Surface and Interface Science

Summary

Surface and interface science examines the properties of materials and products at the surface and at the locations where two or more materials meet. The discipline typically focuses on surface analysis, surface modification, and interface science. Surface modification changes the properties of a surface, such as the hydrophobicity, lubricity, chemistry, or topography, and surface analysis tells what those properties are for the material being studied. Interface science can include studying interactions between materials and how well the materials adhere to one another, as well as how to achieve desired interactions. The interface between a material and a solution or air can also be studied.

Definition and Basic Principles

Surface and interface science encompasses several disciplines, including surface analysis, interface analysis, and surface modification.

Surface analysis is the study of the chemistry, crystal structure, and morphology of surfaces using a combination of various surface-sensitive analytical methods. Surface sensitivity can mean different things for different applications. However, a good working definition would be the uppermost 0.5 to 3 nanometers (nm) of a surface or two to ten layers of atoms. Because surfaces are often modified with films in the tens to hundreds of nanometers range, the uppermost 100 nm can be considered the surface for some applications. Any surface thicker than 100 nm is typically considered bulk material.

Interface analysis is the study of the morphology and chemistry of internal interfaces or regions where two or more materials meet. A good understanding of the interface between two materials will provide scientists and engineers with a wealth of information, including how well these materials are bonded, whether a certain amount of force is likely to separate the materials, whether any new species have been formed, or whether the two materials are diffusing into each other.

Surface modification is the process by which scientists and engineers change the chemistry, physical properties, or topography of a surface to improve the function of the material, such as changing the coefficient of friction, lubricity, hydrophobicity, roughness, or bondability of the material.

Surface analysis and surface modification are both quite sensitive to contamination, so much care must be taken that the surfaces of interest do not come in contact with materials that could potentially transfer to the surfaces. However, even if every precaution is taken to prevent transfer, contamination can occur from simple exposure to air, which is why many surface modifications and analyses are conducted under vacuum. A vacuum is also essential for many of the surface techniques to prevent interactions between air molecules and the species being analyzed.

Background and History

Surface science is a relatively new area of science. The first X-ray photoelectron spectrum was collected in 1907 by P. D. Innes, and the first vacuum photoelectron spectrum was taken in 1930. American physicists Clinton Davisson and Lester Germer pioneered low energy electron diffraction (LEED) in 1927 and observed diffraction patterns on crystalline nickel. Ultra-high vacuum techniques for surface analysis and modification (specifically welding) were invented in the 1950s. In 1968, a surface science division of the American Vacuum Society was proposed, the same year that Auger electron spectroscopy began to be used. German scientist Alfred Benninghoven introduced time-of-flight secondary ion mass spectrometry (TOF-SIMS) in the 1980s, about the same time that scanning tunneling microscopy began to be used.

How It Works

Surface Modification. Surface modification can be accomplished in many ways, including methods that modify the chemistry of a surface, the topography of a surface, or both. Films of material can be grown on a substrate in a vacuum or deposited using an ultrasonic spray-coating, spin-coating, or dip-coating process. Anodization is a process whereby an oxide layer is deposited on a metal using oxidation and reduction reactions. It has the potential to grow oxide films in a very controlled manner. Passivation is a process that changes the oxide layer on a metal to make it more resistant to oxidation. It is typically carried out in an acid solution. Surface topography can be modified using laser or acid etching, which roughens the surface, or electropolishing, which uses electricity to smooth conductive surfaces.

Plasma treatment is a method of modifying the chemistry and often the topography of a surface. It uses a highly ionized, activated gas to react with the molecules of a surface. The plasma gas can vary from an inert gas, such as argon or helium, which would be expected to cause the species at the surface to react with one another, or a polymeric monomer, which could polymerize on the surface and create a thin plasma-treated layer. Plasmas can also be employed to clean surfaces before modification. Chemical vapor deposition is a technique in which the sample is exposed to a vapor that reacts with the surface to modify it.

Surface Analysis Techniques. There are many surface analytical techniques. The most common techniques are complementary to one another, and several are often used in combination to gain all the necessary information about a surface.

Scanning electron microscopy (SEM) is a technique that uses a focused electron beam that scans across the surface of a sample and allows for the construction of a very detailed map of surface topography. The surface sensitivity of this technique varies based on the sample and the instrument parameters, which can make the instrument range from simply surface sensitive to able to gain information up to microns within a sample.

Electron spectroscopy for chemical analysis (ESCA), also known as X-ray photoelectron spectroscopy (XPS), uses X-rays to generate electrons from the surface of a sample. These electrons can then be detected and traced back to whatever elements are at the surface. XPS gives quantitative elemental information about the uppermost 5 to 10 nm of a sample surface and can also provide some information about the types of bonds present. Auger electron spectroscopy (AES) is a surface-sensitive technique that uses an electron beam to gain surface elemental information similar to that obtained by XPS. AES is less quantitative than XPS. However, higher resolution mapping is possible with AES than with XPS.

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is an extremely surface-sensitive analytical technique that uses a beam of liquid metal ions to probe the top one to three molecular layers of a sample, causing the emission of ions from the sample to be detected in a time-of-flight mass spectrometer, which gives detailed information about the specific molecules and atoms at a surface.

White light interferometry, sometimes called profilometry, uses white light interference patterns to quickly construct detailed surface topography maps. It can detect minute surface features in the nanometer range. Spectroscopic ellipsometry is often used to measure the thickness of surface films, such as oxides or plasma treatments on materials. Atomic force microscopy (AFM) and scanning tunneling microscopy (STM) both employ a very sharp tip called a cantilever that rasters across a sample and yields high-resolution images of surface topography. AFM is typically used for nonconducting samples, and STM is useful for conducting samples.

Interface Science. The interface of two materials can be probed using several techniques, including some of the techniques used for surface analysis. Depth profiling is a method by which information is gained from within the sample. If the interface in question is near the surface, several techniques can be used. Both TOF-SIMS and XPS use beams of ions or carbon 60 (C₆₀) (Buckminsterfullerene) to sputter material away from the surface to obtain information about interfacial layers and boundaries. An advantage of these techniques is that they are performed in a vacuum. Therefore, the interface is not exposed to ambient conditions before analysis. Confocal Raman microscopy has the potential to probe interfaces up to 100 microns below the surface of a sample by focusing the laser on the interface and using a pinhole to eliminate out-of-focus data.

Applications and Products

Surface Modification. Surface modification has a very wide range of applications. Surfaces are modified to impart desirable properties and often change properties such as lubricity, surface energy, the coefficient of friction, or the functional groups present on a surface. Self-assembled monolayers can be deposited, and crystals can be grown in a highly controlled manner. Sensors and catalysts can also be fabricated using surface modification. Surface modification is applied across a wide variety of industries, including the automobile, aerospace, medical device, textile, chemical, steel, and electrical industries.

A primary application for surface modification is making materials more corrosion and wear-resistant. Steel is used in everything from skyscrapers to motorcycles, so increasing the stability of this crucial material is a priority for many people in the steel industry and industries that use steel to fabricate other products.

In the medical device community, the surface of a material is extremely important because it is in contact with the body. For example, the components of implants may be plasma-treated to make their surfaces more biocompatible, which reduces cell adhesion and the formation of fibrous tissue around the implant. Implantable metal devices are often passivated to make the device resistant to corrosion when subjected to the aqueous environment inside the body.

In the automobile industry, surfaces of parts that must be bonded to other parts to provide structural integrity to the vehicle are sometimes plasma-treated to promote better adhesion and reliability. The surfaces of exhaust systems are modified with a catalyst that greatly reduces toxic emissions.

Surface Analysis. Surface analysis can be employed in many situations, but it is particularly well suited for the analysis of contamination or surface damage. In many industries, contamination at the nanometer scale can spell disaster for a process. For example, semiconductor materials have very predictable conductive behavior, which is essential for designing microelectronics that work properly. The addition of contaminants to the system will change the behavior of the materials. It can cause failures, so surface analysis must be employed in developing a product and sometimes during manufacturing stages to ensure that the materials are clean and reliable.

Surface analysis is critical in the study of nanomaterials, as these materials and structures are often so minute that the majority of a sample's mass is considered surface material. Less sensitive techniques would be unable to effectively study these extremely small materials.

Interface Science. The interface and interactions between two solid materials are very interesting to materials scientists. However, interface science also includes the study of the interactions between materials and a gas, such as air, or a liquid, such as water. Proteins such as insulin are often sold in solution form. Because proteins are unstable under most conditions, it is important for drug companies to understand the interactions between proteins and the surfaces of the containers in which they are stored.

Adhesion specialists often must determine whether the two materials they are trying to bond are well bonded. Mechanical tests, such as peel tests or scratch tests, are usually employed to assess the integrity of the bond in combination with a chemical analysis that can look for new bond types between the layers.

Careers and Course Work

A wide variety of careers is available in surface science, and the desired career will determine the proper coursework. To work in surface science requires a bachelor of science degree in a core science such as chemistry or physics, and having a graduate degree greatly improves a student's chances of finding work in the field. A bachelor's degree is an adequate qualification for becoming a laboratory technician, which generally involves being trained on the operation of an instrument and performing mainly routine tasks. A master's or doctoral degree and appropriate research experience are the necessary qualifications for a position involving the advanced use and maintenance of surface analysis instruments, experiment design, data analysis, interpretation, and communication, as well as possibly developing new instruments.

Careers in surface science can be found in industry (such as at an automobile company), academia, and government laboratories. Surface and interface science is critical in materials science and engineering and is likely to play a significant role in the field's development. Regardless of the career choice, a strong foundation in basic science, including chemistry, physics, and mathematics, is required.

Social Context and Future Prospects

Surface science is a growing field that will continue to play a major role in the development of materials science and engineering, especially in light of the focus on nanomaterials and the potential of surface science to make a significant contribution to that field. In many industries, the focus is on making devices smaller and smaller, which naturally leads to the surface becoming a more significant part of the device because of the increase in the surface area to volume ratio. As such, a growing area of surface science is the development of micro- and nanoelectromechanical systems (MEMS and NEMS), or building microscopic machines often referred to as nanomachines, which have applications in fields ranging from computing to biochemistry to navigation.

New applications are being discovered for surface modification every day. Plasma treatment is becoming more common in many industries, such as the automobile and medical device industries, because it can make it easier to bond one material to another. The process increases quality and is relatively fast, which suits production in the fast-paced cultures of these industries. As governing agencies such as the Food and Drug Administration increase their scrutiny of medical devices and drug-device combination products concerning biocompatibility and stability, surface analysis will help characterize the surfaces that come into direct contact with a patient.

In the first two decades of the twenty-first century, the proliferation of the terms and data associated with surface and interface science caused significant confusion across related industries and research areas. Scientific papers utilizing surface science or interface science data frequently over- or under-interpreted data from the field. XPS data is required for publication in research concerning electrodes, polymers, and catalysts. However, XPS data is most frequently mischaracterized in the literature. 

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