Search for: All records

Cetyltrimethylammonium bromide (CTAB) has been used to enhance the selectivity of CO2 electrochemical reduction. Traditionally, this selectivity was attributed to repulsion of water molecules due to a CTAB self-assembled monolayer, which forms under negative potential and disassembles at positive voltage due to electrostatic repulsions. In this report, using in operando interface sensitivity sum frequency generation spectroscopy, we investigated the self-assembly behavior of CTAB across a broad electrochemical potential range. We observed that CTAB molecules form a stable monolayer at the Stern layer over the entire potential scan, even when the electrodes are positively charged. Rather than disassembling, the CTAB molecules reorient themselves to balance the electrostatic interactions and the non-covalent hydrophobic effects, the latter being the primary driving force maintaining the monolayer at a positive potential. This finding contrasts the traditional view that CTAB monolayers are absent when the electrodes are positively charged, indicating a stable and ordered monolayer with respect to the electrostatic repulsions at liquid/electrode interfaces. The balance between non-covalent and electrostatic interactions offers a facile and reversible electrochemical method to control the local environment and dominating interactions at the Stern layer of the electrode surface, thus providing a means for engineering a micro-electrochemical environment. 

Becauseof thehighdielectricstrengthofwater, it isextremelydifficult todischargeplasmainacontrollablewayin the aqueous phase. By using lithographically defined electrodes andmetal/dielectric nanoparticles, we create electric field enhancementthatenablesplasmadischargeinliquidelectrolytesatsignificantlyreducedappliedvoltages.Here,weusehighvoltage (10−30kV)nanosecondpulse(20ns)dischargestogenerateatransientplasmaintheaqueousphase.Anelectrodegeometrywitha radiusofcurvatureofapproximately10μm,agapdistanceof300μm,andanestimatedfieldstrengthof5×106V/cmresultedina reductionintheplasmadischargethresholdfrom28to23kV.Asecondstructurehadaradiusofcurvatureofaround5μmanda gapdistanceof100μmhadanestimatedfieldstrengthof9×106V/cmbutdidnotperformaswellasthelargergapelectrodes. Addinggoldnanoparticles(20nmdiameter) insolutionfurther reducedthethresholdforplasmadischargeto17kVduetothe electricfieldenhancementatthewater/goldinterface,withanestimatedE-fieldenhancementof4×.Addingaluminananoparticles decoratedwithPtreducedtheplasmadischargethresholdto14kV. Inthisscenario, theemergenceofatriplepointatthejuncture ofalumina,Pt,andwaterresultsinthecoexistenceofthreedistinctdielectricconstantsatasingularlocation.Thisleadstoanotable concentrationof electric field, effectively aiding in the initiationof plasma discharge at a reduced voltage. To gain amore comprehensive and detailed understanding of the electric field enhancement mechanism, we performed rigorous numerical simulations.Thesesimulationsprovidevaluableinsights intotheintricateinterplaybetweenthelithographicallydefinedelectrodes, thenanoparticles, andthe resultingelectricfielddistribution, enablingus toextract crucial informationandoptimize thedesign parameters forenhancedperformance. 

Understanding and characterizing the intrinsic properties of charge carrier transport across the interfaces in van der Waals heterostructures is critical to their applications in modern electronics, thermoelectrics, and optoelectronics. However, there are very few published cross-plane resistivity measurements of thin samples because these inherently 2-probe measurements must be corrected for contact and lead resistances. Here, we present a method to extract contact resistances and metal lead resistances by fitting the width dependence of the contact end voltages of top and bottom electrodes of different linewidths to a model based on current crowding. These contributions are then subtracted from the total 2-probe cross-plane resistance to obtain the cross-plane resistance of the material itself without needing multiple devices and/or etching steps. This approach was used to measure cross-plane resistivities of a (PbSe)1(VSe2)1 heterostructure containing alternating layers of PbSe and VSe2 with random in-plane rotational disorder. Several samples measured exhibited a 4 order of magnitude difference between cross-plane and in-plane resistivities over the 6–300 K temperature range. We also reported the first observation of charge density wave transition in the cross-plane transport of (PbSe)1(VSe2)1 heterostructure. The device fabrication process is fully lift-off compatible, and the method developed enables the straightforward measurement of the resistivity anisotropy of most thin film materials with nm thicknesses. 

We demonstrate the use of nanosecond pulse transient plasma (NPTP) to improve the control (and acceleration) of the combustion of solid rocket propellants. Here, we fabricate end-burning propellant samples (i.e., grains) with a co-axial center wire electrode using hydroxyl terminated polybutadiene (HTPB), isodecyl pelargonate (IDP), modified diphenyl diisocyanate (MDI), and ammonium perchlorate (AP) as the fuel, plasticizer, curative, and oxidizer, respectively. High voltage (20 kV) nanosecond pulses (20 nsec) produce a streamer discharge that provides electronic throttling of the solid rocket propellant. These studies are carried out over a wide range of oxidizer mass fractions, including those considered insensitive munitions (IM). In addition, real time imaging is performed characterizing the plasma-formation, evolution of the ignition process, and plasma enhanced flamefuel coupling. We believe the plasma-based mechanisms of enhancement are 3-fold: 1.) The plasma provides highly energetic electrons that drive new chemical reaction pathways via highly reactive atomic species such as H, O, and Cl, 2.) The plasma sputters chunks of the solid fuel material up into the flame where it is combusted, producing an agitated flame profile. 3.) The plasma provides increased turbulence and multi-scale mixing due to hydrodynamic effects (i.e., ionic winds), which further improves the combustion process. Having electronic control of the burn rate introduces the ability to “throttle” solid rocket motors and introduce new flight profile options beyond a pre-selected profile such as the typical boost-sustain profile. While we are unable to quantify the burn rate or thrust from these relatively simple observations, we observe clear evidence of the effect of the plasma on the combustion of these solid rocket fuels even at high oxidizer content. 

We have used surface plasmon resonant metal gratings to induce and probe the dielectric response (i.e., electro-optic modulation) of ionic liquids (ILs) at electrode interfaces. Here, the cross-plane electric field at the electrode surface modulates the refractive index of the IL due to the Pockels effect. This is observed as a shift in the resonant angle of the grating (i.e., Δϕ), which can be related to the change in the local index of refraction of the electrolyte (i.e., Δnlocal). The reflection modulation of the IL is compared against a polar (D2O) and a non-polar solvent (benzene) to confirm the electro-optic origin of resonance shift. The electrostatic accumulation of ions from the IL induces local index changes to the gratings over the extent of electrical double layer (EDL) thickness. Finite difference time domain simulations are used to relate the observed shifts in the plasmon resonance and change in reflection to the change in the local index of refraction of the electrolyte and the thickness of the EDL. Simultaneously using the wavelength and intensity shift of the resonance enables us to determine both the effective thickness and Δn of the double layer. We believe that this technique can be used more broadly, allowing the dynamics associated with the potential-induced ordering and rearrangement of ionic species in electrode–solution interfaces.