Issue Date: January 16, 2012
Pushing The Capabilities Of Surface Analysis
News Channels: Analytical SCENE, Materials SCENE, Nano SCENE
Keywords: surface analysis, SIMS, depth profile, XPS, AFM
Whoever first said, “Necessity is the mother of invention,” probably did not have in mind the needs of surface chemical analysis. Even so, innovations in that field, like in much of analytical science, have been driven by the need to learn about complicated systems at an ever finer level of detail.
“It all comes down to getting molecular-scale information about increasingly sophisticated devices,” says Nathan Havercroft, a sales manager with ION-TOF, a laboratory instrument manufacturer based in Münster, Germany. Havercroft’s focus is on using secondary ion mass spectrometry (SIMS) in novel ways—for example, with new types of cluster ion beams—to image electronic devices built up from multiple layers of complex organic materials. Examples of such devices include organic light-emitting diodes (OLEDs), various types of memory chips and sensors, and organic photovoltaic cells.
Other researchers have designed special versions of instruments typically used in high vacuum to interrogate molecular processes that generally take place at high gas pressures. Similar innovations in instrumentation enable researchers to use classic high-vacuum surface science tools to probe chemical reactions in liquids or at the interfaces of liquids with solids or gases. Still other innovations are based on combination instruments that integrate two or more traditionally separate analytical techniques into a single research tool.
The common thread in all of these cases is new twists on surface analysis tools that push the limits of traditional methods. The latest versions of these instruments permit researchers to scrutinize technologically important materials, devices, and chemical processes under conditions that have traditionally prevented up-close investigations. A number of these new instruments have been commercialized recently, with several on display or the subject of presentations last fall at the AVS science and technology conference in Nashville and the Materials Research Society (MRS) meeting in Boston. Those product launches are set to bring these new surface analysis tools into wider use.
For decades, SIMS measurements were made by bombarding surfaces with energetic and often monatomic ions, such as Ar+ and Ga+, and monitoring the ejected (or secondary) ions. As the ion beam slowly sputtered surface atoms and revealed deeper and deeper layers, researchers would measure the signal of one or more of the analyte ions and relate the signal intensity to the penetration depth to create depth profiles for those analytes.
That method for depth profiling works well for measuring impurity or dopant concentration profiles in simple materials. But for the kinds of stratified structures that Havercroft studies, ones built up from multiple nanometer-thick layers of large aromatic compounds, often with molecular masses well above 500 amu, the method works poorly or not at all. Yet optimizing the lifetime, efficiency, and other performance parameters of devices based on these thin-film structures calls for an in-depth knowledge of the layer and interface compositions.
“The trouble is, gallium ions penetrate quite deeply into the surface and often cause intermixing of layers beneath the surface,” Havercroft explains. By destroying the integrity of the molecular layers, the Ar+ and Ga+ ion beams thwart the chances of extracting meaningful data from those layers. Another problem associated with those kinds of ion beams is they fragment large organic molecules, which complicates or prevents analysis.
To get around those problems, SIMS method developers have turned to sputtering with beams of clusters—for example, bismuth clusters and C60. Clusters offer the benefit of spreading the kinetic energy of the beam, which is generally in the range of thousands or tens of thousands of electron volts, across all atoms in the cluster. The upshot, Havercroft explains, is that by reducing the per-atom kinetic energy, the sputtering projectiles don’t penetrate deeply into the material. Rather, the ion beam deposits all the energy in the near-surface region and leaves the subsurface largely intact.
The latest developments in organic depth profiling are based on sputtering with argon clusters that consist of hundreds or thousands of atoms. The clusters form as a result of a cooling process as a jet of high-pressure gas expands into vacuum. Researchers at ION-TOF, which is one of the few companies now selling these new gas-cluster sources, applied this method to dissecting and imaging custom-made OLED structures. The structures (before they were whittled away) consisted of a stack of layers of multiring organic compounds that function as electron-transport and hole-transport media and also included fluorescent, phosphorescent, and insulating layers.
The team sputtered through the layers by rastering a beam of mass-selected argon clusters across an area measuring several hundred square micrometers. They repeatedly zoomed in on the center of the sputter crater with a beam of Bi3+ clusters to analyze the composition of the freshly exposed layers. The team used a dual-beam technique, Havercroft explains, “because it provides the flexibility to independently tune each beam for its job.”
“Tuning,” in this case, means finding the optimum probe conditions by systematically adjusting the beam energy, cluster size, and other parameters while monitoring the effects on sputter yield, resolution, and other measured quantities. The team found that this dual-beam SIMS method, with up to 5,000-atom argon clusters, is well suited to interrogating layered organic structures such as custom-made OLED samples and an off-the-shelf media storage device.
Researchers have also shown recently that SIMS methods not only can be tailored to analyze complex organic structures but also can be customized to image harder-to-analyze cells and tissues. “SIMS analysis of biomaterials is potentially an important application,” asserts Jiro Matsuo of Kyoto University, in Japan. He’s working to overcome commercial instrument limitations that make that type of analysis a formidable challenge.
For example, to overcome the inherently small SIMS signals obtained from biomaterials, Matsuo and coworkers built a customized ion gun that, compared with commercial instruments, delivers a much more tightly focused and more intense beam of argon-cluster ions. And rather than pulsing the ion beam, as is commonly done for standard time-of-flight detection of SIMS ions, they operate the cluster source continuously and measure analyte signals with a so-called orthogonal-acceleration time-of-flight detector.
In a proof-of-concept study, the Kyoto researchers found that they could image rat cerebellum tissue and generate chemical maps of those samples by measuring SIMS signals from phosphatidylcholine, cholesterol, and other compounds. Now the group is working on ways to further boost SIMS signals and interface a compact version of their ion source with standard commercial mass spectrometers.
Another surface analysis tool that is being applied in unconventional ways these days is X-ray photoelectron spectroscopy (XPS). Because of the way photoelectrons are typically collected and analyzed, XPS has long been limited to probing materials under high vacuum. Yet at less than one-billionth of atmospheric pressure, high vacuum is often very different from the working or “operando” conditions under which materials are normally used. As a result, researchers have traditionally been unable to use XPS to examine solid catalysts while they are exposed to high gas pressures and electrodes in contact with liquids. Innovations in instruments and analysis methods are now making those kinds of investigations possible.
At the University of Notre Dame, Franklin (Feng) Tao, an assistant chemistry professor, has been developing a novel ambient-pressure (AP) XPS system and using it to learn about the way catalyst surfaces change under working conditions. Tao acquired instrument development experience as a postdoctoral fellow when he worked with Gabor A. Somorjai and Miquel B. Salmeron at the University of California, Berkeley, and Lawrence Berkeley National Laboratory.
Scientists have known for decades that the surfaces of platinum, a common catalyst material, and other metals undergo structural changes upon exposure to vacuum levels (~1 × 10–7 torr) of reactive gases such as carbon monoxide. But in 2010, the Berkeley group found that upon exposure to 1 torr of CO, platinum’s surface changes in a previously unrecognized way; it forms numerous tiny nanoclusters at the edges of features known as steps. The study, which was based on AP-XPS and AP-scanning tunneling microscopy, also showed that the nanoclusters disappear when the CO pressure is reduced (Science, DOI: 10.1126/science.1182122).
But the Berkeley team’s AP-XPS system relied on a large and costly synchrotron to produce and tune the energy of the necessary X-rays. Now leading his own research group at Notre Dame, Tao has just finished building lower cost versions of these instruments. Unlike the Berkeley system, Tao’s new design features a benchtop X-ray source and an integrated flow-cell reactor that can be charged with reactive gas up to roughly 50 torr. Tao and coworkers tested the new system on ceria, a catalytic material used for automobile emissions control. They found that they could monitor the appearance and disappearance of surface oxygen vacancies upon sequential exposure to hydrogen and oxygen.
The Notre Dame group is also testing its newly built AP-high-temperature scanning tunneling microscope. That instrument features a mini flow-cell reactor that is thermally isolated from the STM head and, like the XPS instrument, can be pressurized to 50 torr. Historically, it has been quite challenging to achieve atomic resolution with an STM operating at high pressure and high temperature simultaneously, Tao says. Yet in unpublished work, Tao shows that the new STM resolved atomic features of a ruthenium model catalyst partially covered with graphene under imaging conditions of 300 °C and 50 torr.
The interface between liquids and gases is another atypical molecular system for XPS analysis. But those interfaces can be sites of technologically important processes that aren’t well understood. Such interfaces occur, for example, in industrial CO2 scrubbers, which capture carbon by treating exhaust or flue gas with concentrated aqueous solutions of basic compounds such as monoethanolamine (MEA) (C&EN, May 2, 2011, page 30).
To probe the liquid-gas interface, Tanza Lewis and John C. Hemminger at UC Irvine, with Bernd Winter at the BESSY synchrotron facility in Berlin and Manfred Faubel at Germany’s Max Planck Institute for Dynamics & Self-Organization, used XPS to study liquid microjets of pure and CO2-treated MEA in vacuum. The group found that the products that form upon reaction with CO2, which include a carbamate species, carbamic acid, and protonated MEA, tend to migrate into the bulk of the solution. In contrast, neutral, unreacted MEA enriches the liquid surface, where it can enhance the CO2 capture rate (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201101250).
Another approach to advancing surface analysis is based on coupling two or more traditionally separate probe methods into an integrated instrument. That strategy often calls for engineering a unique interface between the probes or developing novel ways of making the measurements. Several combination methods have been commercialized by manufacturers of atomic force microscopes.
AFM maker Bruker Nano, for example, teamed up with Renishaw, a Raman spectrometer company, to make an AFM-Raman instrument that was unveiled last month. “AFM provides nanoscale details about a material’s topography as well as mechanical, electrical, and other properties, but it doesn’t provide chemical information in a straightforward universal way,” says Bruker’s Thomas Müller, an AFM product manager. The combined method not only delivers chemically specific nanoscale spectroscopy information but also provides a ready platform for taking advantage of the large enhancements in Raman signals associated with tip-enhanced Raman spectroscopy.
Another way to get nanoscale chemical information is by coupling AFM with infrared spectroscopy. That’s the approach taken recently by Anasys Instruments, a Santa Barbara, Calif.-based company that makes AFMs in which the probe tip functions as an IR detector by sensing rapid thermal expansions in a sample as it absorbs IR light. As with the company’s combination AFM-thermal analysis instruments, the new AFM-IR probes are often used to analyze polymer blends and multilayer polymer films common in food packaging and other applications.
A host of recent instrument and methodology innovations have placed traditionally inaccessible information in the hands of surface analysts. As the trend to produce ever more sophisticated devices and materials continues, the trend to produce ever more capable analytical tools is sure to keep pace.
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