Category Archives: Packaging Materials

Adding hydrogen to graphene


November 3, 2016

Adding hydrogen to graphene could improve its future applicability in the semiconductor industry, when silicon leaves off. Researchers at the Center for Multidimensional Carbon Materials (CMCM), within the Institute for Basic Science (IBS) have recently gained further insight into this chemical reaction. Published in Journal of the American Chemical Society, these findings extend the knowledge of the fundamental chemistry of graphene and bring scientists perhaps closer to realizing new graphene-based materials.

The images show a graphene flake before (a), two minutes (b), and eight minutes (c), after exposure to a solution of lithium and liquid ammonia (Birch-type reaction). Graphene gets gradually hydrogenated starting from the edges. (Reprinted with permission from Zhang X. et al., JACS, Copyright 2016 American Chemical Society) Credit: IBS

The images show a graphene flake before (a), two minutes (b), and eight minutes (c), after exposure to a solution of lithium and liquid ammonia (Birch-type reaction). Graphene gets gradually hydrogenated starting from the edges. (Reprinted with permission from Zhang X. et al., JACS, Copyright 2016 American Chemical Society) Credit: IBS

Understanding how graphene can chemically react with a variety of chemicals will increase its utility. Indeed, graphene has superior conductivity properties, but it cannot be directly used as an alternative to silicon in semiconductor electronics because it does not have a bandgap, that is, its electrons can move without climbing any energy barrier. Hydrogenation of graphene opens a bandgap in graphene, so that it might serve as a semiconductor component in new devices.

While other reports describe the hydrogenation of bulk materials, this study focuses on hydrogenation of single and few-layers thick graphene. IBS scientists used a reaction based on lithium dissolved in ammonia, called the “Birch-type reaction”, to introduce hydrogen onto graphene through the formation of C-H bonds.

The research team discovered that hydrogenation proceeds rapidly over the entire surface of single-layer graphene, while it proceeds slowly and from the edges in few-layer graphene. They also showed that defects or edges are actually necessary for the reaction to occur under the conditions used, because pristine graphene with the edges covered in gold does not undergo hydrogenation.

Using bilayer and trilayer graphene, IBS scientists also discovered that the reagents can pass between the layers, and hydrogenate each layer equally well. Finally, the scientists found that the hydrogenation significantly changed the optical and electric properties of the graphene.

“A primary goal of our Center is to undertake fundamental studies about reactions involving carbon materials. By building a deep understanding of the chemistry of single-layer graphene and a few layer graphene, I am confident that many new applications of chemically functionalized graphenes could be possible, in electronics, photonics, optoelectronics, sensors, composites, and other areas,” notes Rodney Ruoff, corresponding author of this paper, CMCM director, and UNIST Distinguished Professor at the Ulsan National Institute of Science and Technology (UNIST).

van der Pauw measurements with a parameter analyzer are examined followed by a look at Hall effects measurements.

BY MARY ANNE TUPTA, Keithley Instruments Product Line at Tektronix, Cleveland, OH

Semiconductor material research and device testing often involves determining the resistivity and Hall mobility of a sample. The resistivity of a particular semiconductor material primarily depends on the bulk doping used. In a device, the resistivity can affect the capacitance, the series resistance, and the threshold voltage, so it’s important to perform this measurement carefully and accurately.

The resistivity of the semiconductor material is often determined using a four-point probe or Kelvin technique where two of the probes are used to source current and the other two probes are used to measure voltage. Using four probes eliminates measurement errors due to probe resistance, spreading resistance under each probe, and contact resis- tance between each metal probe and the semiconductor material. Because a high impedance voltmeter draws little current, the voltage drops are very small.

One useful Kelvin technique for determining the resistivity of a semiconductor material is the van der Pauw (vdp) method using a parameter analyzer with high input impedance and accurate low current sourcing. This article first looks at van der Pauw measurements with a parameter analyzer followed by a look at Hall effects measurements.

van der Pauw resistivity measurements

The van der Pauw method involves applying a current and measuring voltage using four small contacts on the circumference of a flat, arbitrarily shaped sample of uniform thickness. This method is particularly useful for measuring very small samples because geometric spacing of the contacts is unimportant, meaning that effects due to a sample’s size are irrelevant.

Using this method, the resistivity is derived from a total of eight measurements that are made around the periphery of the sample using the configurations shown in FIGURE 1.

FIGURE 1. van der Pauw resistivity conventions.

FIGURE 1. van der Pauw resistivity conventions.

Once all the voltage measurements are taken, two values of resistivity, ρA and ρB, are derived as follows:

Equation 1

 

where: ρA and ρB are volume resistivities in ohm-cm

ts is the sample thickness in cm

V1–V8 represents the voltages measured by the voltmeter

I is the current through the sample in amperes

fA and fB are geometrical factors based on sample symmetry. They are related to the two resistance ratios QA and QB as shown in the following equations (fA = fB = 1 for perfect symmetry).

QA and QB are calculated using the measured voltages as follows:

Equation 2

Also, Q and f are related as follows:

Equation 3

A plot of this function is shown in FIGURE 2. The value of f can be found from this plot once Q has been calculated.

FIGURE 2. Plot of f vs. Q.

FIGURE 2. Plot of f vs. Q.

Once ρA and ρB are known, the average resistivity (ρAVG) can be determined as follows:

Equation 4

The electrical measurements for determining van der Pauw resistivity require a current source and a voltmeter. To automate measurements, it’s possible to use a programmable switch to switch the current source and the voltmeter to all sides of the sample. However, a parameter analyzer offers greater efficiency.

A parameter analyzer with four source measure units (SMU) and four preamps (for high resistance measurements) is well-suited for performing van der Pauw resis- tivity measurements, and enables measurements of resistances greater than 1012Ω. A key advantage is that each SMU instrument can be configured as a current source or as a voltmeter with no external switching required. This eliminates leakage and offsets errors caused by mechanical switches as well as the need for additional instruments and programming.

For high resistance materials, a current source that can output very small current with a high output impedance is necessary. A differential electrometer with high input impedance is required to minimize loading effects on the sample.

Each terminal of the sample is connected to one SMU instrument, so a parameter analyzer with four SMU instruments is required. A diagram of how the four SMUs are configured for each of the tests is shown in FIGURE 3. For each test, three of the SMU instruments are configured as a current bias and a voltmeter. One of the SMUs applies the test current and the other two SMUs are used as high impedance voltmeters with a test current of zero amps on a low current range (typically 1nA range). The fourth SMU instrument is set to common. The voltage difference is calculated between the two SMU instruments set up as high impedance voltmeters. This measurement setup is duplicated around the sample, with each of the four SMU instruments changing functions in each of the four tests. The test current and voltage differences between the terminals from the four tests are used to calculate resistivity.

FIGURE 3. SMU Instrument Configurations for van der Pauw Measurements.

FIGURE 3. SMU Instrument Configurations for van der Pauw Measurements.

For high resistance samples, it’s necessary to determine the settling time of the measurement. This is done by sourcing current into two terminals of the sample and measuring the voltage difference between the other two terminals. The settling time can be determined by graphing the voltage difference versus the time of the measurement. A timing graph of a very high resistance material is shown in FIGURE 4. Note that settling time needs to be determined every time for different materials; however, it’s not necessary for low resistance materials since they have a short settling time.

FIGURE 4. Voltage vs. time graph of a very high resistance sample.

FIGURE 4. Voltage vs. time graph of a very high resistance sample.

Hall voltage measurements

Hall effect measurements are important to semiconductor material characterization because from the Hall voltage, the conductivity type, carrier density, and mobility can be derived. With an applied magnetic field, the Hall voltage can be measured using the configurations shown in FIGURE 5.

FIGURE 5. Hall voltage measurement configurations.

FIGURE 5. Hall voltage measurement configurations.

With a positive magnetic field, B, current is applied between terminals 1 and 3, and the voltage drop (V2–4+) is measured between terminals 2 and 4. When the current is reversed, the voltage drop (V4–2+) is measured. Next, current is applied between terminals 2 and 4, and the voltage drop (V1–3+) between terminals 1 and 3 is measured. Then the current is reversed and the voltage (V3–1+) is measured again.

Then the magnetic field, B, is reversed and the procedure is repeated again, measuring the four voltages: (V2–4–), (V4–2–), (V1–3–), and (V3–1–).

From the eight Hall voltage measurements, the average Hall coefficient can be calculated as follows:

Equation 5

where: RHC and RHD are Hall coefficients in cm3/C

ts is the sample thickness in cm

V represents the voltages measured by the voltmeter

I is the current through the sample in amperes

B is the magnetic flux in Vs/cm2 (1 Vs/cm2 = 108 gauss)

Once RHC and RHD have been calculated, the average Hall coefficient (RHAVG) can be determined as follows:

Equation 6

From the resistivity (ρAVG) and the Hall coefficient (RHAVG), the mobility (μH) can be calculated:

Equation 7

For successful resistivity measurements, potential sources of errors need to be considered. Here are the errors sources you are most likely to encounter.

Electrostatic Interference — Electrostatic interference occurs when an electrically charged object is brought near an uncharged object. Usually, the effects of the interference are not noticeable because the charge dissi- pates rapidly at low resistance levels. However, high resis- tance materials do not allow the charge to decay quickly and unstable measurements may result. The erroneous readings may be due to either DC or AC electrostatic fields.

To minimize the effects of these fields, an electrostatic shield can be built to enclose the sensitive circuitry. The shield should be made from a conductive material and connected to the low impedance (FORCE LO) terminal of the test instrument. The cabling in the circuit must also be shielded.

Leakage Current — For high resistance samples, leakage current may degrade measurements. The leakage current is due to the insulation resistance of the cables, probes, and test fixturing.

Leakage current may be minimized by using good quality insulators, by reducing humidity, and by using guarding.

A guard is a conductor connected to a low impedance point in the circuit that is nearly at the same potential as the high impedance lead being guarded. Using triax cabling and fixturing will ensure that the high impedance terminal of the sample is guarded. The guard connection will also reduce measurement time since the cable capacitance will no longer affect the time constant of the measurement.

Light — Currents generated by photoconductive effects can degrade measurements, especially on high resistance samples. To prevent this, the sample should be placed in a dark chamber.

Temperature — Thermoelectric voltages may also affect measurement accuracy. Temperature gradients may result if the sample temperature is not uniform. Thermoelectric voltages may also be generated from sample heating caused by the source current. Heating from the source current will more likely affect low resistance samples, because a higher test current is needed to make the voltage measure- ments easier. Temperature fluctuations in the laboratory environment may also affect measurements. Because semiconductors have a relatively large temperature coeffi- cient, temperature variations in the laboratory may need to be compensated for by using correction factors.

Carrier Injection — To prevent minority/majority carrier injection from influencing resistivity measurements, the voltage difference between the two voltage sensing terminals should be kept at less than 100mV, ideally 25mV, since the thermal voltage, kt/q, is approximately 26mV. The test current should be kept as low as possible without affecting the measurement precision.

Conclusion

The van der Pauw technique in conjunction with a parameter analyzer is a proven method for determining the resistivity of very small samples because geometric spacing of the contacts is unimportant. Hall effect measurements are important to semiconductor material characterization for determining conductivity type, carrier density, and mobility. Some parameter analyzers may include built-in configurable tests that include the necessary calculations.

For successful measurements, it’s important to consider potential sources of error including electronics interference, leakage current and environmental factor such as light and temperature. Resistivity can impact the characteristics of a device, serving as reminder of the importance of making accurate and repeatable measurements.

Gallium nitride (GaN) has emerged as one of the most important and widely used semiconducting materials. Its optoelectronic and mechanical properties make it ideal for a variety of applications, including light-emitting diodes (LEDs), high-temperature transistors, sensors and biocompatible electronic implants in humans.

In 2014, three Japanese scientists won the Nobel Prize in physics for discovering GaN’s critical role in generating blue LED light, which is required, in combination with red and green light, to produce white LED light sources.

Now, four Lehigh engineers have reported a previously unknown property for GaN: Its wear resistance approaches that of diamonds and promises to open up applications in touch screens, space vehicles and radio-frequency microelectromechanical systems (RF MEMS), all of which require high-speed, high-vibration technology.

The researchers reported their findings in August in Applied Physics Letters (APL) in an article titled “Ultralow wear of gallium nitride.” The article’s authors are Guosong Zeng, a Ph.D. candidate in mechanical engineering; Nelson Tansu, Daniel E. ’39 and Patricia M. Smith Endowed Chair Professor in the Electrical and Computer Engineering department, and Director of the Center for Photonics and Nanoelectronics (CPN); Brandon A. Krick, assistant professor of mechanical engineering and mechanics; and Chee-Keong Tan ’16 Ph.D., now assistant professor of electrical and computer engineering at Clarkson University.

GaN’s electronic and optical properties have been studied extensively for several decades, said Zeng, the lead author of the APL article, but virtually no studies have been done of its tribological properties, that is, its resistance to the mechanical wear imposed by reciprocated sliding.

“Our group is the first to investigate the wear performance of GaN,” said Zeng. “We have found that its wear rate approaches that of diamonds, the hardest material known.”

Wear rate is expressed in negative cubic millimeters of Newton meters (Nm). The rate for chalk, which has virtually no wear resistance, is on the order of 10 2 mm3/Nm, while that of diamonds is between 10-9 and 10-10, making diamonds eight orders of magnitude more wear resistant than chalk. The rate for GaN ranges from 10¬-7 to 10-9, approaching the wear resistance of diamonds and three to five orders of magnitude more wear resistant than silicon (10-4).

The Lehigh researchers measured the wear rate and friction coefficients of GaN using a custom microtribometer to perform dry sliding wear experiments. They were surprised by the results.

“When performing wear measurements of unknown materials,” they wrote in APL, “we typically slide for 1,000 cycles, then measure the wear scars; [these] experiments had to be increased to 30,000 reciprocating cycles to be measurable with our optical profilometer.

“The large range in wear rates (about two orders of magnitude)…can provide insight into the wear mechanisms of GaN.”

That range in wear resistance, the researchers said, is caused by several factors, including environment, crystallographic direction and, especially, humidity.

“The first time we observed the ultralow wear rate of GaN was in winter,” said Zeng. “These results could not be replicated in summer, when the material’s wear rate increased by two orders of magnitude.”

To determine how the higher summer humidity was affecting GaN’s wear performance, the researchers put their tribometer in a glove box that can be backfilled with either nitrogen or humid air.

“We observed that as we increased the humidity inside the glove box, we also increased the wear rate of GaN,” said Zeng.

Zeng gave a presentation about the Lehigh project in October at the International Workshop on Nitride Semiconductors (IWN 2016) in Orlando, Florida. The session at which he spoke was titled “Wear of Nitride Materials and Properties of GaN-based structures.” Zeng was one of seven presenters at the session and the only one to discuss the wear properties of GaN and other III-Nitride materials.

Tansu, who has studied GaN for more than a decade, and Krick, a tribology expert, became curious about GaN’s wear performance several years ago when they discussed their research projects after a Lehigh faculty meeting.

“Nelson asked me if anyone had ever investigated the friction and wear properties of gallium nitride,” said Krick, “and I said I didn’t know. We checked later and found a wide-open field.”

Tansu said the group’s discovery of GaN’s hardness and wear performance could have a dramatic effect on the electronic and digital device industries. In a device such as a smartphone, he said, the electronic components are housed underneath a protective coating of glass or sapphire. This poses potential compatibility problems which could be avoided by using GaN.

“The wear resistance of GaN,” said Tansu, “gives us the opportunity to replace the multiple layers in a typical semiconductor device with one layer made of a material that has excellent optical and electrical properties and is wear-resistant as well.

“Using GaN, you can build an entire device in a platform without multiple layers of technologies. You can integrate electronics, light sensors and light emitters and still have a mechanically robust device. This will open up a new paradigm for designing devices. And because GaN can be made very thin and still strong, it will accelerate the move to flexible electronics.”

In addition to its unexpectedly good wear performance, said Zeng, GaN also has a favorable radiation hardness, which is an important property for the solar cells that power space vehicles. In outer space, these solar cells encounter large quantities of very fine cosmic dust, along with x-rays and gamma rays, and thus require a wear-resistant coating, which in turn needs to be compatible with the cell’s electronic circuitry. GaN provides the necessary hardness without introducing compatibility issues with the circuitry.

The Lehigh group has begun collaborating with Bruce E. Koel, a surface chemistry expert and professor of chemical and biological engineering at Princeton University, to gain a better understanding of the interaction of GaN and water under contact. Koel was formerly a chemistry professor and vice president for research and graduate studies at Lehigh.

To determine the evolution of wear with GaN, the group has subjected GaN to stresses by running slide tests in which the slide distance and the corresponding number of cycles are varied. The group then uses an x-ray photoelectron spectrometer (XPS), which can identify the elemental composition of the first 12 nanometers of a surface, to scan the unworn surface of the GaN, the scar created by the slide machine, and the wear particles deposited by the slide machine on either side of the scar.

The group plans next to use aberration-corrected transmission electron microscopy to examine the lattice of atoms beneath the scar. Meanwhile, they will simulate a test in which the lattice is strained with water in order to observe the variations caused by deforming energy.

“This is a very new experiment,” said Zeng. “It will enable us to see dynamic surface chemistry by watching the chemical reaction that results when you apply shear, tensile or compressive pressure to the surface of GaN.”

An electric current will not only heat a hybrid metamaterial, but will also trigger it to change state and fade into the background like a chameleon in what may be the proof-of-concept of the first controllable metamaterial device, or metadevice, according to a team of engineers.

“Previous metamaterials work focused mainly on cloaking objects so they were invisible in the radio frequency or other specific frequencies,” said Douglas H. Werner, John L. and Genevieve H. McCain Chair Professor of electrical engineering, Penn State. “Here we are not trying to make something disappear, but to make it blend in with the background like a chameleon and we are working in optical wavelengths, specifically in the infrared.”

Metamaterials are synthetic, composite materials that possess qualities not seen in natural materials. These composites derive their functionality by their internal structure rather than by their chemical composition. Existing metamaterials have unusual electromagnetic or acoustic properties. Metadevices take metamaterials and do something of interest or value as any device does.

“The key to this metamaterial and metadevice is vanadium dioxide, a phase change crystal with a phase transition that is triggered by temperatures created by an electric current,” said Lei Kang, research associate in electrical engineering, Penn State.

The metamaterial is composed of a base layer of gold thick enough so that light cannot pass through it. A thin layer of aluminum dioxide separates the gold from the active vanadium dioxide layer. Another layer of aluminum dioxide separates the vanadium from a gold-patterned layer that is attached to an external electric source. The geometry of the patterned mesh screen controls the functional wavelength range. The amount of current flowing through the device controls the Joule heating effect, the heating due to resistance.

“The proposed metadevice integrated with novel transition materials represents a major step forward by providing a universal approach to creating self-sufficient and highly versatile nanophotonic systems,” the researchers said in today’s (Oct. 27) issue of Nature Communications.

As a proof of concept, the researchers created a .035 inch by .02 inch device and cut the letters PSU into the gold mesh layer so the vanadium dioxide showed through. The researchers photographed the device using an infrared camera at 2.67 microns. Without any current flowing through the device, the PSU stands out as highly reflective. With a current of 2.03 amps, the PSU fades into the background and becomes invisible, while at 2.20 amps, the PSU is clearly visible but the background has become highly reflective.

The response of the vanadium dioxide is tunable by altering the current flowing through the device. According to the researchers, vanadium dioxide can change state very rapidly and it is the device configuration that limits the tuning.

Thermoelectric materials, which can directly convert thermal energy into electrical energy (Seebeck effect), can be effectively used for the development of a clean and environmentally compatible power-generation technology.

Picture of the synthesized bulk CaMgSi thermoelectric material through the procedure developed in this study. CREDIT: TOYOHASHI UNIVERSITY OF TECHNOLOGY.

Picture of the synthesized bulk CaMgSi thermoelectric material through the procedure developed in this study. CREDIT: TOYOHASHI UNIVERSITY OF TECHNOLOGY.

However, these materials are not commonly used for practical applications as they mostly include toxic and/or expensive elements.

Recently, researchers at the Materials Function Control Laboratory at the Toyohashi University of Technology and the Nagoya Institute of Technology have successfully synthesized a new thermoelectric material, CaMgSi, which is an intermetallic compound. The key to this development was the synthesis procedure; bulk CaMgSi intermetallic compound was synthesized by combining mechanical ball-milling (MM) and pulse current sintering (PCS) processes.

“Appearance of thermoelectric property in the intermetallic compound, CaMgSi, has been predicted by both theoretical and experimental studies”, explain the researchers of this work, Nobufumi Miyazaki and Nozomu Adachi. ” However, the biggest issue in front of us was the synthesis of thermoelectric CaMgSi of optimal size “, they continued. In general, alloys are produced by mixing the constituent elements in their molten forms. However, when a temperature is raised up to the melting temperature of Si, Mg vapors; liquids of Ca, Mg, and Si cannot exists at same time.

Associate Professor Yoshikazu Todaka says “To overcome the aforementioned problem, we chose the mechanical ball milling process to mix the elements homogeneously, without melting, and then a chemical reaction between Ca, Mg, and Si was induced using the pulse current sintering process”.

Consequently, the intermetallic compound, CaMgSi, with sufficient size was synthesized. The thermoelectric property of the synthesized CaMgSi exhibited a performance comparable to that of the previously developed Mg-based thermoelectric materials. It is expected that an addition of a fourth element to CaMgSi renders it with superior thermoelectric properties. Interestingly, they found that the novel thermoelectric can exhibit both n- and p-type conductivity with a slight change in the composition of CaMgSi. Such a property for the material is very significant for its application in power-generation modules.

The new thermoelectric material synthesized in this study is composed of lightweight elements, and has a low density of 2.2 g/cm3. Therefore, one of the possible applications of the material is in automobiles to utilize waste heat emitted from engines. These findings could contribute to the development of green energy technology.

Glass fibres do everything from connecting us to the internet to enabling keyhole surgery by delivering light through medical devices such as endoscopes. But as versatile as today’s fiber optics are, scientists around the world have been working to expand their capabilities by adding semiconductor core materials to the glass fibers.

Ursula Gibson, a professor of physics at the Norwegian University of Science and Technology, holds a glass fiber with a semiconductor core. Rapid heating and cooling of this kind of fiber allows the researchers to make functional materials with applications beyond traditional fiber optics. Credit: Nancy Bazilchuk

Ursula Gibson, a professor of physics at the Norwegian University of Science and Technology, holds a glass fiber with a semiconductor core. Rapid heating and cooling of this kind of fiber allows the researchers to make functional materials with applications beyond traditional fiber optics. Credit: Nancy Bazilchuk

Now, a team of researchers has created glass fibers with single-crystal silicon-germanium cores. The process used to make these could assist in the development of high-speed semiconductor devices and expand the capabilities of endoscopes says Ursula Gibson, a physics professor at the Norwegian University of Science and Technology and senior author of the paper.

“This paper lays the groundwork for future devices in several areas,” Gibson said, because the germanium in the silicon core allows researchers to locally alter its physical attributes.

The article, “Laser recrystallization and inscription of compositional microstructures in crystalline SiGe-core fibres,” was published in Nature Communications on October 24.

Melting and recrystallizing

To understand what the researchers did, you need to recognize that silicon and germanium have different melting points. When the two substances are combined in a glass fiber, flecks of germanium-rich material are scattered throughout the fiber in a disorderly way because the silicon has a higher melting point and solidifies, or “freezes” first. These germanium flecks limit the fiber’s ability to transmit light or information. “When they are first made, these fibers don’t look very good,” Gibson said.

But rapidly heating the fiber by moving it through a laser beam allowed the researchers to melt the semiconductors in the core in a controlled fashion. Using the difference in the solidification behavior, the researchers were able to control the local concentration of the germanium inside the fiber depending upon where they focused the laser beam and for how long.

“If we take a fibre and melt the core without moving it, we can accumulate small germanium-rich droplets into a melt zone, which is then the last thing to crystalize when we remove the laser slowly,” Gibson said. “We can make stripes, dots… you could use this to make a series of structures that would allow you to detect and manipulate light.”

An interesting structure was produced when the researchers periodically interrupted the laser beam as it moved along their silicon-germanium fibre. This created a series of germanium-rich stripes across the width of the 150-micrometer diameter core. That kind of pattern creates something called a Bragg grating, which could help expand the capability of long wavelength light-guiding devices. “That is of interest to the medical imaging industry,” Gibson said.

Rapid heating, cooling key

Another key aspect of the geometry and laser heating of the silicon-germanium fibre is that once the fibre is heated, it can also be cooled very quickly as the fibre is carried away from the laser on a moving stage.

Controlled rapid cooling allows the mixture to solidify into a single uniform crystal the length of the fibre — which makes it ideal for optical transmission.

Previously, people working with bulk silicon-germanium alloys have had problems creating a uniform crystal that is a perfect mix, because they have not had sufficient control of the temperature profile of the sample.

“When you perform overall heating and cooling, you get uneven composition through the structure, because the last part to freeze concentrates excess germanium,” Gibson said. “We have shown we can create single crystalline silicon-germanium at high production rates when we have a large temperature gradient and a controlled growth direction.”

Transistors that switch faster

Gibson says the laser heating process could also be used to simplify the incorporation of silicon-germanium alloys into transistor circuits.

“You could adapt the laser treatment to thin films of the alloy in integrated circuits,” she said.

Traditionally, Gibson said, electronics researchers have looked at other materials, such as gallium arsenide, in their quest to build ever-faster transistors. However, the mix of silicon and germanium, often called SiGe, allows electrons to move through the material more quickly than they move through pure silicon, and is compatible with standard integrated circuit processing.

“SiGe allows you to make transistors that switch faster” than today’s silicon-based transistors, she said, “and our results could impact their production.”

A new design for solar cells that uses inexpensive, commonly available materials could rival and even outperform conventional cells made of silicon.

A tandem perovskite solar cell boosts efficiency by absorbing high- and low-energy photons from the sun. Credit: Rongrong Cheacharoen/Stanford University

A tandem perovskite solar cell boosts efficiency by absorbing high- and low-energy photons from the sun. Credit: Rongrong Cheacharoen/Stanford University

Writing in the Oct. 21 edition of Science, researchers from Stanford and Oxford describe using tin and other abundant elements to create novel forms of perovskite – a photovoltaic crystalline material that’s thinner, more flexible and easier to manufacture than silicon crystals.

“Perovskite semiconductors have shown great promise for making high-efficiency solar cells at low cost,” said study co-author Michael McGehee, a professor of materials science and engineering at Stanford. “We have designed a robust, all-perovskite device that converts sunlight into electricity with an efficiency of 20.3 percent, a rate comparable to silicon solar cells on the market today.”

The new device consists of two perovskite solar cells stacked in tandem. Each cell is printed on glass, but the same technology could be used to print the cells on plastic, McGehee added.

“The all-perovskite tandem cells we have demonstrated clearly outline a roadmap for thin-film solar cells to deliver over 30 percent efficiency,” said co-author Henry Snaith, a professor of physics at Oxford. “This is just the beginning.”

Tandem technology

Previous studies showed that adding a layer of perovskite can improve the efficiency of silicon solar cells. But a tandem device consisting of two all-perovskite cells would be cheaper and less energy-intensive to build, the authors said.

“A silicon solar panel begins by converting silica rock into silicon crystals through a process that involves temperatures above 3,000 degrees Fahrenheit (1,600 degrees Celsius),” said co-lead author Tomas Leijtens, a postdoctoral scholar at Stanford. “Perovskite cells can be processed in a laboratory from common materials like lead, tin and bromine, then printed on glass at room temperature.”

But building an all-perovskite tandem device has been a difficult challenge. The main problem is creating stable perovskite materials capable of capturing enough energy from the sun to produce a decent voltage.

A typical perovskite cell harvests photons from the visible part of the solar spectrum. Higher-energy photons can cause electrons in the perovskite crystal to jump across an “energy gap” and create an electric current.

A solar cell with a small energy gap can absorb most photons but produces a very low voltage. A cell with a larger energy gap generates a higher voltage, but lower-energy photons pass right through it.

An efficient tandem device would consist of two ideally matched cells, said co-lead author Giles Eperon, an Oxford postdoctoral scholar currently at the University of Washington.

“The cell with the larger energy gap would absorb higher-energy photons and generate an additional voltage,” Eperon said. “The cell with the smaller energy gap can harvest photons that aren’t collected by the first cell and still produce a voltage.”

The smaller gap has proven to be the bigger challenge for scientists. Working together, Eperon and Leijtens used a unique combination of tin, lead, cesium, iodine and organic materials to create an efficient cell with a small energy gap.

“We developed a novel perovskite that absorbs lower-energy infrared light and delivers a 14.8 percent conversion efficiency,” Eperon said. “We then combined it with a perovskite cell composed of similar materials but with a larger energy gap.”

The result: A tandem device consisting of two perovskite cells with a combined efficiency of 20.3 percent.

“There are thousands of possible compounds for perovskites,” Leijtens added, “but this one works very well, quite a bit better than anything before it.”

Seeking stability

One concern with perovskites is stability. Rooftop solar panels made of silicon typically last 25 years or more. But some perovskites degrade quickly when exposed to moisture or light. In previous experiments, perovskites made with tin were found to be particularly unstable.

To assess stability, the research team subjected both experimental cells to temperatures of 212 degrees Fahrenheit (100 degrees Celsius) for four days.

“Crucially, we found that our cells exhibit excellent thermal and atmospheric stability, unprecedented for tin-based perovskites,” the authors wrote.

“The efficiency of our tandem device is already far in excess of the best tandem solar cells made with other low-cost semiconductors, such as organic small molecules and microcrystalline silicon,” McGehee said. “Those who see the potential realize that these results are amazing.”

The next step is to optimize the composition of the materials to absorb more light and generate an even higher current, Snaith said.

“The versatility of perovskites, the low cost of materials and manufacturing, now coupled with the potential to achieve very high efficiencies, will be transformative to the photovoltaic industry once manufacturability and acceptable stability are also proven,” he said.

Researchers have found an unexpected way to control the thermal conductivity of two-dimensional (2-D) materials, which will allow electronics designers to dissipate heat in electronic devices that use these materials.

2-D materials have a layered structure, with each layer having strong bonds horizontally, or “in plane,” and weak bonds between the layers, or “out of plane.” These materials have unique electronic and chemical properties, and hold promise for use in creating flexible, thin, lightweight electronic devices.

For many of these potential applications, it’s important to be able to dissipate heat efficiently. And this can be tricky. In 2-D materials, heat is conducted differently in plane than it is out of plane.

For example, in one class of 2-D materials, called TMDs, heat is conducted at 100 watts per meter per Kelvin (W/mK) in plane, but at only 2 W/mK out of plane. That gives it a “thermal anisotropy ratio” of about 50.

To better understand the thermal conduction properties of 2-D materials, a team of researchers from North Carolina State University, the University of Illinois at Urbana-Champaign (UI) and the Toyota Research Institute of North America (TRINA) began experimenting with molybdenum disulfide (MoS2), which is a TMD.

The researchers found that, by introducing disorder to the MoS2, they could significantly alter the thermal anisotropy ratio.

The researchers created this disorder by introducing lithium ions between the layers of MoS2. The presence of the lithium ions does two things simultaneously: it puts the layers of the 2-D material out of alignment with each other, and it forces the MoS2 to rearrange the structure of its component atoms.

When the ratio of lithium ions to MoS2 reached 0.34, the in-plane thermal conductivity was 45 W/mK, and the out-of-plane thermal conductivity dropped to 0.4 W/mK- increasing the material’s thermal anisotropy ratio from 50 to more than 100. In other words, heat became more than twice as likely to travel in plane — along the layer, rather than between the layers.

And that was as good as it got. Adding fewer lithium ions made the thermal anisotropy ratio lower. Adding more ions also made it lower. But in both cases, the ratio was affected in a predictable way, meaning that the researchers could tune the material’s thermal conductivity and thermal anisotropy ratio.

“This finding was very counter-intuitive,” says Jun Liu, an assistant professor of mechanical and aerospace engineering at NC State and co-corresponding author of a paper describing the work. “The conventional wisdom has been that introducing disorder to any material would decrease the thermal anisotropy ratio.

“But based on our observations, we feel that this approach to controlling thermal conductivity would apply not only to other TMDs, but to 2-D materials more broadly,” Liu says.

“We set out to advance our fundamental understanding of 2-D materials, and we have,” Liu adds. “But we also learned something that is likely to be of practical use for the development of technologies that make use of 2-D materials.”

University of Alabama at Birmingham researchers will use pressures greater than those found at the center of the Earth to potentially create as yet unknown new materials. In the natural world, such immense forces deep underground can turn carbon into diamonds, or volcanic ash into slate.

Credit: UAB

Credit: UAB

The ability to produce these pressures depends on tiny nanocrystalline-diamond anvils built in a UAB clean room manufacturing facility. Each anvil head is just half the width of an average human hair. The limits of their pressure have not yet been reached as the first 27 prototypes are being tested.

“We have achieved 75 percent of the pressure found at the center of the Earth, or 264 gigapascals, using lab-grown nanocrystalline-diamond micro-anvil,” said Yogesh Vohra, Ph.D., a professor and university scholar of physics in the UAB College of Arts and Sciences. “But the goal is one terapascal, which is the pressure close to the center of Saturn. We are one-quarter of the way there.”

One terapascal, a scientific measure of pressure, is equal to 147 million pounds per square inch.

One key to high pressure is to make the point of the anvil, where the pressure is applied, very narrow. This magnifies the pressure applied by a piston above the micro-anvil, much like the difference of being stepped on by a spiked high heel rather than a loafer.

A more difficult task is how to make an anvil that is able to survive this ultra-high pressure. The solution for the Vohra team is to grow a nanocrystalline pillar of diamond — 30 micrometers wide and 15 micrometers tall — on the culet of a gem diamond. The culet is the flat surface at the bottom of a gemstone.

“We didn’t know that we could grow nanocrystalline diamonds on a diamond base,” Vohra said. “This has never been done before.”

In the 264-gigapascal pressure test at Argonne National Laboratory in Lemont, Illinois, the nanocrystalline diamond showed no sign of deformation. Vohra and colleagues recently reported this result in the American Institute of Physics journal AIP Advances.

“The structure did not collapse when we applied pressure,” Vohra said. “Nanocrystalline diamond has better mechanical properties than gem diamonds. The very small-sized grain structure makes it really tough.”

As more micro-anvils are tested and improved, they will be used to study how transition metals, alloys and rare earth metals behave under extreme conditions. Just as graphitic carbon that is subjected to high pressure and temperature can turn into diamond, some materials squeezed by the micro-anvils may gain novel crystal modifications with enhanced physical and mechanical properties — modifications that are retained when the pressure is released. Such new materials have potential applications in the aerospace, biomedical and nuclear industries.

The micro-anvils are made in a Class 7000 clean room in the UAB Diamond Microfabrication Lab, using maskless lithography and microwave plasma chemical vapor deposition.

Vohra says his research team wants to generate smaller grain sizes in the nanocrystalline diamond, which may make it even stronger; understand how the nanocrystalline diamond is bonded to the gem diamond; and use ion beams to machine the top of the micro-anvil to a hemispherical shape. That shape will mean an even narrower contact point, thus increasing the pressure.

Testing is done at Argonne because it has a very bright synchrotron X-ray source that can probe crystal structure of micron-sized materials under pressure. Vohra and two graduate students travel to Argonne about four times a year.

Researchers at the Center for Multidimensional Carbon Materials (CMCM), within the Institute for Basic Science (IBS) have demonstrated graphene coating protects glass from corrosion. Their research, published in ACS Nano, can contribute to solving problems related to glass corrosion in several industries. Glass has a high degree of both corrosion and chemical resistance. For this reason it is the primary packaging material to preserve medicines and chemicals. However, over time at high humidity and pH, some glass types corrode. Corroded glass loses its transparency and its strength is reduced. As a result, the corrosion of silicate glass, the most common and oldest form of glass, by water is a serious problem especially for the pharmaceutical, environmental and optical industries, and in particular in hot and humid climates.

Although there are different types of glass, ordinary glazing and containers are made of silicon dioxide (SiO2), sodium oxide (Na2O) along with minor additives. Glass corrosion begins with the adsorption of water on the glass surface. Hydrogen ions from water then diffuse into the glass and exchange with the sodium ions present on the glass surface. The pH of the water near the glass surface increases, allowing the silicate structure to dissolve.

Scientists have been looking at how to coat glass to protect it from damage. An ideal protective coating should be thin, transparent, and provide a good diffusion barrier to chemical attack. Graphene with its chemical inertness, thinness, and high transparency makes it very promising as a coating material. Moreover, owing to its excellent chemical barrier properties it blocks helium atoms from penetrating through it. The use of graphene coating is being explored as a protective layer for other materials requiring resistance to corrosion, oxidation, friction, bacterial infection, electromagnetic radiation, etc.

IBS scientists grew graphene on copper using a technique previously invented by Prof. Rodney S. Ruoff and collaborators, and transferred either one or two atom-thick layers of graphene onto both sides of rectangular pieces of glass. The effectiveness of the graphene coating was evaluated by water immersion testing and observing the differences between uncoated and coated glass. After 120 days of immersion in water at 60°C, uncoated glass samples had significantly increased in surface roughness and defects, and reduced in fracture strength. In contrast, both the single and double layer graphene-coated glasses had essentially no change in both fracture strength and surface roughness.

“The purpose of the study was to determine whether graphene grown by chemical vapor deposition on copper foils, a now established method, could be transferred onto glass, and protect the glass from corrosion. Our study shows that even one atom-thick layer of graphene does the trick,” explains Prof. Ruoff, director of the CMCM and Professor at the Ulsan National Institute of Science and Technology (UNIST). “In the future, when it is possible to produce larger and yet higher-quality graphene sheets and to optimize the transfer on glass, it seems reasonably likely that graphene coating on glass will be used on an industrial scale.”