Frank J. Scheltens

Measurement of optical properties in organic photovoltaic materials using monochromated electron energy-loss spectroscopy

The optical and electronic properties of organic materials influence the functionality of all organic electronics. These properties can be measured in bulk materials easily, but determining the opto-electronic properties in thin films and at interfaces is challenging. In this report we describe how these properties can be measured with high spatial resolution using an optimized electron energy-loss spectroscopy (EELS) method in the scanning transmission electron microscope (STEM). EELS spectra were collected for poly(3-hexylthiophene) (P3HT), [6,6]phenyl-C61 butyric acid methyl ester (PCBM), copper phthalocyanine (CuPc), and C60. These organic materials are both susceptible to electron beam damage and commonly utilized in organic photovoltaics (OPVs). In order to prove that these spectra are representative of the pure materials and that the samples have not undergone any significant beam damage, the real and imaginary parts of the complex dielectric function obtained from these spectra have been compared to the same functions obtained using variable angle spectroscopic ellipsometry (VASE), a technique that should not induce any beam damage to the samples. Comparisons of these two data sets reveal good agreement in both measured peak intensities and their corresponding peak energies, thus validating this low-damage EELS acquisition method. EELS spectrum images were acquired from a CuPc/C60 bilayer structure to demonstrate that it is possible to collect spatially resolved EELS data from device-related structures comprised of these beam-sensitive materials.

Absorption Corrections for a Four-Quadrant SuperX EDS Detector

Consider a four-quadrant detector consisting of four circular active regions of area Aq each, placed symmetrically around the sample, as shown schematically in Fig. 1(a). The sample holder is inserted from the right, and the quadrants are 90° apart, oriented symmetrically at ±45° with respect to the primary tilt axis. The specimen tilt angles are labeled  and β, with positive angles corresponding to counterclockwise rotations. Each detector quadrant is located above the plane of the specimen and is tilted from the vertical plane by an angle d; the angle between the line connecting the center of a quadrant D with the eucentric point S and the horizontal plane is labeled d (Fig. 1(b)). The active circular regions have a radius of Rd, and a distance to the eucentric point of rd; the detector opening angle is R = Rd/rd. When R is not negligible, as is the case for a SuperX detector, then the x-ray photon path length inside the sample becomes a function of the position on the detector where the pho-ton hits; the absorption correction factor must thus involve an integration over the detector surface area.

Determining Optical Absorption Coefficients in Beam Sensitive Materials using Monochromated Electron Energy-Loss Spectroscopy

Using electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM), it is possible to measure optoelectronic properties of materials with high spatial and spectral resolutions. In particular, low-loss spectra (energy-losses of less than 50 eV) can be used to extract the real (ε1) and imaginary (ε2) parts of the complex dielectric function [1] over this entire range, which encompasses ultraviolet, visible, and infrared wavelengths. Since ε1 and ε2 are related to the refractive index (n) and the extinction coefficient (κ), both n and κ can also be derived over this entire range with high spatial and high energy resolutions. This makes STEMEELS a very powerful technique for probing the optoelectronic properties of materials.

Correlative 3D Imaging and Characterization of Human Dentine

3D characterization [1, 2] is of particular importance in the study of mineralized tissues such as teeth and bones due to the presence of channels, pores and features that span millimeter, micrometer and nanometer length scales. The major component in human teeth, by weight and volume, is dentine. This hydrated hard tissue encloses the central pulp and has microscopic channels, dentineal tubules that radiate from the pulp to the cementum on the surface of the dentine that connects with the hard outer enamel. The permeability provided by these tubules can cause dental hypersensitivity. The object of our current work is to understand how treatments for dental hypersensitivity act on these tubules.

Composition of Epitaxial ZrO 2 :Y2O3/SrTiO 3 Heterostructures

Thermally activated oxygen ion conductivity in the electrolyte of a solid oxide fuel cell (SOFC) device is critical to its successful operation. The current generation of bulk solid oxide electrolytes requires temperatures in the range of 700 to 1000 ̊C in order to achieve the required level of oxygen conductivity, which leads to longevity issues due to thermal stress and fatigue and the requirement for more costly materials [1]. The search for SOFC materials with enhanced ion conductivity at low temperatures has lead researchers to the arena of strained epitaxial heterostructures. Reports of ion conductivity increases of several orders of magnitude in highly strained multilayers consisting of alternating layers of yttria stabilized zirconia (YSZ) and SrTiO3 (STO) [2] have created a great deal of excitement in the field SOFC development.

Optimized Damage-Reduction 60 keV Monochromated Electron Energy-Loss Spectroscopy Measurements of Optical Properties at the Donor/Acceptor Interface in Organic Photovoltaic Devices

The key to improving the performance of OPVs is to understand the donor/acceptor interface within the device. The two primary areas of understanding that correlate to this interface that can affect the performance of the device are the morphology of the interface and the local electronic structure at the interface [1]. Multiple groups using different characterization techniques have extensively studied the morphology of the interface [2, 3, 4]. However, the local electronic structure at the interface has largely been ignored as these measurements require the use of electron energy-loss spectroscopy (EELS) to measure these optoelectronic properties as the transmission electron microscope is the only instrument that results in the high spatial resolution necessary to probe the interface [5].

Monochromated Electron Energy-Loss Spectroscopy of Organic Photovoltaics

Monochromated electron energy-loss spectroscopy (EELS) is opening up new opportunities for the study of the electronic structure in complex materials. The prospect of mapping band structure with high spatial and energy resolution is an exciting prospect. A particularly challenging task is to realise this in organic materials such as polymer matrix composites, biomaterials and organic electronic materials. For example, the processes that generate current in organic photovoltaics (OPVs) are highly dependent on the microand nano-structure of the devices, especially at the donor-acceptor (D-A) interface. Hence, the structure of this interface is vital to understanding the efficiency of devices, as this knowledge will provide a foundation for the engineering of new OPV devices with improved power conversion efficiency. Scanning transmission electron microscopy (STEM) EELS can be used to probe the nature and structure of interfaces in OPV devices because it is possible to obtain high energy resolution measurements over large ranges of energy-loss (ΔE).

EELS Investigations of Aging Mechanisms in LiFePO4 Cathodes Resulting From Prolonged Electrochemical Cycling

Lithium iron phosphate (LiFePO4) has considerable potential as a cathode in batteries for automotive applications due to its high rate capability, reasonable energy density and environmentally benign nature [1]. However, performance degradation seen after thousands of cycles at high charging-rates (C-Rates) has been a point of major concern [2]. Studies of the aging mechanism suggest that phases (LiFePO4/FePO4) formed in the cathode during discharge influence the aging profile [3]. Previously, we used electron energy loss spectroscopy (EELS) to demonstrate the use of Li-K edge for identifying lithium in the sample with a potential for quantification [4]. This required a modified procedure for focused ion beam (FIB) milling to minimize ion beam damage during sample preparation [5]. We reduced beam dosage in the electron microscope to prevent knock-on damage to the lithium in the sample. Lithium content, combined with information of the oxidation state of iron, can be used to identify the phases formed upon intercalation. The study was able to identify fine variations in the Li-K edge structure, and hence, phase composition in nanoparticles inside the LiFePO4 composite electrode.

Variable Angle Spectroscopic Ellipsometry and Electron Energy-Loss Spectroscopy

Electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) has considerable potential for investigation of interfaces in organic photovoltaic (OPV) devices. In particular, the low-loss region of the EEL spectrum can be used to obtain the complex dielectric function of the material. The complex dielectric function in turn allows us to distinguish between single electron transitions and collective excitations. Spatial mapping of the single electron transitions can then be used to learn about the chemistry and bonding in the vicinity of interfaces between the acceptor and donor interfaces in the OPV samples [1].

High Performance Remote Electron Microscopy

Efforts to explore the feasibility and practicality of operating microscopy instruments from afar have existed for years in various forms [1-4]. The goal of these efforts is to enable the highest possible utilization of a scarce resource—powerful, expensive electron microscopes and related analytical instrumentation. Universities and research organizations require the use of electron microscopes, but may not have the facilities or the capital to acquire and maintain the modern, high-performance, corrected TEM instruments. Past efforts driving remote access sought to utilize users in different time zones to enable a Follow-the-sun work flow [4]. The focus of the current effort is to enable tools to expand local user base and build a critical mass of users by fostering high quality collaborations. Facilitating remote operation to teach and train large groups of students, simultaneously [5-7] helps to build this user base and expand exposure to characterization techniques. Whether an off-campus outreach effort, or a classroom demonstration, these educational efforts seek to build excitement for science, and show the wonderment of “seeing the unseen” to students of all ages. Often an understated benefit of remote operation is assisting staff and researchers in operation and maintenance of the instruments, thus allowing personnel to better utilize facility resources. This last benefit may be a key consideration for a facility deciding to allocate funding to enable such resources.