AVS2012 Session AS+BI-TuA: Surface Analysis of Materials Using Vibrational Techniques (2:00-3:20 pm)/ Multi-Technique Analysis (4:00-6:00 pm)

This work was supported in part by DOE grant DE-FG02-07ER15842

[1] Kim et al., Langmuir 27, 11650 (2011)

[2] Alfonso J. Phys. Chem. C 115, 17077 (2011)

Recent experiments have successfully synthetized MoS_X nanostructures in a controlled manner by evaporating Mo adatoms on the copper sulfide monolayer that forms on Cu(111) upon sulfur preloading[1,2]. Based on STM observations and total-energy calculations based on density functional theory, including ab intio van-der-Waals interactions, several structures for MoS_X/Cu(111) have been proposed. In this study, we investigate the plausibility of those structures and provide elements for further experimental substantiation or refutation. Namely, we perform density-functional-theory calculations (also including ab intio van-der-Waals interactions) of the total energy and the vibrational spectrum of the proposed structure to (1) attest their dynamical stability; (2) compare their thermodynamic stability as obtained from the total free energy; and (3) provide the vibrational frequencies that uniquely fingerprint the proposed structures.

[1] Kim et al., Langmuir 27, 11650 (2011)

[2] Le et al., PRB 85, 075429 (2012)

This work was supported in part by DOE grant DE-FG02-07ER15842

Chemical characterization of inorganic particles becomes more difficult as the particle sizes decrease. For application areas ranging from semiconductor failure analysis to nanotoxicology, the distinct chemical signatures of both the surface and bulk of particles can provide insight into system mechanisms and behavior. New methods that aim to explore the surface chemistry of inorganic nanoparticles for both their elemental and organic overlayer signatures will be presented. Specifically, the “static” nature of time-of-flight-secondary ion mass spectrometry is used to provide mass spectral characterization at the very surfaces and sub-surfaces of well-prepared (via drop-on-demand inkjet printing) and well-characterized (via scanning transmission electron microscopy and ultraviolet-visible spectroscopy) nanoparticle aggregates. This information can potentially be combined with full aggregate analysis using more elementally sensitive dynamic SIMS instrumentation once target species are identified with ToF-SIMS. Both sets of SIMS data can be used to obtain a chemical distribution of signals throughout the particle depths. Additionally, the question of whether the centers of inorganic nanoparticle aggregates are chemically similar to the overall aggregate surfaces will be explored.

Focused ion beams are routinely used for site-specific specimen preparation, nanopatterning, and analysis. It is important to know whether the primary ion beam is present outside the region targeted for ion beam modification. A previous report showed that > 1E12 atoms/cm² of Ga was detected up to several millimeters away from a focused ion beam (FIB) milled feature [1]. In this work, we reproduce this earlier report using a blind study of 2 different state-of-the-art Ga-FIB columns. Each column was used to FIB mill a 100 μm x 100 μm square into a (100) Si wafer at 30 keV with a nominal beam current of 20 nA at constant dose. Time of flight secondary ion mass spectrometry (TOF SIMS) was used to measure Ga depth profiles and Ga surface concentration at a distance up to 6.5 mm from the FIB milled square. In column “A,” > 1E12 atoms/cm² of Ga was detected up to ~ 5 mm from the FIB milled square. Column “B” showed considerably less Ga with > 1E12 atoms/cm² detected within ~ 250 μm from the FIB milled square. The depth profiles show that the Ga concentration was fairly uniform to a depth of ~ 2 nm from the surface for column “A” and ~ 1 nm into the surface for column “B”. Using SRIM [2] simulations we determine that these implantation depths correspond to an ion energy < 500 eV. The consequences of the presence of Ga at long distances from desired FIB milled features will be discussed.

[1] U. Muehle, R. Gaertner, J. Steinhoff, W. Zahn, “Characterisation of Ga-distribution on a silicon wafer

after inline FIB-preparation using inline ToFSIMS,” M. Luysberg, K. Tillmann, T. Weirich (Eds.): EMC 2008, Vol. 1: Instrumentation and Methods, pp. 749–750, DOI: 10.1007/978-3-540-85156-1_375, © Springer-Verlag Berlin Heidelberg 2008

[2] www.srim.org

Extensive surface analysis of available gold nanoparticles (AuNPs) is crucial to understand how their production and functionalization affects their final properties. This information is needed to improve the performance of engineered nanoparticles in research and commercial applications. Ethylene glycol functionality is desirable owing to the benefits such as the reduction of protein adhesion which if not properly controlled can lead to activation of an immune response and/or clearance.

In this work AuNPs ~14nm and ~40nm in diameter are synthesized and functionalized with 1-undacanethiol (HS-CH2)₁₁ terminated with either (OEG)₄OH or (OEG)₄CH₃. The AuNPs were characterized with transmission electron microscopy (TEM), time of flight secondary ion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and low energy ion scattering (LEIS). These studies provided both qualitative and quantitative information about the functionalization of the AuNPs with an OEG containing monolayer.

TEM showed the 14nm AuNPs had a narrower size distribution and more spherical shape than the 40nm AuNPs. ToF-SIMS clearly differentiates the two SAMs based on the C₃H₇O⁺ peak attributed to the CH₃ terminated SAM. Angle-resolved XPS high-resolution C1s spectra from flat gold samples at photoelectron take-off angles of 0˚, 55˚ and 75˚ from the surface normal shows an increase in the ether component and reduction in CH with an increase in take-off angle. The changes in these values are comparable for both SAMs. This illustrates the increased presence of ethylene glycol monomers in the outer surface region and shows little difference between the two types of terminal functional groups. The 40 nm AuNPs show a sightly greater surface OEG concentration than 14 nm AuNPs, possibly indicating a more vertically oriented SAM on the 40 nm AuNPs. FTIR indicates similar crystalline CH2 backbones for all samples, however it appears the structure of OEG head groups are less crystalline on the 14nm AuNPs. This likely results in thicker and/or higher density SAMs on the 40 nm AuNPs compared to the 14nm AuNPs. This is consistent with the nearly identical XPS determined surface elemental compositions determined for OEG SAMs on the two different sized AuNPs. This is contrary to previously XPS results observed for AuNPs functionalized with COOH SAMs [1].

1. Techane, S.D., L.J. Gamble, and D.G. Castner, Multi-technique Characterization of Self-assembled Carboxylic Acid Terminated Alkanethiol Monolayers on Nanoparticle and Flat Gold Surfaces. J Phys Chem C Nanomater Interfaces, 2011. 115(19): p. 9432-9441.