Session Y39: Spectroscopy and Kinetics; Nanoscale Chemical Physics

Bistable phases of diazodiphenylmethane on Cu(111): revelations from Inelastic Tunneling Spectroscopy and ab initio calculations

In the ever-changing field of information technology, molecular-scale electronics can benefit from molecules with switchable stable states whose transitions can be controlled. We have found diazodiphenylmethane (Ph₂CN₂) adsorbed on Cu(111)  to display this bistable configuration. The transition between the states occurs thermally or can be voltage-induced via scanning tunneling microscopy (STM).  With the use of STM, inelastic tunneling microscopy (IETS), and density functional theory (DFT) based calculations, the two distinctly different configurations of the molecule are observed and identified. While both the symmetrical and non-symmetrical orientations of the molecule with respect to the carbon rings are shown to be stable on the Cu surface with a binding energy of -1.58 eV and -1.72 eV respectively, their binding with the surface is not the same. The non-symmetrical phase has a stronger surface interaction and has its frontier orbitals shifted to the fermi level exhibiting metallic characteristics. The frontier orbitals of the symmetrical phase maintain their gas phase characteristics. A red shift in the IR spectra is seen from the symmetrical phase relative to the non-symmetrical phase, while IETS captures features only from the symmetrical phase.   

Substitution of Hydrogens with Fluorine Impact on Second-Generation Molecular Motors

Molecular motors, the essential agents of movement in living organisms, have been the subject of much scientific attention in recent years. We present results of studies of the impact of fluorine substitution in the stator on second-generation molecular motors. From DFT calculations, we predict that fluorine substitution can significantly alter the rotational speed of the motor, depending on the location of the hydrogen atom that is replaced by fluorine. Moreover, the calculations show that fluorine substitution can also affect the motor's stability. Our results demonstrate that the stability can either be enhanced or decreased depending on the location of the hydrogen atom that is substituted with fluorine. This offers valuable insights into the design and optimization of molecular motors for various technological applications, such as nanotechnology and biomedicine. We have identified motors with half-lives in the range of 1-1000 seconds, with high differences in absorption spectrums of isomers, that can be used in liquid crystal applications as well. 

A phenomenological model we have developed describes the effects of fluorine substitution in second-generation molecular motors. This allows for predictive insights without exhaustive calculations, paving the way for optimized motor designs and a better understanding of the mechanism of operation of molecular motors as a whole.   

Enhancing Protein Stability: Tuning Hydrophobic Interactions with Additives

Among the various forces involved in protein folding and stability, hydrophobic interactions are dominant contributors. Introduction of additives to solutions can be used to modulate the strength of hydrophobic interactions, and in turn, the stability of proteins. Here, we investigated the effects of several amino acids and sugars – which are common additives in protein formulations – on the folding and stability of a model hydrophobic polymer. We show that amino acids and sugars modulate hydrophobic interactions through a combination of direct and indirect mechanisms, depending on their concentration. We also find that the amino acid side chain and polar tail play different roles in driving these mechanisms and the nature of the side chain determines the mechanism of action. We demonstrate that the complex and context-dependent effects provided by these amino acids and sugars on protein stability can be explained by the balance between direct and indirect contributions. This work provides insights into the molecular-level mechanisms of additive-induced stabilization and lays the foundation for future studies on rational formulation design.   

Theory of entangled two-photon emission/absorption [E2P- EA] between molecules

Entangled photons are of great interest as promising candidates for numerous applications in quantum information, quantum communication and quantum sensing. We present a comprehensive study of the theory of entangled two-photon emission/absorption (E2P-EA) between a many-level cascade donor and a many-level acceptor (which could be quantum dots or molecules) using second-order perturbation theory. We analyze how dipole orientation, radiative lifetime, energy detuning between intermediate states, separation distance, and entanglement time impact the E2P-EA rate. Our study demonstrates that there are quantum interference effects in the E2P-EA rate expression that lead to oscillations in the rate as a function of entanglement time. Moreover, we show that the E2P-EA rate can be comparable to one-photon emission/absorption (OP-EA) when donor and acceptor are within a few nm.   

Ab initio molecular dynamics study of ion-specific olivine dissolution with implications to CO2 mineralization geosequestration

CO₂ mineralization is the safest CO₂ geosequestration method with the highest sequestration capacity. Even though there have been lab and pilot-scale demonstrations, the complex chemical reaction is still elusive at atomic level. Here, I show that the ab initio molecular dynamics (AIMD) and metadynamics simulations enable quantitative analysis of reaction pathways, thermodynamics, and kinetics of the Mg²⁺ and Ca²⁺ ion dissolutions from CO₂-reactive minerals (olivine endmembers), which are the rate-determining steps for CO₂ mineralization geosequestration. The leaching of Ca²⁺ from the Ca-olivine surface is a ligand exchange process that results in a much lower energy barrier with 10³ times faster dissolution rate compared to the leaching of Mg²⁺, which the tight magnesium sites on the forsterite (Mg-olivine) surface forbid ligand exchange. My results have implications in CO₂ mineralization geosequestration operations including the exploration of the desired mineralogy and the evaluation of enhanced mineralization mechanisms from first principle.   

Estimating Thermally Driven Magnetic Field Fluctuations near Nanoscale Ferromagnets - Understanding Signal Loss in Magnetic Resonance Force Microscopy (MRFM)

Many spin-based quantum computing proposals involve carrying out electron or proton (spin) magnetic resonance very close to a magnet [1]. Additionally, nitrogen-vacancy center imaging and magnetic resonance force microscopy (MRFM) employ nanoscale magnets close to spins [2,3]. One potential problem in these experiments is loss of signal due to unwanted spin-lattice relaxation caused by stochastic magnetic field fluctuations [4]. We present a novel simulation protocol - ground, excite, ring, Fourier transform (GERFT) - to estimate thermomagnetic fluctuations in nanoscale ferromagnets. Using NIST's Object Oriented Micromagnetic Framework (OOMMF) code, we apply a pulsed magnetic field at a point of interest, calculate the resulting transient change in magnetization at each location in the ferromagnet, and use a Fourier transform fluctuation-dissipation theorem relation to compute the power spectral density of field fluctuations at the point of interest. GERFT estimates spin-lattice relaxation times due to fluctuations from common ferromagnetic materials to be 6-7 orders of magnitude longer than would cause significant signal loss in MRFM experiments. The insights provided by this model will lead to higher resolution spin imaging experiments and better designed quantum spintronic devices.

1. Loss, et al. Physical Review X (2013) 3: 041023

2. Yacoby, et al. Nature Nanotechnology (2014) 9: 279-284.

3. Rugar, et al. PNAS (2009) 106(5): 1313-1317.

4. Rugar, et al. Phys. Rev. Lett. (2001) 87: 277602.   

Reliable deposition of gold nanoparticle projectiles launched from an ion trap

We have succeeded in depositing single gold nanoparticles initially levitated in an ion trap onto a substrate with sticking probabilities around 90%.  Our research uses a two-part approach to study the material properties of nanoparticles: first, optical and thermodynamic measurements are made while the particle is levitated in an electric field trap in high vacuum; next, as a complement to the levitated measurements, we deposit the particle on a substrate that can be removed for study via microscopy.  The nanoparticles (of diameter 250 nm) are given electric charge and delivered to the trap from a liquid suspension via electrospray ionization, and a single particle is studied using laser scattering.  Next, the particle is expelled from the trap with velocity of order 10-20 m/s.  A laser liquefies the particle during travel to promote sticking at the substrate.  The location of the particle on the substrate is determined using camera imaging.  In the talk, we will present preliminary data on the focusing of the particle’s deposition location using an electrostatic lens.   

Topological insulator single molecule circuits formed with neutral organic radicals.

Incorporation of open-shell compounds into single molecule circuits can increase the scope of observable phenomena due to expanded electronic degrees of freedom in such molecules. For example, recent work has shown that organic diradical cations can exhibit orders of magnitude greater conductance than their neutral versions due to the topological edge states associated with radical pairs. Yet challenges and questions about how to create stable radical junctions and control their electronic properties still remain. Neutral diradicals which possess open-shell character but do not require oxidation are a promising class of molecules that have not been probed in molecular junctions in ambient conditions to date. Here, I will present our recent single molecule conductance measurements and electronic structure calculations on a class of such molecules in solution at room temperature on gold. We addapt the Su-Schrieffer-Heeger (SSH) model to our diradicals chemical structures and show good agreement with experimental results. Our work provides insights into molecular structures that promote radical character in molecular circuits, leading to higher conductance, and paves the way towards engineering single molecule junction properties.   

Impact of Structural Changes to the Electronic Properties of PbS Colloidal Quantum Dots via DFT Study

Correlating morphologic stoichiometry, atomic coordination, and bonding character in PbX (X=S,Se,Te) colloidal quantum dots to their allowable low energy optical transitions (<0.5eV) could pave the way for new infrared photodetector architechtures. This work specifically employed three toy models to isolate, within the PbS core, the impacts of atomic coordination, cation-rich structures, and lattice distortions. Analysis of the electron localication function, density of states, and charge density suggests metavalent bonding occurs in PbS nanocrystals. Metavalent bonding is a recently recognized novel bonding type with characteristics which lie between covalent and metallic; in PbS it is primarily studied in the bulk crystalline structure. Density functional theory results support the persistence of this bonding type in confined PbS nanocrystals. It is observed that low-coordinated Pb and S atoms on the surface impact the overall electronic environment of the nanocrystal and is exacerbated by the presence of Peirels distortions. The Peirels distortions lower the overall energy, reducing the number of states near the Fermi energy which reduces the number of states available for optical transitions. These results correlate with recent experimental observations in which a Pb_xCl_y shell was shown to result in longer bond lengths and enhanced optical absorption.   

Nanoscale fluorescence microscopy maps cathodic pitting corrosion reactions on carbon steel

Fluorescent molecules that are sensitive to pH, metal ions, or electron transfer have recently been studied as real-time indicators of local metal corrosion. However, the fluorescence imaging systems used in previous studies were diffraction-limited and thus did not provide information below the single-micron scale size of a corrosion pit. We employ a state-of-the-art fluorescence microscope and single-molecule localization microscopy (SMLM) to access the nanoscale positions of the corrosion-sensitive fluorescent molecule resazurin within one corrosion pit. We expose A109 carbon steel to a chloride solution containing resazurin. Resazurin will accept electrons and become strongly fluorescent resorufin. We then map the positions of these fluorescence "turn-on" events relative to a corrosion pit. We complement our SMLM with reflection microscopy, scanning electron microscopy energy dispersive spectroscopy (SEM-EDS), and optical profilometry to correlate traditional methods of assessing pitting corrosion rates with our new technique. Our fluorescence microscopy quantifies the rate of electron transfer to the cathodic site within metastable pits that form in tens of minutes. The reaction rates observed by fluorescence correlates to sites with 30% increased oxygen content via SEM-EDS, depths of 0.5 um via profilometry, and 50% increases in pit diameter from reflection microscopy. Our work enables real-time observation of the cathodic pitting corrosion reaction rate on the nanoscale. In the future, this technique will allow us to examine how anode and cathode corrosion reactions are coupled on the nanoscale and test different proposed reaction pathways for pitting corrosion.   

Simultaneous spectroscopic pH detection and single-molecule, super-resolution fluorescence microscopy of corrosion in situ

The ability to acquire local pH information is key to understanding corrosion processes, as detection of areas with low pH can inform where active corrosion reactions are taking place. Typical methods to sense pH introduce larger, bulk scale pH sensors or use electrochemical techniques which add external factors that can influence the corrosion process. Here we probe the pH of active corrosion sights in situ by adding spectral analysis to fluorescence microscopy of nM concentrations of fluorophores that spectrally respond to corrosion reactions. Single-molecule, super-resolution microscopy is supplemented with the use of a diffraction grating to allow simultaneous measurement of single-molecule activity and the individual spectra of each molecule. While probing corrosion reduction reactions via the turn-on fluorophores resazurin and resorufin, their spectra can shift due to localized changes in pH, which can be observed in real time. We measure localized pH drops near active corrosion pits identified via super-resolution microscopy to 500 nm spatial and 20 nm spectral resolutions. These results show that it is possible to obtain localized pH information alongside simultaneous single-molecule, super-resolution microscopy of active corrosion pits in situ. In addition, correlation between pit localization via pH sensing and super-resolution microscopy techniques using the same fluorophores lends support to both techniques' ability to detect active single corrosion pits.   

SrₓBa₁₋ₓFeO₃₋ᵧ as an Improved Oxygen Carrier for Chemical Looping Air Separation: A Computational and Experimental Study

Chemical looping air separation (CLAS) is a promising method to generate pure carbon-dioxide from fuel combustion with a pure oxygen stream, which is produced through the capture, and targeted release, of oxygen from the atmosphere using a solid oxide carrier. The performance of this process depends on the redox characteristics of the oxide carrier. Using experimental oxygen-temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA), we show that Sr₀․₂₅Ba₀․₇₅FeO₃ has improved oxygen storage capacity (OSC), oxidation, and reduction kinetics over pristine SrFeO₃ at temperatures ranging from 300-500 °C. The redox energetics computed by using first-principles density functional theory (DFT) calculations also depict the measured trend establishing it as an important descriptor of the measured performance. The SrₓBa₁₋ₓFeO₃₋ᵧ depicts a Sr-substitution and oxygen stoichiometry induced structural phase transition from hexagonal with higher OSC at low temperatures up to T=400 °C to pseudo-cubic phase. We also identify the mechanism leading to this structural phase transition by performing electronic and vibrational structural calculations.