Co-reporter:Andrew R. Attar;Kirsten Schnorr;Kristina D. Closser;Aditi Bhattacherjee;C. D. Pemmaraju;David Prendergast
Science 2017 Volume 356(Issue 6333) pp:54-59
Publication Date(Web):07 Apr 2017
DOI:10.1126/science.aaj2198
X-ray vision catches Woodward-Hoffmann
The celebrated Woodward-Hoffmann (W-H) rules rationalize a variety of rapid bond rearrangements in organic molecules. The key insight involved symmetry conservation in the electronic journey from reactant to product. Attar et al. now report femtosecond x-ray absorption spectra and accompanying simulation studies that track shifts in carbon electronic states during one such reaction: the photochemical ring opening of cyclohexadiene to hexatriene (see the Perspective by Sension). The smooth evolution that occurs in the vicinity of the pericyclic minimum provides direct affirmation of the W-H framework. Moreover, the use of a convenient tabletop apparatus bodes well for future x-ray studies of ultrafast electronic dynamics.
Science, this issue p. 54; see also p. 31
Co-reporter:Jie Ma;Letian Dou;Natalie A. Gibson;Minliang Lai;Lin-Wang Wang;Andrew B. Wong;Peidong Yang;Samuel W. Eaton
PNAS 2016 Volume 113 (Issue 8 ) pp:1993-1998
Publication Date(Web):2016-02-23
DOI:10.1073/pnas.1600789113
The rapidly growing field of nanoscale lasers can be advanced through the discovery of new, tunable light sources. The emission
wavelength tunability demonstrated in perovskite materials is an attractive property for nanoscale lasers. Whereas organic–inorganic
lead halide perovskite materials are known for their instability, cesium lead halides offer a robust alternative without sacrificing
emission tunability or ease of synthesis. Here, we report the low-temperature, solution-phase growth of cesium lead halide
nanowires exhibiting low-threshold lasing and high stability. The as-grown nanowires are single crystalline with well-formed
facets, and act as high-quality laser cavities. The nanowires display excellent stability while stored and handled under ambient
conditions over the course of weeks. Upon optical excitation, Fabry–Pérot lasing occurs in CsPbBr3 nanowires with an onset of 5 μJ cm−2 with the nanowire cavity displaying a maximum quality factor of 1,009 ± 5. Lasing under constant, pulsed excitation can be
maintained for over 1 h, the equivalent of 109 excitation cycles, and lasing persists upon exposure to ambient atmosphere. Wavelength tunability in the green and blue regions
of the spectrum in conjunction with excellent stability makes these nanowire lasers attractive for device fabrication.
Co-reporter:Erika R. Warrick, Wei Cao, Daniel M. Neumark, and Stephen R. Leone
The Journal of Physical Chemistry A 2016 Volume 120(Issue 19) pp:3165-3174
Publication Date(Web):February 10, 2016
DOI:10.1021/acs.jpca.5b11570
An attosecond pulse is used to create a wavepacket in molecular nitrogen composed of multiple bound and autoionizing electronic states of Rydberg and valence character between 12 and 16.7 eV. A time-delayed, few-femtosecond, near-infrared (NIR) laser pulse is used to couple individual states in the wavepacket to multiple neighboring states, resulting in time-dependent modification of the absorption spectrum and revealing both individual quantum beats of the wavepacket and the energy shifts of the excited states in the presence of the strong NIR field. The broad bandwidth of the attosecond pulse and high energy resolution of the extreme ultraviolet spectrometer allow the simultaneous observation of time-dependent dynamics for many individual vibrational levels in each electronic state. Quantum beating with periods from 1.3 to 12 fs and transient line shape changes are observed among vibrational levels of a progression of electronically autoionizing Rydberg states leading to the excited A 2Πu N2+ ion core. Vibrational levels in the valence b 1Πu state exhibit 50 fs oscillation periods, revealing superpositions between individual vibrational levels within this state. Comparisons are made to previous studies of electronic wavepackets in atoms that highlight similarities to atomic behavior yet illustrate unique contributions of the diatomic molecular structure to the wavepacket, including the influence of different electronic potentials and vibrational-level-specific electronic dynamics.
Co-reporter:Mihai E. Vaida
The Journal of Physical Chemistry C 2016 Volume 120(Issue 5) pp:2769-2776
Publication Date(Web):January 20, 2016
DOI:10.1021/acs.jpcc.5b11161
Wide band gap heterostructures can be accurately fabricated at room temperature to exhibit remarkable electrical properties that facilitate the transfer of electrons across the heterojunction while blocking the transfer of holes. The present investigation focuses on engineering the electronic structure of a TiO2 overlayer on a p-type doped Si(100) substrate by controlling the concentration of TiO2 oxygen vacancies. TiO2 films are deposited on p-Si(100) in an ultrahigh vacuum setup by evaporation of Ti atoms at a constant rate in a variable O2 atmosphere. The concentration of oxygen vacancies and consequently the degree of n-type doping of TiO2 is tuned by controlling the oxygen background pressure during the TiO2 formation. To investigate the electronic structure and the concentration of defects in the TiO2 layer as well as to characterize the TiO2/p-Si(100) band alignment we used photoelectron spectroscopy employing femtosecond extreme ultraviolet laser pulses produced via high harmonic generation. Furthermore, using a pump–probe technique in conjunction with photoemission spectroscopy, an ultrafast electron injection from the p-Si(100) substrate into the defect-rich TiO2 layer is observed with a time constant of 450 fs, as well as the subsequent charge carrier recombination. The latter is revealed to be affected by the oxygen defects when investigated with femtosecond resolution. No charge transfer is observed when defect-poor TiO2 films are prepared on the p-Si(100) substrate. This might be attributed to a change in the energy band alignment at the TiO2/Si(100) interface that reduces the built-in potential across the heterojunction and consequently reduces the driving force responsible for the injection of electrons into the TiO2 layer.
Co-reporter:Youn Jue Bae, Natalie A. Gibson, Tina X. Ding, A. Paul AlivisatosStephen R. Leone
The Journal of Physical Chemistry C 2016 Volume 120(Issue 51) pp:29484-29490
Publication Date(Web):December 7, 2016
DOI:10.1021/acs.jpcc.6b09780
The fluorescence intermittency of single CdSe/CdS quantum dots (QDs) with different shell sizes is studied using the conventional bin and threshold method and the statistically more rigorous method, change point analysis (CPA). The on-state truncation time (τc) is a critical value used to interpret the dynamics of charge trapping in single QDs; however, changing the bin size and threshold in blink traces significantly modifies τc. Herein, we use the CPA method to minimize the bias that binning and thresholding introduces and find that a widely used assumption that there is only one on and one off state is questionable. We observe that 12 out of 17 QDs exhibit more than two intensity levels and find that the τc values of individual levels differ from the values obtained when the levels are combined, i.e., when one assumes there is only one on and one off state as in the conventional bin and threshold method. For instance, one QD has τc values of 0.5 (0.1) and 2.0 (0.2) s from two different intensity levels, whereas when the levels are combined into only one on state, τc is found to be 7 (1) s. The CPA method is found to be more suitable for studying multilevel emission in QDs than the conventional bin and threshold method.
Co-reporter:Jessica F. Lockyear, Martin Fournier, Ian R. Sims, Jean-Claude Guillemin, Craig A. Taatjes, David L. Osborn, Stephen R. Leone
International Journal of Mass Spectrometry 2015 Volume 378() pp:232-245
Publication Date(Web):15 February 2015
DOI:10.1016/j.ijms.2014.08.025
•The reaction of C2H with 1,3-butadiene was probed in a flow tube.•Product analysis was by tunable synchrotron VUV ionization.•The product photoionization spectrum reveals fulvene is the major product.•The maximum amount of benzene plus 1,3-hexadien-5-yne is 45%.Products formed in the reaction of C2H radicals with 1,3-butadiene at 4 Torr and 298 K are probed using photoionization time-of-flight mass spectrometry. The reaction takes place in a slow-flow reactor, and products are ionized by tunable vacuum-ultraviolet light from the Advanced Light Source. The principal reaction channel involves addition of the radical to one of the unsaturated sites of 1,3-butadiene, followed by H-loss to give isomers of C6H6. The photoionization spectrum of the C6H6 product indicates that fulvene is formed with a branching fraction of (57 ± 30)%. At least one more isomer is formed, which is likely to be one or more of 3,4-dimethylenecyclobut-1-ene, 3-methylene-1-penten-4-yne or 3-methyl-1,2-pentadien-4-yne. An experimental photoionization spectrum of 3,4-dimethylenecyclobut-1-ene and simulated photoionization spectra of 3-methylene-1-penten-4-yne and 3-methyl-1,2-pentadien-4-yne are used to fit the measured data and obtain maximum branching fractions of 74%, 24% and 31%, respectively, for these isomers. An upper limit of 45% is placed on the branching fraction for the sum of benzene and 1,3-hexadien-5-yne. The reactive potential energy surface is also investigated computationally. Minima and first-order saddle-points on several possible reaction pathways to fulvene + H and 3,4-dimethylenecyclobut-1-ene + H products are calculated.
Co-reporter:Andrew R. Attar; Aditi Bhattacherjee
The Journal of Physical Chemistry Letters 2015 Volume 6(Issue 24) pp:5072-5077
Publication Date(Web):December 4, 2015
DOI:10.1021/acs.jpclett.5b02489
Femtosecond extreme ultraviolet (XUV) pulses produced by high harmonic generation are used to probe the transition-state region in the 266 nm photodissociation of CH3I by the real-time evolution of core-to-valence transitions near the iodine N-edge at 45–60 eV. During C–I bond breaking, new core-to-valence electronic states appear in the spectra, which decay concomitantly with the rise of the atomic iodine resonances of I(2P3/2) and I*(2P1/2). The short-lived features are assigned to repulsive valence-excited transition-state regions of 3Q0 and 1Q1, which can connect to transient core-excited states via promotion of 4d(I) core electrons. A simplified one-electron transition picture is described that accurately predicts the relative energies of the transient states observed. The transition-state resonances reach a maximum at ∼40 fs and decay to complete C–I dissociation in ∼90 fs, representing the shortest-lived chemical transition state observed by core-level, XUV, or X-ray spectroscopy.
Co-reporter:L. Robert Baker, Chang-Ming Jiang, Stephen T. Kelly, J. Matthew Lucas, Josh Vura-Weis, Mary K. Gilles, A. Paul Alivisatos, and Stephen R. Leone
Nano Letters 2014 Volume 14(Issue 10) pp:5883-5890
Publication Date(Web):September 15, 2014
DOI:10.1021/nl502817a
Charge carrier dynamics in Co3O4 thin films are observed using high harmonic generation transient absorption spectroscopy at the Co M2,3 edge. Results reveal that photoexcited Co3O4 decays to the ground state in 600 ± 40 ps in liquid methanol compared to 1.9 ± 0.3 ns in vacuum. Kinetic analysis suggests that surface-mediated relaxation of photoexcited Co3O4 may be the result of hole transfer from Co3O4 followed by carrier recombination at the Co3O4–methanol interface.
Co-reporter:Mihai E. Vaida, Stephen R. Leone
Chemical Physics 2014 Volume 442() pp:41-47
Publication Date(Web):17 October 2014
DOI:10.1016/j.chemphys.2014.01.015
Highlights
- •
The ultrafast photodissociation of CH3Br in the red wing of the A-band is studied.
- •
Femtosecond XUV pulses are used to track the CH3Br photodissociation.
- •
The 3Q0 state is the most likely dissociation pathway at 266 nm.
- •
The measured dissociation time is in a good agreement with the literature estimation.
Co-reporter:Chang-Ming Jiang ; L. Robert Baker ; J. Matthew Lucas ; Josh Vura-Weis ; A. Paul Alivisatos
The Journal of Physical Chemistry C 2014 Volume 118(Issue 39) pp:22774-22784
Publication Date(Web):August 27, 2014
DOI:10.1021/jp5071133
The identities of photoexcited states in thin-film Co3O4 and the ultrafast carrier relaxation dynamics of Co3O4 are investigated with oxidation-state-specific pump–probe femtosecond core level spectroscopy. A thin-film sample is excited near the 2.8 eV optical absorption peak, and the resulting spectral changes at the 58.9 eV M2,3-edge of cobalt are probed in transient absorption with femtosecond high-order harmonic pulses generated by a Ti/sapphire laser. The initial transient state shows a significant 2 eV redshift in the absorption edge compared to the static ground state, which indicates a reduction of the cobalt valence charge. This is confirmed by a charge transfer multiplet spectral simulation, which finds the experimentally observed extreme ultraviolet (XUV) spectrum matches the specific O2–(2p) → Co3+(eg) charge-transfer transition, out of six possible excitation pathways involving Co3+ and Co2+ in the mixed-valence material. The initial transient state has a power-dependent amplitude decay (190 ± 10 fs at 13.2 mJ/cm2) together with a slight redshift in spectral shape (535 ± 33 fs), which are ascribed to hot carrier relaxation to the band edge. The faster amplitude decay is possibly due to a decrease of charge carrier density via an Auger mechanism, as the decay rate increases when more excitation fluence is used. This study takes advantage of the oxidation-state-specificity of time-resolved XUV spectroscopy, further establishing the method as a new approach to measure ultrafast charge carrier dynamics in condensed-phase systems.
Co-reporter:Denisia M. Popolan-Vaida, Kevin R. Wilson, and Stephen R. Leone
The Journal of Physical Chemistry A 2014 Volume 118(Issue 45) pp:10688-10698
Publication Date(Web):October 20, 2014
DOI:10.1021/jp5085247
The gas-phase reaction of iodine atoms with hydrocarbon molecules is energetically unfavorable, and there is no direct evidence for iodinated product formation by either H abstraction or I addition reactions at ambient temperature. Here we consider the possible heterogeneous reaction of I atoms with submicrometer droplets composed of a saturated alkane, squalane (Sq), and an unsaturated alkene, squalene (Sqe). The investigations are performed in an atmospheric pressure photochemical flow tube reactor in conjunction with a vacuum ultraviolet photoionization aerosol mass spectrometer and a scanning mobility particle sizer. Squalane, a branched alkane, is unreactive toward I atoms within the signal-to-noise, and an upper limit of the effective reactive uptake coefficient is estimated to be γISq ≤ 8.58 × 10–7. In contrast, the reaction of I atoms with unsaturated submicrometer squalene droplets results in observable iodinated squalene products. The effective reactive uptake coefficient of I atom with squalene particles is determined to be γISqe = (1.20 ± 0.52) × 10–4 at an average I concentration of 1.5 × 1014 molecules·cm–3.
Co-reporter:Scott G. Sayres, Erik R. Hosler, and Stephen R. Leone
The Journal of Physical Chemistry A 2014 Volume 118(Issue 37) pp:8614-8624
Publication Date(Web):June 9, 2014
DOI:10.1021/jp503468u
Orbital alignment measurements and theory are used to examine the role of electron correlation during atomic strong-field double ionization (795 nm, (1–5) × 1014 W cm–2). High-order harmonic, transient absorption spectroscopy is used to measure the angular distributions of singly and doubly tunnel-ionized xenon atomic states via 4d core to 5p valence shell transitions between 55 and 60 eV. The experimental MJ alignment distributions are compared to results of a rate-equation model based on sequential ionization, previously developed for coherent electron motion, and now applied to account for the alignment prepared by tunneling ionization. The hole generated in the 2P3/2 state of Xe+ is measured to be entirely composed of |MJ| = 1/2, in agreement with theory. The result is a higher degree of alignment than previously reported. Because the model neglects effects of electron–ion recollision, the theory predicts a high degree of alignment in both spin-parallel (triplet) and antiparallel (singlet) terms of Xe2+. However, the alignment generated with linearly polarized light is observed to be spin-state dependent. The measured alignments for triplet spin states (3P2 has |MJ| = [0 : 1 : 2] of [27±6 : 45±11 : 29±0] and 3P1 has |MJ| = [0 : 1] of [56±2 : 44±2]) are in good agreement with the expectations of theory, which are [33 : 53 : 14] and [66 : 33], respectively. The results validate the rate equation model for sequential tunnel ionization. However, the alignment extracted for a singlet state is greatly diminished: 1D2 is measured to be [18±1 : 39±2 : 43 ± 2] compared to theoretical expectation of [60 : 39 : 1] for |MJ| = [0 : 1 : 2]. The poor agreement with the sequential ionization model suggests that the alignment of 1D2 is strongly influenced by the high propensity for the liberated first electron to return to and recollide with its parent atomic orbital. Therefore, although the influence of electron recollision appears minor in the triplet states and suggests sequential ionization, electron correlation between the ionic core and the first ionized electron cannot be ignored in the singlet state. Singlet states are likely to be generated through nonsequential double ionization over the intensity range where the experiments are performed.
Co-reporter:Martin Schultze;Krupa Ramasesha;C.D. Pemmaraju;S.A. Sato;D. Whitmore;A. Gandman;James S. Prell;L. J. Borja;D. Prendergast;K. Yabana;Daniel M. Neumark
Science 2014 Volume 346(Issue 6215) pp:1348-1352
Publication Date(Web):12 Dec 2014
DOI:10.1126/science.1260311
Abstract
Electron transfer from valence to conduction band states in semiconductors is the basis of modern electronics. Here, attosecond extreme ultraviolet (XUV) spectroscopy is used to resolve this process in silicon in real time. Electrons injected into the conduction band by few-cycle laser pulses alter the silicon XUV absorption spectrum in sharp steps synchronized with the laser electric field oscillations. The observed ~450-attosecond step rise time provides an upper limit for the carrier-induced band-gap reduction and the electron-electron scattering time in the conduction band. This electronic response is separated from the subsequent band-gap modifications due to lattice motion, which occurs on a time scale of 60 ± 10 femtoseconds, characteristic of the fastest optical phonon. Quantum dynamical simulations interpret the carrier injection step as light-field–induced electron tunneling.
Co-reporter:Amy A. Cordones and Stephen R. Leone
Chemical Society Reviews 2013 vol. 42(Issue 8) pp:3209-3221
Publication Date(Web):11 Jan 2013
DOI:10.1039/C2CS35452G
Semiconductor nanocrystal optical and charge transport properties are largely influenced by the trapping of charge carriers on the nanocrystal surface. Charge trapping increases the non-radiative exciton decay pathways, thus decreasing the fluorescence quantum yield, and it also impedes efficient charge transfer at the nanocrystal interface. On a single nanocrystal basis, charge trapping causes interruptions in the otherwise continuous fluorescence known as fluorescence intermittency or blinking. In this tutorial review we examine the relationship between charge trapping and fluorescence blinking. The fluorescence microscopy techniques and statistical analysis methods used to measure single nanocrystal blinking are introduced. The development of numerous blinking mechanisms is reviewed, as is the physical nature of charge trapping sites. An overview of blinking experiments used to probe specific mechanisms for charge carrier trapping is presented. Finally, a summary and outlook are offered. Although the detailed mechanism is not fully understood, blinking experiments are found to provide direct evidence for several charge trapping mechanisms and report on changes to the nature and distribution of charge trapping sites.
Co-reporter:Phillip M. Nagel, Joseph S. Robinson, Bruce D. Harteneck, Thomas Pfeifer, Mark J. Abel, James S. Prell, Daniel M. Neumark, Robert A. Kaindl, Stephen R. Leone
Chemical Physics 2013 Volume 414() pp:106-111
Publication Date(Web):12 March 2013
DOI:10.1016/j.chemphys.2012.03.013
Abstract
Electron photoemission from lithographically prepared gold nanopillars using few-cycle, 800 nm laser pulses is measured. Electron kinetic energies are observed that are higher by up to tens of eV compared to photoemission from a flat gold surface at the same laser intensities. In addition, ionization from the nanopillar sample scales like a two-photon process, while three photons are needed to overcome the work function taking into account the shortest wavelength within the laser bandwidth. A classical electron acceleration model consisting of nonlinear ionization followed by field acceleration qualitatively reproduces the electron kinetic energy data and suggests average enhanced electric fields due to the nanopillars that are between 25 and 39 times greater than the experimentally used laser fields. Implications for plasmon-enhanced attosecond streaking are discussed.
Co-reporter:Zhi-Heng Loh and Stephen R. Leone
The Journal of Physical Chemistry Letters 2013 Volume 4(Issue 2) pp:292-302
Publication Date(Web):December 21, 2012
DOI:10.1021/jz301910n
Recent technical advances in ultrafast laser sources enable the generation of femtosecond and attosecond soft X-ray pulses in tabletop laser setups as well as accelerator-based synchrotron and free-electron laser sources. These new light sources can be harnessed via pump–probe spectroscopy to elucidate ultrafast quantum dynamics in atoms, molecules, and condensed matter with unprecedented time resolution and chemical sensitivity. Employing such ultrashort pulses in transient X-ray absorption spectroscopy combines the unique advantages of core-level absorption probing of chemical environments and oxidation states with the ability to obtain ultimately freeze-frame snapshots of electronic and nuclear dynamics. In this Perspective, we provide an overview of the progress in applying the recently developed technique of femtosecond to attosecond time-resolved soft X-ray transient absorption spectroscopy to the study of ultrafast phenomena, including some of our own efforts to elucidate the interaction of intense laser pulses with atoms and molecules in the strong-field, nonperturbative limit. Possible avenues for future work are outlined.
Co-reporter:Josh Vura-Weis, Chang-Ming Jiang, Chong Liu, Hanwei Gao, J. Matthew Lucas, Frank M. F. de Groot, Peidong Yang, A. Paul Alivisatos, and Stephen R. Leone
The Journal of Physical Chemistry Letters 2013 Volume 4(Issue 21) pp:3667-3671
Publication Date(Web):October 15, 2013
DOI:10.1021/jz401997d
Oxidation-state-specific dynamics at the Fe M2,3-edge are measured on the sub-100 fs time scale using tabletop high-harmonic extreme ultraviolet spectroscopy. Transient absorption spectroscopy of α-Fe2O3 thin films after 400 nm excitation reveals distinct changes in the shape and position of the 3p → valence absorption peak at ∼57 eV due to a ligand-to-metal charge transfer from O to Fe. Semiempirical ligand field multiplet calculations of the spectra of the initial Fe3+ and photoinduced Fe2+ state confirm this assignment and exclude the alternative d–d excitation. The Fe2+ state decays to a long-lived trap state in 240 fs. This work establishes the ability of time-resolved extreme ultraviolet spectroscopy to measure ultrafast charge-transfer processes in condensed-phase systems.Keywords: extreme ultraviolet”; hematite; high-harmonic generation; ligand-to-metal charge transfer; transient absorption; X-ray;
Co-reporter:Hsiang-An Chen ; Cheng-Lun Hsin ; Yu-Ting Huang ; Ming Lee Tang ; Scott Dhuey ; Stefano Cabrini ; Wen-Wei Wu ▽
The Journal of Physical Chemistry C 2013 Volume 117(Issue 43) pp:22211-22217
Publication Date(Web):March 25, 2013
DOI:10.1021/jp312363x
The variation in localized surface plasmon resonances of single Au nanodisks (diameter 100 nm and height 25 nm) on 0–13 graphene layers is investigated using dark-field scattering spectroscopy to obtain the graphene electric field screening length. For nanodisks (NDs) with and without underlying graphene layers on a SiO2 (300 nm)/Si substrate, the plasmon resonance red shifts from 604 to 620 nm with increasing graphene layers. The spectra of the plasmonic nanostructures obey an exponential saturation function versus increasing number of layers of graphene from 0 to 13. As a conducting film, the graphene layers screen the electric field generated by the plasmonic resonance of the Au NDs in the vicinity of the interface, and the red shifts of the resonance wavelength are explained in the framework of the electromagnetic field coupling between in-plane antiparallel image dipoles in the graphene layers and the ND dipole. A screening length of 1.2 ± 0.2 nm, equivalent to 3–4 graphene layers, is experimentally obtained, in good agreement with the measurement by field-effect transistors and theoretical calculation in doped graphene. The resonance shift of plasmonic nanostructures on a layered graphene system provides an alternative and convenient method for screening length measurement of graphene films.
Co-reporter:Amy A. Cordones, Kenneth L. Knappenberger Jr., and Stephen R. Leone
The Journal of Physical Chemistry B 2013 Volume 117(Issue 16) pp:4241-4248
Publication Date(Web):September 11, 2012
DOI:10.1021/jp3041549
Memory effects in single nanocrystal fluorescence blinking are investigated as a function of the on-state kinetics for CdSe/ZnS quantum dots and CdSe nanorods. The on-state duration probability distributions for single nanocrystal blinking traces are characterized by an inverse power law, which crosses over to exponential decay for long on-state durations. The correlations of subsequent on-state durations (Rlog,on) are found to decrease for nanocrystals that display earlier crossover times and smaller power law coefficients. Specifically, Rlog,on increases from 0.14 ± 0.02 to a saturation value of 0.44 ± 0.01 for nanocrystals with average crossover times of ∼100 ms to more than 5.0 s, respectively. The results represent the first link between memory effects and blinking kinetics and are interpreted in the framework of two competing charge trapping mechanisms. A slow fluctuation-based trapping mechanism leads to power-law-distributed on durations and significant memory effects; however, the additional contribution of an ionization-induced trapping pathway is found to induce crossover to exponential decay and decreased memory. Monte Carlo simulations of nanocrystal blinking based on the two trapping mechanisms reproduce the experimental results, suggesting that the power law component and the memory effects correlate with a fluctuation-based mechanism. This effect is found to be universal, occurring for two nanocrystal morphologies and in blinking data measured using a wide range of continuous and pulsed excitation conditions.
Co-reporter:Jessica F. Lockyear, Oliver Welz, John D. Savee, Fabien Goulay, Adam J. Trevitt, Craig A. Taatjes, David L. Osborn, and Stephen R. Leone
The Journal of Physical Chemistry A 2013 Volume 117(Issue 43) pp:11013-11026
Publication Date(Web):October 3, 2013
DOI:10.1021/jp407428v
The products formed in the reaction between the methylidene radical (CH) and acrolein (CH2═CHCHO) are probed at 4 Torr and 298 K employing tunable vacuum-ultraviolet synchrotron light and multiplexed photoionization mass-spectrometry. The data suggest a principal exit channel of H loss from the adduct to yield C4H4O, accounting for (78 ± 10)% of the products. Examination of the photoionization spectra measured upon reaction of both CH and CD with acrolein reveals that the isomeric composition of the C4H4O product is (60 ± 12)% 1,3-butadienal and (17 ± 10)% furan. The remaining 23% of the possible C4H4O products cannot be accurately distinguished without more reliable photoionization spectra of the possible product isomers but most likely involves oxygenated butyne species. In addition, C2H2O and C3H4 are detected, which account for (14 ± 10)% and (8 +10, −8)% of the products, respectively. The C2H2O photoionization spectrum matches that of ketene and the C3H4 signal is composed of (24 ± 14)% allene and (76 ± 22)% propyne, with an upper limit of 8% placed on the cyclopropene contribution. The reactive potential energy surface is also investigated computationally, and specific rate coefficients are calculated with RRKM theory. These calculations predict overall branching fractions for 1,3-butadienal and furan of 27% and 12%, respectively, in agreement with the experimental results. In contrast, the calculations predict a prominent CO + 2-methylvinyl product channel that is at most a minor channel according to the experimental results. Studies with the CD radical strongly suggest that the title reaction proceeds predominantly via cycloaddition of the radical onto the C═O bond of acrolein, with cycloaddition to the C═C bond being the second most probable reactive mechanism.
Co-reporter:Amy A. Cordones ; Marcus Scheele ; A. Paul Alivisatos
Journal of the American Chemical Society 2012 Volume 134(Issue 44) pp:18366-18373
Publication Date(Web):October 17, 2012
DOI:10.1021/ja3071732
Fluorescence intermittency and excited-state decay measurements are carried out on single CdSe–CdS core–shell quantum dots (QD) stabilized with chalcogenidometalates (ChaMs, In2Se42–, or Sn2S64–)−. The results are used to probe the nature and distribution of charge trapping sites in the QD local environment. A comparison is made between capping by a neutral organic ligand (oleylamine) and a small inorganic ligand with high charge density (ChaMs). Overall, shorter on-state durations and longer off-state durations are observed for the ChaMs. These results indicate an increased density of charge trapping sites and increased stabilization of surface-trapped charges. By varying the thickness of the CdS shell, we identified hole trapping by the ligand as the dominant charging mechanism in ChaM-capped QDs. Faster excited-state decay rates are measured for the ChaM-capped QDs, highlighting the role of strongly stabilized trapped charges in this system. Using cyclic voltammetry measurements of the ChaMs, an energy level diagram is constructed relating the ChaMs and CdSe–CdS-QDs that explains their superior performance as active layers in photodetectors.
Co-reporter:Teresa J. Bixby, Amy A. Cordones, Stephen R. Leone
Chemical Physics Letters 2012 Volume 521() pp:7-11
Publication Date(Web):10 January 2012
DOI:10.1016/j.cplett.2011.11.051
Abstract
Fluorescence-intermittency statistics of CdSe/ZnS core/shell quantum dots (QDs) are measured for the first time in TPD. Comparison to blinking behavior in PMMA reveals significant differences in the statistics such that longer on and off durations are observed for quantum dots in TPD. Further, in both matrices the on- and off-duration probability density distributions deviate from power law behavior at longer durations. The observed trends are consistent with increased accessibility to charge-stabilizing states in PMMA relative to TPD as well as the presence of an extrinsic hole density in TPD.
Co-reporter:Terefe G. Habteyes, Scott Dhuey, Erin Wood, Daniel Gargas, Stefano Cabrini, P. James Schuck, A. Paul Alivisatos, and Stephen R. Leone
ACS Nano 2012 Volume 6(Issue 6) pp:5702
Publication Date(Web):May 30, 2012
DOI:10.1021/nn301885u
Drastic chemical interface plasmon damping is induced by the ultrathin (∼2 nm) titanium (Ti) adhesion layer; alternatively, molecular adhesion is implemented for lithographic fabrication of plasmonic nanostructures without significant distortion of the plasmonic characteristics. As determined from the homogeneous linewidth of the resonance scattering spectrum of individual gold nanorods, an ultrathin Ti layer reduces the plasmon dephasing time significantly, and it reduces the plasmon scattering amplitude drastically. The increased damping rate and decreased plasmon amplitude are due to the dissipative dielectric function of Ti and the chemical interface plasmon damping where the conduction electrons are transferred across the metal–metal interface. In addition, a pronounced red shift due to the Ti adhesion layer, more than predicted using electromagnetic simulation, suggests the prevalence of interfacial reactions. By extending the experiment to conductively coupled ring-rod nanostructures, it is shown that a sharp Fano-like resonance feature is smeared out due to the Ti layer. Alternatively, vapor deposition of (3-mercaptopropyl)trimethoxysilane on gently cleaned and activated lithographic patterns functionalizes the glass surface sufficiently to link the gold nanostructures to the surface by sulfur–gold chemical bonds without observable plasmon damping effects.Keywords: adhesion layer; chemical interface; lithography; molecular adhesion; plasmon damping
Co-reporter:Fabien Goulay, Adam J. Trevitt, John D. Savee, Jordy Bouwman, David L. Osborn, Craig A. Taatjes, Kevin R. Wilson, and Stephen R. Leone
The Journal of Physical Chemistry A 2012 Volume 116(Issue 24) pp:6091-6106
Publication Date(Web):January 9, 2012
DOI:10.1021/jp2113126
The reaction of the methylidyne radical (CH) with acetaldehyde (CH3CHO) is studied at room temperature and at a pressure of 4 Torr (533.3 Pa) using a multiplexed photoionization mass spectrometer coupled to the tunable vacuum ultraviolet synchrotron radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory. The CH radicals are generated by 248 nm multiphoton photolysis of CHBr3 and react with acetaldehyde in an excess of helium and nitrogen gas flow. Five reaction exit channels are observed corresponding to elimination of methylene (CH2), elimination of a formyl radical (HCO), elimination of carbon monoxide (CO), elimination of a methyl radical (CH3), and elimination of a hydrogen atom. Analysis of the photoionization yields versus photon energy for the reaction of CH and CD radicals with acetaldehyde and CH radical with partially deuterated acetaldehyde (CD3CHO) provides fine details about the reaction mechanism. The CH2 elimination channel is found to preferentially form the acetyl radical by removal of the aldehydic hydrogen. The insertion of the CH radical into a C–H bond of the methyl group of acetaldehyde is likely to lead to a C3H5O reaction intermediate that can isomerize by β-hydrogen transfer of the aldehydic hydrogen atom and dissociate to form acrolein + H or ketene + CH3, which are observed directly. Cycloaddition of the radical onto the carbonyl group is likely to lead to the formation of the observed products, methylketene, methyleneoxirane, and acrolein.
Co-reporter:Amy A. Cordones, Teresa J. Bixby, and Stephen R. Leone
Nano Letters 2011 Volume 11(Issue 8) pp:3366-3369
Publication Date(Web):July 6, 2011
DOI:10.1021/nl2017674
Fluorescence decay times measured during the off-state of single CdSe/ZnS quantum dot blinking are found to decrease with increasing off-state duration, contradicting the charging model widely considered to explain the blinking phenomenon. The change in the nonradiative process of a short off-state duration compared to a long one is investigated here through simultaneous measurement of fluorescence decay and blinking behavior. The results are investigated in the framework of two models based on fluctuating trapping rates.
Co-reporter:Terefe G. Habteyes, Scott Dhuey, Stefano Cabrini, P. James Schuck, and Stephen R. Leone
Nano Letters 2011 Volume 11(Issue 4) pp:1819-1825
Publication Date(Web):March 22, 2011
DOI:10.1021/nl200585b
Quadrupole plasmon and (octupolar) Fano resonances are induced in lithographically fabricated theta-shaped ring−rod gold nanostructures. The optical response is characterized by measuring the light scattered by individual nanostructures. When the nanorod is brought within 3 nm of the ring wall, a weak quadrupolar resonance is observed due to capacitive coupling, and when a necklike conductive bridge links the nanorod to the nanoring the optical response changes dramatically bringing the quadrupolar resonance into prominence and creating an octupolar Fano resonance. The Fano resonance is observed due to the destructive interference of the octupolar resonance with the overlapping and broadened dipolar resonance. The quadrupolar and Fano resonances are further enhanced by capacitive coupling (near-field interaction) that is favored by the theta-shaped arrangement. The interpretation of the data is supported by FDTD simulation.
Co-reporter:Fabien Goulay, Satchin Soorkia, Giovanni Meloni, David L. Osborn, Craig A. Taatjes and Stephen R. Leone
Physical Chemistry Chemical Physics 2011 vol. 13(Issue 46) pp:20820-20827
Publication Date(Web):14 Oct 2011
DOI:10.1039/C1CP22609F
The reaction of ethynyl radical (C2H) with allene (C3H4) at room temperature is investigated using an improved synchrotron multiplexed photoionization mass spectrometer (MPIMS) coupled to tunable vacuum ultraviolet (VUV) synchrotron radiation from the Advanced Light Source at the Lawrence Berkeley National Laboratory (LBNL). The orthogonal-accelerated time-of-flight mass spectrometer (OA-TOF) compared to the magnetic sector mass spectrometer used in a previous investigation of the title reaction (Phys. Chem. Chem. Phys., 2007, 9, 4291) enables more sensitive and selective detection of low-yield isomeric products. The C5H4 isomer with the lowest ionization energy, pentatetraene, is now identified as a product of the reaction. Pentatetraene is predicted to be formed based on recent ab initio/RRKM calculations (Phys. Chem. Chem. Phys., 2010, 12, 2606) on the C5H5 potential energy surface. However, the computed branching fraction for pentatetraene is predicted to be five times higher than that for methyldiacetylene, whereas experimentally the branching fraction of pentatetraene is observed to be small compared to that of methyldiacetylene. Although H-atom assisted isomerization of the products can affect isomer distribution measurements, isomerization has a negligible effect in this case. The kinetic behavior of the several C5H4 isomers is identical, as obtained by time-dependent photoionization spectra. Even for high allene concentrations (and hence higher H-atom concentrations) no decay of the pentatetraene fraction is observed, indicating that H-assisted isomerization of pentatetraene to methyldiacetylene does not account for the difference between the experimental data and the theoretical branching ratios.
Co-reporter:Christine J. Koh, Chen-Lin Liu, Christopher W. Harmon, Daniel Strasser, Amir Golan, Oleg Kostko, Steven D. Chambreau, Ghanshyam L. Vaghjiani, and Stephen R. Leone
The Journal of Physical Chemistry A 2011 Volume 115(Issue 18) pp:4630-4635
Publication Date(Web):April 20, 2011
DOI:10.1021/jp200633b
Isolated ion pairs of a conventional ionic liquid, 1-Ethyl-3-Methyl-Imidazolium Bis(trifluoromethylsulfonyl)imide ([Emim+][Tf2N–]), and a reactive hypergolic ionic liquid, 1-Butyl-3-Methyl-Imidazolium Dicyanamide ([Bmim+][Dca–]), are generated by vaporizing ionic liquid submicrometer aerosol particles for the first time; the vaporized species are investigated by dissociative ionization with tunable vacuum ultraviolet (VUV) light, exhibiting clear intact cations, Emim+ and Bmim+, presumably originating from intact ion pairs. Mass spectra of ion pair vapor from an effusive source of the hypergolic ionic liquid show substantial reactive decomposition due to the internal energy of the molecules emanating from the source. Photoionization efficiency curves in the near threshold ionization region of isolated ion pairs of [Emim+][Tf2N–] ionic liquid vapor are compared for an aerosol source and an effusive source, revealing changes in the appearance energy due to the amount of internal energy in the ion pairs. The aerosol source has a shift to higher threshold energy (∼0.3 eV), attributed to reduced internal energy of the isolated ion pairs. The method of ionic liquid submicrometer aerosol particle vaporization, for reactive ionic liquids such as hypergolic species, is a convenient, thermally “cooler” source of isolated intact ion pairs in the gas phase compared to effusive sources.
Co-reporter:Amy A. Cordones ; Teresa J. Bixby
The Journal of Physical Chemistry C 2011 Volume 115(Issue 14) pp:6341-6349
Publication Date(Web):March 7, 2011
DOI:10.1021/jp2001223
Fluorescence intermittency of single CdSe/ZnS core/shell quantum dot particles is investigated over a wide range of excitation intensities and at two excitation wavelengths. Deviation from previously observed power law behavior in both the on- and off-duration probability distributions is observed at both excitation wavelengths, one near the band gap and one 350 meV above the band gap. Increasing the excitation intensity modifies the off-duration probability distribution such that the probability of long off-time events decreases, an effect that is observed to saturate at an average number of excitons created per pulse of one, ⟨N⟩ ≈ 1. The on-duration probability distribution is well-described by a power law for short on-time events, crossing over to an exponential distribution for long on-time events. Increasing the excitation intensity induces this crossover to occur at earlier times, saturating at ⟨N ⟩ ≈ 2. The different intensity-dependent trends for the on- and off-duration probability distributions are evidence that multiple mechanisms govern the blinking statistics, as no single mechanism can account for the different saturation behaviors of the off and on durations as a function of excitation intensity. These mechanisms are assumed to be one based on a light-induced diffusion of both the excited-state and trap state energies (i.e., diffusion-controlled electron transfer) and one that relies on the absorption of multiple photons (i.e., Auger ionization induced trapping).
Co-reporter:Stephen R. Leone, Musahid Ahmed and Kevin R. Wilson
Physical Chemistry Chemical Physics 2010 vol. 12(Issue 25) pp:6564-6578
Publication Date(Web):23 Apr 2010
DOI:10.1039/C001707H
Scientists at the Chemical Dynamics Beamline of the Advanced Light Source in Berkeley are continuously reinventing synchrotron investigations of physical chemistry and chemical physics with vacuum ultraviolet light. One of the unique aspects of a synchrotron for chemical physics research is the widely tunable vacuum ultraviolet light that permits threshold ionization of large molecules with minimal fragmentation. This provides novel opportunities to assess molecular energetics and reaction mechanisms, even beyond simple gas phase molecules. In this perspective, significant new directions utilizing the capabilities at the Chemical Dynamics Beamline are presented, along with an outlook for future synchrotron and free electron laser science in chemical dynamics. Among the established and emerging fields of investigations are cluster and biological molecule spectroscopy and structure, combustion flame chemistry mechanisms, radical kinetics and product isomer dynamics, aerosol heterogeneous chemistry, planetary and interstellar chemistry, and secondary neutral ion-beam desorption imaging of biological matter and materials chemistry.
Co-reporter:Satchin Soorkia, Craig A. Taatjes, David L. Osborn, Talitha M. Selby, Adam J. Trevitt, Kevin R. Wilson and Stephen R. Leone
Physical Chemistry Chemical Physics 2010 vol. 12(Issue 31) pp:8750-8758
Publication Date(Web):12 May 2010
DOI:10.1039/C002135K
The reaction of the ground state methylidyne radical CH (X2Π) with pyrrole (C4H5N) has been studied in a slow flow tube reactor using Multiplexed Photoionization Mass Spectrometry coupled to quasi-continuous tunable VUV synchrotron radiation at room temperature (295 K) and 363 K, at 4 Torr (533 Pa). Laser photolysis of bromoform (CHBr3) at 248 nm (KrF excimer laser) is used to produce CH radicals that are free to react with pyrrole molecules in the gaseous mixture. A signal at m/z = 79 (C5H5N) is identified as the product of the reaction and resolved from 79Br atoms, and the result is consistent with CH addition to pyrrole followed by H-elimination. The photoionization efficiency curve unambiguously identifies m/z = 79 as pyridine. With deuterated methylidyne radicals (CD), the product mass peak is shifted by +1 mass unit, consistent with the formation of C5H4DN and identified as deuterated pyridine (d-pyridine). Within detection limits, there is no evidence that the addition intermediate complex undergoes hydrogen scrambling. The results are consistent with a reaction mechanism that proceeds via the direct CH (CD) cycloaddition or insertion into the five-member pyrrole ring, giving rise to ring expansion, followed by H atom elimination from the nitrogen atom in the intermediate to form the resonance stabilized pyridine (d-pyridine) molecule. Implications to interstellar chemistry and planetary atmospheres, in particular Titan, as well as gas-phase combustion processes, are discussed.
Co-reporter:Satchin Soorkia, Adam J. Trevitt, Talitha M. Selby, David L. Osborn and Craig A. Taatjes, Kevin R. Wilson, Stephen R. Leone
The Journal of Physical Chemistry A 2010 Volume 114(Issue 9) pp:3340-3354
Publication Date(Web):February 8, 2010
DOI:10.1021/jp911132r
The rate coefficient for the reaction of the ethynyl radical (C2H) with 1-butyne (H—C≡C—CH2—CH3) is measured in a pulsed Laval nozzle apparatus. Ethynyl radicals are formed by laser photolysis of acetylene (C2H2) at 193 nm and detected via chemiluminescence (C2H + O2 → CH (A2Δ) + CO2). The rate coefficients are measured over the temperature range of 74−295 K. The C2H + 1-butyne reaction exhibits no barrier and occurs with rate constants close to the collision limit. The temperature-dependent rate coefficients can be fit within experimental uncertainties by the expression k = (2.4 ± 0.5) × 10−10(T/295 K)−(0.04±0.03) cm3 molecule−1 s−1. Reaction products are detected at room temperature (295 K) and 533 Pa using a multiplexed photoionization mass spectrometer (MPIMS) coupled to the tunable vacuum ultraviolet synchrotron radiation from the Advanced Light Source at the Lawrence Berkeley National Laboratory. Two product channels are identified for this reaction: m/z = 64 (C5H4) and m/z = 78 (C6H6) corresponding to the CH3-loss and H-loss channels, respectively. Photoionization efficiency (PIE) curves are used to analyze the isomeric composition of both product channels. The C5H4 products are found to be exclusively linear isomers composed of ethynylallene and methyldiacetylene in a 4:1 ratio. In contrast, the C6H6 product channel includes two cyclic isomers, fulvene 18(±5)% and 3,4-dimethylenecyclobut-1-ene (DMCB) 32(±8)%, as well as three linear isomers, 2-ethynyl-1,3-butadiene 8(±5)%, 3,4-hexadiene-1-yne 28(±8)%, and 1,3-hexadiyne 14(±5)%. Within experimental uncertainties, we do not see appreciable amounts of benzene and an upper limit of 10% is estimated. Diacetylene (C4H2) formation via the C2H5-loss channel is also thermodynamically possible but cannot be observed due to experimental limitations. The implications of these results for modeling of planetary atmospheres, especially of Saturn’s largest moon Titan and the relationships to combustion reactions, are discussed.
Co-reporter:Adam J. Trevitt, Fabien Goulay, Craig A. Taatjes, David L. Osborn and Stephen R. Leone
The Journal of Physical Chemistry A 2010 Volume 114(Issue 4) pp:1749-1755
Publication Date(Web):December 31, 2009
DOI:10.1021/jp909633a
Low-temperature rate coefficients are measured for the CN + benzene and CN + toluene reactions using the pulsed Laval nozzle expansion technique coupled with laser-induced fluorescence detection. The CN + benzene reaction rate coefficient at 105, 165, and 295 K is found to be relatively constant over this temperature range, (3.9−4.9) × 10−10 cm3 molecule−1 s−1. These rapid kinetics, along with the observed negligible temperature dependence, are consistent with a barrierless reaction entrance channel and reaction efficiencies approaching unity. The CN + toluene reaction is measured to have a rate coefficient of 1.3 × 10−10 cm3 molecule−1 s−1 at 105 K. At room temperature, nonexponential decay profiles are observed for this reaction that may suggest significant back-dissociation of intermediate complexes. In separate experiments, the products of these reactions are probed at room temperature using synchrotron VUV photoionization mass spectrometry. For CN + benzene, cyanobenzene (C6H5CN) is the only product recorded with no detectable evidence for a C6H5 + HCN product channel. In the case of CN + toluene, cyanotoluene (NCC6H4CH3) constitutes the only detected product. It is not possible to differentiate among the ortho, meta, and para isomers of cyanotoluene because of their similar ionization energies and the ∼40 meV photon energy resolution of the experiment. There is no significant detection of benzyl radicals (C6H5CH2) that would suggest a H-abstraction or a HCN elimination channel is prominent at these conditions. As both reactions are measured to be rapid at 105 K, appearing to have barrierless entrance channels, it follows that they will proceed efficiently at the temperatures of Saturn’s moon Titan (∼100 K) and are also likely to proceed at the temperature of interstellar clouds (10−20 K).
Co-reporter:Yohannes Abate, Adam Schwartzberg, Daniel Strasser, Stephen R. Leone
Chemical Physics Letters 2009 Volume 474(1–3) pp:146-152
Publication Date(Web):25 May 2009
DOI:10.1016/j.cplett.2009.04.026
Abstract
Apertureless near-field scanning optical microscopy (ANSOM) is used to image optical near-field light scattering from uncapped gold nanoparticles and gold nanoparticles capped with the cationic surfactant cetyl trimethyl ammonium bromide (CTAB). The measurements investigate the gold-particle size-dependent signals and the modification of those signals by the spacer layer of commonly used CTAB in the visible at λ = 633 nm. Imaging of capped nanoparticles by apertureless near-field microscopy opens the possibility to predict quantitative layer thicknesses of capping agents on the surface of nanoparticles, as well as the effect of capping layers on the optical scattering properties of nanoparticles.
Co-reporter:Yaroslav E. Romanyuk, Daniel Kreier, Yi Cui, Kin Man Yu, Joel W. Ager III, Stephen R. Leone
Thin Solid Films 2009 Volume 517(Issue 24) pp:6512-6515
Publication Date(Web):30 October 2009
DOI:10.1016/j.tsf.2009.03.207
The effect of silicon nitridation on structural quality, indium incorporation, and electrical properties of the InGaN/Si heterojunctions is investigated. A series of InxGa1 − xN (x = 0–0.32) thin films are grown directly on Si(111) substrates, with and without a SixNy surface layer, by plasma-assisted molecular beam epitaxy. The crystalline quality is higher and the indium incorporation is increased when the InxGa1 − xN thin films are grown with the intentional SixNy buffer. These observations are explained by the reduced local elastic stress at the interface and N-polarity of the surface, both of which promote the incorporation of In. The obtained n-InxGa1 − xN/p-Si (x = 0.2–0.3) heterojunctions exhibit a nearly ohmic behavior, and the series resistance is higher for the SixNy-containing junctions. Our results suggest that unlike the amorphous SixNy region spontaneously formed during direct deposition of III-nitrides on Si, the SixNy layer obtained by high-temperature annealing of Si(111) in nitrogen atmosphere is beneficial to the InxGa1 − xN deposition.
Co-reporter:Adam J. Trevitt, Fabien Goulay, Giovanni Meloni, David L. Osborn, Craig A. Taatjes, Stephen R. Leone
International Journal of Mass Spectrometry 2009 280(1–3) pp: 113-118
Publication Date(Web):
DOI:10.1016/j.ijms.2008.07.033
Co-reporter:Kenneth L. Knappenberger Jr., Daryl B. Wong, Wei Xu, Adam M. Schwartzberg, Abraham Wolcott, Jin Z. Zhang and Stephen R. Leone
ACS Nano 2008 Volume 2(Issue 10) pp:2143
Publication Date(Web):September 13, 2008
DOI:10.1021/nn800421g
The influence of excitation wavelength and embedding media on fluorescence blinking statistics of 4 nm × 20 nm cadmium selenide (CdSe) nanorods is investigated. Photon antibunching (PAB) experiments confirm nonclassical emission from single CdSe nanorods that exhibit a radiative lifetime of 26 ± 13 ns. The blinking data show behaviors that can be categorized into two classes: excitation near the energy of the band gap and at energies exceeding 240 meV above the band gap. Excitation at the band gap energy (λ ≥ 560 nm) results in more pronounced “on” time probabilities in the distribution of “on” and “off” events, while those resulting from excitation exceeding the band gap by 240 meV or more (λ ≤ 560 nm) are 200 times less likely to display continuous “on” fluorescence persisting beyond 4 s. The “off” time statistics are also sensitive to the excitation wavelength, showing a similar, but inversely correlated, effect. To understand better the excitation-wavelength dependence, fluorescence measurements are obtained for single nanorods deposited both on a bare microscope coverslip and embedded in 1-ethyl-3-methylimidizolium bis(trifluoromethylsulfonyl)imide room-temperature ionic liquid (RTIL). The embedding RTIL medium has a significant influence on the resultant fluorescence statistics only when the excitation energy exceeds the 240 meV threshold. The results are explained by a threshold to access nonemissive trap states, attributed to self-trapping of hot charge carriers at the higher photon excitation energies.Keywords: CdSe nanorod; fluorescence intermittency; ionic liquids; photon antibunching; single molecule; trap-state quenching
Co-reporter:Astrid M. Müller, Jürgen Plenge, Stephen R. Leone, Sophie E. Canton, Bruce S. Rude, John D. Bozek
Journal of Electron Spectroscopy and Related Phenomena 2006 Volume 154(1–2) pp:32-37
Publication Date(Web):December 2006
DOI:10.1016/j.elspec.2006.09.002
The Al 2p and Br 3d inner-shell photoelectron spectra of aluminum tribromide monomer and dimer vapor were measured at 90 and 95 eV photon energy, respectively, to determine the core electron binding energies of the atoms in the two molecular species. While AlBr3 has three identical Br atoms, Al2Br6 exhibits four terminal and two bridging Br atoms. The species are identified by their distinct valence photoelectron spectra. Comparison of the observed Al 2p1/2 and Al 2p3/2 electron binding energies of AlBr3 with those of Al2Br6 shows that there is a chemical shift of (0.15 ± 0.03) eV to lower energy in the dimer. In Al2Br6, an assignment is proposed in which the Br 3d3/2 and Br 3d5/2 binding energies of terminal Br atoms are (1.18 ± 0.03) eV lower than those of bridging Br atoms. This assignment assumes that both types of Br atoms have similar cross-sections for ionization. With this result, the Br 3d3/2 and Br 3d5/2 binding energies of Br atoms in AlBr3 are (0.81 ± 0.03) eV lower than those of bridging Br atoms of the dimer but (0.37 ± 0.03) eV higher than those of terminal Br atoms of the dimer. The obtained chemical shifts are considered in terms of the binding relations and electron density distributions in both molecules. Chemical shifts that are larger than a few hundred millielectron volts, as observed in the Al2Br6/AlBr3 system, offer potential to study the dissociation dynamics of the dimer in a femtosecond visible or ultraviolet-pump/XUV-probe experiment.
Co-reporter:Jodi M. Szarko, Jae Kyu Song, Charles Wesley Blackledge, Ingmar Swart, Stephen R. Leone, Shihong Li, Yiping Zhao
Chemical Physics Letters 2005 Volume 404(1–3) pp:171-176
Publication Date(Web):7 March 2005
DOI:10.1016/j.cplett.2005.01.063
Abstract
The properties of zinc oxide (ZnO) nanotetrapod lasers are characterized by a novel ultrafast two-color pump/stimulated emission probe technique. Single legs of tetrapod species are isolated by a microscope objective, pumped by 267 nm pulses, and subjected to a time-delayed 400 nm optical injection pulse, which permits investigation of the ultrafast carrier dynamics in the nanosize materials. With the optical injection pulse included, a large increase in the stimulated emission at 400 nm occurs, which partially depletes the carriers at this wavelength and competes with the normal 390 nm lasing. At the 390 nm lasing wavelengths, the optical injection causes a decrease in the stimulated emission due to the energetic redistribution of the excited carrier depletion, which occurs considerably within the time scale of the subpicosecond duration of the injection pulse. The effects of the optical injection on the spectral gain are employed to probe the lasing dynamics, which shows that the full-width at half-maximum of the lasing time is 3 ps.
Co-reporter:Zhi-Heng Loh, Chris H. Greene, Stephen R. Leone
Chemical Physics (23 June 2008) Volume 350(Issues 1–3) pp:
Publication Date(Web):23 June 2008
DOI:10.1016/j.chemphys.2007.11.005
Femtosecond high-order harmonic transient absorption spectroscopy is used to observe electromagnetically induced transparency-like behavior as well as induced absorption in the extreme ultraviolet by laser dressing of the He 2s2p (1Po) and 2p2 (1Se) double excitation states with an intense 800 nm field. Probing in the vicinity of the 1s2 → 2s2p transition at 60.15 eV reveals the formation of an Autler–Townes doublet due to coherent coupling of the double excitation states. Qualitative agreement with the experimental spectra is obtained only when optical field ionization of both double excitation states into the N = 2 continuum is included in the theoretical model. Because the Fano q-parameter of the unperturbed probe transition is finite, the laser-dressed He atom exhibits both enhanced transparency and absorption at negative and positive probe energy detunings, respectively.
Co-reporter:Amy A. Cordones and Stephen R. Leone
Chemical Society Reviews 2013 - vol. 42(Issue 8) pp:NaN3221-3221
Publication Date(Web):2013/01/11
DOI:10.1039/C2CS35452G
Semiconductor nanocrystal optical and charge transport properties are largely influenced by the trapping of charge carriers on the nanocrystal surface. Charge trapping increases the non-radiative exciton decay pathways, thus decreasing the fluorescence quantum yield, and it also impedes efficient charge transfer at the nanocrystal interface. On a single nanocrystal basis, charge trapping causes interruptions in the otherwise continuous fluorescence known as fluorescence intermittency or blinking. In this tutorial review we examine the relationship between charge trapping and fluorescence blinking. The fluorescence microscopy techniques and statistical analysis methods used to measure single nanocrystal blinking are introduced. The development of numerous blinking mechanisms is reviewed, as is the physical nature of charge trapping sites. An overview of blinking experiments used to probe specific mechanisms for charge carrier trapping is presented. Finally, a summary and outlook are offered. Although the detailed mechanism is not fully understood, blinking experiments are found to provide direct evidence for several charge trapping mechanisms and report on changes to the nature and distribution of charge trapping sites.
Co-reporter:Satchin Soorkia, Craig A. Taatjes, David L. Osborn, Talitha M. Selby, Adam J. Trevitt, Kevin R. Wilson and Stephen R. Leone
Physical Chemistry Chemical Physics 2010 - vol. 12(Issue 31) pp:NaN8758-8758
Publication Date(Web):2010/05/12
DOI:10.1039/C002135K
The reaction of the ground state methylidyne radical CH (X2Π) with pyrrole (C4H5N) has been studied in a slow flow tube reactor using Multiplexed Photoionization Mass Spectrometry coupled to quasi-continuous tunable VUV synchrotron radiation at room temperature (295 K) and 363 K, at 4 Torr (533 Pa). Laser photolysis of bromoform (CHBr3) at 248 nm (KrF excimer laser) is used to produce CH radicals that are free to react with pyrrole molecules in the gaseous mixture. A signal at m/z = 79 (C5H5N) is identified as the product of the reaction and resolved from 79Br atoms, and the result is consistent with CH addition to pyrrole followed by H-elimination. The photoionization efficiency curve unambiguously identifies m/z = 79 as pyridine. With deuterated methylidyne radicals (CD), the product mass peak is shifted by +1 mass unit, consistent with the formation of C5H4DN and identified as deuterated pyridine (d-pyridine). Within detection limits, there is no evidence that the addition intermediate complex undergoes hydrogen scrambling. The results are consistent with a reaction mechanism that proceeds via the direct CH (CD) cycloaddition or insertion into the five-member pyrrole ring, giving rise to ring expansion, followed by H atom elimination from the nitrogen atom in the intermediate to form the resonance stabilized pyridine (d-pyridine) molecule. Implications to interstellar chemistry and planetary atmospheres, in particular Titan, as well as gas-phase combustion processes, are discussed.
Co-reporter:Fabien Goulay, Satchin Soorkia, Giovanni Meloni, David L. Osborn, Craig A. Taatjes and Stephen R. Leone
Physical Chemistry Chemical Physics 2011 - vol. 13(Issue 46) pp:NaN20827-20827
Publication Date(Web):2011/10/14
DOI:10.1039/C1CP22609F
The reaction of ethynyl radical (C2H) with allene (C3H4) at room temperature is investigated using an improved synchrotron multiplexed photoionization mass spectrometer (MPIMS) coupled to tunable vacuum ultraviolet (VUV) synchrotron radiation from the Advanced Light Source at the Lawrence Berkeley National Laboratory (LBNL). The orthogonal-accelerated time-of-flight mass spectrometer (OA-TOF) compared to the magnetic sector mass spectrometer used in a previous investigation of the title reaction (Phys. Chem. Chem. Phys., 2007, 9, 4291) enables more sensitive and selective detection of low-yield isomeric products. The C5H4 isomer with the lowest ionization energy, pentatetraene, is now identified as a product of the reaction. Pentatetraene is predicted to be formed based on recent ab initio/RRKM calculations (Phys. Chem. Chem. Phys., 2010, 12, 2606) on the C5H5 potential energy surface. However, the computed branching fraction for pentatetraene is predicted to be five times higher than that for methyldiacetylene, whereas experimentally the branching fraction of pentatetraene is observed to be small compared to that of methyldiacetylene. Although H-atom assisted isomerization of the products can affect isomer distribution measurements, isomerization has a negligible effect in this case. The kinetic behavior of the several C5H4 isomers is identical, as obtained by time-dependent photoionization spectra. Even for high allene concentrations (and hence higher H-atom concentrations) no decay of the pentatetraene fraction is observed, indicating that H-assisted isomerization of pentatetraene to methyldiacetylene does not account for the difference between the experimental data and the theoretical branching ratios.
Co-reporter:Stephen R. Leone, Musahid Ahmed and Kevin R. Wilson
Physical Chemistry Chemical Physics 2010 - vol. 12(Issue 25) pp:NaN6578-6578
Publication Date(Web):2010/04/23
DOI:10.1039/C001707H
Scientists at the Chemical Dynamics Beamline of the Advanced Light Source in Berkeley are continuously reinventing synchrotron investigations of physical chemistry and chemical physics with vacuum ultraviolet light. One of the unique aspects of a synchrotron for chemical physics research is the widely tunable vacuum ultraviolet light that permits threshold ionization of large molecules with minimal fragmentation. This provides novel opportunities to assess molecular energetics and reaction mechanisms, even beyond simple gas phase molecules. In this perspective, significant new directions utilizing the capabilities at the Chemical Dynamics Beamline are presented, along with an outlook for future synchrotron and free electron laser science in chemical dynamics. Among the established and emerging fields of investigations are cluster and biological molecule spectroscopy and structure, combustion flame chemistry mechanisms, radical kinetics and product isomer dynamics, aerosol heterogeneous chemistry, planetary and interstellar chemistry, and secondary neutral ion-beam desorption imaging of biological matter and materials chemistry.
Co-reporter:Fabien Goulay, David L. Osborn, Craig A. Taatjes, Peng Zou, Giovanni Meloni and Stephen R. Leone
Physical Chemistry Chemical Physics 2007 - vol. 9(Issue 31) pp:NaN4300-4300
Publication Date(Web):2006/12/15
DOI:10.1039/B614502G
The reactions of the ethynyl radical (C2H) with propyne and allene are studied at room temperature using an apparatus that combines the tunability of the vacuum ultraviolet radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory with time-resolved mass spectrometry. The C2H radical is prepared by 193-nm photolysis of CF3CCH and the mass spectrum of the reacting mixture is monitored in time using synchrotron–photoionization with a dual-sector mass spectrometer. Analysis using photoionization efficiency curves allows the isomer-specific detection of individual polyynes of chemical formula C5H4 produced by both reactions. The product branching ratios are estimated for each isomer. The reaction of propyne with ethynyl gives 50–70% diacetylene (H–CC–CC–H) and 50–30% C5H4, with a C5H4-isomer distribution of 15–20% ethynylallene (CH2CCH–CCH) and 85–80% methyldiacetylene (CH3–CC–CCH). The reaction of allene with ethynyl gives 35–45% ethynylallene, 20–25% methyldiacetylene and 45–30% 1,4-pentadiyne (HCC–CH2–CCH). Diacetylene is most likely not produced by this reaction; an upper limit of 30% on the branching fraction to diacetylene can be derived from the present experiment. The mechanisms of polyynes formation by these reactions as well as the implications for Titan’s atmospheric chemistry are discussed.
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