ToF-SIMS analysis of ultrathin films and their fragmentation patterns.

Organic thin films are of great interest due to their intriguing interfacial and functional properties, especially for device applications such as thin-film transistors and sensors. As their thickness approaches single nanometer thickness, characterization and interpretation of the extracted data become increasingly complex. In this study, plasma polymerization is used to construct ultrathin films that range in thickness from 1 to 20 nm, and time-of-flight secondary ion mass spectrometry coupled with principal component analysis is used to investigate the effects of film thickness on the resulting spectra. We demonstrate that for these cross-linked plasma polymers, at these thicknesses, the observed trends are different from those obtained from thicker films with lower degrees of cross-linking: contributions from ambient carbon contamination start to dominate the mass spectrum; cluster-induced nonlinear enhancement in secondary ion yield is no longer observed; extent of fragmentation is higher due to confinement of the primary ion energy; and the size of the primary ion source also affects fragmentation (e.g., Bi1 versus Bi5). These differences illustrate that care must be taken in choosing the correct primary ion source as well as in interpreting the data.

Ultraviolet/Ozone as a Tool To Control Grafting Density in Surface-Initiated Controlled-Radical Polymerizations via Ablation of Bromine.

We used an ultraviolet-ozone (UVO) cleaner to create substrates for atom-transfer radical polymerization (ATRP) with varying surface initiator coverage. We collected complementary time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) measurements to investigate the precise chemical origin of the variation in grafting density. At short exposure times, the atomic composition underwent minor changes except for the relative amount of bromine. At longer UVO exposure times, there is clear evidence of exposure-dependent surface initiator oxidation. We interpret these data as evidence of a bromine ablation process within the UVO cleaner, with additional oxidative modification of the rest of the surface. We then used these substrates to create a series of poly(methyl methacrylate) (PMMA) brushes varying in grafting density, demonstrating the utility of this tool for the control of polymer brush density. The measured brush grafting densities were correlated with the bromine concentration measured by both ToF-SIMS and XPS. XPS and brush thicknesses correlated strongly, following an exponential decay with a half-life of 18 ± 1 s.

After oxidation, zinc nanoparticles lose their ability to enhance responses to odorants.

Electrical responses of olfactory sensory neurons to odorants were examined in the presence of zinc nanoparticles of various sizes and degrees of oxidation. The zinc nanoparticles were prepared by the underwater electrical discharge method and analyzed by atomic force microscopy and X-ray photoelectron spectroscopy. Small (1.2 ± 0.3 nm) zinc nanoparticles significantly enhanced electrical responses of olfactory neurons to odorants. After oxidation, however, these small zinc nanoparticles were no longer capable of enhancing olfactory responses. Larger zinc oxide nanoparticles (15 nm and 70 nm) also did not modulate responses to odorants. Neither zinc nor zinc oxide nanoparticles produced olfactory responses when added without odorants. The enhancement of odorant responses by small zinc nanoparticles was explained by the creation of olfactory receptor dimers initiated by small zinc nanoparticles. The results of this work will clarify the mechanisms for the initial events in olfaction, as well as to provide new ways to alleviate anosmia related to the loss of olfactory receptors.

Vapor-Liquid-Solid Etch of Semiconductor Surface Channels by Running Gold Nanodroplets.

We show that Au nanoparticles spontaneously move across the (001) surface of InP, InAs, and GaP when heated in the presence of water vapor. As they move, the particles etch crystallographically aligned grooves into the surface. We show that this process is a negative analogue of the vapor-liquid-solid (VLS) growth of semiconductor nanowires: the semiconductor dissolves into the catalyst and reacts with water vapor at the catalyst surface to create volatile oxides, depleting the dissolved cations and anions and thus sustaining the dissolution process. This VLS etching process provides a new tool for directed assembly of structures with sublithographic dimensions, as small as a few nanometers in diameter. Au particles above 100 nm in size do not exhibit this process but remain stationary, with oxide accumulating around the particles.

Strategies for potential age dating of fingerprints through the diffusion of sebum molecules on a nonporous surface analyzed using time-of-flight secondary ion mass spectrometry.

Age dating of fingerprints could have a significant impact in forensic science, as it has the potential to facilitate the judicial process by assessing the relevance of a fingerprint found at a crime scene. However, no method currently exists that can reliably predict the age of a latent fingerprint. In this manuscript, time-of-flight secondary ion imaging mass spectrometry (TOF-SIMS) was used to measure the diffusivity of saturated fatty acid molecules from a fingerprint on a silicon wafer. It was found that their diffusion from relatively fresh fingerprints (t ≤ 96 h) could be modeled using an error function, with diffusivities (mm(2)/h) that followed a power function when plotted against molecular weight. The equation x = 0.02t(0.5) was obtained for palmitic acid that could be used to find its position in millimeters (where the concentration is 50% of its initial value or c0/2) as a function of time in hours. The results show that on a clean silicon substrate, the age of a fingerprint (t ≤ 96 h) could reliably be obtained through the extent of diffusion of palmitic acid.

Test Sample for the Spatially Resolved Quantification of Illicit Drugs on Fingerprints Using Imaging Mass Spectrometry.

A novel test sample for the spatially resolved quantification of illicit drugs on the surface of a fingerprint using time-of-flight secondary ion mass spectrometry (ToF-SIMS) and desorption electrospray ionization mass spectrometry (DESI-MS) was demonstrated. Calibration curves relating the signal intensity to the amount of drug deposited on the surface were generated from inkjet-printed arrays of cocaine, methamphetamine, and heroin with a deposited-mass ranging nominally from 10 pg to 50 ng per spot. These curves were used to construct concentration maps that visualized the spatial distribution of the drugs on top of a fingerprint, as well as being able to quantify the amount of drugs in a given area within the map. For the drugs on the fingerprint on silicon, ToF-SIMS showed great success, as it was able to generate concentration maps of all three drugs. On the fingerprint on paper, only the concentration map of cocaine could be constructed using ToF-SIMS and DESI-MS, as the signals of methamphetamine and heroin were completely suppressed by matrix and substrate effects. Spatially resolved quantification of illicit drugs using imaging mass spectrometry is possible, but the choice of substrates could significantly affect the results.

Designing high-performance PbS and PbSe nanocrystal electronic devices through stepwise, post-synthesis, colloidal atomic layer deposition.

We report a simple, solution-based, postsynthetic colloidal, atomic layer deposition (PS-cALD) process to engineer stepwise the surface stoichiometry and therefore the electronic properties of lead chalcogenide nanocrystal (NC) thin films integrated in devices. We found that unlike chalcogen-enriched NC surfaces that are structurally, optically, and electronically unstable, lead chloride treatment creates a well-passivated shell that stabilizes the NCs. Using PS-cALD of lead chalcogenide NC thin films we demonstrate high electron field-effect mobilities of ∼4.5 cm(2)/(V s).

Low-temperature plasma for compositional depth profiling of crosslinking organic multilayers: comparison with C60 and giant argon gas cluster sources.

RATIONALE: For organic electronics, device performance can be affected by interlayer diffusion across interfaces. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) can resolve buried structures with nanometer resolution, but instrument artifacts make this difficult. Low-temperature plasma (LTP) is suggested as a way to prepare artifact-free surfaces for accurate determination of chemical diffusion. METHODS: A model organic layer system consisting of three 1 nm delta layers of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) separated by three 30 nm layers of tris(8-hydroxyquinolinato)aluminum (Alq3) was used to evaluate the effectiveness of LTP etching for the preparation of crater edge surfaces for subsequent compositional depth profile analysis. This was compared with depth profiles obtained using an instrument equipped with an argon cluster sputter source. RESULTS: The quality of the depth profiles was determined by comparing the depth resolutions of the BCP delta layers. The full width at half maximum gave depth resolutions of 6.9 nm and 6.0 nm using LTP, and 6.2 nm and 5.8 nm using argon clusters. In comparison, the 1/e decay length of the trailing edge gave depth resolutions of 2.0 nm and 1.8 nm using LTP, and 3.5 nm and 3.4 nm using argon clusters. CONCLUSIONS: The comparison of the 1/e decay lengths showed that LTP can determine the thickness and composition of the buried structures without instrument artifacts. Although it does suffer from contaminant deposition, LTP was shown to be a viable option for preparing crater edges for a more accurate determination of buried structures.

Visualizing mass transport in desorption electrospray ionization using time-of-flight secondary ion mass spectrometry: a look at the geometric configuration of the spray.

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to visualize the transport of analyte molecules desorbed onto a silicon wafer collection substrate by desorption electrospray ionization (DESI). The effect of spray incidence angle, tip height, and probe distance on the concentration and the spatial distribution of desorbed analyte molecules were investigated with the objective of identifying DESI operational parameters that provide more reproducible results by achieving steady ion transmission and minimized material loss. An incidence angle between 25° and 35° with respect to the plane of the surface provided the best compromise between maximizing ambient MS signal and achieving the best reliability. Glancing incidence angles provided higher ambient MS signals through a tighter dispersion of the secondary droplet plume, but run-to-run variability of as much as 40%. On the other hand, steeper incidence angles led to a widening of the lateral dispersion of the secondary droplets and decreased analyte desorption. For all incidence angles, shorter probe distances were preferred since the resulting tighter dispersion of the secondary droplets produced higher ion transmission and therefore higher ambient MS signals. Tip height was found to correlate with the spot size (footprint) of the spray on the surface; changing the tip height from (1 to 2 to 3) mm changed the diameter of the spray impact area from (1.3, 1.8, to 2.1) mm, respectively. For shorter probe to MS inlet distances, larger tip heights increased the ambient MS signal due to increased analyte desorption while maintaining a tighter dispersion of the secondary droplet plume. Equally important to optimizing instrument configuration was the understanding that the deposition of a sample onto the surface resulted in a coffee ring, where the diameter was larger than the spot size of the spray. Higher tip heights may be preferred for a more consistent analyte response since all or a large fraction of the analyte could be sampled to reduce variability in ambient MS response. The study showed that ToF-SIMS can be used as a unique tool for characterizing the transport of desorbed analyte molecules for DESI, and potentially offers insight into new interface designs for improved transmission of analyte into the mass spectrometer.

Visualizing mass transport in desorption electrospray ionization using time-of-flight secondary ion mass spectrometry.

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to determine the effect of ambient probe incidence angle on the amount and direction of analyte molecules transported from the sample surface for desorption electrospray ionization (DESI). Incidence angle was critical to both the lateral dispersion and vertical take-off angles of analyte molecules desorbed from the surface; as the incidence angle was increased from 30° to 45° to 60° (relative to the sample surface), the lateral dispersion angle decreased from 79° to 71° to 62°, respectively, while the vertical take-off angle decreased dramatically from 12° to 6° to 4°, respectively. As for the amount of material transported, the ToF-SIMS normalized secondary ion intensity of the molecular ion (peak counts per total spectrum counts) showed a significant decrease in the signal when the incidence angle was made steeper, decreasing from 8.1 × 10(-3) to 4.2 × 10(-3) to 7.5 × 10(-4), respectively. The ambient mass spectrometer interfaced with DESI also showed a similar analyte response, where the intensity of the molecular ion decreased from 1.6 × 10(7) counts to 3.3 × 10(6) counts to 5.4 × 10(5) counts, respectively. Overall, a steeper incidence angle was characterized by smaller amount of material desorption and tighter dispersion in both lateral and vertical directions. The study showed how ToF-SIMS can be used as a unique tool for characterizing the transport of desorbed analyte molecules in ambient ionization mass spectrometry, potentially offering new interface designs for optimal analyte collection.

Extensive hydrogen incorporation is not necessary for superconductivity in topotactically reduced nickelates.

A key open question in the study of layered superconducting nickelate films is the role that hydrogen incorporation into the lattice plays in the appearance of the superconducting state. Due to the challenges of stabilizing highly crystalline square planar nickelate films, films are prepared by the deposition of a more stable parent compound which is then transformed into the target phase via a topotactic reaction with a strongly reducing agent such as CaH2. Recent studies, both experimental and theoretical, have introduced the possibility that the incorporation of hydrogen from the reducing agent into the nickelate lattice may be critical for the superconductivity. In this work, we use secondary ion mass spectrometry to examine superconducting La1-xXxNiO2 / SrTiO3 (X = Ca and Sr) and Nd6Ni5O12 / NdGaO3 films, along with non-superconducting NdNiO2 / SrTiO3 and (Nd,Sr)NiO2 / SrTiO3. We find no evidence for extensive hydrogen incorporation across a broad range of samples, including both superconducting and non-superconducting films. Theoretical calculations indicate that hydrogen incorporation is broadly energetically unfavorable in these systems, supporting our conclusion that extensive hydrogen incorporation is not generally required to achieve a superconducting state in layered square-planar nickelates.

TOF-SIMS 3D imaging of native and non-native species within HeLa cells.

In this study, a non-native chemical species, bromodeoxyuridine (BrdU), was imaged within single HeLa cells using time-of-flight secondary ion mass spectrometry (TOF-SIMS). z-corrected 3D images were reconstructed that accurately portray the distribution of intracellular BrdU as well as other intracellular structures. The BrdU was localized to the nucleus of cells, whereas structures composed of CxHyOz(-) species were located in bundles on the periphery of cells. The CxHyOz(-) subcellular features had a spatial resolution at or slightly below a micrometer (900 nm), as defined by the distance between the 16% and 84% intensities in a line scan across the edge of the features. Additionally, important parameters influencing the quality of the HeLa cell 3D images were investigated. Atomic force microscopy measurements revealed that the HeLa cells were sputtered at a rate of approximately 4 nm per 10(13) C60(+) ions/cm(2) at 10 keV and a 45° incident angle. Optimal 3D images were acquired using a Bi3(+) liquid metal ion gun operating in the simultaneous high mass and spatial resolution mode.

Bandlike transport in strongly coupled and doped quantum dot solids: a route to high-performance thin-film electronics.

We report bandlike transport in solution-deposited, CdSe QD thin-films with room temperature field-effect mobilities for electrons of 27 cm(2)/(V s). A concomitant shift and broadening in the QD solid optical absorption compared to that of dispersed samples is consistent with electron delocalization and measured electron mobilities. Annealing indium contacts allows for thermal diffusion and doping of the QD thin-films, shifting the Fermi energy, filling traps, and providing access to the bands. Temperature-dependent measurements show bandlike transport to 220 K on a SiO(2) gate insulator that is extended to 140 K by reducing the interface trap density using an Al(2)O(3)/SiO(2) gate insulator. The use of compact ligands and doping provides a pathway to high performance, solution-deposited QD electronics and optoelectronics.

Exploring the surface sensitivity of TOF-secondary ion mass spectrometry by measuring the implantation and sampling depths of Bi(n) and C60 ions in organic films.

The surface sensitivity of Bi(n)(q+) (n = 1, 3, 5, q = 1, 2) and C(60)(q+) (q = 1, 2) primary ions in static time-of-flight secondary ion mass spectrometry (TOF-SIMS) experiments were investigated for molecular trehalose and polymeric tetraglyme organic films. Parameters related to surface sensitivity (impact crater depth, implantation depth, and molecular escape depths) were measured. Under static TOF-SIMS conditions (primary ion doses of 1 × 10(12) ions/cm(2)), the 25 keV Bi(1)(+) primary ions were the most surface sensitive with a molecular escape depth of 1.8 nm for protein films with tetraglyme overlayers, but they had the deepest implantation depth (~18 and 26 nm in trehalose and tetraglyme films, respectively). The 20 keV C(60)(+2) primary ions were the second most surface sensitive with a slightly larger molecular escape depth of 2.3 nm. The most important factor that determined the surface sensitivity of the primary ion was its impact crater depth or the amount of surface erosion. The most surface sensitive primary ions, Bi(1)(+) and C(60)(+2), created impact craters with depths of 0.3 and 1.0 nm, respectively, in tetraglyme films. In contrast, Bi(5)(+2) primary ions created impact craters with a depth of 1.8 nm in tetraglyme films and were the least surface sensitive with a molecular escape depth of 4.7 nm.

Ambient low temperature plasma etching of polymer films for secondary ion mass spectrometry molecular depth profiling.

The feasibility of a low temperature plasma (LTP) probe as a way to prepare polymer bevel cross sections for secondary ion mass spectrometry (SIMS) applications was investigated. Poly(lactic acid) and poly(methyl methacrylate) films were etched using He LTP, and the resulting crater walls were depth profiled using time-of-flight secondary ion mass spectrometry (ToF-SIMS) to examine changes in chemistry over the depth of the film. ToF-SIMS results showed that while exposure to even 1 s of plasma resulted in integration of atmospheric nitrogen and contaminants to the newly exposed surface, the actual chemical modification to the polymer backbone was found to be chemistry-dependent. For PLA, sample modification was confined to the top 15 nm of the PLA surface regardless of plasma exposure dose, while measurable change was not seen for PMMA. The confinement of chemical modification to 15 nm or less of the top surface suggests that LTP can be used as a simple method to prepare cross sections or bevels of polymer thin films for subsequent analysis by surface-sensitive molecular depth profiling techniques such as SIMS, X-ray photoelectron spectroscopy (XPS), and other spatially resolved mass spectrometric techniques.

Engineering Magnetic Anisotropy and Emergent Multidirectional Soft Ferromagnetism in Ultrathin Freestanding LaMnO3 Films.

The combination of small coercive fields and weak magnetic anisotropy makes soft ferromagnetic films extremely useful for nanoscale devices that need to easily switch spin directions. However, soft ferromagnets are relatively rare, particularly in ultrathin films with thicknesses of a few nanometers or less. We have synthesized large-area, high-quality, ultrathin freestanding LaMnO3 films on Si and found unexpected soft ferromagnetism along both the in-plane and out-of-plane directions when the film thickness was reduced to 4 nm. We argue that the vanishing magnetic anisotropy between the two directions is a consequence of two coexisting magnetic easy axes in different atomic layers of the LaMnO3 film. Spectroscopy measurements reveal a change in Mn valence from 3+ in the film interior to approximately 2+ at the surfaces where considerable hydrogen infiltration occurs due to the water dissolving process. First-principles calculations show that protonation of LaMnO3 decreases the Mn valence and switches the magnetic easy axis from in-plane to out-of-plane as the Mn valence approaches 2+ from its 3+ bulk value. Our work demonstrates that ultrathin freestanding films can exhibit functional properties that are absent in homogeneous materials, concomitant with their convenient compatibility with Si-based devices.

Deep depth profiling using gas cluster secondary ion mass spectrometry: Micrometer topography development and effects on depth resolution.

Secondary ion mass spectrometry using the argon cluster primary ion beam enables molecular compositional depth profiling of organic thin films with minimal loss of chemical information or changes in sputter rate. However, for depth profiles of thicker organic films (> 10 µm of sputtered depth) we have observed the rapid formation of micron-scale topography in the shape of pillars that significantly affect both the linearity of the sputter yield and depth resolution. To minimize distortions in the 3D reconstruction of the sample due to this topography, a step-wise, staggered sample rotation was employed. By using polymer spheres embedded in an organic film, it was possible to measure the depth resolution at the film-sphere interface as a function of sputtered depth and observe when possible distortions in the 3D image occurred. In this way, it was possible to quantitatively measure the effect of micron-scale topography and sample rotation on the quality of the depth profile.

Visualizing shed skin cells in fingerprint residue using dark-field microscopy.

This proof-of-concept study shows that dark-field microscopy provides sufficient contrast for cell visualization in fingerprints with high sebum content. Although the application is limited to smooth surfaces that do not scatter light, such as polyethylene terephthalate (PET), it was able to measure the number of cells deposited within a fingerprint residue and the reduction in cell transfer with repeated skin contact. On a PET surface, at roughly 5 N of contact force, a typical finger transfers several hundred cells onto the surface. Over subsequent finger contacts onto a clean PET surface, this number decreased exponentially until a steady state was reached, which is characterized by the transfer of (78 ± 36) cells or (0.46 ± 0.21) cells/mm2 when normalized for fingerprint area. High uncertainty in cell transfer was due to: the highly variable nature of a human finger (where the number of loose cells varies from person to person and from day to day depending on what they touch) and difficulties in controlling the contact force and finger movement such as twisting during deposition (where twisting of the finger can expose a new patch of skin to the substrate, increasing the number of cell transfer). Plasma etching was also explored as an effective way to validate dark-field microscopy for cell counting. Although limited to inorganic substrates due to etching effects, exposing the fingerprint for less than 10 min can remove a majority of the sebum while keeping the cells intact for a before-and-after comparison using light microscopy.

Development of surface functionalization strategies for 3D-printed polystyrene constructs.

There is a growing interest in 3D printing to fabricate culture substrates; however, the surface properties of the scaffold remain pertinent to elicit targeted and expected cell responses. Traditional 2D polystyrene (PS) culture systems typically require surface functionalization (oxidation) to facilitate and encourage cell adhesion. Determining the surface properties which enhance protein adhesion from media and cellular extracellular matrix (ECM) production remains the first step to translating 2D PS systems to a 3D culture surface. Here we show that the presence of carbonyl groups to PS surfaces correlated well with successful adhesion of ECM proteins and sustaining ECM production of deposited human mesenchymal stem cells, if the surface has a water contact angle between 50° and 55°. Translation of these findings to custom-fabricated 3D PS scaffolds reveals carbonyl groups continued to enhance spreading and growth in 3D culture. Cumulatively, these data present a method for 3D printing PS and the design considerations required for understanding cell-material interactions. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B:2566-2578, 2019.

Electron Doping BaZrO3 via Topochemical Reduction.

We report the topochemical reduction of epitaxial thin films of the cubic perovskite BaZrO3. Reduction with calcium hydride yields n-type conductivity in the films, despite the wide band gap and low electron affinity of the parent material. X-ray diffraction studies show concurrent loss of out-of-plane texture with stronger reducing conditions. Temperature-dependent transport studies on reduced films show insulating behavior (decreasing resistivity with increasing temperature) with a combination of thermally activated and variable-range hopping transport mechanisms. Time-dependent conductivity studies show that the films are stable over short periods, with chemical changes over the course of weeks leading to an increase in electrical resistance. Neutron reflectivity and secondary ion mass spectrometry indicate that the source of the carriers is most likely hydrogen incorporated from the reducing agent occupying oxygen vacancies and/or interstitial sites. Our studies introduce topochemical reduction as a viable pathway to electron-dope and meta-stabilize low electron affinity and work function materials.