Virginia Manner

CdSe Quantum-Dot-Sensitized Solar Cell with ∼100% Internal Quantum Efficiency

We have constructed and studied photoelectrochemical solar cells (PECs) consisting of a photoanode prepared by direct deposition of independently synthesized CdSe nanocrystal quantum dots (NQDs) onto a nanocrystalline TiO(2) film (NQD/TiO(2)), aqueous Na(2)S or Li(2)S electrolyte, and a Pt counter electrode. We show that light harvesting efficiency (LHE) of the NQD/TiO(2) photoanode is significantly enhanced when the NQD surface passivation is changed from tri-n-octylphosphine oxide (TOPO) to 4-butylamine (BA). In the PEC the use of NQDs with a shorter passivating ligand, BA, leads to a significant enhancement in both the electron injection efficiency at the NQD/TiO(2) interface and charge collection efficiency at the NQD/electrolyte interface, with the latter attributed mostly to a more efficient diffusion of the electrolyte through the pores of the photoanode. We show that by utilizing BA-capped NQDs and aqueous Li(2)S as an electrolyte, it is possible to achieve ∼100% internal quantum efficiency of photon-to-electron conversion, matching the performance of dye-sensitized solar cells.

Role of solvent-oxygen ion pairs in photooxidation of CdSe nanocrystal quantum dots

Understanding the mechanisms for photodegradation of nanocrystal quantum dots is an important step toward their application in real-world technologies. A usual assumption is that photochemical modifications in nanocrystals, such as their photooxidation, are triggered by absorption of a photon in the dot itself. Here, we demonstrate that, contrary to this commonly accepted picture, nanocrystal oxidation can be initiated by photoexcitation of solvent-oxygen ion pairs that relax to produce singlet oxygen, which then reacts with the nanocrystals. We make this conclusion on the basis of photolysis studies of solutions of CdSe nanocrystals. Our measurements indicate a sharp spectral onset for photooxidation, which depends on solvent identity and is 4.8 eV for hexane and 3.4 eV for toluene. Importantly, the photooxidation onset correlates with the position of a new optical absorption feature, which develops in a neat solvent upon its exposure to oxygen. This provides direct evidence that nanocrystal photooxidation is mediated by excitation of solvent-oxygen pairs and suggests that the stability of the nanocrystals is defined by not only the properties of their surfaces (as has been commonly believed) but also the properties of their environment, that is, of the surrounding solvent or matrix.

Quantum dot sensitized solar cell

Photoelectrochemical solar cells (PECs) have been constructed and studied, the cells comprising a photoanode prepared by direct deposition of independently synthesized nanocrystal quantum dots (NQDs) onto a nanocrystalline metal oxide film, aqueous electrolyte and a counter electrode. It has been shown that the light harvesting efficiency (LHE) of the NQD/metal oxide photoanode is significantly enhanced when the NQD surface passivation is changed to a smaller ligand (e.g. butylamine (BA)). In the PEC the use of NQDs with a shorter passivating ligand leads to a significant enhancement in both the electron injection efficiency at the NQD/metal oxide interface and charge collection efficiency at the NQD/electrolyte interface.

Effect of organic passivation on photoinduced electron transfer across the quantum dot/TiO2 interface

We report a study of the internal quantum efficiency (IQE) of CdSe quantum-dot (QD)-sensitized solar cells prepared by direct adsorption of pre-synthesized QDs, passivated with either tri-n-octylphosphine oxide (TOPO) or n-butylamine (BA), onto a nanocrystalline TiO(2) film.

Shock compression of formic acid

Simple molecules such as formic acid, HCOOH, have been suggested to play important roles in the origin of life due to their high pressure and temperature chemistry. The hydrogen bonding characteristics and polymerization of HCOOH under static high pressure have been recently investigated using both molecular dynamics calculations and experimental work. These works suggest that symmetric hydrogen bonding of HCOOH (forming a linear chain polymer where all C–O bonds are equivalent) occurs at 16 – 21 GPa at room temperature. In order to examine the shock compression behavior of this simple carboxylic acid, we present a series of gas gun-driven plate impact experiments on formic acid with shock inputs in the range of 5.5 – 23.0 GPa. Using in-situ electromagnetic gauges, shock wave profiles (particle velocities) were measured at multiple positions as a function of shock input pressure, providing valuable information about its unreacted equation of state. No easily recognizable shock-induced reactions were obser...

Molecular dynamics simulations of the first reactions in nitrate ester-based explosives

Reactive quantum molecular dynamics (QMD) simulations have been performed to understand features of the response of a series of PETN derivatives under impact tests. Liquid PETN and the PETN derivatives PETN-CH, PETN-CMe, and PETN-CNH2 were studied at ambient density with initial temperatures of 1200 and 1400 K, that is, under ‘cook-off’-like conditions, to mimic the low pressure conditions accessed in drop weight tests. The reactive QMD simulations were performed using semi-empirical density functional tight binding theory (DFTB). Our DFTB parameterization was validated against published reaction energies for PETN. The QMD simulations revealed notable differences in the initial chemical events and performance of the PETN-based materials that we correlate with recent experimental data.Reactive quantum molecular dynamics (QMD) simulations have been performed to understand features of the response of a series of PETN derivatives under impact tests. Liquid PETN and the PETN derivatives PETN-CH, PETN-CMe, and PETN-CNH2 were studied at ambient density with initial temperatures of 1200 and 1400 K, that is, under ‘cook-off’-like conditions, to mimic the low pressure conditions accessed in drop weight tests. The reactive QMD simulations were performed using semi-empirical density functional tight binding theory (DFTB). Our DFTB parameterization was validated against published reaction energies for PETN. The QMD simulations revealed notable differences in the initial chemical events and performance of the PETN-based materials that we correlate with recent experimental data.

Numerical Simulations of Near-Field Blast Effects using Kinetic Plates

Numerical simulations using two hydrocodes were compared to near-field measurements of blast impulse associated with ideal and non-ideal explosives to gain insight into testing results and predict untested configurations. The recently developed kinetic plate test was designed to measure blast impulse in the near-field by firing spherical charges in close range from steel plates and probing plate acceleration using laser velocimetry. Plate velocities for ideal, non-ideal and aluminized explosives tests were modeled using a three dimensional hydrocode. The effects of inert additives in the explosive formulation were modeled using a 1-D hydrocode with multiphase flow capability using Lagrangian particles. The relative effect of particle impact on the plate compared to the blast wave impulse is determined and modeling is compared to free field pressure results.

High-Pressure Far-Infrared Spectroscopic Studies of Hydrogen Bonding in Formic Acid

Simple molecules such as HCOOH, or formic acid, are suggested to have played important roles in planetary physics due to their possibility for high pressure and temperature chemistry under impact conditions. In this study, we have investigated the effect of pressure (up to 50 GPa) on H-bonding and reactivity of formic acid using synchrotron far infrared spectroscopy. Based on the pressure-induced changes to H-bond ν(O–H···O) stretching and γ(O–H···O) deformations, we observe significant reorganization of H-bonding network beginning at ∼20 GPa. This is in good agreement with reports of symmetrization of H-bonds reported at 16–21 GPa from X-ray diffraction and Raman spectroscopy studies as well as molecular dynamics simulations. With further increase in pressure, beyond 35 GPa, formic acid undergoes a polymerization process that is complete beyond 45 GPa. Remarkably, upon decompression, the polymeric phase reverts to the crystalline high-pressure phase at 8 GPa.

Fast reactions of aluminum and explosive decomposition products in a post-detonation environment

In order to determine the reaction behavior of Al in RDX or HMX/cast-cured binder formulations shortly after the passage of the detonation, a series of cylinder tests was performed on formulations comprising of varying binder systems and either 3.5 μm spherical Al or LiF (an inert salt with a similar molecular weight and density to Al). In these studies, both detonation velocity and cylinder expansion velocity are measured in order to determine exactly how and when Al contributes to the explosive event, particularly in the presence of oxidizing/energetic binders. The U.S. Army Research, Development and Engineering Laboratory at Picatinny have recently coined the term “combined effects“ explosives for materials such as these; as they demonstrate both high metal pushing capability and high blast ability. This study is aimed at developing a fundamental understanding of the reaction of Al with explosives decomposition products, where both the detonation and early post-detonation environment are analyzed. Reac...

Understanding and manipulating the sensitivity of nitrate ester explosives

Understanding the factors that influence sensitivity is critical for the design and screening of new energetic molecules and materials, but it is often difficult to isolate a single variable to analyze its effect. Pentaerythritol tetranitrate (PETN) is a very common nitrate ester explosive that has been widely studied due to its use in military and commercial explosives. We have developed PETN derivatives with modified sensitivity characteristics by substituting the -CCH2ONO2 moiety with other substituents, including -CH, -CNH2, -CNH3X, -CCH3, or -PO. We relate the handling sensitivity properties of each PETN derivative to its structure, and discuss the potential roles of the central atom, oxygen balance, thermal stability, heat capacity, crystal structure, and inter- and intramolecular hydrogen bonding on impact sensitivity. Reactive molecular dynamics (MD) simulations of the C, H, N, O-based PETN-derivatives have been performed under cook-off conditions that provide insights into how the substituents change the initial chemistry and decomposition paths.Understanding the factors that influence sensitivity is critical for the design and screening of new energetic molecules and materials, but it is often difficult to isolate a single variable to analyze its effect. Pentaerythritol tetranitrate (PETN) is a very common nitrate ester explosive that has been widely studied due to its use in military and commercial explosives. We have developed PETN derivatives with modified sensitivity characteristics by substituting the -CCH2ONO2 moiety with other substituents, including -CH, -CNH2, -CNH3X, -CCH3, or -PO. We relate the handling sensitivity properties of each PETN derivative to its structure, and discuss the potential roles of the central atom, oxygen balance, thermal stability, heat capacity, crystal structure, and inter- and intramolecular hydrogen bonding on impact sensitivity. Reactive molecular dynamics (MD) simulations of the C, H, N, O-based PETN-derivatives have been performed under cook-off conditions that provide insights into how the substituents ch...