Daniel E. Blumling

Bio: Daniel E. Blumling is an academic researcher from Pennsylvania State University. The author has contributed to research in topics: Quantum dot & Thermal ionization. The author has an hindex of 7, co-authored 9 publications receiving 131 citations. Previous affiliations of Daniel E. Blumling include Florida State University & Tennessee Wesleyan College.

Oxygen-containing gas-phase diatomic trications and tetracations: ReOz+, NbOz+ and HfOz+ (z = 3, 4)

Abstract: Three oxygen-containing gas-phase diatomic trications ReO3+, NbO3+ and HfO3+ as well as the diatomic tetracation NbO4+ have been observed by mass spectrometry at non-integer m/z values. These unusual triply charged molecular ion species, together with the corresponding diatomic dications ReO2+, NbO2+ and HfO2+, were produced by energetic, high-current oxygen (16O−) ion beam sputtering of rhenium, niobium and hafnium metal samples, respectively, whose surfaces were dynamically oxidized by oxygen primary ion incorporation. In addition, NbOz+ (z ≤ 4) were generated by intense femtosecond laser excitation and photofragmentation (Coulomb explosion) of NbxOyclusters and were detected through Time-of-Flight Mass Spectrometry (TOF). Our experimental results confirm previous reports on the detection of NbO4+, NbO3+, NbO2+, HfO3+ and HfO2+ with Atom Probe mass spectrometry, whereas ReO3+ and ReO2+ apparently had not been observed before. In addition, these multiply charged molecular ions have been studied theoretically for the first time. Ab initio calculations of their electronic structures show that the diatomic trications ReO3+, NbO3+ and HfO3+ are long-lived metastable gas-phase species, with bond lengths of 1.61 A, 1.62 A and 1.86 A, respectively. They present large potential barriers with respect to dissociation of more than 2.7 eV. The corresponding diatomic dications are thermochemically stable molecules with very large dissociation energies (>3.5 eV). Our calculations predict the diatomic tetracation ReO4+ to be a metastable ion species in the gas phase. We compute a potential barrier toward fragmentation of 0.6 eV; its formation requires a quadruple adiabatic ionization energy of 85.7 eV. Even though our calculations show that NbO4+ is a weakly bound (dissociation barrier ∼0.1 eV) metastable molecule, it is here identified via linear time-of-flight mass spectrometry.

Synthesis, characterization, and self-organization of dendrimer-encapsulated HgTe quantum dots.

Abstract: Mercury telluride (HgTe) quantum dots (QDs) were synthesized in methanol at 5 degrees C using generation 5 (G5) and 7 (G7) polyamidoamine (PAMAM) dendrimers, which function both as nucleation sites and as nanoparticle stabilizers. Transmission electron microscopy (TEM) data indicate these particles were slightly oblate, with an average aspect ratio of 1.3 +/- 0.1 and a minor axis of 2.6 +/- 0.3 nm. The crystal phase was determined to be coloradoite (cubic system) by analysis of the electron diffraction pattern. Absorption maxima for HgTe QDs ranged from 950 to 970 nm, depending on the dendrimer generation and concentration. QD size distribution was optimized by careful variation of the Hg(2+):dendrimer surface group molar ratio for both G5 and G7 dendrimers. An increase in molar ratio from 1:0.5 to 1:4 resulted in a decrease in the half-width at half-maximum (HWHM) of the HgTe bandgap absorption from 68 +/- 3 to 52 +/- 2 nm, indicating a size distribution focusing of 23 +/- 4%. Second-derivative analysis of HgTe QD FTIR absorption spectra suggested that the quantum dots were fully encapsulated by a single G7 dendrimer, whereas multiple G5 dendrimers were necessary to stabilize a single nanoparticle. TEM and FTIR data revealed that the HgTe QDs form two-dimensional necklace-type arrays through a self-organization process, which proceeds through interpenetration of dendritic arms. TEM data further indicated that the average nanonecklace contained 10-15 QDs with an average inter-QD separation of 1.3 +/- 0.7 nm and a total chain length of 46 +/- 6 nm.

Photodissociation of thioglycolic acid studied by femtosecond time-resolved transient absorption spectroscopy

Abstract: Steady-state and time-resolved spectroscopies were employed to study the photodissociation of both the neutral (HS-CH(2)-COOH) and doubly deprotonated ((-)S-CH(2)-COO(-)) forms of thioglycolic acid (TGA), a common surface-passivating ligand used in the aqueous synthesis and organization of semiconducting nanostructures. Room temperature UV-Vis absorption spectroscopy indicated strong absorption by the S(1) and S(2) excited states at 250 nm and 185 nm, respectively. The spectrum also contained a weaker absorption band that extended to approximately 550 nm, which was assigned to the π(CO) (*)←n(O) transition. Femtosecond time-resolved transient absorption spectroscopy was performed on TGA using 400 nm excitation and a white-light continuum probe to provide the temporally and spectrally resolved data. Both forms of TGA underwent a photoinduced dissociation from the excited state to form an α-thiol-substituted acyl radical (α-TAR, S-CH(2)-CO(●)). For the acidic form of TGA, radical formation occurred with an apparent time constant of 60 ± 5 fs; subsequent unimolecular decay took 400 ± 60 fs. Similar kinetics were observed for the deprotonated form of TGA (70 ± 10 fs radical formation; 420 ± 40 fs decay). The production of the α-TAR was corroborated by the observation of its characteristic optical absorption. Time-resolved data indicated that the photoinduced dissociation of TGA via cleavage of the C-OH bond occurred rapidly (≤100 fs). The prevalence of TGA in aqueous semiconducting nanoparticles makes its absorption in the visible spectral region and subsequent dissociation key to understanding the behavior of nanoscale systems.

Strong-field ionization and dissociation studies on small early transition metal carbide clusters via time-of-flight mass spectrometry.

Abstract: In this work, we report experimental results from the strong-field ionization and subsequent Coulomb explosion of narrow distributions of small (<40 atoms) heteronuclear clusters composed of transi...

Mid-infrared HgTe colloidal quantum dot photodetectors

Abstract: Researchers show that thin films containing HgTe quantum dots with diameters of around 10 nm exhibit a photoresponse in the mid-infrared that extends to wavelengths as long as 5 µm. Such films could become the basis of a new form of low-cost mid-infrared photodetector.

Narrow bandgap colloidal metal chalcogenide quantum dots: synthetic methods, heterostructures, assemblies, electronic and infrared optical properties

Abstract: The chemistry, material processing and fundamental understanding of colloidal semiconductor nanocrystals (quantum dots) are advancing at an astounding rate, bringing the prospects of widespread commercialization of these novel and exciting materials ever closer. Interest in narrow bandgap nanocrystals in particular has intensified in recent years, and the results of research worldwide point to the realistic prospects of applications for these materials in solar cells, infrared optoelectronics (e.g. lasers, optical modulators, photodetectors and photoimaging devices), low cost/large format microelectronics, and in biological imaging and biosensor systems to name only some technologies. Improvements in fundamental understanding and material quality are built on a vast body of experience spread over many different methods of colloidal synthetic growth, each with their own strengths and weaknesses for different materials and sometimes with regard to particular applications. The nanocrystal growth expertise is matched by a rapidly expanding, and highly interdisciplinary, understanding of how best to assemble these materials into films or hybrid composites and thereby into useful devices, and again there are many different strategies that can be adopted. In this review we have attempted to survey and compare the recent work on colloidal synthesis, film and nanocrystal composite material fabrication, concentrating on narrow bandgap chalcogenide materials and some of their topical applications in the solar energy and biological fields. Since these applications are attracting rising interest across a wide range of disciplines, from the biological sciences, device engineering, and materials processing fields as well as the physics and synthetic chemistry communities, we have endeavoured to make the review of these narrow bandgap nanomaterials both comprehensive and accessible to newcomers to the area.

Exciton-LO-phonon couplings in spherical semiconductor microcrystallites

Abstract: Exciton-LO-phonon couplings in CdS x Sc 1-x semiconductor microcrystallites (x=0.12±0.05) are investigated by measuring the temperature dependence of the width and energy of excitons by electroabsorption. The LO phonons are shown semiquantitatively to contribute to the experimentally obtained temperaure dependencies of the width and energy of excitons. The dependence of the coupling constant (the Huang-Rhys parameter) on the radius of microcrystallites is calculated for CdSe and GaAs microcrystallites. The phonon confinement effects are considered with "free-standing" and "rigid"-boundary conditions. As for the exciton state, nonparabolicity of the conduction band and the valence-band mixing are considered in order to obtain a precise exciton wave function, which is crucially important in calculating the Huang-Rhys parameter in a microcrystallite. The exciton-confined-optical-phonon interaction Harniltonian is constructed for a microcrystallite. It is found that the Huang-Rhys parameters have a minimum at a radius of 70 A for CdSe and 270 A for GaAs microcrystallites. The size dependence of the Huang-Rhys parameter is also calculated for a microcrystallite with an extra charge at the spherical-particle center. The lowest (s,S 3/2 ) state in the trapped state is found to have small transition probability and g values of 1 in CdSe (R =30 A) and 0.01 in GaAs? = 100 A). The higher states are found to have larger transition probability and g values of 0.7 in CdSe (R =30 A) and 0.01 in GaAs (R = 100 A). These results suggest that large g values observed experimentally in CdS and CdSe microcrystallites originate from extrinsic effects such as the presence of charged point defects inside the microcrystallite.