystalline scintillator thin films (a few microns thick) which were tested in order to determine what characterizations could be made concerning a material’s ultimate potential as a scintillator prior to pursuing crystal growth. Thin films of CaF2(Eu), CeF3, and CeCl3, all known scintillators, were produced by vapor deposition. The hygroscopic CeCl3 was coated with multiple polymer-aluminum oxide bi-layers. Emission spectra peak wavelengths and decay times agreed with single crystal values. The films were too thin to measure gamma photopeaks, but using alpha energy deposition peaks, one could compare the relative photon yield/MeV between materials. The values obtained appear to give a relevant indication of a material’s light yield potential. The technique also appears useful for quickly determining the proper dopant amount for a given material.
2009. "High-Pressure Effects on the Electronic Structure of Anthracene Single Crystals: Role of Nonhydrostaticity." Journal of Physical Chemistry A 113(8):1489-1496. Abstract Optical spectroscopy methods were used to examine the effect of nonhydrostaticity on the electronic structure of anthracene single crystals compressed statically to 9 GPa. Two pressure-transmitting media, nitrogen (hydrostatic) and water (nonhydrostatic above ~ 5.5 GPa), were utilized. It was found that nonhydrostatic compression generates several new features both in the absorption and fluorescence spectra: (i) formation of new absorption and fluorescence bands, (ii) deviations in pressure shift of fluorescence peaks, (iii) extensive broadening of vibrational peaks, and (iv) irreversible changes in the spectra shape upon pressure unloading. Furthermore, the time-resolved fluorescence decay curves measured at the wavelength corresponding to the new fluorescence band show clear initial increase. These new features are accompanied by inhomogeneous color changes and macroscopic lines on the (001) plane of the crystal. All of the changes are discussed and correlated with microscopic transformations in the crystal. It is demonstrated that nonhydrostatic compression in anthracene crystal introduces inelastic changes in the form of dislocations along [110] and [1 10 ] directions. These dislocations lead to the development of dimeric structures and, consequently, to various changes in the electronic response of the compressed anthracene crystal.
2008. "The Effects of Aging on the Luminescence of PEG-Coated Water-Soluble ZnO Nanoparticles Solutions." Journal of Physical Chemistry C 112(37):14292-14296. doi:10.1021/jp803649k Abstract Water-soluble ZnOnanoparticles coated with polyethylene glycol biscarboxymethyl (PEG(COOH)2)were prepared in ethanol/water solutions. The ZnOnanoparticles have a hexagonal structure with an average size of 10 nm. Three different luminescence bands are observed from the nanoparticle solutions: green emission at 530 nm from surface states or defects, UV emission at 380 nm from the ZnOexcitons, and an emission band at around 338 nm from the PEG(COOH)2. The fresh as-prepared samples have very strong green emission at 530 nm from surface states or defects but very weak excitonic emission at 380 nm. After dilution with ethanol, the green emission decreases in intensity and the excitonic emission increases. In the diluted samples, the excitonic luminescence intensity increases with storage time. This intensity increase is attributed to surface passivation by CH3COO-ligands resulting from precursor reactions in the ethanol solvent.
2008. "Laser and Electrical Current Induced Phase Transformation of In2Se3 Semiconductor thin film on Si(111) ." Applied Physics A, Materials Science and Processing 93(1):93-98. doi:10.1007/s00339-008-4776-8 Abstract Phase transformation of thin film (~30 nm) In2Se3/Si(111) (amorphous crystalline) was performed by resistive annealing and the reverse transformation (crystalline amorphous) was performed by nanosecond laser annealing. As an intrinsic-vacancy, binary chalcogenide semiconductor, In2Se3 is of interest for non-volatile phase-change memory. Amorphous InxSey was deposited at room temperature on Si(111) after pre-deposition of a crystalline In2Se3 buffer layer (6.4 Å). Upon resistive annealing to 380°C, the film was transformed into a y-In2Se3 single crystal with its {0001} planes parallel to the Si (111) substrate and parallel to Si , as evidenced by scanning tunneling microscopy, low energy electron diffraction, and x-ray diffraction. Laser annealing with 20 nanosecond pulses (0.1 milliJoules/pulse) re-amorphized the region exposed to the laser beam, as observed with photoemission electron microscopy (PEEM). The amorphous phase in PEEM appears dark, likely due to abundant defect levels inhibiting electron emission from the amorphous InxSey film.
2008. "X-Ray Luminescence of LaF3:Tb3+ and LaF3:Ce3+, Tb3+ Water Soluble Nanoparticles." Journal of Applied Physics 103(6):Art. No. 063105. doi:10.1063/1.2890148 Abstract Utilizing scintillation nanoparticles as agents for photodynamic therapy for cancer treatment necessitates the use of biocompatible and water soluble nanoparticles. In this article, we report the synthesis and X-ray luminescence of water soluble Ce and Tb doped LaF3 nanoparticles. The nanoparticles are conjugated with folic acid and meso-tetra (o-carboxyphenyl) porphyrin. X-ray luminescence is observed from the nanoparticles in both powder and solution samples. More importantly, singlet oxygen has been detected from the conjugated system following X-ray excitation. These preliminary observations indicate that water-soluble scintillation nanoparticles can be potentially used in photodynamic therapy for deep-tissue cancer treatment.
2008. "Investigation of Water-Soluble X-ray Luminescence Nanoparticles for Photodynamic Activation." Applied Physics Letters 92(4):Art. No. 043901. Abstract In this letter, we report the synthesis of LaF3:Tb3+-MTCP (meso-Tetra(4-carboxyphenyl) porphine) 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 in the nanoparticle-MTCP conjugates. Our pilot studies indicate that water-soluble scintillation nanoparticles can be potentially used to activate photodynamic therapy as a promising deep cancer treatment.
2008. "Synthesis and Optical Properties of Sulfide Nanoparticles Prepared in Dimethylsulfoxide." Journal of Nanoscience and Nanotechnology 8(11):5646-5651. Abstract Many methods have been reported for the formation of sulfide nanoparticles by the reaction of metallic salts with sulfide chemical sources in aqueous solutions or organic solvents. Here, we report the formation of sulfide nanoparticles in dimethylsulfoxide (DMSO) by boiling metallic salts without sulfide sources. The sulfide sources are generated from the boiling of DMSO and react with metallic salts to form sulfide nanoparticles. In this method DMSO functions as a solvent and a sulfide source as well as a stabilizer for the formation of the nanoparticles. The recipe is simple and economical making sulfide nanoparticles formed in this way readily available for many potential applications.
2008. "A Study of H and D doped ZnO epitaxial films grown by pulsed laser deposition." Journal of Applied Physics 104(5):Article no. 053711. doi:10.1063/1.2975219 Abstract We examine the crystal structure, electrical and optical properties of ZnO epitaxial films grown by pulsed laser deposition in a H2 or D2 ambient. We compared with pure ZnO films grown in O2 and vacuum. N-type electrical conductivity is enhanced by two to three orders of magnitude as a result of growing in H2 or D2. Temperature dependent Hall effect measurements reveal small (a few meV) carrier activation energies, along with carrier concentrations of 2-7 x 1018 cm-3, and mobilities of 20-40 cm2/Vs in ZnO films doped with H or D in the 1018 cm-3 range. We have modeled the low-temperature electrical properties of H- and D-doped ZnO films using variable range hopping and surface layer conductivity models, but our data do not fit well with these models. Rather, it appears that growth in H2 or D2 promotes the formation of an exceedingly shallow or conduction-band degenerate donor state, possibly associated with H or D substitution at O sites in the lattice.
2008. "Electronic Energy Transfer on CaO Surfaces." Journal of Chemical Physics 129(12):124704. doi:10.1063/1.2980049 Abstract We excite low-coordinated surface sites of nanostructured CaO samples using tunable UV laser pulses and observe hyperthermal O-atom emission indicative of an electronic excited-state desorption mechanism. The O-atom yield increases dramatically with photon energy, between 3.75 and 5.4 eV, below the bulk absorption threshold. The peak of the kinetic energy distribution does not increase with photon energy in the range 3.9 to 5.15 eV. These results are analyzed in the context of a laser desorption model developed previously for nanostructured MgO samples. The data are consistent with desorption induced by exciton localization at corner-hole trapped surface sites following either direct corner excitation or diffusion and localization of excitons from higher coordinated surface sites.
2008. "Energy and Site Selectivity in O-Atom Photodesorption from Nanostructured MgO." Surface Science 602(11):1968-1973. doi:10.1016/j.susc.2008.03.046 Abstract Electronic excitation of wide gap ionic solids can induce desorption of neutral atoms with distinct hyperthermal and thermal kinetic energy distributions. Hyperthermal atomic desorption results from electronic surface excitation while thermal desorption is initiated primarily by bulk excitation. Calculations indicate that surface-localized transitions can be excited independently from bulk transitions using selected photon energies. The photon energy required to excite specific surface sites depends upon the site coordination with successively lower energies required to excite terrace, step, and corner sites. Here, we excite low-coordinated surface sites of nanostructured MgO samples using 4.7 eV UV laser pulses and observe dominant hyperthermal O-atom emission. We then selectively excite bulk sites of nanostructured MgO, using a 7.9 eV laser, and observe dominant thermal O-atom desorption. These results are analyzed in terms of laser desorption models developed previously for alkali halide crystals. We propose a multi-step mechanism for hyperthermal O-atom desorption, under surface selective excitation, based on hole trapping at 3C (corner) O-atom sites followed by exciton decomposition. The proposed “hole plus exciton” model has similarities to the surface exciton desorption model, established for alkali halides, but is more complex and requires more steps. Nonetheless, the principle of site-specific photoreaction, established for alkali halide crystals, is clearly extendable to a prototypical metal oxide.
2007. "Photoemission Electron Microscopy of TiO2 Anatase Films Embedded with Rutile Nanocrystals." Advanced Functional Materials 17(13):2133-2138. doi:10.1002/adfm.200700146 Abstract Photoemission electron microscopy (PEEM) excited by x-ray and UV sources is used to investigate epitaxial anatase thin films embedded with rutile nanocrystals, a model system for the study of heterocatalysis on mixed-phase TiO2. Both excitation sources show distinct contrast between the two TiO2 phases, however, the contrast is reversed. Rutile nanocrystals appear darker than the anatase film in X-ray PEEM images but brighter in UV-PEEM images. Topography-induced contrast is dominant X-ray PEEM imaging, whereas work function contrast, dominates for UV-PEEM. Work function contrast results from the differences in work function and surface defect state densities between the two phases near the Fermi level. This assertion is confirmed by UPS data that shows the rutile work function to be 0.2 eV lower and a greater occupied valence band density-of-states in rutile (100) than in anatase (001). Since the boundaries between rutile nanocrystals and the anatase film are clearly resolved, these results indicate that PEEM studies of excited state dynamics and heterocatalysis are possible at chemically intriguing mixed-phase TiO2 interfaces and grain boundaries.
2007. "Study of Copper Diffusion Through Ruthenium Thin Film by Photoemission Electron Microscopy." Applied Physics Letters 90:111906. doi:10.1063/1.2712832 Abstract Photoemission electron microscopy (PEEM) is employed to study Cu diffusion in real time through a Ru barrier in a Cu/Ru bilayer system. The PEEM images display large contrast between Cu and Ru due of the differences in work function between the two metals, making PEEM an ideal methodology to study diffusion in real time. At low temperature (175-290 °C), Cu mainly diffuses through the defective sites in the Ru film. Uniform diffusion of Cu through a Ru thin film occurs at approximately 300 °C. The results are confirmed by X-ray photoemission spectroscopy (XPS) depth profiling and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) analysis.
2007. "Upconversion Luminescence of Colloidal CdS and ZnCdS Semiconductor Quantum Dots." Journal of Physical Chemistry C 111(44):16261-16266. doi:10.1021/jp074416b Abstract Strong upconversion luminescence is observed from colloidal CdS and CdZnS quantum dots dispersed in hexanes. The nanocrystals were synthesized via non-hot-injection approaches at relatively low temperature, exhibiting cubic crystal structures and narrow size distribution. The upconversion luminescence emission spectra of these binary and ternary nanocrystal ensembles studied are similar in both peak shape and position compared to their corresponding photoluminescence emission spectra. The upconversion luminescence exhibits a near quadratic laser power dependence. Furthermore, the upconversion and photoluminescence show similar decay dynamics. Accordingly, our study indicates that the upconversion luminescence is due to two-photon excitation.
2007. "Pressure Behaviour of the UV and Green Emission Bands in ZnO Micro-rods." Physica Status Solidi B, Basic Research 244(1):87-92. doi:10.1002/pssb.200672545 Abstract The pressure behavior of the ultraviolet (UV) and green emission bands in ZnO tetrapod-like micro-rods has been investigated at 300 and 70 K, respectively. The pressure coefficient of the UV band at 300 K is 24.5 meV/GPa, consistent with that of the band gap of bulk ZnO. However, the pressure coefficient of the green band is 25 meV/GPa, far larger than previous literature reports. The green band in this work comes originates from Cu-related emission, as confirmed by the fine structure observed in the spectra at 10 K. The pressure coefficients of four pho-non replicas of the free exciton emission (FX) at 70 K are 21.0, 20.2, 19.8, and 19.3 meV/GPa, respectively. The energy shift rate of the FX emission and the LO phonon energies is then determined to be 21.4 and 0.55 meV/GPa. The pressure coefficient of the neutral donor bound exciton (D0X) transition is 20.5 meV/GPa, only 4% smaller than that of FX. This confirms that the D0X emission corresponds to excitons bound to neutral shallow donors.
2007. "Synthesis and Photoexcited Charge Carrier Dynamics of beta-FeOOH Nanorods." Applied Physics Letters 90(10):Art. No. 103504. doi:10.1063/1.2711395 Abstract Akaganeite(B-FeOOH) nanorods of dimensions 15 nm diameter and 200 nm length were prepared by aqueous synthesis. Charge carrier dynamics following femtosecond excitation displays three timescales. The first is a sub-picosecond decay of initially excited carriers to the band edge followed by trapping or nonradiative decay within 2 ps. The trapped electrons and holes persist for significantly longer times (at least tens-of-ps), similar to previous results from a-Fe2O3 materials. The short carrier lifetimes in these materials are attributed to fast trapping to Fe d-d and midgap states.
2007. "Electronic Energy Relaxation and Luminescence Decay Dynamics of Eu3+ in Zn2SiO4:Eu3+." Journal of Luminescence 126(2):491-496. doi:10.1016/j.jlumin.2006.09.004 Abstract Abstract: Electronic energy relaxation and decay dynamics of Eu3+ in Zn2SiO4:Eu3+ phosphors displays evidence of intra-ion energy transfer from the 5D1 to the 5D0 manifold. The energy transfer timescale does not depend on Eu3+ concentration, or the addition of Mn2+ as a co-dopant and is estimated to be about 11 microseconds in Zn2SiO4. Evidence for Eu3+ electronic energy transfer has also been observed in Eu-doped MgS as well as Eu3+ encapsulated in zeolite-Y. The energy transfer timescale in these other materials is shorter than in Zn2SiO4, most likely due to differences in Eu3+ surroundings or site symmetry.
2007. "N incorporation and electronic structure in N-doped TiO2(110) rutile." Surface Science 601(7):1754-1762. doi:10.1016/j.susc.2007.01.051 Abstract Epitaxial TiO2-xNx film growth under anion-rich conditions is characterized by nearly balanced incorporation rates for substitutional N (NO) and interstitial Ti (Tii). Tii donors fully compensate and stabilize N3-, but preclude the formation of p-type material. Hybridization occurs between Tii(IV) and NO3-, but the value of x is limited to ~0.02 under these conditions. Tii(IV)-NO3- states occur above the valence band maximum of pure TiO2, riving rise to enhanced optical absorption in the visible up to ~2.5 eV. Much higher NO and Tii concentrations result from using cation-rich conditions.
2007. "Properties of Structurally Excellent N-doped TiO2 Rutile." Chemical Physics 339(1-3):27-35. doi:10.1016/j.chemphys.2007.04.024 Abstract We have used plasma-assisted molecular beam epitaxy to synthesize structurally near-perfect crystalline films of TiO2-xNx rutile for the first time. These materials allow the properties of TiO2-xNx to be elucidated without the interfering effects of oxygen vacancy defects. In the absence of such defects, the extent of N incorporation in the lattice is limited to 2 ± 1 at. % of the anions. Substitutional N (NO) exhibits a -3 formal charge due to charge transfer from shallow-donor interstitial Ti(III), which forms during epitaxial growth. Hybridization between NO and adjacent lattice Ti ions occurs, resulting in new states off the top of the rutile valence band and an apparent band gap reduction of ~ 0.5 eV. It is not yet known if these new states result in mobile electron-hole pair creation upon irradiation, but experiments are planned to answer this important question.
2007. "Study of Martensitic Phase transformation in a NiTiCu Thin Film Shape Memory Alloy Using Photoelectron Emission Microscopy." Advanced Functional Materials 17(1):161-167. doi:10.1002/adfm.200600611 Abstract The thermally-induced martensitic phase transformation in a polycrystalline NiTiCu thin film shape memory alloy was probed by photoelectron emission microscopy (PEEM). In situ PEEM images reveal distinct changes in microstructure and photoemission intensity at the phase transition temperatures. In particular, images of the low temperature, martensite phase are brighter than that of the high temperature, austenite phase, due to the relatively lower work function of the martensite. Ultra-violet photoelectron spectroscopy shows that the effective work function changes by about 0.16 eV during thermal cycling. In situ PEEM images also show that the network of trenches observed on the room temperature film disappear suddenly during heating and reappear suddenly during subsequent cooling. These trenches are also characterized by atomic force microscopy at selected temperatures. We describe implications of these observations with respect to the spatial distribution of phases during thermal cycling in this thin film shape memory alloy.
2007. "An In Situ Study of the Martensitic Transformation in Shape Memory Alloys Using Photoemission Electron Microscopy." Journal of Nuclear Materials 361(2-3):306-312. doi:10.1016/j.jnucmat.2006.12.008 Abstract Thermally-induced martensitic phase transformations in polycrystalline CuZnAl and thin-film NiTiCu shape memory alloys were probed using photoemission electron microscopy (PEEM). Ultra-violet photoelectron spectroscopy shows a reversible change in the apparent work function during transformation, presumably due to the contrasting surface electronic structures of the martensite and austenite phases. In situ PEEM images provide information on the spatial distribution of these phases and the evolution of the surface microstructure during transformation. PEEM offers considerable potential for improving our understanding of martensitic transformations in shape memory alloys in real time.
2006. "Laser-Induced Oxygen Vacancy Formation and Diffusion on TiO2(110) Surfaces Probed by Photoemission Electron Microscopy." Physica Status Solidi. C 3(10):3598-3602. Abstract Photoemission electron microscopy is used to probe photon-induced oxygen vacancies generated on TiO2 (110)-(1×2) surfaces. An increased oxygen vacancy concentration within the irradiated region leads to an increase of local photoelectron emission. The local oxygen deficient region can be compensated by exposing the surface to molecular oxygen at 1×10-5 Torr, or via surface diffusion at 450 K in vacuum. The surface diffusion coefficient was estimated to be on the order of 10-12 m2/s. Photoemission electron microscopy allows in situ studies of surface electronic defect formation and removal.
2006. "Introduction to Photoelectron Emission Microscopy: Principles and Applications." Journal of Chinese Electron Microscopy Society 25(1):15-25. Abstract In the past decade, photoelectron emission microscopy (PEEM) has undergone major instrument development and become commercially available. PEEM probes photoelectrons in a high-contrast imaging technique that is sensitive to the surface electronic structure. In this paper we illustrate the principles of PEEM and analyze important PEEM contrast mechanisms. We briefly summarize the applications of PEEM to areas such as surface structure analysis, surface chemistry, magnetism, and semiconductor device characterization. Two important new directions in PEEM development are multiphoton and time-resolved PEEM. Multiphoton PEEM is capable of imaging materials with work functions greater than the incident photon energy, while time-resloved PEEM enables study of fast relaxation dynamics of surface intermediate states. We discuss our recent progress on implementing femtosecond time-solved PEEM and multiphoton PEEM to investigate the silver nanostructured film coated on silicon. The multiphoton images consist of some “hot spots” with far greater photonelectron intensity than is observed in single-photon derived images. We surmise that this is due to the highly-selective excitation of surface localized plasmon of silver. To illustrate the utility of PEEM, we also describe an in-situ thermal-induced structural phase transformation of CuZnA1 shape memory alloy.
2006. "In Situ Photoelectron Emission Microscopy of a Thermally Induced Martensitic Transformation in a CuZnAI Shape Memory Alloy." Applied Physics Letters 88(9):Art. No. 091910. Abstract Photoemission electron microscopy, in conjunction with photoemission spectroscopy, reflectivity, and surface roughness measurements, is used to study the thermally-induced martensitic transformation in a CuZnAI shape memory alloy. Real-time phase transformation is observed as a nearly instantaneous change of photoelectron intensity, accompanied by microstructural deformation and displacement due to the shape memory effect. The difference in the photoelectron intensity before and after the phase transformation is attributed to the concomitant change of work function as measured by photoelectron spectroscopy. Photoemission electron microscopy is shown to be a valuable new technique facilitating the study of phase transformations in shape memory alloys, and provides real-time information on microstructural changes and phase-dependent electronic properties.
2006. "Excited Carrier Dynamics of α-Cr2O3/α-Fe2O3 Core-Shell Nanostructures." Journal of Physical Chemistry B 110(34):16937-16940. doi:10.1021/jp062507n S1520-6106(06)02507-7 Abstract In this work α-Cr2O3/α-Fe2O3 core-shell polycrystalline nanostructures were synthesized using α-Cr2O3 nanoparticles as seed crystals during aqueous nucleation. The formation of α-Fe2O3 polycrystallites on α-Cr2O3 surfaces were confirmed by x-ray diffraction, transmission electron microscopy, and energy dispersive x-ray analysis. The excited state relaxation dynamics of as-grown core-shell structures and \pure" α-Fe2O3 particles of the same size were measured using femtosecond transient absorption spectroscopy. The results show the carrier lifetimes decay within a few picoseconds regardless of sample. This is likely due to fast recombination/trapping of carriers to defects and iron d-states.
2006. "Temperature and pressure dependences of the Copper-related Green Emission in ZnO Microrods." Journal of Applied Physics 100(1):013107-1 - 013107-6. doi:10.1063/1.2206705 Abstract We investigate the temperature and pressure dependence of the green, Cu2+-related photoluminescence (PL) from tetrapod-like ZnO micro-rods. The temperature dependence of the green emission energy follows the changes in the band gap from 10-200 K, but deviates from this behavior above 200 K. The pressure dependence of the defect-related green band (25±5 meV/GPa) is similar to that of the band gap of ZnO, which is far larger than reported previously.
2006. "Pressure Dependence of the Near-Band-Edge Photoluminescence from ZnO Microds at Low Temperature." The Journal of Physics and Chemistry of Solids 67(11):2376-2381. Abstract The temperature and pressure dependences of band-edge photoluminescence from ZnO mico-rods have been investigated. The energy separation between the free exciton (FX) and its first order phonon replica (FX-1LO) decreases at a rate of kBT with increasing temperature. The intensity ratio of the FX-1LO to the bound exciton (BX) emission is found to decrease slightly with increasing pressure. All of the exciton emission peaks show a blue shift with increasing pressure. The pressure coefficient of the FX transition, longitudinal optical (LO) phonon energy, and binding energy of BX are estimated to be 21.4, 0.5, and 0.9 meV/GPa, respectively.
2006. "Luminescence Temperature and Pressure Studies of Zn2SiO4 Phosphors Doped with Mn2+ and Eu3+ Ions." Journal of Luminescence 116(1-2):117-126. Abstract Zn2SiO4:Mn2+, Zn2SiO4:Mn3+, and Zn2SiO4:Mn2+, Eu3+ phosphors were prepared by a sol-gel process and their luminescence spectra were investigated. The emission bands from intra-ion transitions of Mn2+ and Eu3+ samples were studied as a function of pressure. The pressure coefficient of Mn2+ emission was found to be -25.3±0.5 and -28.5±0.9 neV/GPa for Zn2SiO4:Mn2+ and Zn2SiO4:Mn2+, Eu3+, emission shows only weak pressure dependence. The pressure dependences of the Mn2+ and Eu3+ emissions in Zn2SiO4:Mn2+, Eu3+ are slightly different from that in Zn2SiO4:Mn2+ and Zn2SiO4:Mn3+ samples, which can be attributed to the co-doping of Mn2+ and Eu3+ ions. The Mn2+ emission in the two samples, however, exhibits analogous temperature dependence and similar luminescence lifetimes, indicating no energy transfer from Mn2+ to Eu3+ occurs.
2006. "Comparison of Water-Soluble CdTe Nanoparticles Synthesized In Air and In Nitrogen." Journal of Physical Chemistry B 110(34):16992-17000. Abstract It is commonly believed that high quality CdTe nanoparticles with strong luminescence can only be prepared under the protection of an inert gas such as nitrogen or argon. Here we report the preparation of highly luminescent CdTe nanoparticles in air and compare their luminescence properties with CdTe nanoparticles made in nitrogen. We find that both CdTe nanoparticles made in air and in nitrogen exhibit strong photoluminescence as well as upconversion luminescence at room temperature. However, differences do exist between the particles made in air and in nitrogen. In particular, the particles prepared in air display a faster growth rate, grow to larger sizes, and display stronger electron coupling relative to the particles prepared in nitrogen. X-ray photoelectron spectroscopy analysis indicates that the oxygen content in the nanoparticles synthesized in air is higher that in particles synthesized in N2, likely resulting in a higher availability of excess free cadmium. Cytotoxicology measurements reveal that the particles made in air appear slightly more toxic, possibly due to the excess of free cadmium.
2006. "Probing Electron Transfer Dynamics at MgO Surfaces by Mg-Atom Desorption." Journal of Physical Chemistry B 110(37):18093-18096. doi:10.1021/jp064092b Abstract Desorption of a weakly bound adsorbate from a porous solid was studied for the case of N2 on amorphous solid water (ASW). Porous ASW films of different thickness were grown on Pt(111) by ballistic deposition. N2 adsorption and desorption kinetics were monitored mass-spectrometrically. Temperature programmed desorption spectra show that with the increasing film thickness, the N2 desorption peak systematically shifts to higher temperatures. The results are explained and quantitatively reproduced by a simple model, which assumes that the N2 transport within the film is faster than the depletion rate to vacuum. The local coverage at the pore mouth determines the desorption rate. For thick ASW films (>1 μm), the assumption of the fast equilibration within the film is shown to be no longer valid due to diffusion limitations. The mechanisms of the adsorbate transport are discussed.
2006. "Carrier Dynamics in a-Fe2O3 (0001) Thin Films and Single Crystals Probed by Femtosecond Transient Absorption and Reflectivity." Journal of Applied Physics 99(5):Article: 053521 (6 pages). Abstract Femtosecond transient reflectivity and absorption is used to measure the carrier lifetimes in α-Fe2O3 thin films and hematite single crystals. The results from the thin films show that initially excited hot electrons relax to the bandedge within 300 femtoseconds and then recombine with holes or trap within 5 pioseconds. The trapped electrons have a lifetime of hundreds of picoseconds. 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. In hematite, the transient reflectivity displays oscillations due to the formation of longitudinal acoustic phonons generated following absorption of the ultrashort excitation pulse.
2006. "Utilizing Nanofabrication to Construct Strong, Luminescent Materials." Nanotechnology 17(10):2595-2601. doi:10.1088/0957-4484/17/10/025 Abstract Luminescent materials have been utilized widely in applications from lighting to sensing. The new development of technologies based on luminescence properties requires the materials to have high luminescence efficiency and mechanical strength. In this article, we report the fabrication of luminescent materials possessing high mechanical strength by nanofabrication with polyvinyl alcohol used as a stabilizer or coupling agent. X-ray diffraction and high resolution transmission microscope observations reveal that the nanocomposite sample contains ZnS and ZnO nanoparticles as well as kozoite and sodium nitrate. The mechanical strength and hardness of these nanocomposite materials are higher than polycarbonate and some carbon nanotube reinforced nanocomposites. Strong luminescence is observed in the new nanocomposites and the luminescence intensity does not degrade following up to 30 minutes of X-ray irradiation. Our results indicate that nanofabrication may provide a good method to improve the mechanical strength of luminescent materials for some applications in which high strength luminescent materials are needed.
2006. "The Origin of X-ray Luminescence from CdTe Nanoparticles in CdTe/BaFBr:Eu2+ Nanocomposite Phosphors." Journal of Applied Physics 99(3):Art. No. 034302. Abstract Intense X-ray luminescence from CdTe nanoparticles is observed when CdTe particles are encapsulated into BaFBr:Eu2+ phosphors. In contrast, negligible X-ray luminescence is observed from the unencapsulated nanoparticles, either in solution or in solid form. The origin of the X-ray luminescence is attributed to the effective energy transfer from Eu2+ ions to CdTe nanoparticles in the nanocomposite materials. The X-ray luminescence of these nanocomposites is dose dependent and the emission wavelength is size-adjustable which may allow use as a new type of dosimeter for both in vitro and in vivo applications.
2005. "Synthesis and Luminescence of ZnMgS:Mn2+ Nanoparticles." Journal of Nanoscience and Nanotechnology 5(9):1465-1471. Abstract Efficient green emission from ZnMgS:Mn2+ nanoparticles prepared by co-doping Mg2+ and Mn2+ ions into ZnS lattices has been observed. The synthesis is carried out in aqueous solution, followed by a post-annealing process, thus showing the features of less complexity, low cost, and easy incorporation of dopants. In comparison with the emission of ZnS:Mn2+ nanoparticles, which is located generally around 590 nm, the photoluminescence of ZnMgS:Mn2+ nanoparticles is blue-shifted by 14 nm in wavelength, leading to the enhanced green emission. The X-ray diffraction, electron spin resonance, and pressure dependent photoluminescence measurements suggest that the change of the crystal field caused by Mg2+ ionic doping and the lower symmetry in the nanoparticles may account for the blue-shift of the photoluminescence. The ZnMgS:Mn2+ nanoparticles with 1% Mn2+ doping exhibit the strongest luminescence, which could potentially meet the requirements for the construction of green light emitting diodes.
2005. "A Mechanism of Photo-Induced Desorption of Oxygen Atoms From MgO Nano-Crystals." Surface Science 593(1-3):210-220. Abstract In a series of recent experimental and theoretical papers we reported the results of our studies of photo-induced hyper-thermal halogen atom desorption from alkali halide surfaces. There we demonstrated that the yield, electronic state and velocity distributions of desorbed atoms can be controlled by carefully choosing parameters of photo-irradiation such as laser photon energy and pulse power [ ]. To achieve laser control over desorption process one must have clear understanding of possible desorption mechanisms and parameters responsible for their selective excitation. For alkali halides, as it has been shown through a combination of theory and experiment, such selectively is observed if the laser energy is tuned to preferentially excite surface excitons. If similar mechanisms could be demonstrated for a wider variety of materials, this approach could become a new method for controlling surface processes and hence modifying surface structures on an atomic scale. In this paper we report the first experimental observation of the hyper-thermal oxygen atom emission from an of MgO nano-clusters and thin films using frequency selected laser pulses oxide surface and investigate theoretically the mechanisms of this process. On this way we demonstrate a new concept that can be applied to studying surface reactions and desorption of binary oxides.
2005. "Upconversion Luminescence of CdTe Nanoparticles." Physical Review. B, Condensed Matter 71:165304. doi:10.1103/PhysRevB.71.165304 Abstract Efficient upconversion luninescence is observed from CdTe nanoparticles in solution and precipitated as solids. In the solids, the upconversion luminescence spectrum is significantly red-shifted relative to the normal photoluminescence, whereas in solution, there is very little spectral shift. The upconversion luminescence exhibits a near quadratic laser power dependencem, both at room temperature and at 10K. Both the upconversion and photoluminescence show similar decay dynamics with the solid samples showing shorter lifetimes compared to the solutions. This lifetime shortening is attributed to surface states is the likely upconversion excitation mechanism in these particles and that phonon populated defeat do not contribute to the upconversion.
2005. "Laser Control of Desorption Through Selective Surface Excitation." Journal of Physical Chemistry B 109(42):19563-19578. doi:10.1021/jp0523672 Abstract We review recent developments in controlling photo-induced desorption processes of alkali halides and magnesium oxide. We focus primarily on hyperthermal desorption of halogen (and oxygen) atoms and show that the yield, electronic state, and velocity distributions of desorbed atoms can be selected using tunable laser excitation. We demonstrate that the observed control is due to preferential excitation of surface excitons. This approach takes advantage of energetic differences between surface and bulk exciton states and probes the surface exciton directly. We demonstrate that desorption of these materials leads to controlled modification of their surface geometric and electronic structures. Laser desorption can serve as a solid-state source of halogen and oxygen atoms in well defined electronic states and with controlled velocity distributions. The latter can be used for studying chemical processes in the gas phase and at surfaces. We demonstrate that the exciton mechanism of desorption developed for alkali halides can be extended to cubic oxide surfaces.
2005. "Surface Electronic Properties and Site-Specific Laser Desorption Processes of Highly Structured Nanoporous MgO Thin Films." Surface Science 593(1-3):242-247. Abstract The surface electronic properties of metal oxides critically depends on low-coordinated sites, such as kinks, corners and steps [1]. In order to characterize experimentally those surface states as well as their role for laser desorption processes, we prepare defect enriched surfaces by growing thin MgO films using reactive ballistic deposition [2] on crystalline dielectric substrates. With samples held at room temperature, the resulting MgO films are highly textured and consist of porous columns with column lengths ranging from tens of nanometers up to six micrometers. Measurements by x-ray photoelectron spectroscopy (XPS) are carried out in-situ for MgO films, vacuum-cleaved MgO crystals, and water vapor exposed samples. In the case of thin films, we observe O 1s spectra with a significant shoulder feature at 2.3 eV higher binding energy (HBE) than the corresponding peak at 530.0 eV representing regular lattice oxygen. We evaluate this feature in terms of non-stoichiometric oxygen and formation of an oxygen-rich layer at the topmost surface of the MgO columns. In contrast, no HBE-features are detectable from clean single crystal MgO surfaces, while the hydroxyl O 1s band peaks at 531.6 eV. Under excitation with 266-nm-laser-pulses, known to be resonant with low-coordinated surface anions [3], we observe preferential depletion of defective oxygen-states (HBE signal) and temporary restoration of ideal surface stoichiometry. Furthermore, auxiliary signals are observed on several micrometer thick films, acting like satellites to major photoelectron-peaks (O 1s, Mg 2s, and Mg 2p) but shifted by approximately 4 eV towards lower kinetic energy. These features are depleted by UV-light exposure, pointing to the occurrence of surface-charge imbalance, accompanied by photon stimulated charge-transfer reactions. These results are in line with desorption experiments of neutrals, stimulated by laser excitation at 266 nm. According to the low-coordination nature of nanoporous MgO films, we find that the laser fluence required for oxygen desorption is much lower in comparison to the single-crystal MgO (100) surface. The detected neutral oxygen desorption occurs with a single photon power dependence from nanoporous MgO thin films. This contrasts to the two-photon power dependence observed from MgO single crystals. In fact, the single photon energy of the laser lies within the charge-transfer transition in surface ion pairs with three-coordinated anions (~ 4.7 eV), while a two-photon absorption regime easily lies within the four-coordinated anion charge transfer transition near 5.4 eV [3]. In summary, our XPS studies and laser desorption experiments indicate new surface-site-specific excitation regimes, which might eventually allow for site-specific manipulation of surface morphology.
2005. "Upconversion Luminescence from CdSe Nanoparticles." Journal of Chemical Physics 122(22):224708 (7 pages). Abstract 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 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 surface or trap states work as emitters rather than as intermediate states for upconversion luminescence and that two-photon absorption is the likely excitation mechanism.
2005. "Structure and Luminescence of BaFBr:Eu2+ and BaFBr:Eu2+, Tb3+ Phosphors and Thin Films." Journal of Applied Physics 97(8):083506 (8 p.). Abstract The inclusionof Tb3+ ions doped into the storage phosphor BaFBr: Eu2+ results in increased purity of the host material as well as an improvement in the linearity of the photostimulated luminescence intensity versus the X-ray irradiation dose. Films produced by pulsed laser deposition (PLD) also show similar results, indicating that PLD is a powerful technique for producing high quality, high purity thin films. As a result, the PSL dose response is more linear in the thin films than in the powder samples. The emission intensity of Eu2+ in the phosphors increases with the increasing time of X-ray irradiation while the emission intensity of Tb3+ remains almost constant. This result may be explained by considering the reduction of impurity Eu3+ ions following trapping of the photoexcited electrons. The results demonstrate that these materials are not only potentially useful for medical imaging but also show promise for use in radiation dosimetry.
2005. "Fabrication and Luminescence of ZnS:Mn2+ Nanoflowers." Journal of Nanoscience and Nanotechnology 5(9):1309-1322. Abstract Visually striking nanoflowers composed of ZnS:Mn2+ nanoparticles are prepared and characterized. The configurations of these fractal structures are very sensitive to both the pH values of the particle solutions from which they are precipitated and the substrates on which they are deposited. At pH 2.2, the fractal structures resemble trees without leaves; at pH 7.7, they are tree-like with four arms and at pH 11.0 they resemble trees with 6 arms. High resolution transmission microscopy reveals that the nanoflowers are composed of ZnS:Mn2+ nanoparticles of 2-5 nm in size. X-ray photoelectron spectral data indicate that the sample compositions of nitrogen, chlorine, and sulfur vary gradually with pH values of the solutions. These changes may have an impact on both the fractal configuration and the luminescence properties. The emission spectra of the particle solutions at pH of 2.2 and 11.0 are similar with the emission maximum at 475 nm. As the pH value approaches 7.7, the emission spectral maximum shifts to longer wavelengths. At a pH value of 7.7, the emission peak wavelength is the reddest, 520 nm. The emission peak of the nanoflowers at a pH value of 9.3 is 510 nm, while the emission spectrum of the nanoflowers at 5.2 has two peaks at 500 nm and 440 nm, respectively. These blue-green emissions are attributed to defects and are the dominant luminescence from the nanoflowers. The emission from Mn2+ dopant is only observed in the delayed spectra of the fractal solid samples.
2005. "Structure, Band Offsets and Photochemistry at Epitaxial ⍺-Cr₂O₃/⍺-Fe₂O₃ Heterojunctions." Surface Science 587(3):L197-L207. Abstract We test the hypothesis that electron-hole pair separation following light absorption enhances photochemistry at oxide/oxide heterojunctions which exhibit a type II or staggered band alignment. We have used hole-mediated photodecomposition of trimethyl acetic acid chemisorbed on surfaces of heterojunctions made from epitaxial ⍺-Cr₂O₃ on ⍺-Fe₂O₃(0001) to monitor the effect of UV light of wavelength 385 nm (3.2 eV) in promoting photodissociation. Absorption of photons of energies between the bandgaps of ⍺-Cr₂O₃ (Eg = 4.8 eV) and ⍺-Fe₂O₃ (Eg = 2.1 eV) is expected to be strong only in the ⍺-Fe₂O₃ layer. The staggered band alignment should then promote the segregation of holes (electrons) to the ⍺-Cr₂O₃ (⍺-Fe₂O₃) layer. Surprisingly, we find that the ⍺-Cr₂O₃ surface alone promotes photodissociation of the molecule at hv = 3.2 eV, and that any effect of the staggered band alignment, if present, is masked. We propose that the inherent photoactivity of the ⍺-Cr₂O₃ (0001) surface results from the creation of bound excitons in the surface which destabilize the chemisorption bond in the molecule, resulting in photodecomposition.
2004. "Cryogenic Laser Induced Fluorescence Characterization of U(VI) in Hanford Vadose Zone Pore Waters." Environmental Science and Technology 38:5591-5597. Abstract Ambient and liquid helium temperature laser-induced time-resolved uranyl fluorescence spectroscopy was applied to study the speciation of aqueous uranyl solutions containing carbonate and phosphate and two porewater samples obtained by ultra-centrifugation of U(VI)-contaminated sediments. The significantly enhanced fluorescence signal intensity and spectral resolution found at liquid helium temperature allowed, for the first time, direct fluorescence spectroscopic observation of the higher aqueous uranyl complexes with carbonate: UO2(CO3)22-, UO2(CO3)34- and (UO2)2(OH)3CO3-. The porewater samples were non-fluorescent at room temperature. However, at liquid helium temperature, both porewater samples displayed strong, well-resolved fluorescence spectra. Comparisons of the spectroscopic characteristics of the porewaters with those of the standard uranyl-carbonate complexes confirmed that U(VI) in the porewaters existed primarily as UO2(CO3)34-. A small amount of the dicalcium-urano-tricarbonate complex, Ca2UO2(CO3)3, was also observed that was consistent with thermodynamic calculation. The U(VI)-carbonate complex is apparently the mobile species responsible for the subsurface migration of U(VI), even though the majority of the in-ground U(VI) inventory at the site from which the samples were obtained exists as intragrain U(VI)-silicate precipitates.
2004. "Determination of Surface Exciton Energies by Velocity Resolved Atomic Desorption." Surface Science 564(1-3):62-70. Abstract We have developed a new method for determining surface exciton band energies in alkali halides based on velocity-resolved atomic desorption (VRAD). Using this new method, we predict the surface exciton energies for K1, KBr, KC1, and NaC1 within +0.15 eV. Our data, combined with the available EELS data for alkali fluorides, demonstrate a universal linear correlation with the inverse inter-atomic distance in these materials. The results suggest that surface excitons exist in all alkali halides and their excitation energies can be predicted from the known bulk exciton energies and the obtained correlation plot.
2004. "Laser-Induced Damage of Calcium Fluoride." Journal of Undergraduate Research Volume IV, 2004:60-65. Abstract Radiation damage of materials has long been of fundamental interest, especially since the growth of laser technology. One such source of damage comes from UV laser light. Laser systems continue to move into shorter wavelength ranges, but unfortunately are limited by the damage threshold of their optical components. For example, semiconductor lithography is making its way into the 157nm range and requires a material that can not only transmit this light (air cannot), but also withstand the highly energetic photons present at this shorter wavelength. CaF2, an alkaline earth halide, is the chosen material for vacuum UV 157 nm excimer radiation. It can transmit light down to 120 nm and is relatively inexpensive. Although it is readily available through natural and synthetic sources, it is often times difficult to find in high purity. Impurities in the crystal can result in occupied states in the band gap that induce photon absorption [2] and ultimately lead to the degradation of the material. In order to predict how well CaF2 will perform under irradiation of short wavelength laser light, one must understand the mechanisms for laser-induced damage. Laser damage is often a two-step process: initial photons create new defects in the lattice and subsequent photons excite these defects. When laser light is incident on a solid surface there is an initial production of electron-hole (e-h) pairs, a heating of free electrons and a generation of local heating around optically absorbing centers [3]. Once this initial excitation converts to the driving energy for nuclear motion, the result is an ejection of atoms, ions and molecules from the surface, known as desorption or ablation [3]. Secondary processes further driving desorption are photoabsorption, successive excitations of self-trapped excitons (STE’s) and defects, and ionization of neutrals by incident laser light [3]. The combination of laser-induced desorption and the alterations to the electronic and geometrical structure of the lattice result in defect formation. In the material CaF2 some of these defects take the form of F-centers, an electron trapped at a halogen vacancy [4], and H-centers, a F2- molecular ion at a single lattice site [5]. While the F-centers are stable, the H-centers are transient but can form into aggregates that are stable. There are many different configurations the defects can take based on the relative position of F and H centers in the lattice and this is extensively discussed in literature [1,4,5]. Once these defects have formed they cause further absorption of light, which ultimately induces particle emission and the production of even more defects. Various forms of laser-induced damage of CaF2 have been studied. For example, the mechanism for photon-stimulated desorption (PSD) of F+ from CaF2 (111) is discussed in ref. 6 and the energy threshold, distribution and kinetics governing electron-stimulated desorption (ESD) is investigated in ref. 7. The desorption of neutral Ca and F atoms has also been explored [1]. In this paper I focus on the emission of ions and neutrals from CaF2 under the irradiation of pulsed laser light at 266 nm, in addition to a brief study of its purity and transmittance.
2004. "Upconversion luminescence of Eu3+ and Mn2+ in ZnS:Mn2+, Eu3+ codoped nanoparticles." Journal of Applied Physics 95(2):667-672. Abstract 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.
2004. "Optical Properties and Potential Applications of Doped Semiconductor Nanoparticles." Journal of Nanoscience and Nanotechnology 4(8):919-947. Abstract Recent studies on the optical properties, in particular luminescence, of a variety of doped semiconductor nanoparticles are reviewed. The effects of quantum confinement, temperature, and pressure on luminescent properties are discussed. In addition, electroluminescence, cathodoluminescence, magnetoluminescence and related applications involving doped semiconductor nanoparticles are presented. A new phenomenon, upconversion luminescence of doped nanoparticles, is reviewed and its potential applications are discussed. While more research efforts are necessary in order to fully understand the fundamentals and explore the great technological potential behind doped nanoparticles, recent results already show that this is a new and exciting field with applications in many fields. In particular, the emerging field of "spintronics", where spin states are exploited in analogy to conventional electronic states, is discussed and the advantages of using doped semiconductor nanoparticles are elucidated.
2004. "Full-Color Emission From In2S3 and In2S3:Eu3+ Nanoparticles." Journal of Physical Chemistry B 108(32):11927-11934. Abstract Blue (425 nm) and green (510 nm) emission is observed from In2S3 nanoparticles which are synthesized using a novel method. Both the blue and green emissions have large Stokes shifts of 62 and 110 nm, respectively. Excitation with longer wavelength photons causes the blue emission to shift to longer wavelength while the green emission wavelength remains unchanged. The lifetimes of both green and blue emissions are similar to reported values for excitonic recombination. When doped with Eu3+, in addition to the broad blue and green emissions, a red emission at 615 nm attributed to Eu3+ is observed. Temperature dependences on nanoparticle thin films indicate that with increasing temperature, the green emission wavelength remains constant, however, the blue emission shifts toward longer wavelengths. Based on these observations, the blue emission is attributed to exciton recombination and the green emission to Indium interstitial defects. These nanoparticles show full-color emission with high efficiency, fast lifetime decays, good stability, and are relatively simple to prepare thus making them a new type of phosphor with potential applications in lighting, flat-panel displays, and communications.
2004. "Laser Control of Product Electronic State: Desorption from Alkali Halides." Journal of Chemical Physics 120(5):2456-2463. Abstract We demonstrate laser control of the electronic product state distribution of photodesorbed halogen atoms from alkali halide crystals. Our general model of surface exciton desorption dynamics is developed into a simple method for laser control of the relative halogen atom spin orbit laser desorption yield. By tuning the excitation laser photon energy in a narrow region of the absorption threshold, the relative C1(2P1/2) yield can be made to vary from near 0 to 80% from KCI and from near 0 to 60% for NaC1. We described the physical properties necessary to obtain a high degree of product state control and the limitation induced when these requirements are not met. These results demonstrate that laser control can be applied to solid state surface reactions and provide strong support for surface exciton-based desorption models.
2003. "The Structures of Fluorene-(H2O)(1,2) Determined by Rotational Coherence Spectroscopy ." Journal of Chemical Physics 119(4):1970-1977. Abstract Rotational coherence spectroscopy (RCS), via time-correlated single photon counting, and two-color resonant two-photon ionization (R2PI) time-of-flight mass spectrometry, have been used to characterize fluorene-(water)1,2 (FL-(water)1,2) van der Waals clusters generated in supersonic jets. Rotational coherence traces have been obtained at excitation energies corresponding to several resonant features in the S1S0 R2PI spectra of FL-(H2O)1,2. RCS simulations and diagonalization of the moment of inertia tensor have been used to obtain the S1 excited state rotational constants and structures of FL-(H2O)1,2 that are consistent with the experimental rotational coherence traces. The RCS results indicate that: (i) the water molecule in FL-H2O bridges the central five-membered ring of fluorene and hydrogen bonds to both aromatic sites; (ii) the water molecules in FL-(H2O)2 form a water dimer that is oriented along the long axis of fluorene and is hydrogen-bonded to both aromatic sites. The S1S0 R2PI spectra of FL-(D2O)1,2 and FL-HDO have also been obtained. The transition is a doublet in the R2PI spectra of FL-H2O, FL-D2O, and a singlet in the R2PI spectrum of FL-HDO. The presence of this doublet in the FL-H2O/D2O spectra, and the absence of such a splitting in the FL-HDO spectrum, is an indication of nearly free internal rotation of the water molecule on a potential energy surface that changes upon electronic excitation. Lastly, the use of RCS and psec time-resolved fluorescence as a tool for assigning features in R2PI spectra that are of ambiguous origin due to fragmentation of higher mass clusters into lower mass channels is demonstrated.
2003. "Surface Electronic Spectra Detected by Atomic Desorption." Surface Science 544(1):L683-L688. Abstract Using continuously tunable laser excitation of KI we measure the velocity profiles and the yield of desorbing hyperthermal iodine atoms as a function of photon energy. Based on the theoretical model of desorption we demonstrate that these spectra display a signature of a surface exciton and constitute a new sensitive method of surface specific desorption spectroscopy. Our results demonstrate that creation of surface excitions can be a much more general phenomenon than was previously thought based on extant spectroscopic measurements.
2003. "Photon stimulated desorption from KI: Laser control of I-atom velocity distributions." Surface Science 528 (1-3):219-223. Abstract Abstract: Irradiation of cleaved KI single crystals with photons near the bulk absorption threshold between 5.1 and 6.0 eV produces hyper-thermal I(2P3/2) emission with a minor thermal component, and with little spin orbit excited I(2P1/2) emission. The I-atom kinetic energy distribution may be tuned by choice of photon energy indicating that control of photon energy and pulse power can produce I-atoms in selectable quantities and velocity distributions.
2003. "Full-Color Emission and Temperature Dependence of the Luminescence in Poly-P -Phenylene Ethynylene-ZnS:Mn2+ Composite Particles." Journal of Physical Chemistry B 107(27):6544-6551. Abstract Synthesis of a nanocomposite material composed of anionic poly (phenylene ethynylene) (aPPE) polymer particles and ZnS:Mn2+ nanoparticles is described and its luminescence properties investigated. aPPE particles have two emissions, one in the blue (460 nm), which are assigned to the 0-0 transition and an excimer, respectively. ZnS:Mn2+ nanoparticle have an emission at 596 nm which is due to the 4T1-6A1 transition of Mn2+ and an emission at 706 nm which is ascribed to a defect-related luminescence. The blue, green, yellow, and red emissions make the composite a potential material for full-color displays. More interesting, the relative intensities of the different emissions may be varied by changing the excitation energy. Photoluminescence excitation and emission spectra as well as observations on luminescence lifetimes indicate that there is negligible energy transfer from the polymer particles to the ZnS:Mn2+ nanoparticle. Temperature studies reveal that the ZnS:Mn2+ particles in the nanocomposite have a significantly reduced thermal quenching energy relative to bare ZnS:<n2+ nanoparticles. In addition, between room temperature and 90° C, the luminescence of the ZnS:Mn2+ nanoparticles at 596 nm increases in intensity with increasing temperature. This surprising phenomenon is attributed to thermo luminescence and thermal curing of the particle surface upon heating.
2002. "Femtosecond Time-Resolved Photo-Stimulated Desorption from Ionic Crystals." Applied Surface Science 186(1-4):339-344. Abstract We have used the pump-probe technique to measure the positive ion yield, from ionic crystals, as a function of time-delay between two femtosecond laser pulses. The two-pulse technique allows direct observation of solid state and surface dynamics on a femtosecond timescale. We find the ion yield, from 265 nm irradiated MgO and KBr, depends critically on the time delay between pulses. For example, the Mg+ desorption yield displays three distinct features; a coherence peak followed by a fast rise and decay features. In contrast, the yield of K+ from KBr displays only the coherence peak and picosecond decay features. The observed thresholds suggest that, although the nanosecond laser ion desorption mechanism may be dominated by defect photoabsorption, significant electron-hole pair production may contribute to the femtosecond laser desorption mechanism. By determining the ultrafast time-dependence we hope to reveal the mechanism of the laser ion desorption for both regimes.
2002. "Solid-State Halogen Atom Source for Chemical Dynamics and Etching." Applied Physics Letters 81(6):1140-1142. Abstract We describe a solid state Br atom source for surface etching, kinetics, or reaction dynamics studies. Pulsed laser irradiation of crystalline KBr, near the bulk absorption threshold at 6eV, produces hyperthermal Br atmos in dense plumes. The Br atom density and velocities may be controlled by choice of laser pulse power and photon energy. Single and multiple pulse excitation of KBr produces Br and Br* in controllable quantities and velocities, thus providing an attractive UHV compatible solid-state radical atom source. The solid-state atom source is in principle extensible to other halogens using other alkali halides and perhaps other materials.
2002. "Control of Laser Desorption Using Tunable Single Pulses and Pulse Pairs." Journal of Chemical Physics 116(18):8144-8151. Abstract Abstract: We desorb ground state Br and spin-orbit excited Br* atoms from KBr single crystals using single pulses and sequential pulse pairs of tunable nanosecond laser radiation. Irradiation of cleaved KBr crystals, near the bulk absorption threshold produces hyper-thermal Br emission without a significant thermal component, and with little spin orbit excited Br* emission. The Br kinetic energy distribution may be controlled either by choice of photon energy or by excitation of transient defect centers created within the crystal. In this latter scheme, a first laser pulse generates transient centers within the bulk crystal and in the vicinity of the surface, and a second delayed laser pulse then excites the transient centers leading to atomic desorption. The Br* to Br yield ratio is significantly enhanced using two-pulse excitation as compared to resonant single-pulse desorption. Single and multiple pulse excitation of KBr produces Br and Br* in controllable quantities, velocities, and spin state distributions.
2002. "Temperature and Pressure Dependences of the Mn2+ and Donor-Acceptor Emissions in ZnS:Mn2+ Nanoparticles." Journal of Applied Physics 92(4):1950-1955. Abstract 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 (4T1 ? 6A1 ) 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 using 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. The relatively weak pressure dependence of the DA emission is attributed to the increase of the binding energies and the localisation of the defect wavefunctions 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.
2002. "Structure, Luminescence, and Dynamics of Eu2O3 Nanoparticles in MCM-41." Journal of Physical Chemistry B 106(28):7034-7041. Abstract The structure, luminescence spectroscopy, and lifetime decay dynamics of Eu2O3 nanoparticles formed in MCM-41 have been investigated. Both X-ray diffraction and high resolution transmission electron microscope observations indicate that Eu2O3 nanoparticles of monoclinic structure are formed inside channels of MCM-41 by heating at 140o C. However, heat treatment at 600 and 700o C causes migration of Eu2O3 from the MCM-41 channels forming nanoparticles of cubic structure outside of the MCM-41 channels. The feature of the hypersensitive 5D0 ? 7F2 emission profile of Eu3+ is used to follow the structural changes. Photoluminescence lifetimes show the existence of short (< 1 microsecond) and long (microsecond to millisecond) components for each sample. The fast decay is attributed to quenching by surface states of the nanoparticles or energy transfer to the MCM-41 while the longer time decays show the effects of concentration quenching. The monoclinic sample prepared at 140o C shows a higher luminescence intensity than the cubic samples or the bulk powder.
2002. ""Photostimulated Luminescence and Dynamics of Agl and Ag Nanoclusters in Zeolites"." Physical Review. B, Condensed Matter 65:245404-1-8. Abstract The photoluminescence and photostimulated luminescence of Ag and AgI nanoclusters formed in zeolite-Y are studied using fluorescence spectroscopy. The photoluminescence spectra of AgI nanoclusters show emission from both AgI and Ag nanoclusters, while the in the photostimulated luminescence, only the emission of Ag clusters is observed. While the photoluminescence from both Ag and AgI particles displays both sub-nanosecond and microsecond lifetimes, the emission from photostimulated luminescence shows very short, picosecond lifetimes. A model which ascribes the photostimulated luminescence to recombination of electrons trapped in the zeolite with Ag in close proximity to the trap site is proposed. The appearance of strong photostimulated luminescence with short decays in these systems demonstrates that nanoparticles have potential for digital storage and medical radiology applications.
2001. "Ground-State Proton Transfer Tautomer of Al(III)-Salicylate Complexes in Ethanol Solution." Journal of Physical Chemistry A 105(5):942-950. Abstract The tautomerization of salicylate anion in the presence of A1(III) in ethanol was studied by UV ? visible absorption spectroscopy and fluorescence spectroscopy, anisotropy, and lifetime measurements from 100 to 298 K. Complexation with A1(III) causes an equilibrium shift from the normal form of the salicylate anion toward the tautomer form, demonstrating that the presence of a highly charged cation, A1(III), stabilizes the tautomer form of salicylate. Spectra and fluorescence lifetimes of salicylate and other salicyl derivatives in the presence of A1(III) reveal three types of A1(III)-salicylate complexes. In type I complexes, salicylate binds to A1(III) through the carboxylate group, preserving the intramolecular hydrogen bond between the carbonyl oxygen and the phenol group, as indicated by the largely Stokes-shifted fluorescence emission following the excited state pr oton transfer process. In type II complexes, salicylate binds to A1(III) through the carboxylate group, but the phenol proton is oriented away from the carbonyl oxygen so that the complex shows short wavelength fluorescence emission characteristic of substituted phenolic compounds. In type III compleses, A1(III) stabilizes and binds to the tautomer form of salicylate through the phenolate oxyten, in which salicylate exists in its proton transferred tautomer form. Absorption spectra recorded at temperatures between 100 K and 298 K indicate that the type III tautomer complex is energetically favored at low temperature, although type I is the dominant species at room temperature. All three types of complexes are interconvertible above the ethanol glass transition temperature. However, below the glass transition temperature interconversion ceases, indicating large amplitude atomic motion is involved in the conversion.
2001. "Anthracene as the Origin of the Red-Shifted Emission from Commercial Zone-Refined Phenanthrene Sorbed on Mineral Surfaces." Journal of Physical Chemistry A 105(25):6020-6023. Abstract The Origin of the Red-Shifted Fluorescence Emission from Commercial Zone-refined Phenanthrene and Its Application as a Spectral Probe for Phenanthrene Association on Mineral Surface
2001. "A Fluorescence Spectroscopic Study of Phenanthrene Sorbed on Porous Silica." Environmental Science and Technology 35(13):2710-2716. Abstract A Fluorescence Spectroscopic Study of Phenanthrene Sorbed on Porous Silica
2001. "Temperature Dependence of Up-Conversion Luminescence and Photoluminescence of Mn2+ in ZnS:Mn2+ Nanoparticles." Journal of Nanoscience and Nanotechnology 1(3):295-301. Abstract The photoluminescence (excited at both 300 nm and 383.5 nm) and up-conversion luminescence (excited at 767 nm) of the Mn2+4T1??6A1 transition in both bulk and ZnS:Mn2+ nanoparticles have been measured as a function of temperature. The Mn2+ emission spectra shift monotonically to longer wavelengths at lower temperatures, while the intensity change of the luminescence is more complex. The complicated temperature behavior is explained by considering the processes of non-radioation relaxation via phonon coupling, exciton thermal dissaciation (binding energy), energy transfer, carrier trapping, and the temperature change of the absorption spectra. The fact that the temperature dependence of the 767 nm excited up-conversion luminescence is the same as the 383.5 nm excited photoluminescence in both bulk and nanoparticles supports the conclusion that the up-conversion luminescence is due to two-photon absorption.
2001. "Selective laser desorption of ionic surfaces: resonant surface excitation of KBr." Journal of Chemical Physics 115(20):9463 - 9472. Abstract We demonstrate evidence of selective laser-induced desorption of ground state Br(2P3/2) and spin-orbit excited state Br*(P1/2) atoms from single crystals at 6.4 eV. Laser excitation tuned selectively to a surface resonance below the first bulk absorption band excites surfaces states preferentially leading to surface specific reaction 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 hyper-thermal Br(2P3/2) emissions without a significant thermal or Br*(2P1/2) component. The hyper-thermal emission is shown theoretically to be characteristic of surface induced reaction of exciton decomposition while thermal Br emission is attributed to bulk photoreaction.
2001. "Up-Conversion Luminescence of Mn2+ in ZnS:Mn2+ Nanoparticles." Physical Review. B, Condensed Matter 64:041202-1-4(R). Abstract Strong anti-Stokes luminescence of Mn2+ is observed in ZnS:Mn2+ bulk and nanoparticles. The anti-Stokes emission band excited at 767 nm is red-shifted from the Stokes emission excited at 300 nm with the bulk and the particles encapsulated in zeolite-USY as exceptions. In the nanoparticles, the decay lifetimes of the anti-Stokes emission excited at 767 nm are shorter than the 300 nm excited Stokes lifetimes, while in the bulk, the two decays are almost identical. When the Stokes emission is obtained by excitation at 383.5 nm, which is the sum energy of two-photons at 767 nm, the emission spectra and the lifetimes of the two types of luminescence are almost identical. The power dependence of the Stokes emission is linear, while that of the anti-Stokes emission is quadratic. Based on those observations along with the temperature dependence of the anti-Stokes luminescence intensity, it is concluded that two-photon excitation is responsible for the anti-Stokes luminescence of Mn2+ in ZnS:Mn2+ nanoparticles.
2001. "Evidence for a Surface Exciton in KBr via Laser Desorption ." Physical Review. B, Condensed Matter 63(12):125423(6). Abstract We demonstrate that direct photoexcitation of the KBr surface exciton leads to desorption of hyperthermal neutral bromine atoms. We have for the first time produced separately the hyperthermal and the near-thermal components of neutral halogen emission from an alkali halide. The source of hyperthermal bromine emission is attributed to decay of a surface exciton excited at photon energies below that of the bulk exciton. We further demonstrate that the frequently observed near-thermal component is derived from excitation within the bulk crystal. Our experimental data provides strong support to a theoretical emission model previously described in the literature.
2000. "Kinetics and Mechanism of Surface Reaction of Salicylate on Alumina in Colloidal Aqueous Suspension ." Geochimica et Cosmochimica Acta 64(7):1159-1172. Abstract N/A
2000. "Luminescence Decay Kinetics of Mn2+ Doped ZnS Nanoclusters Grown in Reverse Micelles." Physical Review. B, Condensed Matter 62(3):2021-2028. Abstract We report the synthesis and luminescence decay kinetics of 1.2 nm Mn2+ doped ZnS nanoclusters grown in reverse micelles. The preparation method produces small particles with narrow size distribution and fluorescence bands near 400-450 nm and 585 nm. Time-dependent fluorscence decay measurements using picosecond, nanosecond and millisecond techniques reveal relaxation processes on all three time scales. In the doped sample the red emission detected at 600 nm exhibits an 1-2 ms decay in addition to faster decay with time constants on the order of hundreds of ps, a few ns and tens of us. While the slow decay is the same as that of bulk Mn2+ doped ZnS, the fast decays are present for both doped and undoped samples and are unique to nanocluster ZnS, which are attributed deep trap state emission. The blue luminescence near 400-450 nm is attributed to shallow trap state emission of ZnS. These results help to resolve the controversy in the literature regarding the lifetime of the Mn2+ emission in Mn-doped ZnS nanoparticles.
2000. "The Discovery and Study of Nanocrystalline TiO2-(MoO3) Core-Shell Materials." Journal of the American Chemical Society 122(21):5138-5146. Abstract Here we report the synthesis of a series of new nanocrystalline TiO2-(MoO3) core-shell materials whose photoabsorption energy (PE, the energy required to excite TiO2-core valence band electrons to MoO3-shell conduction band states) properties are correlated with both the nanoparticle size and the degree of chemical interaction between the TiO2 core and the MoO3 shell. The TiO2-(MoO3) nanoparticle size can be readily adjusted from 80 to 40 A, and in this series, the PE decreases from 2.88 to 2.60 eV with decreasing particle size. The systematic PE red-shift exhibited by the core-shell materials is ascribed to the change in the relative position of the MoO3-shell conduction band as it evolves from less than a monolayer to a two monolayer shell.
2000. "Time-Resolved Femtosecond Laser-Induced Desorption from Magnesium Oxide and Lithium Fluoride Single Crystals." Surface Science 451(1-3):166-173. Abstract We have used the pump-probe technique to measure the photostimulated positive ion yield, as a function of time-delay between two sub-threshold femtosecond laser pulses. We find the ion yield, from 265 nm femtosecond irradiated MgO and LiF, depends critically on the laser pulse delay two-pulse experiments. For example, single pulse excitation of MgO produces a variety of ions including Mg+, MgO+, and H+. If the femtosecond laser pulse is split into two sub-threshold beams and then re-combined with a variable time delay, the Mg+ desorption yield displays three distinct lifetimes and persists for laser delays of over 100 picoseconds. A pulse delay of only 500 femtoseconds nearly eliminates ion desorption except for Mg+. In contrast, for LiF the majority of Li+ yields decays rapidly, largely within the femtosecond pulse duration. However, a weak but measurable decay component of approximately 2 picoseconds is indicated. We hypothesize the nonresonant two-photon excitation contributes to the ultrfast desorption mechanism through the creation of electron/hole pairs and allows direct observation of the surface electron/hole pair trapping dynamics and measurment of the surface free carrier lifetime.
2000. "Time-resolved Femtosecond Laser Induced Desorption from Magnesium Oxide and Lithium Fluoride Single Crystals ." Surface Science 451:166-173.
1999. "Time-Resolved Fluorescence Anisotropies in Mixed Surfactant Solutions." Journal of Colloid and Interface Science 218(1):260-264. Abstract Time-resolved fluorescence anisotropy decays of solutions of Triton X-114(TX-114) with various amounts of sodium dodecyl sulfate(SDS) were measured using emission both from the surfactant itself and from added perylene. In the former case, the monomer and aggregate species of the surfactant were spectroscopically isolated and were shown to have significantly different rotational correlation times. The rotational diffusion of perylene in micellar TX-114 with small amounts of added SDS appeared to have a component with a very short correlation time. The anisotropy decay curves showed the existence of limiting anisotropies, indicating hindered probe rotation in the micellar environment. At higher SDS concentrations, the fast-decaying component slowed down and the limiting anisotropy decreased substantially, suggesting some migration of the probe to the interior of the micelle.
1999. "Ultrafast and Nanosecond Laser Induced Desorption of Positive Ions from Lithium Fluoride Single Crystals." Applied Physics A, Materials Science and Processing 69:S153-S157. Abstract We compare desorption of positive ions from litium floride (LiF) single crystals following pulsed laser excitation using eith femtosecond (~300fs, 265 nm) or nanosecond (3ns, 266 nm) sources.
1999. "Femtosecond Time-Resolved Laser Induced Desorption of Positive Ions from MgO." Applied Physics A, Materials Science and Processing 69:S389-S393. Abstract We have used the pump-probe technique to measure the photostimulated positive ion yield, as a function of time-delay between two sub-threshold femtosecond laser pulses. We find the ion yield, from UV femtosecond irratdiated MgO, depends critically on the laser pulse delay, delta-t, in two-pulse experiments. n single pulse experiments, excitation of MgO produces a variety of ions including Mg+, MgO+, and a significant yield of H+. In contrast, if the femtosecond laser pulse is split into two sub-threshold beams and then re-combined with a variable time delay, the ion yield may be drastically may be drastically altered depending on the delay between pulses. The Mg+ desorption yield displays three distinct liftimes and persists for laser delays of over 100 picosecond.
1999. "Ground-state proton-transfer tautomer of the salicylate anion." Journal of Physical Chemistry A 103(48):9644-9653. Abstract Gound-state proton-transfer tautomer of the salicylate anion