Masayuki Nishi

Looking at a Small World

Just as satellites orbiting the Earth can reveal atmospheric conditions, surface structures and sometimes even mysterious ancient ruins, imaging technologies of a different scale can provide new insights into targets of scientific interest. Microscopes, like satellites, can allow us to view structures that would be invisible to the unaided eye. Studying small structures is important because it enables researchers to understand mechanisms for fascinating phenomena and develop new functional materials.

The world that some microscopes can show us might be beyond your expectations. In common microscopes, light strikes and stimulates matter while being partially reflected and scattered. This stimulus-and-response allows us to learn the shape and morphology of that matter. There are many different ways that stimuli and responses can be combined, and some combinations may provide additional “chemical information”, such as what elements and chemical bonds are present. For example, you can discover what elements are present in the structure of a material by using an electron beam for a stimulus and characteristic X-rays for a response, even though individual particles of the elements cannot be seen directly. Another combination that can reveal what chemical bonds are present is a monochromatic laser light for a stimulus and weak components of scattered light with colors that are slightly different from the original for a response. This method is called Raman spectroscopy. Raman is the name of the person who experimentally demonstrated this scattering of light and was awarded the Nobel Prize in Physics in 1930. Due to its usefulness in identifying molecules and materials, and in analyzing their structures, Raman spectroscopy is now widely used in materials science. Raman spectroscopy using a microscope is called “micro-Raman spectroscopy”. Changing the position of the focal spot in and/or on a target while acquiring Raman signals at each position results in mappable chemical information. The smaller the focal spot, the higher the spatial resolution. However, a single spot of roughly a few hundred nanometers is the smallest theoretical size obtainable by a common micro-Raman system, owing to the diffraction limit.

Tip-enhanced Raman spectroscopy (TERS) allows spatial resolutions beyond the above-mentioned diffraction limit. Several research groups independently reported on their success in performing TERS in 2000. TERS combines the micro-Raman system with a scanning probe microscope (SPM). Among SPMs, the atomic force microscope (AFM) has the least restrictions on targets to be measured. AFMs are quite different from common microscopes. They do not focus a beam but are like touching the surface of the target with a tiny fingertip mere nanometers in width. They scan a surface with the physical probe and image the relative height distribution of the surface, including nanoparticles on a substrate. TERS uses an electric field that is enhanced and confined at just around the tip apex of the physical probe in response to light irradiation (Fig. 1). Theoretically, Raman scattering is an inefficient process and is commonly weak in intensity. However, enhancement allows for the detection of very weak Raman signals from tiny volumes that would otherwise be undetectable. The tip apex is usually made of gold or silver, which is effective in generating a special electric field of this type in response to irradiation of visible laser light common in Raman measurement. The size and shape of the gold or silver tip play a crucial role in the electric-field confinement and enhancement. Commercially available AFM-TERS probes are produced by vapor deposition and have gold or silver coating on AFM probes, which are usually made of silicon.

Dr. Nishi encountered TERS by chance. He previously researched the optical properties of rare-earth doped oxide glasses, glass ceramics and ceramics. Such materials are commonly produced by high-temperature processes; for example, glasses are produced by melting raw materials followed by quenching. It was his growing curiosity about using chemical processes at near-room temperatures to produce functional materials that prompted another research. One result of this research was a shape-controlled synthesis of gold micro and nanoparticles. One day, his student was observing the gold nanoparticles using a scanning electron microscope and found that a small portion of the gold nanoparticles had gathered into lines. The student asked him about the phenomenon. Dr. Nishi was not sure of the reason but was at least certain that the lines of gold nanoparticles had grown in a way that was different from the other main products of gold in different shapes and sizes. He realized that revealing the mechanism behind this phenomenon will lead to the development of a new area-selective metal growth method. After this, Dr. Nishi and his student began their investigation. Through new findings resulting from collaborative works with students and professors, their area-selective growth method was developed, simplified and sophisticated over time. Dr. Nishi eventually decided to apply this method to the preparation of AFM-TERS probes. They were able to area-selectively grow gold nanoparticles and even a single gold nanoparticle at the tip apex of silicon AFM probes (Fig. 2) and successfully performed TERS using these probes.

AFM-TERS is a powerful tool with a high spatial resolution, but the substances that AFM-TERS can measure are still very limited because of its insufficient enhancement factor. Dr. Nishi and his collaborators are working on new AFM-TERS probes with new features, including a higher enhancement factor to discover more secrets hidden in small worlds.