8 research outputs found
Qualitative Multiplatform Microanalysis of Individual Heterogeneous Atmospheric Particles from High-Volume Air Samples
High-resolution
microscopic analysis of individual atmospheric
particles can be difficult, because the filters upon which particles
are captured are often not suitable as substrates for microscopic
analysis. Described here is a multiplatform approach for microscopically
assessing chemical and optical properties of individual heterogeneous
urban dust particles captured on fibrous filters during high-volume
air sampling. First, particles embedded in fibrous filters are transferred
to polished silicon or germanium wafers with electrostatically assisted
high-speed centrifugation. Particles are clustered in an array of
deposit areas, which allows for easily locating the same particle
with different microscopy instruments. Second, particles with light-absorbing
and/or light-scattering behavior are identified for further study
from bright-field and dark-field light-microscopy modes, respectively.
Third, particles identified from light microscopy are compositionally
mapped at high definition with field-emission scanning electron microscopy
and energy-dispersive X-ray spectroscopy. Fourth, compositionally
mapped particles are further analyzed with focused ion-beam (FIB)
tomography, whereby a series of thin slices from a particle are imaged,
and the resulting image stack is used to construct a three-dimensional
model of the particle. Finally, particle chemistry is assessed over
two distinct regions of a thin FIB slice of a particle with energy-filtered
transmission electron microscopy (TEM) and electron energy-loss spectroscopy
associated with scanning transmission electron microscopy (STEM)
Visualization of Phase Evolution in Model Organic Photovoltaic Structures <i>via</i> Energy-Filtered Transmission Electron Microscopy
The morphology of the active layer in an organic photovoltaic bulk-heterojunction device is controlled by the extent and nature of phase separation during processing. We have studied the effects of fullerene crystallinity during heat treatment in model structures consisting of a layer of poly(3-hexylthiophene) (P3HT) sandwiched between two layers of [6,6]-phenyl-C<sub>61</sub>-butyric acid methyl ester (PCBM). Utilizing a combination of focused ion-beam milling and energy-filtered transmission electron microscopy, we monitored the <i>local</i> changes in phase distribution as a function of annealing time at 140 °C. In both cases, dissolution of PCBM within the surrounding P3HT was directly visualized and quantitatively described. In the absence of crystalline PCBM, the overall phase distribution remained stable after intermediate annealing times up to 60 s, whereas microscale PCBM aggregates were observed after annealing for 300 s. Aggregate growth proceeded vertically from the substrate interface <i>via</i> uptake of PCBM from the surrounding region, resulting in a large PCBM-depleted region in their vicinity. When precrystallized PCBM was present, amorphous PCBM was observed to segregate from the intermediate P3HT layer and ripen the crystalline PCBM underneath, owing to the far lower solubility of crystalline PCBM within P3HT. This process occurred rapidly, with segregation already evident after annealing for 10 s and with uptake of nearly all of the amorphous PCBM by the crystalline layer after 60 s. No microscale aggregates were observed in the precrystallized system, even after annealing for 300 s
Optical Dark-Field and Electron Energy Loss Imaging and Spectroscopy of Symmetry-Forbidden Modes in Loaded Nanogap Antennas
We have produced large numbers of hybrid metal–semiconductor nanogap antennas using a scalable electrochemical approach and systematically characterized the spectral and spatial character of their plasmonic modes with optical dark-field scattering, electron energy loss spectroscopy with principal component analysis, and full wave simulations. The coordination of these techniques reveal that these nanostructures support degenerate transverse modes which split due to substrate interactions, a longitudinal mode which scales with antenna length, and a symmetry-forbidden <i>gap-localized transverse</i> mode. This gap-localized transverse mode arises from mode splitting of transverse resonances supported on both antenna arms and is confined to the gap load enabling (i) delivery of substantial energy to the gap material and (ii) the possibility of tuning the antenna resonance <i>via</i> active modulation of the gap material’s optical properties. The resonant position of this symmetry-forbidden mode is sensitive to gap size, dielectric strength of the gap material, and is highly suppressed in air-gapped structures which may explain its absence from the literature to date. Understanding the complex modal structure supported on hybrid nanosystems is necessary to enable the multifunctional components many seek
Multimodal Characterization of the Morphology and Functional Interfaces in Composite Electrodes for Li–S Batteries by Li Ion and Electron Beams
We
report the characterization of multiscale 3D structural architectures
of novel polyÂ[sulfur-<i>random</i>-(1,3-diisopropenylbenzene)]
copolymer-based cathodes for high-energy-density Li–S batteries
capable of realizing discharge capacities >1000 mAh/g and long
cycling lifetimes >500 cycles. Hierarchical morphologies and interfacial
structures have been investigated by a combination of focused Li ion
beam (LiFIB) and analytical electron microscopy in relation to the
electrochemical performance and physicomechanical stability of the
cathodes. Charge-free surface topography and composition-sensitive
imaging of the electrodes was performed using recently introduced
low-energy scanning LiFIB with Li<sup>+</sup> probe sizes of a few
tens of nanometers at 5 keV energy and 1 pA probe current. Furthermore,
we demonstrate that LiFIB has the ability to inject a certain number
of Li cations into the material with nanoscale precision, potentially
enabling control of the state of discharge in the selected area. We
show that chemical modification of the cathodes by replacing the elemental
sulfur with organosulfur copolymers significantly improves its structural
integrity and compositional homogeneity down to the sub-5-nm length
scale, resulting in the creation of (a) robust functional interfaces
and percolated conductive pathways involving graphitic-like outer
shells of aggregated nanocarbons and (b) extended micro- and mesoscale
porosities required for effective ion transport
Transient Thermometry and High-Resolution Transmission Electron Microscopy Analysis of Filamentary Resistive Switches
We present data on the filament size
and temperature distribution in Hf<sub>0.82</sub>Al<sub>0.18</sub>O<sub><i>x</i></sub>-based Resistive Random Access Memory
(RRAM) devices obtained by transient thermometry and high-resolution
transmission electron microscopy (HRTEM). The thermometry shows that
the temperature of the nonvolatile conducting filament can reach temperatures
as high as 1600 K at the onset of RESET at voltage of 0.8 V and power
of 40 μW. The size of the filament was estimated at about 1
nm in diameter. Hot filament increases the temperature of the surrounding
high resistivity oxide, causing it to conduct and carry a significant
fraction of the total current. The current spreading results in slowing
down the filament temperature increase at higher power. The results
of thermometry have been corroborated by HRTEM analysis of the as-fabricated
and switched RRAM devices. The functional HfAlO<sub><i>x</i></sub> layer in as-fabricated devices is amorphous. In devices that
were switched, we detected a small crystalline region of 10–15
nm in size. The crystallization temperature of the HfAlO<sub><i>x</i></sub> was determined to be 850 K in an independent annealing
experiment. The size of the crystalline region agrees with thermal
modeling based on the thermometry data. Scanning transmission electron
microscopy (TEM) coordinated with electron energy loss spectroscopy
could not detect changes in the chemical makeup of the filament
Formation of the Conducting Filament in TaO<sub><i>x</i></sub>‑Resistive Switching Devices by Thermal-Gradient-Induced Cation Accumulation
The distribution of tantalum and
oxygen ions in electroformed and/or switched TaO<sub><i>x</i></sub>-based resistive switching devices has been assessed by high-angle
annular dark-field microscopy, X-ray energy-dispersive spectroscopy,
and electron energy-loss spectroscopy. The experiments have been performed
in the plan-view geometry on the cross-bar devices producing elemental
distribution maps in the direction perpendicular to the electric field.
The maps revealed an accumulation of +20% Ta in the inner part of
the filament with a 3.5% Ta-depleted ring around it. The diameter
of the entire structure was approximately 100 nm. The distribution
of oxygen was uniform with changes, if any, below the detection limit
of 5%. We interpret the elemental segregation as due to diffusion
driven by the temperature gradient, which in turn is induced by the
spontaneous current constriction associated with the negative differential
resistance-type <i>I</i>–<i>V</i> characteristics
of the as-fabricated metal/oxide/metal structures. A finite-element
model was used to evaluate the distribution of temperature in the
devices and correlated with the elemental maps. In addition, a fine-scale
(∼5 nm) intensity contrast was observed within the filament
and interpreted as due phase separation of the functional oxide in
the two-phase composition region. Understanding the temperature-gradient-induced
phenomena is central to the engineering of oxide memory cells
In Situ X‑ray Scattering Studies of the Influence of an Additive on the Formation of a Low-Bandgap Bulk Heterojunction
The
evolution of the morphology of a high-efficiency, blade-coated,
organic-photovoltaic (OPV) active layer containing the low band gap
polymer polyÂ[(4,8-bisÂ[5-(2-ethylhexyl)Âthiophene-2-yl]ÂbenzoÂ[1,2-b:4,5-b′]Âdithiophene)-2,6-diyl-<i>alt</i>-(4-(2-ethylhexanoyl)-thienoÂ[3,4-<i>b</i>]Âthiophene))-2,6-diyl]
(PBDTTT-C-T) is examined by in situ X-ray scattering. In situ studies
enable real-time characterization of the effect of the processing
additive 1,8-diiodoocatane (DIO) on the active layer morphology. In
the presence of DIO, X-ray scattering indicates that domain structure
radically changes and increases in purity, while X-ray diffraction
reveals little change in crystallinity/local order. The solidification
behavior of this active layer differs dramatically from those that
strongly crystallize such as polyÂ(3-hexylthiophene) and small molecule
containing systems, exposing significant diversity in the solidification
routes relevant to high-efficiency polymer–fullerene OPV processing.
In the presence of DIO, we find quantitative agreement between the
evolution of the phase structure revealed by small-angle X-ray scattering
and the binodal phase structure of a simple Flory–Huggins model
Comparing Matched Polymer:Fullerene Solar Cells Made by Solution-Sequential Processing and Traditional Blend Casting: Nanoscale Structure and Device Performance
Polymer:fullerene bulk heterojunction
(BHJ) solar cell active layers
can be created by traditional blend casting (BC), where the components
are mixed together in solution before deposition, or by sequential
processing (SqP), where the pure polymer and fullerene materials are
cast sequentially from different solutions. Presently, however, the
relative merits of SqP as compared to BC are not fully understood
because there has yet to be an equivalent (composition- and thickness-matched
layer) comparison between the two processing techniques. The main
reason why matched SqP and BC devices have not been compared is because
the composition of SqP active layers has not been accurately known.
In this paper, we present a novel technique for accurately measuring
the polymer:fullerene film composition in SqP active layers, which
allows us to make the first comparisons between rigorously composition-
and thickness-matched BHJ organic solar cells made by SqP and traditional
BC. We discover that, in optimal photovoltaic devices, SqP active
layers have a very similar composition as their optimized BC counterparts
(≈44–50 mass % PCBM). We then present a thorough investigation
of the morphological and device properties of thickness- and composition-matched
P3HT:PCBM SqP and BC active layers in order to better understand the
advantages and drawbacks of both processing approaches. For our matched
devices, we find that small-area SqP cells perform better than BC
cells due to both superior film quality and enhanced optical absorption
from more crystalline P3HT. The enhanced film quality of SqP active
layers also results in higher performance and significantly better
reproducibility in larger-area devices, indicating that SqP is more
amenable to scaling than the traditional BC approach. X-ray diffraction,
UV–vis absorption, and energy-filtered transmission electron
tomography collectively show that annealed SqP active layers have
a finer-scale blend morphology and more crystalline polymer and fullerene
domains when compared to equivalently processed BC active layers.
Charge extraction by linearly increasing voltage (CELIV) measurements,
combined with X-ray photoelectron spectroscopy, also show that the
top (nonsubstrate) interface for SqP films is slightly richer in PCBM
compared to matched BC active layers. Despite these clear differences
in bulk and vertical morphology, transient photovoltage, transient
photocurrent, and subgap external quantum efficiency measurements
all indicate that the interfacial electronic processes occurring at
P3HT:PCBM heterojunctions are essentially identical in matched-annealed
SqP and BC active layers, suggesting that device physics are surprisingly
robust with respect to the details of the BHJ morphology