Xiao-Fei Yu

Improving the Molecular Ion Signal Intensity for In Situ Liquid SIMS Analysis

AbstractIn situ liquid secondary ion mass spectrometry (SIMS) enabled by system for analysis at the liquid vacuum interface (SALVI) has proven to be a promising new tool to provide molecular information at solid–liquid and liquid–vacuum interfaces. However, the initial data showed that useful signals in positive ion spectra are too weak to be meaningful in most cases. In addition, it is difficult to obtain strong negative molecular ion signals when m/z>200. These two drawbacks have been the biggest obstacle towards practical use of this new analytical approach. In this study, we report that strong and reliable positive and negative molecular signals are achievable after optimizing the SIMS experimental conditions. Four model systems, including a 1,8-diazabicycloundec-7-ene (DBU)-base switchable ionic liquid, a live Shewanella oneidensis biofilm, a hydrated mammalian epithelia cell, and an electrolyte popularly used in Li ion batteries were studied. A signal enhancement of about two orders of magnitude was obtained in comparison with non-optimized conditions. Therefore, molecular ion signal intensity has become very acceptable for use of in situ liquid SIMS to study solid–liquid and liquid–vacuum interfaces. Graphical Abstractᅟ

Visible and near-infrared planar waveguide structure of polycrystalline zinc sulfide from C ions implantation.

We report the fabrication of a planar waveguide in polycrystalline zinc sulfide by 6.0 MeV C ions implantation with a fluence of 5 × 10¹⁴ ion/cm² at room temperature. The near-field light intensity profiles in the visible and near-infrared bands are measured by the end-face coupling method with different laser sources. Investigation of the Raman spectra demonstrates that the microstructure of the polycrystalline zinc sulfide has no significant change after C ion implantation. The absorption spectra show that the implantation processes have no influence on the visible and infrared bands.

Low Propagation Loss of Single-Mode Planar Waveguides on MgF

The single-mode planar waveguides were fabricated on MgF2 crystals by 6.0 MeV carbon ion implantation with the fluence of 1 × 1015 ions/cm2. The guiding modes were measured by the prism-coupling method at the wavelength of 632.8 nm. The near-field intensity distributions were obtained by the end-face coupling method, and the simulation of the light propagation process was performed simultaneously for comparison. The thermal stability was investigated by annealing treatment at different temperatures ranging from 260 °C to 400 °C in air. After suitable annealing, the minimum propagation loss of the waveguide can be reduced to 0.4 dB/cm.

_{\bf 2}

Ion-solvent interactions in nonaqueous electrolytes are of fundamental interest and practical importance, yet debates regarding ion preferential solvation and coordination numbers persist. In this work, in situ liquid SIMS was used to examine ion-solvent interactions in three representative electrolytes, i.e., lithium hexafluorophosphate (LiPF6) at 1.0 M in ethylene carbonate (EC)-dimethyl carbonate (DMC) and lithium bis(fluorosulfonyl)imide (LiFSI) at both low (1.0 M) and high (4.0 M) concentrations in 1,2-dimethoxyethane (DME). In the positive ion mode, solid molecular evidence strongly supports the preferential solvation of Li+ by EC. Besides, from the negative spectra, we also found that PF6- forms association with EC, which has been neglected by previous studies due to the relatively weak interaction. In both LiFSI in DME electrolytes, however, no evidence shows that FSI- is associated with DME. Furthermore, strong salt ion cluster signals were observed in the 1.0 M LiPF6 in EC-DMC electrolyte, suggesting that a significant amount of Li+ ions stay in the vicinity of anions. In sharp comparison, weak ion cluster signals were detected in dilute LiFSI in DME electrolyte, suggesting most ions are well separated, in agreement with our molecular dynamics simulation results. These findings indicate that with virtues of little bias on detecting positive and negative ions and the capability of directly analyzing concentrated electrolytes, in situ liquid SIMS is a powerful tool that can provide key evidence for improved understanding on the ion-solvent interactions in nonaqueous electrolytes. Therefore, we anticipate wide applications of in situ liquid SIMS on investigations of various ion-solvent interactions in the near future.

Crystals

RATIONALE During in situ liquid secondary ion mass spectrometry (SIMS) analysis, the primary ion beam is normally scanned on a very small area to collect signals with high ion doses (1014 to 1016 ions/cm2 ). As a result, beam damage may become a concern when compared with the static limit of SIMS analysis, in which the dose is normally less than 1012 ions/cm2 . Therefore, a comparison of ion yields in in situ liquid SIMS analysis versus traditional static SIMS analysis of corresponding dry samples is of great interest. METHODS In this study, a dipalmitoylphosphatidylcholine (DPPC) liposome solution was used as a model system. Both liquid sample and dry sample were examined. Secondary ion yields using three primary ion species (Bi+ , Bi3+ and Bi3++ ) with various beam currents were investigated. RESULTS Usable ion yields for both positive and negative characteristic signals (including molecular ions and characteristic fragment ions) were achievable based on optimized experimental conditions for in situ liquid SIMS analysis. The ion yield of the key DPPC molecular ion was comparable to that of traditional static SIMS, and unexpected low fragmentation was observed. The flexible structure of the liquid plays an important role for these observations. CONCLUSIONS Therefore, beam damage may not be a concern in in situ liquid SIMS analysis if proper experimental conditions are used.

Investigation of Ion–Solvent Interactions in Nonaqueous Electrolytes Using in Situ Liquid SIMS

We report on the fabrication and optical properties of planar and channel waveguides in a CdS crystal using a C ion implantation technique combined with a standard photolithographic technique. The prism-coupling and the end-face coupling methods separately measure the guiding modes and the near-field intensity distribution of the light at 633 and 1539 nm. The refractive index profiles of the planar and channel waveguides are reconstructed at 633 and 1539 nm. The finite difference beam propagation method is used to simulate the guided mode profiles. There is excellent agreement between the measured and simulated modes at 633 and 1539 nm.

An Investigation of the Beam Damage Effect on In Situ Liquid SIMS Analysis

In this work, the optical properties of potassium titanyl phosphate (KTP) waveguides in the visible and near-infrared region are reported. The KTP waveguides were fabricated using 550 keV proton implantation at room temperature, and the refractive index profiles of the implanted region in the visible and near-infrared region were reconstructed. The profiles of the guided modes were measured through the end-face coupling method with both 632.8 and 1539 nm laser sources and then compared with the simulation results using the beam propagation method. Optical transmission and Raman spectra in the original substrate and waveguide active region were measured to study microstructural changes. The propagation loss of the TM0-mode at 632.8 nm was also measured.

Planar and Channel Waveguide Structures in CdS Crystals at 633 and 1539 nm

Bacterial biofilms are surface-associated communities that are vastly studied to understand their self-produced extracellular polymeric substances (EPS) and their roles in environmental microbiology. This study outlines a method to cultivate biofilm attachment to the System for Analysis at the Liquid Vacuum Interface (SALVI) and achieve in situ chemical mapping of a living biofilm by time-of-flight secondary ion mass spectrometry (ToF-SIMS). This is done through the culturing of bacteria both outside and within the SALVI channel with our specialized setup, as well as through optical imaging techniques to detect the biofilm presence and thickness before ToF-SIMS analysis. Our results show the characteristic peaks of the Shewanella biofilm in its natural hydrated state, highlighting upon its localized water cluster environment, as well as EPS fragments, which are drastically different from the same biofilms dehydrated state. These results demonstrate the breakthrough capability of SALVI that allows for in situ biofilm imaging with a vacuum-based chemical imaging instrument.

Visible and near-infrared optical properties of a proton-implanted KTP waveguide

We report the formation of two waveguide layers in a lithium niobate crystal by irradiation with swift heavy Kr ions with high (GeV) energies and ultralow fluences. The micro-Raman spectra are measured at different depths in the irradiated layer and show that the high electronic energy loss can cause lattice damage along the ion trajectory, while the nuclear energy loss causes damage at the end of the ion track. Two waveguide layers are formed by confinement with two barriers associated with decreases in the refractive index that are caused by electronic and nuclear energy losses, respectively.

In Situ Characterization of Shewanella oneidensis MR1 Biofilms by SALVI and ToF-SIMS

A planar waveguide structure in a chalcohalide glass was fabricated by dual-energy C ion implantation with energies of 5.5 and 6.0 MeV at fluences of 7.0×10¹⁴ and 8.0×10¹⁴ions cm⁻², respectively. A waveguide with a thickness of 5.9 μm was formed. SRIM 2013 was used to simulate the defect distribution fabricated by C ion implantation. Images of the polished end face of the C-implanted chalcohalide glass were measured with a metallographic microscope using reflected polarized light. The micro-Raman spectra were measured in air. The near-field intensity distributions were investigated at visible (633 nm) and near-infrared (1300, 1400, and 1539 nm) bands.