PacSurf2016 Session NM-WeM: Nanocharacterization

The electronic structure of heterointerfaces is known to determine the performance of semiconductor devices. This led to the series rigorous studies on interfaces, i.e., Fermi level pinning or Schottky barrier formation, for decades. However, such studies were not easy and hence the results not conclusive, due to factors such as the reproducibility in preparing identical interfaces or limitation in the availability of tools that could characterize the interfaces on an atomic scale. Kelvin probe force microscopy (KFM) is an atomic force microscopy-based technique that provides us with the opportunity to tackle such problems by mapping the surface potential and topograph on nanometer scale simultaneously. Here, we report the application of KFM for the study of 1) InAs quantum dots (QDs) grown on GaAs(001) and 2) Si heterojunction (SHJ) solar cell structure.

We previously reported that the I-V characteristics of the InAs QD on GaAs(001) vary as a function of QD size, and that the interface may be ohmic-like when the QD diameter is about 100 nm while it exhibited Schottky diode-like behavior when the diameter is about 20 nm [1]. The phenomenon was naively interpreted in terms of a balance between Fermi level pinning in the conduction band of the InAs eventually overcoming that at mid gap for GaAs surrounding the InAs QD as its size is enlarged. Here, we performed KFM on QDs whose diameters are around 20-30 nm. We observe that a ring-like dip of surface potential is formed at the peripheral of the QDs (Fig.1) in agreement with previous work [2]. We converted the surface potential map into band alignment by using the known electron affinity of the bulk materials, as a function of depth from the surface. We find that the Fermi level of the dip region lies in the conduction band suggesting the presence of a source of electric contact, while the Fermi level in the wetting layer and the majority of the QD lies in the band gap (Fig.2).

We also performed KFM measurements on the cleaved interfaces of SHJ solar cell and related structures. The SHJ structure we employed is ITO/p-aSi/i-aSi/n-cSi(001). We observe that while the workfunction, and thus the band alignment, of i-aSi/n-cSi(001) interface is abrupt, p-aSi/i-aSi is not, and also that ITO/p-aSi interface may be blurred as a result of interdiffusion (Fig.3). This is in strong contrast from the reference sample Ag/Si(001) from which we observe a nearly ideal Schottky-like band alignment.

We will discuss the possible mechanisms and implication of these results.

[1] I. Tanaka, I. Kamiya, H. Sakaki, et al., Appl. Phys. Lett. 74, 844 (1999).

[2] S. Ono and T. Takahashi, Jpn. J. Appl. Phys. 44, 6213 (2005).

Liquid phase synthesis is a powerful method for the formation of uniform sized nanoparticles and nanoparticles with a faceted morphology. General strategies for the formation of nanoparticles and through chemical synthesis will be outlined. The results presented will include the formation of catalytic metals such as gold core palladium shell nanoparticles, below and branched nanostructures of gold, palladium and ruthenium. The growth mechanism of how the particles form will also be presented along with HRTEM observations. Biomedical and catalytic applications will be discussed.

We are using soft X-ray scanning transmission X-ray microscopy (STXM) [1] to investigate Cu thin film and dendritic electrochemical growth on a Au surface under variable pH conditions. Electrochemical reduction of Cu(II) under acid conditions is commonly used to deposit copper for integrated circuit interconnects, and in many other technologically important applications of thin films and nanoscale Cu metal. Improved understanding of the fundamental steps of the reaction and how to control undesirable phenomena such as dendritic growth is needed to further optimize this process. The reduction of Cu(II) to Cu(0) proceeds via a Cu(I) intermediate, which can be detected under weakly acidic to neutral conditions. We have developed a novel device for in situ STXM studies of electrochemical reactions which can operate under flow or static conditions [2]. The ability to change electrolyte during an experiment is being used to better understand the role of Cu(I) species in the deposition process. Spectromicroscopy at the Cu 2p and O 1s edges is used to analyze initial and final states, follow the process in situ, and search for intermediate species. The apparatus and techniques for spectro-electrochemical-microscopy will be described and results will be presented from deposition & stripping of copper from CuCl₂(aq) and CuSO₄(aq) electrolytic solutions as a function of pH.

STXM performed using BL10ID1 at CLS and BL 5.3.2.2 at ALS . Research supported by NSERC and the Catalyst Research for Polymer Electrolyte Fuel Cells (CaRPE-FC) network.

References

[1] A.P. Hitchcock, Soft X-ray Imaging and Spectromicroscopy in Handbook on Nanoscopy, eds.

G. Van Tendeloo, D. Van Dyck and S. J. Pennycook 2012. (Wiley)

[2] A.P. Hitchcock, Z. Qin, S.M. Rosendahl, V. Lee , M. Reynolds and H. Hosseinkhannazer, Electro-deposition of Cu studied with in situ electrochemical scanning transmission x-ray microscopy Am. Inst. Phys. Conf Series 1696 (2016) 02003 (1-5)

Polymer electrolyte membrane (PEM) fuel cell performance and materials degradation, particularly associated with the cathode catalyst layer (CCL), can be directly attributed to the structure and chemistry of individual material components, as well as their uniformity/homogeneity within a CCL. The individual material constituents used to form the CCL within the membrane electrode assemblies (MEAs), e.g., the electrocatalyst (Pt or Pt-alloys), catalyst support (C-based), and ionomer films (proton conductor), and especially the critical interfaces that are formed between these various constituents, are critically importance in controlling fuel cell performance. Understanding the specific microstructural characteristics of the individual materials within the CCL, and how the materials interact during fuel cell operation/aging, is important for identifying materials optimization parameters that can significantly enhance performance and durability.

Research efforts at Oak Ridge National Laboratory are focused on the high-resolution microstructural and microchemical characterization of MEAs fabricated using different Pt-based electocatalysts and catalyst loadings, carbon-based support materials, and ionomer solutions, as well as the same MEAs subjected to accelerated stress tests (ASTs) designed to degrade specific MEA components and assess durability. High-resolution analytical microscopy methods are used to study the distribution and chemistry of materials and interfaces within CCLs, which are combined with high-resolution imaging and 3D electron tomography techniques to provide unprecedented insight into the structure and interfaces (ionomer/support, ionomer/catalyst, catalyst/support, ionomer/pore) in “real” MEAs before and after aging. This presentation will focus on understanding materials distributions within the CCL as a function of processing variables, e.g., initial ionomer and/or ink chemistry, electrocatalyst (type, loading, and dispersion), and the type of carbon support used. The stability of the ionomer films, electrocatalyst, and support structures in CCLs after ASTs will be evaluated.

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Research sponsored by (1) the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy and (2) Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences (CNMS), which is a U.S. Department of Energy, Office of Science User Facility.

Metal carbides/nitrides have demonstrated unique properties for various catalytic reactions and are attractive as earth-abundant alternatives to precious group metals (PGM). However, synthesis of nanoscale transition metal carbides and nitrides has proven difficult, and obtaining structures with high surface areas and controlled morphologies have been elusive. Additional challenges arise from the complex nature of these nanomaterials and there is limited characterization available in the literature. Through unique synthetic approaches, we have created a wide array of unsupported and supported nickel-based nanomaterials, and utilized bulk and surface characterization techniques to identify synthesis-composition-property relationships.

Aluminum oxyhydroxide (boehmite) nanoparticles are used for a wide variety of applications including catalysis and adsorption. It has been shown that boehmite nanoparticles with 010 plane as the largest exposed surface area have increased catalysis and adsorption rate, due to the relatively high OH density of this plane. Additionally synthesizing these nanoparticles in a hierarchical structures instead as mono-dispersed, has been reported to increase the reactivity even further. In this study we utilize cryo-transmission electron microscopy (TEM) and in-situ liquid TEM to achieve fundamental insights into the mechanisms of nanoparticle aggregation and reactivity. Although there is a lot of theoretical debate around the preferential interacting zones of AlOOH crystalline nanoparticles in solution, there has been no experimental confirmation to date. Our initial results show that the particles aggregate along a preferred crystal orientation (the main flat (010)), irrespective of pH and solution content, forming large stacked agglomerates and decreasing the exposed surface area.

Aluminum oxide nanoparticles formed by the dehydration of boehmite are also widely used in industrial catalysis including petrochemical, chemical, and automotive due to their stability at high temperatures. Although the shape of the nanoparticle during the thermal treatment of boehmite remains the same the terminating planes of course change with the removal of water. This enabled us to study any change in the aggregation due to the terminating chemistry without the shape of the nanoparticle itself being a possible variable.

Infrared Photo-induced Force Microscopy (IR PiFM) is based on an atomic force microscopy (AFM) platform that is coupled to a widely tunable mid-IR laser. PiFM measures the dipole induced at or near the surface of a sample by an excitation light source by detecting the dipole-dipole force that exists between the induced dipole in the sample and the mirror image dipole in the metallic AFM tip. This interaction is strongly affected by the optical absorption spectrum of the sample, thereby providing a significant spectral contrast mechanism which can be used to differentiate between chemical species. Due to its AFM heritage, PiFM acquires both the topography and spectral images concurrently and naturally provides information on the relationship between local chemistry and topology with sub 10 nm spatial resolution on a variety of samples. PiFM spectral images surpass spectral images that are generated via other techniques such as scanning transmission X-ray microscopy (based on synchrotron source), micro confocal Raman microscopy, and electron microscopes, both in spatial resolution and chemical specificity. The breadth of the capabilities of PiFM will be highlighted by presenting data on various polymer systems. By enabling imaging at the nm-scale with chemical specificity, PiFM provides a powerful new analytical method for deepening our understanding of nanomaterials and facilitating technological applications of such materials.

Irradiation treatment of CNC was performed at doses of 5-20 kGy followed by reaction with a redox group of ascorbic acid (AA) and hydrogen peroxide in order to promote the formation of radicals on the macromolecule to enhanced the interaction of CNC with gallic acid. Our results have demonstrated that both of these pre-treatments induce the formation of carboxylic acid groups on CNC surface by conductometric titration. Derivates of carboxylic acid are known as food stabilizers and preservers due to their antiradical and antimicrobial effects. Since gallic acid has carboxilic acid groups, it was found that its interaction with pre-teated CNC with gamma-irradiation in the presence of redox group (CNC-g-GA) can impart to native CNC antiradical properties. A solution of 20% of CNC-g-GA was then added to a 1% (w/w) gellan in order to produced antiradical gellan-based film packaging. A significant improvement of the tensile strength (TS), the tensile modulus (TM) and the elongation at break (EB) and the water resistance was observed when CNC-g-GA was added to the film packaging formulation (p < 0.05). These results bring to a new application of antiradical film packaging or coatings which could be applied for food industry to preserve fruit and vegetables during storage.