Christopher D. Zangmeister

Crumpled Nanopaper from Graphene Oxide

Graphene oxide (GO) in aqueous solution was aerosolized and rapidly dried to produce crumpled nanopaper-like sheets. Online size selection and aerosol mass analysis was used to determine the fractal dimension (D) of crumpled GO nanosheets as 2.54 ± 0.04. That is identical to macroscale materials, such as crumpled balls of paper and foil. Thermal reduction of crumpled GO nanosheets did not change D, even after loss of nearly 25% of the nanosheets mass. We demonstrate that D is able to be tuned by altering solvent conditions. A 10% acetone mixture increased D to 2.68 ± 0.02. Calculations of the confinement force show that crumpling of GO nanosheets is driven by the capillary force associated with rapid solvent loss.

Origin of discrepancies in inelastic electron tunneling spectra of molecular junctions

We report inelastic electron tunneling spectroscopy (IETS) of multilayer molecular junctions with and without incorporated metal nanoparticles. The incorporation of metal nanoparticles into our devices leads to enhanced IET intensity and a modified line shape for some vibrational modes. The enhancement and line-shape modification are both the result of a low lying hybrid metal nanoparticle-molecule electronic level. These observations explain the apparent discrepancy between earlier IETS measurements of alkane thiolate junctions by Kushmerick et al. [Nano Lett. 4, 639 (2004)] and Wang et al. [Nano Lett. 4, 643 (2004)].

Dependence of soot optical properties on particle morphology: measurements and model comparisons.

We report the first mass-specific absorption and extinction cross sections for size- and mass-selected laboratory-generated soot aerosol. Measurement biases associated with aerosols possessing multiple charges were eliminated using mass selection to isolate singly charged particles for a specified electrical mobility diameter. Aerosol absorption and extinction coefficients were measured using photoacoustic and cavity ring-down spectroscopy techniques, respectively, for lacey and compacted soot morphologies. The measurements show that the mass-specific absorption cross sections are proportional to particle mass and independent of morphology, with values between 5.7 and 6 m(2) g(-1). Mass-specific extinction cross sections were morphology dependent and ranged between 12 and 16 m(2) g(-1) for the lacey and compact morphologies, respectively. The resulting single-scattering albedos ranged from 0.5 to 0.6. Results are also compared to theoretical calculations of light absorption and scattering from simulated particle agglomerates. The observed absorption is relatively well modeled, with minimum differences between the calculated and measured mass absorption cross sections ranging from ∼ 5% (lacey soot) to 14% (compact soot). The model, however, was unable to satisfactorily reproduce the measured extinction, underestimating the single-scattering albedo for both particle morphologies. These discrepancies between calculations and measurements underscore the need for validation and refinement of existing models of light scattering and absorption by soot agglomerates.

Different mechanisms govern the two-phase Brust-Schiffrin dialkylditelluride syntheses of Ag and Au nanoparticles.

Here we report the first unambiguous identification of the chemical structures of the precursor species involving metal (Au and Ag) ions and Te-containing ligands in the Brust-Schiffrin syntheses of the respective metal nanoparticles, through which the different reaction pathways involved are delineated.

Demonstration of Molecular Assembly on Si (100) for CMOS-Compatible Molecule-Based Electronic Devices

In this work, we establish the potential of a UV-promoted direct attachment of alkanes with alcohol and thiol linkers to the silicon (100) surfaces for use in molecular electronic devices with increased potential for integration with existing CMOS technologies. Characterization of the self-assembled monolayers via Fourier transform infrared spectroscopy, spectroscopic ellipsometry, and X-ray photoemission spectroscopy shows that the films assembled on the Si (100) are comparable in quality, aliphatic monolayer coverage, and extent of substrate oxidation to those assembled on the more extensively studied Si (111) crystal face. Simple Si (100)-based electronic devices fabricated with the monolayers exhibited molecule-dependent electrical characteristics. These data highlight the effectiveness of the assembly on Si (100), the ability to fabricate enclosed Si (100)-based molecular devices, and the potential for the future integration of these devices with more conventional technologies.

Practical limitations of aerosol separation by a tandem differential mobility analyzer–aerosol particle mass analyzer

ABSTRACT A cavity ring-down spectrometer and condensation particle counter were used to investigate the limitations in the separation of singly and multiply charged aerosol particles by a tandem differential mobility analyzer (DMA) and aerosol particle mass analyzer (APM). The impact of particle polydispersity and morphology was investigated using three materials: nearly monodisperse polystyrene latex nanospheres (PSL); polydisperse, nearly spherical ammonium sulfate (AS), and polydisperse lacey fractal soot agglomerates. PSL and AS particles were easily resolved as a function of charge. For soot, the presence of multiply charged particles severely affects the isolation of the singly charged particles. In cases where the DMA–APM was unable to fully resolve the singly charged particles of interest, the peak mass deviated by up to 13% leading to errors in the mass specific extinction cross section of over 100%. For measurements of nonspherical particles, nonsymmetrical distributions of concentration as a function of mass were a sign of the presence of multiply charged particles. Under these conditions, the effects of multiply charged particles can be reduced by using a second charge neutralizer after the DMA and prior to the APM. Dilution of the aerosol stream serves to decrease the total number concentration of particles and does not remove the contributions of multiply charged particles.

Direct Measurements of Mass-Specific Optical Cross Sections of Single-Component Aerosol Mixtures

The optical properties of atmospheric aerosols vary widely, being dependent upon particle composition, morphology, and mixing state. This diversity and complexity of aerosols motivates measurement techniques that can discriminate and quantify a variety of single- and multicomponent aerosols that are both internally and externally mixed. Here, we present a new combination of techniques to directly measure the mass-specific extinction and absorption cross sections of laboratory-generated aerosols that are relevant to atmospheric studies. Our approach employs a tandem differential mobility analyzer, an aerosol particle mass analyzer, cavity ring-down and photoacoustic spectrometers, and a condensation particle counter. This suite of instruments enables measurement of aerosol particle size, mass, extinction and absorption coefficients, and aerosol number density, respectively. Taken together, these observables yield the mass-specific extinction and absorption cross sections without the need to model particle morphology or account for sample collection artifacts. Here we demonstrate the technique in a set of case studies which involve complete separation of aerosol by charge, separation of an external mixture by mass, and discrimination between particle types by effective density and single-scattering albedo.

Packing density of rigid aggregates is independent of scale

Significance Particle aggregates are one of the most ubiquitous structural arrangements in the universe, making up large interstellar planetary seedlings and comets, microscale powdered pharmaceuticals, and nanoscale atmospheric particles. The packing of aggregates determines structure and, in many cases, chemical, physical, and mechanical properties. Here we show that packing in weakly compacted aggregated materials is scale invariant over many orders of magnitude. We demonstrate that the effective density of compacted soot aggregates and comets are equivalent, and conclude that the packing of aggregated materials appear to be remarkably independent of the aggregating forces, mechanisms, or initial conditions. Large planetary seedlings, comets, microscale pharmaceuticals, and nanoscale soot particles are made from rigid, aggregated subunits that are compacted under low compression into larger structures spanning over 10 orders of magnitude in dimensional space. Here, we demonstrate that the packing density (θf) of compacted rigid aggregates is independent of spatial scale for systems under weak compaction. The θf of rigid aggregated structures across six orders of magnitude were measured using nanoscale spherical soot aerosol composed of aggregates with ∼17-nm monomeric subunits and aggregates made from uniform monomeric 6-mm spherical subunits at the macroscale. We find θf = 0.36 ± 0.02 at both dimensions. These values are remarkably similar to θf observed for comet nuclei and measured values of other rigid aggregated systems across a wide variety of spatial and formative conditions. We present a packing model that incorporates the aggregate morphology and show that θf is independent of both monomer and aggregate size. These observations suggest that the θf of rigid aggregates subject to weak compaction forces is independent of spatial dimension across varied formative conditions.

Measurement of Gas and Aerosol Phase Absorption Spectra across the Visible and Near-IR Using Supercontinuum Photoacoustic Spectroscopy.

We demonstrate a method to measure the absorption spectra of gas and aerosol species across the visible and near-IR (500 to 840 nm) using a photoacoustic (PA) spectrometer and a pulsed supercontinuum laser source. Measurements of gas phase absorption spectra were demonstrated using H2O(g) as a function of relative humidity (RH). The measured absorption intensities and peak shapes were able to be quantified and compared to spectra calculated using the 2012 High Resolution Transmission (HITRAN2012) database. Size and mass selected nigrosin aerosol was used to measure absorption spectra across the visible and near-IR. Spectra were measured as a function of aerosol size/mass and show good agreement to Mie theory calculations. Lastly, we measured the broadband absorption spectrum of flame generated soot aerosol at 5% and 70% RH. For the high RH case, we are able to quantifiably separate the soot and water absorption contributions. For soot, we observe an enhancement in the mass specific absorption cross section ranging from 1.5 at 500 nm (p < 0.01) to 1.2 at 840 nm (p < 0.2) and a concomitant increase in the absorption Ångström exponent from 1.2 ± 0.4 (5% RH) to 1.6 ± 0.3 (70% RH).

Measured Wavelength-Dependent Absorption Enhancement of Internally Mixed Black Carbon with Absorbing and Nonabsorbing Materials

Optical absorption spectra of laboratory generated aerosols consisting of black carbon (BC) internally mixed with nonabsorbing materials (ammonium sulfate, AS, and sodium chloride, NaCl) and BC with a weakly absorbing brown carbon surrogate derived from humic acid (HA) were measured across the visible to near-IR (550 to 840 nm). Spectra were measured in situ using a photoacoustic spectrometer and step-scanning a supercontinuum laser source with a tunable wavelength and bandwidth filter. BC had a mass-specific absorption cross section (MAC) of 7.89 ± 0.25 m(2) g(-1) at λ = 550 nm and an absorption Ångström exponent (AAE) of 1.03 ± 0.09 (2σ). For internally mixed BC, the ratio of BC mass to the total mass of the mixture was chosen as 0.13 to mimic particles observed in the terrestrial atmosphere. The manner in which BC mixed with each material was determined from transmission electron microscopy (TEM). AS/BC and HA/BC particles were fully internally mixed, and the BC was both internally and externally mixed for NaCl/BC particles. The AS/BC, NaCl/BC, and HA/BC particles had AAEs of 1.43 ± 0.05, 1.34 ± 0.06, and 1.91 ± 0.05, respectively. The observed absorption enhancement of mixed BC relative to the pure BC was wavelength dependent for AS/BC and decreased from 1.5 at λ = 550 nm with increasing wavelength while the NaCl/BC enhancement was essentially wavelength independent. For HA/BC, the enhancement ranged from 2 to 3 and was strongly wavelength dependent. Removal of the HA absorption contribution to enhancement revealed that the enhancement was ≈1.5 and independent of wavelength.