Mechanosensation device by molybdenum disulfide (MoS2)

In a breakthrough invention, scientists have successfully executed an experiment of piezoelectricity and the piezotronic effect in an atomically thin material, molybdenum disulfide (MoS₂), resulting in a unique electric generator and mechanosensation devices; although the piezoelectric effect in this material had already been predicted theoretically.

Piezoelectricity is defined as a phenomenon in which pressure generates an electrical voltage in a material or vice-versa. But no experimental observation of piezoelectricity has been made yet for few atom thickness material. The observation of molybdenum disulfide material has unfolded the potential for new types of mechanically controlled electronic devices.

In an interesting application of this phenomenon, this material could be made as a wearable device, integrated into clothing, to convert energy from your body movement to electricity and power wearable sensors or medical devices, or perhaps supply enough energy to charge cell phone.

There are two keys to using molybdenum disulfide for generating current: using an odd number of layers and flexing it in the proper direction. The material is highly polar, but an even number of layers cancels out the piezoelectric effect. The material's crystalline structure also is piezoelectric in only certain crystalline orientations.

Group of researchers placed thin flakes of MoS₂ on flexible plastic substrates and determined how their crystal lattices were oriented using optical techniques. They then patterned metal electrodes onto the flakes and measure current flow as the samples were mechanically deformed. They monitored the conversion of mechanical to electrical energy, and observed voltage and current outputs.

The researchers also noted that the output voltage reversed sign when they changed the direction of applied strain, and that it disappeared in samples with an even number of atomic layers, confirming theoretical predictions published last year. The presence of piezotronic effect in odd layered MoS₂ was also observed for the first time.

To be piezoelectric, a material must break central symmetry. A single atomic layer of MoS₂ has such a structure, and should be piezoelectric. However, in bulk MoS₂, successive layers are oriented in opposite directions, and generate positive and negative voltages that cancel each other out and give zero net piezoelectric effect.

In fact, MoS₂ is just one of a group of 2D semiconducting materials known as transition metal dichalcogenides, all of which are predicted to have similar piezoelectric properties. These are part of an even larger family of 2D materials whose piezoelectric materials remain unexplored. The research could lead to complete atomic-thick nanosystems that are self-powered by harvesting mechanical energy from the environment. This study also reveals the piezotronic effect in two-dimensional materials for the first time, which greatly expands the application of layered materials for human-machine interfacing, robotics, MEMS, and active flexible electronics.

Mechanical Device invented to measure the mass of a single molecule

A team led by scientists at the California Institute of Technology (Caltech) have made the first-ever mechanical device that can measure the mass of individual molecules one at a time.

This new technology, the researchers say, will eventually help doctors diagnose diseases, enable biologists to study viruses and probe the molecular machinery of cells, and even allow scientists to better measure nanoparticles and air pollution.

The device, which is only a couple millionths of a meter in size, consists of a tiny, vibrating bridge-like structure. When a particle or molecule lands on the bridge, its mass changes the oscillating frequency in a way that reveals how much the particle weighs.

The new instrument is based on a technique Roukes and his colleagues developed over the last 12 years. In work published in 2009, they showed that a bridge-like nanoelectromechanical device could indeed measure the masses of individual particles, which were sprayed onto the apparatus. The difficulty, however, was that the measured shifts in frequencies depended not only on the particle's actual mass, but also on where the particle landed. Without knowing the particle's landing site, the researchers had to analyze measurements of about 500 identical particles in order to pinpoint its mass.

But with the new and improved technique, the scientists need only one particle to make a measurement. To do so, the researchers analyzed how a particle shifts the bridge's vibrating frequency. All oscillatory motion is composed of so-called vibrational modes. If the bridge just shook in the first mode, it would sway side to side, with the center of the structure moving the most. The second vibrational mode is at a higher frequency, in which half of the bridge moves sideways in one direction as the other half goes in the opposite direction, forming an oscillating S-shaped wave that spans the length of the bridge. There is a third mode, a fourth mode, and so on. Whenever the bridge oscillates, its motion can be described as a mixture of these vibrational modes.

The team found that by looking at how the first two modes change frequencies when a particle lands, they could determine the particle's mass and position. Traditionally, molecules are weighed using a method called mass spectroscopy, in which tens of millions of molecules are ionized -- so that they attain an electrical charge -- and then interact with an electromagnetic field. By analyzing this interaction, scientists can deduce the mass of the molecules.

The problem with this method is that it does not work well for more massive particles which have a harder time gaining an electrical charge. As a result, their interactions with electromagnetic fields are too weak for the instrument to make sufficiently accurate measurements.

The new device, on the other hand, does work well for large particles. In fact, the researchers say, it can be integrated with existing commercial instruments to expand their capabilities, allowing them to measure a wider range of masses.

The researchers demonstrated how their new tool works by weighing a molecule called immunoglobulin, an antibody produced by immune cells in the blood. By weighing each molecule, which can take on different structures with different masses in the body, the researchers were able to count and identify the various types of immunoglobulin. Not only was this the first time a biological molecule was weighed using a nanomechanical device, but the demonstration also served as a direct step toward biomedical applications. Future instruments could be used to monitor a patient's immune system or even diagnose immunological diseases.

In the more distant future, the new instrument could give biologists a view into the molecular machinery of a cell. Proteins drive nearly all of a cell's functions, and their specific tasks depend on what sort of molecular structures attach to them -- thereby adding more heft to the protein -- during a process called posttranslational modification. By weighing each protein in a cell at various times, biologists would now be able to get a detailed snapshot of what each protein is doing at that particular moment in time.

Another advantage of the new device is that it is made using standard, semiconductor fabrication techniques, making it easy to mass-produce. That's crucial, since instruments that are efficient enough for doctors or biologists to use will need arrays of hundreds to tens of thousands of these bridges working in parallel. 

Nanomechanical oscillator

Physicists, have shown how a nanomechanical oscillator can be used for detection and amplification of feeble radio waves or microwaves. A measurement using such a tiny device, resembling a miniaturized guitar string, can be performed with the least possible disturbance.

The researchers cooled the nanomechanical oscillator, thousand times thinner than a human hair, down to a low temperature near the absolute zero at -273 centigrade. Under such extreme conditions, even nearly macroscopic sized objects follow the laws of quantum physics which often contradict common sense. In the Low Temperature Laboratory experiments, the nearly billion atoms comprising the nanomechanical resonator were oscillating in pace in their shared quantum state.

The scientists had fabricated the device in contact with a superconducting cavity resonator, which exchanges energy with the nanomechanical resonator. This allowed amplification of their resonant motion. This is very similar to what happens in a guitar, where the string and the echo chamber resonate at the same frequency. Instead of the musician playing the guitar string, the energy source was provided by a microwave laser.

WANDA: Reducing the Human Error!

Berkeley Lab scientists have established a revolutionary nanocrystal-making robot, capable of producing nanocrystals with extreme precision. This one-of-a-kind robot provides colloidal nanocrystals with custom-made properties for electronics, biological labeling and luminescent devices.

This robotic engineer is named WANDA (Workstation for Automated Nanomaterial Discovery and Analysis) and was developed in collaboration with Symyx Technologies at the Molecular Foundry, a U.S. Department of Energy User Facility located at Berkeley Lab. By automating the synthesis of these nanocrystals, WANDA overcomes the issues facing traditional techniques, which can be laborious and are difficult to reproduce from one laboratory to the next. What's more, WANDA's synthetic prowess can help researchers sift through a large, diverse pool of materials for specific applications. Such a combinatorial approach has been used for decades in the pharmaceutical industry and now is being applied to nanomaterials at the Foundry.

WANDA makes nanocrystals of exceptional quality - every time - optimized for different applications. It can also be used to discover new nanocrystal compositions with advantageous properties.

WANDA's liquid-handling robotics prepares and initiates reactions by injecting nanocrystal precursor chemicals into an array of reactors. After a series of reactions is complete, the structural and optical properties of these nanocrystals can be screened rapidly, also using automated methods. WANDA is kept inside a nitrogen-filled chamber, designed to keep oxygen and water from interacting with reactive precursor chemicals and freshly formed nanocrystals. Since this robot is controlled by software protocols, novice users can direct WANDA to perform complex workflows that traditionally require extensive chemistry experience.

Scientists have directed WANDA to produce and optimize a diverse set of nanomaterials under conditions analogous to those employed in traditional flask-based chemistry. Starting with widely studied and practically useful nanomaterials; such as cadmium selenide quantum dots, whose size can be adjusted to emit different colors of visible light. Scientists showed how WANDA can optimize the size, crystal structure and luminescence properties of different nanocrystals.

Scientists are expecting a revolutionary change in the way nanoscience research is performed. Not only does WANDA enable the optimization and mass production of nanoparticles users need, but this robot also facilitates experiments that give us a deeper understanding into the chemistry and physics of nanoscale materials.

MEMS: AN ULTIMATE CONQUEST

MEMS are poised to become the most important technology of the 21st century, but only if packaging issues can be resolved. Substantial progress continues in the key areas of lower-cost ceramics, molded plastic cavities and wafer-level packaging.

MEMS are the ultimate enabling technology for integrating virtually any phenomena — motion, light, sound, radio, chemistry and computation — all on a single chip. High-volume commercial products sense complex motion, measure the slightest pressures, and propel fluids. Newer products receive and send light beams, others detect specific molecules, and some deal with several "senses" simultaneously. If the logic circuit is the brain, MEMS add the eyes, nose, ears, touch, taste buds and super-human sensing ability. MEMS also bring control and manipulation, the hands and fingers that enable chips to operate on matter. This merging of motion, sensing, control and computation represents a gigantic leap in technology.

MEMS technology is essentially the grand unification of mechanical, photonic, electrical and electronic subcomponents into an integrated system that could be the most important technologies of the century. Developers are also adding optics and photonics to expand the increasingly popular subset known as "MOEMS," or optical MEMS. We now have a potent and versatile convergence of the world's technologies onto and into a single remarkable chip. We can build incredibly small machines that perform the same mechanical, optical, electrical and electronic functions found in massive equipment from our macro world. It's like shrinking everyday machinery onto tiny chips. MEMS are the ultimate system-on-a-chip (SoC), the new embodiment of the old cliché, "Shrink the world onto a chip.

A PARADOX

MEMS present a "new" or "old" paradox. MEMS remain a hot "new" technology that achieved notoriety during this decade. So is it new? Surprisingly, MEMS technology is as mature as the IC. But MEMS are as modern as nanotechnology. And the two, which are often grouped together, may converge to become nanoelectromechanical systems (NEMS). The nanotech concept is often attributed to the Nobel-prize-winning physicist Richard Feynman and his famous 1959 lecture, "There's Plenty of Room at the Bottom." But Feynman was also talking about MEMS when he offered up the first MEMS challenge and prize to the person who could make an operating electric motor only 1/64 in.3. Less than a decade later, patents were filed that detailed the basic MEMS principles.

Modern MEMS and ICs travel in parallel universes. Most MEMS are a clever extension of semiconductor technology, where modified fab processes create mechanical and macro world structures on wafers, along with integrated circuitry. MEMS build on the huge semiconductor industry, tapping into that massive knowledge base, synergistically incorporating solid-state electronics where appropriate. But MEMS take us far beyond electronics, with chips that handle multiple energy forms: mechanical forces, light waves and radio waves, as well as conventional electrical energy. MEMS technology can provide the highest level of integration and functionality yet devised. Motion, light, sound, heat, wireless, reactive molecules and electrons can all converge on these unusual chips. MEMS are "convergence central" for all science and technology. The key is extreme integration of dissimilar systems in a miniature form factor to deliver unique, valuable and practical functionality. Most importantly, MEMS use the massively parallel productivity of wafer-level manufacturing. Because MEMS can shrink motors, generators, pumps and even turbines down to the semiconductor scale, it can literally build a factory on a chip.

Motion sensors remain the fastest growing commercial segment, with frequent new product launches by both established and new suppliers. Versatile MEMS accelerometers are trusted with the life-and-death situation of airbag deployment. These chips sense sudden deceleration to instantly "decide" when to deploy an airbag. Newer MEMS gyroscopes even prevent rollovers. Inertial sensors continue to find new applications in a wide range of industries. While airbag systems remain the primary market, many newer applications, such as disk drive free-fall detectors and innovative consumer products, are gaining marketshare. MEMS motion sensors have movedinto consumer applications, including cell phone pedometers, game controllers and the iPhone formotion-based input.

The addition of the photon brings us to MOEMS, which have added versatility.

Packaging, issues

MEMS pose the greatest packaging challenge, but there is a paradox. Early products such as inkjets and airbag sensors use reasonably simple packaging solutions. But emerging areas (i.e., bio-MEMS) have special requirements that need novel approaches. These packages will need to handle fluids. But, let's begin with the general requirements for MEMS, with the understanding that packaging can be device specific. Unfortunately, device-specific solutions mean that progress is slower and standardization is challenging.

MEMS chips can be sensitive to mechanical shock before they are packaged, and are especially vulnerable to particulate contamination from wafer-sawing residue. Some foundries, therefore, do some or all of the packaging. The MEMS chip "motion zone" must be protected during singulation with a mask or a wafer-l

evel technique. The most universal MEMS packaging requirement is protection without restricting mechanical action. However, we cannot simply adopt solutions developed during the past 50 years for "pure" electronic chips. Electronic devices are overmolded with encapsulant contacting the chip face, so this approach would "freeze" moving parts in MEMS.

MEMS have adopted one type of electronic package — the hermetic cavity. Hermetic packages, however, add substantial cost. While early MEMS chips were packaged in somewhat standard ceramic packages, cost impeded success. Progress is being made in reducing the cost of ceramic packages such as QFN. Although full hermeticity can ensure reliability, the added cost can torpedo a product and sink a company. So the hermeticity debate rages on. Which MEMS need full hermeticity? Not all. MEMS devices that handle fluids, especially aqueous, certainly do not need a hermetic enclosure. In fact, inkjet chips have most of their active surface exposed to the environment to "fire" ink at the target.

Another issue is the need for more than "electron plumbing." Even simple inkjets chips have two inputs — electrons and ink fluid — and other chips have even more. But MEMS inertial sensors are somewhat unique, only requiring electrical I/Os, which makes the interface simpler. These chips still need motion, space or "head room." Although early accelerometer packages were hermetic cavities, device makers and packaging partners came up with more cost-effective designs.

Packaging classes

Full hermetic — The full hermetic package has admirably served the electronics and optoelectonics industries. This package class began as the cathode ray tube (CRT), then the vacuum tube and, finally, the semiconductor package. Early opto- and electronic devices could not operate in air, but very few systems require a vacuum today. The hermetic packages evolved from glass to ceramic and metal, and are still used for MEMS.

Non-hermetic plastic package — Plastic packages became mainstream with the breakthrough introduction of the dual inline package (DIP). The chip is typically attached to a metal leadframe by wire bonding, followed by epoxy overmolding. Molding compound — a blend of solid epoxy resins, hardeners, fillers and additives — is forced into a mold that hold the leadframe assembly and is heated to polymerize the material. The molding compound makes direct contact with the die, wire bonds and leadframe. The ball grid array (BGA) uses a similar method, but a different substrate. Overmolding cannot be used on unprotected MEMS chips, but capped devices can be packaged using this method.

Near-hermetic package — The component packaging industry offered two extremes: the full hermetic and non-hermetic packages. Package designers have suggested an intermediate design called the near-hermetic package (NHP). NHPs seek designs that are "good enough" and "cheap enough" for MEMS and other cavity-requiring devices. The concept is still evolving and can have product-specific performance.

Wafer-level partial packaging — This type of packaging is formed while devices are still in wafer format. A considerable number of wafer-level packaging (WLP) methods have emerged for electronic devices, but MEMS are just getting started. The dominant MEMS wafer-level process is capping, a partial packaging process that requires subsequent steps. Several companies have been capping accelerometers for many years , but the caps are passive (no electrical paths). Passive caps must not block chip bond pads, so the most common solution is to singulate caps first using a multi-step process, and later singulate the MEMS wafer. Double singulation, however, adds cost.

Full WLP — The alternative is to fully cap the MEMS device and provide an interconnect structure through the chip or cap. This allows the cap layer to be singulated during the MEMS sawing step. The patent area shows that considerable attention is being expended to create an interconnect structure through the cap or chip. Many inventions have been filed for full WLP MEMS, and this high patent activity indicates WLP will soon emerge.

Materials

Although newer ceramic materials and designs have reduced cost, plastic packaging remains the low-cost leader. Plastic packaging is primarily based on epoxy that accounts for most of that market. Advantages are lowest cost, high versatility and ease of automation. The area-molded QFN has reduced cost even further, making the five-cent package a reality. The MEMS industry has sought ways to adapt cost-reducing plastics with modest success. Two basic strategies are used: a molded cavity design or protection over the mechanical zone.

While overmolding can't be used on bare MEMS chips, it is acceptable for some capped devices. Injection molding, however, is ideal for producing 3-D shapes and is used to make cavity packages. Injection molding uses thermoplastic resins rather than thermoset epoxies. Several high-temperature thermoplastics, with properties superior to epoxies, are available. Liquid crystal polymer, for example, has a good moisture barrier and low flammability without flame-retardants. Thermoplastics are also "no waste" materials that can be remelted, reused and recycled, but withstand soldering

Plastic packages for MEMS

A pioneer in MEMS and MEMS packaging, Analog Devices (Norwood, Mass.) explored plastic cavity packages for its inertial sensors. Chips are first capped at the wafer level so the motion zone is hermetically enclosed. The capped wafer is protected from both particulate contamination and moisture, so package hermeticity is less critical. The capped MEMS devices can be overmolded with epoxy. But products requiring higher performance are placed into molded cavity packages because overmolded stress, caused by shrinkage, detunes the MEMS sensor. The loaded plastic package is completed by sealing a lid with adhesive, ultrasonic welding or a laser. Although wafer capping adds cost, it solves several problems, including contamination from sawing.

MEMS devices cannot be capped using today's technology because they require gas or fluid input. These may be candidates for injection-molded plastic cavity packages. Thermoplastic packages may have higher barrier performance than non-hermetic epoxies, but they don't achieve full hermeticity, even though they can pass the fine leak test.

Other packaging issues

A vacuum may not be required or even acceptable for some devices, but in-package atmosphere control may be essential. A MEMS package enclosure and adhesives can release materials (outgas), and this affects MEMS performance. Phenomena associated with chip mechanics can also be a problem. Getters, anti-stiction additives and lubricants can help. Getters trap moisture and particles, while lubricants and anti-stiction agents can improve performance and reliability. Stiction, a combination of sticking and friction, occurs when smooth planar surfaces make contact and become permanently locked together by short-range atomic forces — a problem for most MEMS chips. Coatings with low surface energy and hydrophobicity can help. The film must be very thin and is applied by vacuum coating or other methods. Many patents deal with stiction and typically involve low surface energy coatings.

Bio-MEMS, fluidic packaging

Bio-MEMS devices are being developed and tested at universities, clinics and companies. They can sense in-body parameters, analyze body motion, measure tiny forces, identify bio-agents, pump and control fluids, administer drugs, and perform other actions that have great value for the medical and biological fields. Bio-MEMS of the future may become the most important MEMS class, with "medical practitioners" becoming permanent inhabitants of your body. Bio-MEMS packaging requirements will be very challenging because some devices will handle fluids that require fluid channels and couplings. Work is well underway to develop fluidic couplings for packages. These couplings can be fabricated from silicon or plastics.

Packaging trends

The fully hermetic package was the original "box" for MEMS, but it was costly. Lower-cost ceramic QFNs have replaced the larger, more expensive traditional packages in many products. Wafer-level capping is a strong trend for inertial sensors, and the technology is evolving. Capped chips are now used in plastic packages, both overmolded and injection-molded cavity styles. Injection-molded packages have key advantages, but still lack full hermeticity. They offer low cost and extraordinary precision at small dimensions. Micro-molding can produce extremely small, precise and complex structures, even nanoscale features. These attributes may not yet be needed, but they will prove valuable for future advanced MEMS, especially biomedical products.

Full WLP is another important direction for MEMS, but is still emerging. Recent patents and applications indicate the MEMS industry is getting ready to launch full WLPs. Capping processes are already well established for passive caps (protection-only caps, no vias), but connector feed-through packages are on the way. Connection strategies include vias-through-chip or vias-through-cap, and such packages could be rolling out of the foundries soon. This brings up supply chain logistics, such as where MEMS will be packaged. At the foundry? Most capping is already done at the fab.

Plastic cavity packaging, which represents only a small segment today, will evolve for specialty applications. A lack of full hermeticity will remain an impediment to wide-scale use unless hermetic barrier properties can be achieved, which is a definite possibility. Future packaging development will concentrate on both plastic molded packages and full WLPs, but will the two methods compete? There may be some competition, but the technologies are so different that each can solve problems for different products and coexist peacefully.

There is also a place for commercial MEMS packaging. Several companies already offer MEMS packages. Some companies, like RJR Polymers (Oakland, Calif.) and Quantum Leap Packaging (Wilmington, Mass.), focus on injection-molded plastic cavity packages. Others, such as Hymite (Copenhagen, Denmark), offer full hermetic systems. Larger companies, like Amkor (Chandler, Ariz.), also provide MEMS packaging services. IP-centric companies, such as Tessera Technologies (San Jose), offer solutions for MEMS and MOEMS. But it remains to be seen how the industry will solve the myriad of packaging issues that relate to both technology and business.

Conclusion

MEMS have the potential to become a hallmark technology of the 21st century. The ability to sense, analyze, compute, control and synthesize materials at the chip level can provide new and powerful products. Sensors are the big market today, but many other areas are quickly evolving, including bio-MEMS and other advanced areas.

Bio-MEMS will be an important technology in the future, and will play a role in the changing field of health care. Clinics are testing implanted blood pressure sensors, wellness monitors and even autonomous drug dispensers. The day will come when MEMS "medibots" continuously monitor and repair the human body, powered by bloodstream nutrients. MOEMS will also expand into very large and very small imaging products, but rather quickly.

Substantial packaging challenges remain, but considerable progress is on the way. WLP is probably the end game for many MEMS devices, but others will need discrete packages that will include plastics. MEMS will continue to grow at a steady, sustainable rate that will reduce the cost of devices and packages. MEMS are already using nanotechnology, and these two technologies will converge to some degree within a decade. When nanodevices are finally commercialized, especially nanoelectronics, they will adopt well-suited MEMS packaging technology.

Researchers create the first THERMAL NANOMOTOR in the world

The motor functions as a nanotransporter by moving and rotating cargo from one end of the carbon nanotube to the other.Researchers from the UAB Research Park have created the first nanomotor that is propelled by changes in temperature. A carbon nanotube is capable of transporting cargo and rotating like a conventional motor, but is a million times smaller than the head of a needle. This research opens the door to the creation of new nanometric devices designed to carry out mechanical tasks and which could be applied to the fields of biomedicine or new materials.
The "nanotransporter" consists of a carbon nanotube - a cylindrical molecule formed by carbon atoms - covered with a shorter concentric nanotube which can move back and forth or act as a rotor. A metal cargo can be added to the shorter mobile tube, which could then transport this cargo from one end to the other of the longer nanotube or rotate around its axis.Researchers are able to control these movements by applying different temperatures at the two ends of the long nanotube. The shorter tube thus moves from the warmer to the colder area and is similar to how air moves around a heater. This is the first time a nanoscale motor is created that can use changes in temperature to generate and control movements.The movements along the longer tube can be controlled with a precision of less than the diameter of an atom. This ability to control objects at nanometre scale can be extremely useful for future applications in nanotechnology, e.g. in designing nanoelectromechanical systems with great technological potential in the fields in biomedicine and new materials.