Nature loves crystals. Salt, snowflakes and quartz are three examples of crystals – materials characterized by the lattice-like arrangement of their atoms and molecules.

Industry loves crystals, too. Electronics are based on a special family of crystals known as semiconductors, most famously silicon.

To make semiconductors useful, engineers must tweak their crystalline lattice in subtle ways to start and stop the flow of electrons.

Semiconductor engineers must know precisely how much energy it takes to move electrons in a crystal lattice. This energy measure is the band gap.

Semiconductor materials like silicon, gallium arsenide and germanium each have a band gap unique to their crystalline lattice. This energy measure helps determine which material is best for which electronic task.

Now an interdisciplinary team at Stanford has made a semiconductor crystal with a variable band gap. Among other potential uses, this variable semiconductor could lead to solar cells that absorb more energy from the sun by being sensitive to a broader spectrum of light.

The material itself is not new. Molybdenum disulfide, or MoS2, is a rocky crystal, like quartz, that is refined for use as a catalyst and a lubricant.

But in Nature Communications, Stanford mechanical engineer Xiaolin Zheng and physicist Hari Manoharan proved that MoS2 has some useful and unique electronic properties that derive from how this crystal forms its lattice.

Molybdenum disulfide is what scientists call a monolayer: A molybdenum atom links to two sulfurs in a triangular lattice that repeats sideways like a sheet of paper. The rock found in nature consists of many such monolayers stacked like a ream of paper. Each MoS2 monolayer has semiconductor potential.

“From a mechanical engineering standpoint, monolayer MoS2 is fascinating because its lattice can be greatly stretched without breaking,” Zheng said.

By stretching the lattice, the Stanford researchers were able to shift the atoms in the monolayer. Those shifts changed the energy required to move electrons. Stretching the monolayer made MoS2 something new to science and potentially useful in electronics: an artificial crystal with a variable band gap.

“With a single, atomically thin semiconductor material we can get a wide range of band gaps,” Manoharan said. “We think this will have broad ramifications in sensing, solar power and other electronics.”

Scientists have been fascinated with monolayers since the Nobel Prize-winning discovery of graphene, a lattice made from a single layer of carbon atoms laid flat like a sheet of paper.

In 2012, nuclear and materials scientists at MIT devised a theory that involved the semiconductor potential of monolayer MoS2.

With any semiconductor, engineers must tweak its lattice in some way to switch electron flows on and off. With silicon, the tweak involves introducing slight chemical impurities into the lattice.

In their simulation, the MIT researchers tweaked MoS2 by stretching its lattice. Using virtual pins, they poked a monolayer to create nanoscopic funnels, stretching the lattice and, theoretically, altering MoS2’s band gap.

Band gap measures how much energy it takes to move an electron. The simulation suggested the funnel would strain the lattice the most at the point of the pin, creating a variety of band gaps from the bottom to the top of the monolayer.

The MIT researchers theorized that the funnel would be a great solar energy collector, capturing more sunlight across a wide swath of energy frequencies.

When Stanford postdoctoral scholar Hong Li joined the mechanical engineering department in 2013, he brought this idea to Zheng. She led the Stanford team that ended up proving all of this by literally standing the MIT theory on its head.

Instead of poking down with imaginary pins, the Stanford team stretched the MoS2 lattice by thrusting up from below. They did this – for real rather than in simulation – by creating an artificial landscape of hills and valleys underneath the monolayer.

They created this artificial landscape on a silicon chip, a material they chose not for its electronic properties, but because engineers know how to sculpt it in exquisite detail. They etched hills and valleys onto the silicon. Then they bathed their nanoscape with an industrial fluid and laid a monolayer of MoS2 on top.

Evaporation did the rest, pulling the semiconductor lattice down into the valleys and stretching it over the hills.

Alex Contryman, a PhD student in applied physics in Manoharan’s lab, used scanning tunneling microscopy to determine the positions of the atoms in this artificial crystal. He also measured the variable band gap that resulted from straining the lattice this way.

The MIT theorists and specialists from Rice University and Texas A&M University contributed to the Nature Communications paper.

Team members believe this experiment sets the stage for further innovation on artificial crystals.

“One of the most exciting things about our process is that is scalable,” Zheng said. “From an industrial standpoint, MoS2 is cheap to make.”

Added Manoharan: “It will be interesting to see where the community takes this.”

“We have opened the door to a new room,” says Professor Christof Wöll, Director of KIT Institute of Functional Interfaces (IFG). “This new application of metal-organic framework compounds is the beginning only. The end of this development line is far from being reached,” the physicist emphasizes.

Metal-organic frameworks, briefly called MOFs, consist of two basic elements, metal node points and organic molecules, which are assembled to form microporous, crystalline materials. For about a decade, MOFs have been attracting considerable interest of researchers, because their functionality can be adjusted by varying the components. “A number of properties of the material can be changed,” Wöll explains. So far, more than 20,000 different MOF types have been developed and used mostly for the storage or separation of gases.

The team of scientists under the direction of KIT has now produced MOFs based on porphyrines. These porphyrine-based MOFs have highly interesting photophysical properties: Apart from a high efficiency in producing charge carriers, a high mobility of the latter is observed. Computations made by the group of Professor Thomas Heine from Jacobs University Bremen, which is also involved in the project, suggest that the excellent properties of the solar cell result from an additional mechanism – the formation of indirect band gaps – that plays an important role in photovoltaics. Nature uses porphyrines as universal molecules e.g. in hemoglobin and chlorophyll, where these organic dyes convert light into chemical energy. A metal-organic solar cell produced on the basis of this novel porphyrine-MOF is now presented by the researchers in the journal Angewandte Chemie (Applied Chemistry). The contribution is entitled “Photoinduzierte Erzeugung von Ladungsträgern in epitaktischen MOF-Dünnschichten: hohe Leistung aufgrund einer indirekten elektronischen Bandlücke?” (photo-induced generation of charge carriers in epitactic MOF-thin layers: high efficiency resulting from an indirect electronic band gap?).

“The clou is that we just need a single organic molecule in the solar cell,” Wöll says. The researchers expect that the photovoltaic capacity of the material may be increased considerably in the future by filling the pores in the crystalline lattice structure with molecules that can release and take up electric charges.

By means of a process developed at KIT, the crystalline frameworks grow in layers on a transparent, conductive carrier surface and form a homogeneous thin film, so-called SURMOFs.

“The SURMOF process is suited in principle for a continuous manufacturing process and also allows for the coating of larger plastic carrier surfaces,” Wöll says.

Thanks to their mechanical properties, MOF thin films of a few hundred nanometers in thickness can be used for flexible solar cells or for the coating of clothing material or deformable components. While the demand for technical systems converting sunlight into electricity is increasing, organic materials represent a highly interesting alternative to silicon that has to be processed at high costs before it can be used for the photoactive layer of a solar cell.

For the first time in the long and vaunted history of scanning electron microscopy, the unique atomic structure at the surface of a material has been resolved. This landmark in scientific imaging was made possible by a new analytic technique developed by a multi-institutional team of researchers, including scientists from the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab).

“We’ve developed a reasonably direct method for determining the atomic structure of a surface that also addresses the very challenging problem of buried interfaces,” says Jim Ciston, a staff scientist with the National Center for Electron Microscopy (NCEM) at the Molecular Foundry, a DOE Office of Science User Facility. “Although surface atoms represent a minuscule fraction of the total number of atoms in a material, these atoms drive a large portion of the material’s chemical interactions with its environment.”

Ciston is the lead and corresponding author of a paper describing this new analytical method in the journal Nature Communications. The article is titled “Surface Determination through Atomically Resolved Secondary Electron Imaging.” Other co-authors are Hamish Brown, Adrian D’Alfonso, Pratik Koirala, Colin Ophus, Yuyuan Lin, Yuya Suzuki, Hiromi Inada, Yimei Zhu, Les Allen, and Laurence Marks.

Most materials interact with other materials through their surfaces, which are often different in both structure and chemistry from the bulk of the material. Many important processes take place at surfaces, ranging from the catalysts used for the generation of energy-dense fuels from sunlight and carbon dioxide, to how bridges and airplanes rust.

“In essence, the surface of every material can act as its own nanomaterial coating that can greatly change its chemistry and behavior,” Ciston says. “To understand these processes and improve material performance it is vital to know how the atoms are arranged at surfaces. While there are now many good methods for obtaining this information for rather flat surfaces, when the surfaces are rough most currently available tools are limited in what they can reveal.”

“The beauty of this technique is that we can image surface atoms and bulk atoms simultaneously,” says co-author Zhu, a scientist at Brookhaven National Laboratory. “Currently none of any existing methods can achieve this.”

Scanning electron microscopy (SEM) is an excellent technique for studying surfaces but typically provides information only about topology at nanoscale resolution. A highly promising new version of scanning electron microscopy, called “high-resolution scanning electron microscopy,” or HRSEM, extends this resolution to the atomic scale and provides information on both surface and bulk atoms simultaneously, retaining much of the surface sensitivity of traditional SEM through secondary electrons.

Secondary electrons are the result of a highly energized beam of electrons striking a material and causing atoms in the material to emit energy in the form of electrons rather than photons. As a large portion of secondary electrons are emitted from the surface of a material in addition to its bulk they are good resources for obtaining information about atomic surface structure. However, the surface selectivity of HRSEM has never been fully exploited.

“Even though powerful instruments have been available for several years, progress in materials science applications has been slow due to an inability to directly interpret the surface and bulk components of HRSEM images independently,” Ciston says. “This difficulty stemmed from the lack of a fully-developed theoretical framework to understand SEM image formation at the atomic scale.”

Existing secondary electron image simulation methods had to be extended to take into account contributions from valence orbitals in the material, he says, and also the effect of dielectric screening on the efficiency of generating signal from those valence orbitals.

To verify the effectiveness of their new theoretical framework, Ciston, Allen, Marks and their colleagues collected and analyzed in detail a series of HRSEM images of a particular arrangement of atoms at the surface of strontium titanate. These experiments were coupled with careful secondary electron image simulations, density functional theory calculations, and aberration-corrected high resolution transmission electron microscopy.

“Conventional transmission electron microscopy images are well-understood and were needed to confirm that we actually had the correct structure and that the new HRSEM theory was on the right track,” Ciston says. “Taken collectively, the analysis enabled us to unambiguously reference surface information to information from the bulk crystal.”

The excellent agreement between calculations and experimental results showed that HRSEM is a highly promising tool for surface structure determination, including the challenging topic of bulk/surface registration. From their demonstration, the collaboration discovered that previously reported atomic surface structures for strontium titanate with a “6×2 periodicity” are wrong, having failed to detect an unusual seven-fold coordination within a typically high surface coverage of titanium oxide groups.

“We started this work by investigating a well-studied material, but new technique is so powerful that we had to revise much of was already thought to be well-known,” Ciston says.

Co-author Allen, a scientist with Melbourne University in Australia, who led the theoretical and modeling aspects of the new imaging technique, adds: “we now have a sophisticated understanding of what the images mean”.

Perhaps the first target for applying this new HRSEM surface analytic technique will be the study of surface structures on the facets of nanoparticles. The surface structures of nanoparticle facets are extremely challenging to image in the plan view (seen from above) using electron microscopy, a deficit that needs to be corrected as Ciston explains.

“Plan view geometry is important because surface structures will often develop multiple domains, and we need to be sure we’re not projecting through multiple structures and orientations,” he says. “This is a very challenging problem since scanning probe techniques cannot usually address nanoparticle surfaces at atomic resolution, and surface X-ray diffraction requires large, single crystal surfaces.”

Says co-author Marks, a professor of materials science and engineering at Northwestern University, “We are also quite excited by the possibilities of applying these to corrosion problems. The cost to industry and the military of corrosion is enormous, and we need to understand everything that is taking place to produce materials which will last longer.”