Caiyan Lu

Direct Observation of Ion Distributions near Electrodes in Ionic Polymer Actuators Containing Ionic Liquids

The recent boom of energy storage and conversion devices, exploiting ionic liquids (ILs) to enhance the performance, requires an in-depth understanding of this new class of electrolytes in device operation conditions. One central question critical to device performance is how the mobile ions accumulate near charged electrodes. Here, we present the excess ion depth profiles of ILs in ionomer membrane actuators (Aquivion/1-butyl-2,3-dimethylimidazolium chloride (BMMI-Cl), 27 μm thick), characterized directly by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) at liquid nitrogen temperature. Experimental results reveal that for the IL studied, cations and anions are accumulated at both electrodes. The large difference in the total volume occupied by the excess ions between the two electrodes cause the observed large bending actuation of the actuator. Hence we demonstrate that ToF-SIMS experiment provides great insights on the physics nature of ionic devices.

Molecular depth profiling of buried lipid bilayers using C(60)-secondary ion mass spectrometry.

An organic delta layer system made of alternating Langmuir-Blodgett multilayers of barium arachidate (AA) and barium dimyristoyl phosphatidate (DMPA) was constructed to elucidate the factors that control depth resolution in molecular depth profile experiments. More specifically, one or several bilayers of DMPA (4.4 nm) were embedded in relatively thick (51 to 105 nm) multilayer stacks of AA, resulting in a well-defined delta layer model system closely resembling a biological membrane. 3-D imaging time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profile analysis was performed on this system using a focused buckminsterfullerene (C(60)) cluster ion beam. The delta layer depth response function measured in these experiments exhibits similar features as those determined in inorganic depth profiling, namely an asymmetric shape with quasi-exponential leading and trailing edges and a central Gaussian peak. The effects of sample temperature, primary ion kinetic energy, and incident angle on the depth resolution were investigated. While the information depth of the acquired SIMS spectra was found to be temperature independent, the depth resolution was found to be significantly improved at low temperature. Ion induced mixing is proposed to be largely responsible for the broadening, rather than topography, as determined by atomic force microscopy (AFM); therefore, depth resolution can be optimized using lower kinetic energy, glancing angle, and liquid nitrogen temperature.

Molecular Depth Profiling by Wedged Crater Beveling

Time-of-flight secondary ion mass spectrometry and atomic force microscopy are employed to characterize a wedge-shaped crater eroded by a 40-keV C(60)(+) cluster ion beam on an organic film of Irganox 1010 doped with Irganox 3114 delta layers. From an examination of the resulting surface, the information about depth resolution, topography, and erosion rate can be obtained as a function of crater depth for every depth in a single experiment. It is shown that when measurements are performed at liquid nitrogen temperature, a constant erosion rate and reduced bombardment induced surface roughness is observed. At room temperature, however, the erosion rate drops by ∼(1)/(3) during the removal of the 400 nm Irganox film and the roughness gradually increased to from 1 nm to ∼4 nm. From SIMS lateral images of the beveled crater and AFM topography results, depth resolution was further improved by employing glancing angles of incidence and lower primary ion beam energy. Sub-10 nm depth resolution was observed under the optimized conditions on a routine basis. In general, we show that the wedge-crater beveling is an important tool for elucidating the factors that are important for molecular depth profiling experiments.

Label-free optical detection of peptide synthesis on a porous silicon scaffold/sensor.

Mesoporous porous silicon (PSi) microcavity sensors are used to conduct conventional solid-phase peptide synthesis. The sensor optical response provides a convenient means to monitor the synthesis reaction in a nondestructive manner. Measurements indicate that peptide synthesis occurs only when the PSi sensor/scaffold is amine-terminated using, for example, the amino silane or deprotected acid-labile Rink linker. Equivalent coupling efficiencies of the first amino acid to both amine terminations are observed. Kinetic studies indicate that coupling reactions are 90% complete in 1 h. Quantitative analysis of the optical response following the synthesis of homo-oligopeptides (4-mers) suggests that coupling efficiencies and/or optical thickness changes depend on the peptide length. The synthesis of the cell adhesive oligopeptide (RGD) was monitored by the optical sensor response and validated by the cell culture of primary dermal fibroblasts. Secondary ion mass spectrometry (SIMS) analysis successfully detected peptide on the silicon wafer adjacent to the PSi. Our findings suggest the potential to exploit the high surface area, efficient coupling, and intrinsic optical detection properties of PSi for label-free high-throughput screening.

Cluster Secondary Ion Mass Spectrometry and the Temperature Dependence of Molecular Depth Profiles

The quality of molecular depth profiles created by erosion of organic materials by cluster ion beams exhibits a strong dependence upon temperature. To elucidate the fundamental nature of this dependence, we employ the Irganox 3114/1010 organic delta-layer reference material as a model system. This delta-layer system is interrogated using a 40 keV C(60)(+) primary ion beam. Parameters associated with the depth profile such as depth resolution, uniformity of sputtering yield, and topography are evaluated between 90 and 300 K using a unique wedge-crater beveling strategy that allows these parameters to be determined as a function of erosion depth from atomic force microscope (AFM) measurements. The results show that the erosion rate calibration performed using the known Δ-layer depth in connection with the fluence needed to reach the peak of the corresponding secondary ion mass spectrometry (SIMS) signal response is misleading. Moreover, we show that the degradation of depth resolution is linked to a decrease of the average erosion rate and the buildup of surface topography in a thermally activated manner. This underlying process starts to influence the depth profile above a threshold temperature between 210 and 250 K for the system studied here. Below that threshold, the process is inhibited and steady-state conditions are reached with constant erosion rate, depth resolution, and molecular secondary ion signals from both the matrix and the Δ-layers. In particular, the results indicate that further reduction of the temperature below 90 K does not lead to further improvement of the depth profile. Above the threshold, the process becomes stronger at higher temperature, leading to an immediate decrease of the molecular secondary ion signals. This signal decay is most pronounced for the highest m/z ions but is less for the smaller m/z ions, indicating a shift toward small fragments by accumulation of chemical damage. The erosion rate decay and surface roughness buildup, on the other hand, exhibit a rather sudden delayed onset after erosion of about 150 nm, indicating that a certain damage level must be reached in order to influence the erosion dynamics. Only after that onset does the depth resolution become compromised, indicating that the temperature reduction does not significantly influence parameters like ion-beam mixing or the altered-layer thickness. In general, the wedge-crater beveling protocol is shown to provide a powerful basis for increased understanding of the fundamental factors that affect the important parameters associated with molecular depth profiling.

Ionization effects in molecular depth profiling of trehalose films using buckminsterfullerene (C60) cluster ions

Salts play a mysterious role in desorption mass spectrometry, especially in biological samples.[1] We used trehalose films doped with a peptide as a well defined model system to investigate the ionization effects in organic molecular depth profiling. Sodium salts at 1% level were added into the solution used to produce the trehalose films, and depth profiles were obtained with a C60 ion source. The results show that the protonated molecular ion signal from the peptide and the quasimolecular ion signal of trehalose are significantly suppressed by the addition of salts, whereas the signals representing salt clusters and salt adducts of trehalose are formed in both positive and negative modes. The formation of protonated molecular ions is found to correlate with the ratio between protonated and bare water ions, suggesting that the latter can be used as an indicator for the accumulation of protons liberated by the ion bombardment. In experiments where no salt was added, it is shown that the surface variation of the protonated molecular ion signal strongly depends upon the water content of the trehalose film.

Fundamental studies of molecular depth profiling using organic delta layers as model systems

Alternating Langmuir-Blodgett multilayers of barium arachidate (AA) and barium dimyristoyl phosphatidate (DMPA) were used to elucidate the factors that control depth resolution in molecular depth profiling experiments. More specifically, thin (4.4 nm) layers of DMPA were embedded in relatively thick (~50 nm) multilayer stacks of AA, resulting in a well-defined delta-layer model system closely resembling a biological membrane. This system was subjected to a three-dimensional imaging depth profile analysis using a focused buckminsterfullerene (C60) cluster ion beam. The depth response function measured in these experiments exhibits similar features as those determined in inorganic depth profiling: namely, an asymmetric shape with quasi-exponential leading and trailing edges and a central Gaussian peak. The magnitude of the corresponding characteristic rise and decay lengths is found to be 5 and 16 nm, respectively, while the total half width of the response function characterizing the apparent depth resolution was about 29 nm. Ion-induced mixing is proposed to be largely responsible for the broadening, rather than topography, as determined by atomic force microscopy.

Ion distribution in ionic electroactive polymer actuators

Ionic electroactive polymer (i-EAP) actuators with large strain and low operation voltage are extremely attractive for applications such as MEMS and smart materials and systems. In-depth understanding of the ion transport and storage under electrical stimulus is crucial for optimizing the actuator performance. In this study, we show the dominances of ion diffusion charge and we perform direct measurements of the steady state ion distribution in charged and frozen actuators by using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). High temperature actuators that consist Aquivion ionomer membrane and high melting temperature ionic liquid 1-butyl-2,3-dimethylimidazolium chloride (BMMI-Cl]) served in this study. Electrical impedance, I-V characteristics, and potential step charging of the actuator are characterized at 25°C and 100°C. The conductivity of the actuator is 0.3mS/cm at 100°C and 2.9μS/cm at 25°C, respectively. The electrochemical window of the device is 3V and a 2mm tip displacement is observed under 2.5V 0.2Hz at 100°C. A semi-quantitative depth profile of the relative ion concentration in charged and frozen actuators is measured by ToF-SIMS. The result shows that, unlike semiconductors, ions do not deplete from the electrodes with same signs. Due to a strong cluster effect between the ions, Cl- and BMMI+ accumulate near both cathode and anode. Furthermore, the profile indicates that the ion size difference causes the BMMI+ space charge layers (~6um) much thicker than those of Cl- (~0.5um).