A trio of new instruments helps researchers sharpen their focus on the world of atoms and molecules
Few advances in modern science match the potential of nanotechnology to deliver so much from so little.
Nanotechnology involves the manipulation of materials ranging in size from single atoms to several tens of nanometers. One nanometer (nm) is equal to one-billionth of a meter, or about one hundred-thousandth the diameter of a human hair.
Out of these tiny chunks of material, researchers have created almost invisible tubes, particles, pillars, wires and a host of other shapes with special functions.
Iron nanoparticles, for example, can be designed to decontaminate groundwater, while gold nanoparticles may help chemotherapy drugs confine their damage to cancer cells. In sunscreens, nanoparticles block UV rays without leaving a white residue on the skin. In lithium-ion batteries, nanoparticle-based electrodes help power electric cars.
“Nano-whiskers” stitched into fabrics make them lightweight as well as water- and stain-repellent. Nanocatalysts can transform biofeedstocks into fuels, while carbon nanotubes arranged into dense “forests” are being tested for their ability to store hydrogen.
The guiding principle of nanotechnology, says Christopher Kiely, is that a material’s properties – chemical, optical, electrical, thermal and magnetic – can change when it is shrunk to the nanoscale. Normally inert gold, for example, morphs into a catalyst at the nanoscale.
To control the structure and composition of nanomaterials, and to fine-tune and optimize their properties, says Kiely, who directs Lehigh’s Nanocharacterization Lab, requires the ability to observe, measure and manipulate the nanoworld of atoms and molecules.
This, in turn, requires increasingly sophisticated instruments.
Lehigh has long possessed some of the world’s best microscopy and spectroscopy tools. Its collection of electron microscopes is one of the most extensive in American academia. Lehigh was the first university to acquire two aberration-corrected electron microscopes, which can pinpoint the position and chemical identity of individual atoms.
The university’s array of spectroscopy instruments is similarly impressive. Its high-resolution X-ray photoelectron spectrometer (HR-XPS) combines with a new high-sensitivity, low-energy ion-scattering spectrometer (HS-LEIS) to provide an unprecedented view of the surface and subsurface that govern a material’s properties and its reactivity.
In the past two years, Lehigh has acquired funding for several new instruments that will improve researchers’ ability to investigate and control the nanoworld.
- A new JEM-ARM200F aberration-corrected scanning transmission electron microscope, with features customized by Lehigh microscopists, will image atoms with unprecedented resolution. Its low-voltage operation-range improved spectrometry will allow the study of sensitive organic materials, including carbon nanotubes, graphene, polymers and biomaterials.
- The new HS-LEIS, the world’s most sensitive spectrometer for identifying surface atoms, offers a 3,000-fold improvement in sensitivity over conventional spectrometers and also allows for elemental 2-D surface mapping.
- A custom-made NTEGRA marries an atomic force microscope (AFM) with an inverted optical microscope, allowing a specimen to be probed from above by the AFM as it is being observed or optically stimulated by the light microscope.
The new instruments, says Kiely, have the potential to help researchers observe nanomaterials in more dynamic environments, to watch as they react with other materials, and to see how they respond to heat, light and mechanical stress.
These in turn will allow researchers to obtain a more accurate picture of the behavior of objects in the nanoworld.
“We are very adept at making and observing nano-things,” says Kiely. “We have good recipes for making nanoparticles, nanorods, nanowires and nanopillars. And we have improved our ability to examine these things with electron microscopy and spectroscopy and determine their structure and chemistry.”
“However, we are much less adept at taking an individual nanoparticle or nanotube and measuring its physical properties because it is just too small to manipulate and probe.
“We need better tools for analyzing these nanomaterials, and that’s what these new instruments provide.”
Angstrom-level imaging and analysis
The new JEM-ARM200F aberration-corrected STEM, says Masashi Watanabe, enables researchers to correlate the structure and chemistry of materials with 3-D resolution at the angstrom (0.1nm) level.
“This capability will enable us to develop new materials and characterize their properties with unprecedented accuracy,” says Watanabe, an associate professor of materials science and engineering.
The features include the most sophisticated detectors for electron energy loss spectroscopy (EELS) and X-ray energy dispersive spectroscopy (XEDS), says Watanabe. These will allow composition analysis at atomic resolution while improving stability, data acquisition speed and image quality.
The new STEM obtains an improved signal from samples with an electron “gun,” or source, that is 10 times brighter than that of any other STEM and an X-ray detector whose collection angle captures four times more signal.
Lehigh is purchasing the JEM-ARM200F from JEOL Ltd. in Japan with an NSF grant and with university matching funds. The new instrument replaces the existing JEOL 2200FS STEM, which was purchased in 2004 and was the first aberration-corrected electron microscope acquired by an American university.
The JEM-ARM200F operates at voltages as low as 60 kV, in contrast with the 200 kV minimum of the JEOL 2200FS. The lower voltages, says Watanabe, will enable the study of carbon-based and other “soft” materials that can be easily damaged by the bombardment of higher-energy electron beams.
“When we first considered purchasing the new STEM, the lowest operating voltage possible was 120 kV. JEOL said this could be reduced to 80 kV. We asked for 60 kV, which would allow us to characterize many more materials. The energy threshold of 80 kV is still too high for some organic materials that require gentle imaging conditions.”
Another piece of ancillary equipment in the new STEM is an electron tomography stage that tilts in increments of one-half degree, making it possible to take as many as 720 2-D images of the same object. When combined, these images can provide a 3-D reconstruction of a nano-object.
“The ability to obtain 3-D reconstructions is critical for determining the location of individual nanoclusters on a support in a catalyst material,” says Watanabe.
The new STEM, which is expected to be delivered in early 2012, achieves greater stability with a larger column. Improved shielding isolates it more effectively from outside air movements, changes in temperature and acoustical waves, and electrical interference.
Similar JEM-ARM200F instruments are being installed at several other universities, says Watanabe, but they lack many of the special features possessed by Lehigh’s STEM.
“I’m sure this new instrument configuration will become standard in the next couple of years. We’re proud to be the first university to have the prototype.”
Playing pool with atoms
While electron microscopy alone can achieve angstrom-level resolution, says Israel Wachs, optical spectrometers are uniquely suited to detect the random signals given off by the amorphous surfaces where material properties are determined and where catalytic activity takes place.
Two instruments give Lehigh an unparalleled ability to study surfaces, says Wachs, professor of chemical engineering and director of the Operando Molecular Spectroscopy and Catalysis Research Lab. A third records critical events that occur in nanoseconds.
The university’s Scienta ESCA 300, one of the world’s most powerful high-resolution X-ray photoelectron spectrometers (HR-XPS), complements the high sensitivity-low energy ion-scattering spectrometer (HS-LEIS), which Lehigh recently purchased with an NSF grant.
LEIS, says Wachs, “is the only technique that can identify the atoms on the outermost layer of a solid surface. XPS provides very useful chemical information from the top 10-20 atomic layers.
“These techniques combine data from the surface and near subsurface, giving a new perspective on material surfaces while establishing the basic relationships between a material’s structure and its performance. They will assist greatly in designing advanced materials.”
The physical principles behind HS-LEIS are similar to those of a game of billiards. Like a cue ball, noble gas ions are fired at the surface of a sample. An ion interacts with a sample’s surface atom the same way a cue ball strikes another pool ball – it bounces straight back or is deflected at an angle. In the process, a fraction of its energy is transferred to the surface atom.
The amount of energy lost is directly related to the atomic weight of the surface atom. The spectrometer measures the energy of the rebounding noble gas ions to determine the identity of the atom from which it was scattered.
A toroidal energy analyzer in Lehigh’s HS-LEIS spectrometer includes a position-sensitive detector and time-of-flight mass filter that provide a 3,000-fold improvement in sensitivity over its predecessors. It also allows for elemental 2-D surface mapping that complements the elemental 2-D near-surfaceregion mapping capabilities of the ESCA 300.
A new Fourier Transform-Infrared (FT-IR) 8700 spectrometer enhances Lehigh’s surface analysis capabilities. Lehigh is one of the first research facilities to acquire this instrument, which collects signals in as little as 10 nanoseconds and can study liquid-solid and gas-solid interfaces.
The FT-IR 8700 provides molecular-level information critical to the photocatalytic splitting of water into oxygen and hydrogen, a clean fuel. The splitting occurs in just the nanoseconds that it takes for light-excited electrons to hop from the valence to the conduction band of a solid semiconductor mixed oxide material, and back.
“Many photocatalytic reactions and chemical processes happen in time scales on the order of nanoseconds,” says Charles A. Roberts, a Ph.D. student in chemical engineering. “FT-IR lets us monitor the rapid electron and chemical transformations that occur during these processes.”
Two views are better than one
To rapidly obtain a 3-D picture of a material’s surface and the location, width, height and depth of its bumps and indentations, scientists rely on atomic force microscopy (AFM).
An atomic force microscope consists of a probe, or needle, that scans a surface like an old record player stylus, measuring the height and recording the position of its topographical features. The needle can also detect surface modulations, magnetic and chemical forces, and atomic and electronic structure.
The resulting representation, says Richard Vinci, resembles a hiker’s topographical map. “AFM is a wonderful visualization tool to imagine what a surface looks like,” says Vinci, professor of materials science and engineering. “You have to imagine because you never really see the surface; what you see is a computer reconstruction of what the surface looks like based on the interaction between the surface and the moving probe.”
Unlike an electron microscope, which operates in a vacuum, an AFM can characterize materials in liquid or air and is thus well-suited to study bio- and nano-materials.
Vinci and Slava V. Rotkin, associate professor of physics in the College of Arts and Sciences, recently acquired an NTEGRA-Spectra, which couples an AFM manufactured by the Russian company NT-MDT with an optical microscope made by Olympus.
By positioning an AFM atop an inverted optical microscope, the NTEGRA allows researchers to examine materials in multiple ways simultaneously. One option is to examine a specimen from below with the optical microscope while probing it from above with AFM. Another is to stimulate a specimen with a laser through the Olympus optics while the AFM measures its properties from above.
“This new instrument was made to our specifications by NT-MDT,” says Vinci. “There is probably no other instrument in the world that is identical to our set-up. The NTEGRA actually contains no new part or component; it is exceptional because of the manner in which existing components are configured.”
The NTEGRA also has fluid cell capabilities to examine biological specimens, says Vinci.
“The NTEGRA is very complex. It is designed to be used for highly customized experiments that can last several weeks.“
What sets Lehigh’s NTEGRA apart from similar instruments, says Vinci, is its ability to simultaneously control the position of a specimen like a nanoparticle, the position of the AFM and the position of the stimulating laser.
Rotkin plans to use the NTEGRA to conduct studies of DNA-wrapped carbon nanotubes (CNTs) utilizing total internal reflection fluorescence (TIRF) in combination with AFM.
“TIRF is often used to look at live cells,” says Rotkin. “We want to look at CNTs, which are smaller than cells, as they sit inside a cell. CNTs are used in medical applications, so we need to find out whether they are harmful to cells.”
The NTEGRA, says Rotkin, is ideally suited to overcome one of the daunting challenges posed by CNTs – locating them on a substrate so they can be studied. It achieves this by combining TIRF and AFM.
“Our microscope can shine laser light at a large enough angle so that only nanoscale objects on top of the cover slip scatter light. If nothing is on the surface, the light is reflected and you see nothing.
If there’s an object on the surface, it scatters light and you see the object.
“The NTEGRA will enable us to investigate a surface with the AFM tip, find the nano-object, then examine it by focusing a light beam on the tip. Often you cannot see the object because it’s too small. The tip is like a big road sign saying ‘Here is your nanotube!’ We can run the TIRF experiment when we know where the object is and where the AFM tip is.
“In short, we will use AFM to locate the object and TIRF to probe the object optically.”
This three-way alignment – of a specimen, and the access to that specimen by both optical microscope and AFM tip – gives Lehigh’s NTEGRA its singular qualities, says Vinci.
“The simultaneous alignment of these three things is critical to the study of CNTs,” he says. “No other tool can do this as well as the new NTEGRA. Other manufacturers’ tools are excellent but they cannot match the NTEGRA’s capabilities.”
