An article in ("Negative capacitance in a ferroelectric capacitor") describes the first direct observation of a long-hypothesized but elusive phenomenon called “negative capacitance.” The work describes a unique reaction of electrical charge to applied voltage in a ferroelectric material that could open the door to a radical reduction in the power consumed by transistors and the devices containing them. Capacitance is the ability of a material to store an electrical charge. Ordinary capacitors—found in virtually all electronic devices—store charge as a voltage is applied to them. The new phenomenon has a paradoxical response: when the applied voltage is increased, the charge goes down. Hence its name, negative capacitance. “This property, if successfully integrated into transistors, could reduce the amount of power they consume by at least an order of magnitude, and perhaps much more,” says the paper’s lead author Asif Khan. That would lead to longer-lasting cell phone batteries, less energy-consumptive computers of all types, and, perhaps even more importantly, could extend by decades the trend toward faster, smaller processors that has defined the digital revolution since its birth. 
The atomic structure of a ferroelectric material exhibits the so-called “negative capacitance” effect. If successfully built into transistors, it could drastically reduce the electricity needed to run computer processors and other transistor-dependent devices. (Illustration: Suraj S. Cheema) Without a major breakthrough of this sort, the trend toward miniaturization and increased function is threatened by the physical demands of transistors operating at a nano scale. Even though the tiny switches can be made ever smaller, the amount of power they need to be turned on and off can be reduced only so much. That limit is defined by what is known as the Boltzmann distribution of electrons—often called the Boltzmann Tyranny. Because they must be fed an irreducible amount of electricity, ultra-small transistors that are packed too tightly cannot dissipate the heat they generate to avoid self-immolation. In another decade or so, engineers will exhaust options for packing more computing power into ever tinier spaces, a consequence viewed with dread by device manufacturers, sensor developers, and a public addicted to ever smaller and more powerful devices. The new research, conducted at UC Berkeley under the leadership of CITRIS researcher and associate professor of electrical engineering and computer sciences Sayeef Salahuddin, provides a possible way to overcome the Boltzmann Tyranny. It relies on the ability of certain materials to store energy intrinsically and then exploit it to amplify the input voltage. This could, in effect, potentially “trick” a transistor into thinking that it has received the minimum amount of voltage necessary to operate. The result: less electricity is needed to turn a transistor on or off, which is the universal operation at the core of all computer processing. The material used to achieve negative capacitance falls in a class of crystalline materials called ferroelectrics, which was first described in the 1940s. These materials have long been researched for memory applications and commercial storage technologies. Ferroelectrics are also popular materials for frequency control circuits and many microelectromechanical systems (MEMS) applications. However, the possibility of using these materials for energy efficient transistors was first proposed by Salahuddin in 2008, right before he joined Berkeley as an assistant professor. Over the past six years, Khan—one of Salahuddin’s first graduate students at Berkeley—has used pulse lasers to grow many kinds of ferroelectric materials and has devised and revised ingenious ways to test for their negative capacitance. In addition to transforming the way transistors work, negative capacitance could also potentially be used to develop high-density memory storage devices, super capacitors, coil-free oscillators and resonators, and for harvesting energy from the environment. Exploiting the negative capacitance of ferroelectrics is one in a list of strategies for reducing the per-joule cost of storing a single bit of information, says UC Berkeley professor of materials science, engineering, and physics Ramamoorthy Ramesh, another of the paper’s authors. Ramesh’s decades of seminal work on ferroelectric materials and device structures for manipulating them underlies the group’s findings. “We have just launched a program called the attojoule-per-bit program. It is an effort to reduce the total energy consumed for manipulating a bit to one attojoule (10-18),” says Ramesh. To achieve that kind of per-bit energy consumption, we need to take advantage of all possible pathways. The negative capacitance of ferroelectrics is going to be a very important one,” he says. This work was enabled by access to CITRIS’s Marvell Nanofabrication Laboratory, a research facility on the UC Berkeley campus that specifically encourages the exploration of new materials and processes. One of the most advanced academic nanofabrication labs of its type in the world, the NanoLab is the birthplace of other game-changing technologies, such as the three-dimensional FinFET transistor that has led the way to scaling far beyond the limits of ordinary transistors. “Today,” says professor Ming Wu, Marvell NanoLab Faculty Director, “every single transistor built for next-generation microprocessors or computers is FinFET.” “CITRIS’s Marvell NanoLab has state-of-the-art equipment for making semiconductor devices and integrated circuits,” says Wu. “But we take these tools and capabilities and apply them to materials that are so new that industry fabrication labs would not touch them. New materials like these negative capacitance ferroelectrics are not only welcome here, they are actively encouraged.” “The next step,” says Salahuddin, “is to try to make actual transistors such that they can exploit the new phenomenon, We need to make sure they are compatible with silicon processing, that they are manufacturable, and that the measurement techniques we’ve now proven in principle are practical and scalable.”
The atomic structure of a ferroelectric material exhibits the so-called “negative capacitance” effect. If successfully built into transistors, it could drastically reduce the electricity needed to run computer processors and other transistor-dependent devices. (Illustration: Suraj S. Cheema) Without a major breakthrough of this sort, the trend toward miniaturization and increased function is threatened by the physical demands of transistors operating at a nano scale. Even though the tiny switches can be made ever smaller, the amount of power they need to be turned on and off can be reduced only so much. That limit is defined by what is known as the Boltzmann distribution of electrons—often called the Boltzmann Tyranny. Because they must be fed an irreducible amount of electricity, ultra-small transistors that are packed too tightly cannot dissipate the heat they generate to avoid self-immolation. In another decade or so, engineers will exhaust options for packing more computing power into ever tinier spaces, a consequence viewed with dread by device manufacturers, sensor developers, and a public addicted to ever smaller and more powerful devices. The new research, conducted at UC Berkeley under the leadership of CITRIS researcher and associate professor of electrical engineering and computer sciences Sayeef Salahuddin, provides a possible way to overcome the Boltzmann Tyranny. It relies on the ability of certain materials to store energy intrinsically and then exploit it to amplify the input voltage. This could, in effect, potentially “trick” a transistor into thinking that it has received the minimum amount of voltage necessary to operate. The result: less electricity is needed to turn a transistor on or off, which is the universal operation at the core of all computer processing. The material used to achieve negative capacitance falls in a class of crystalline materials called ferroelectrics, which was first described in the 1940s. These materials have long been researched for memory applications and commercial storage technologies. Ferroelectrics are also popular materials for frequency control circuits and many microelectromechanical systems (MEMS) applications. However, the possibility of using these materials for energy efficient transistors was first proposed by Salahuddin in 2008, right before he joined Berkeley as an assistant professor. Over the past six years, Khan—one of Salahuddin’s first graduate students at Berkeley—has used pulse lasers to grow many kinds of ferroelectric materials and has devised and revised ingenious ways to test for their negative capacitance. In addition to transforming the way transistors work, negative capacitance could also potentially be used to develop high-density memory storage devices, super capacitors, coil-free oscillators and resonators, and for harvesting energy from the environment. Exploiting the negative capacitance of ferroelectrics is one in a list of strategies for reducing the per-joule cost of storing a single bit of information, says UC Berkeley professor of materials science, engineering, and physics Ramamoorthy Ramesh, another of the paper’s authors. Ramesh’s decades of seminal work on ferroelectric materials and device structures for manipulating them underlies the group’s findings. “We have just launched a program called the attojoule-per-bit program. It is an effort to reduce the total energy consumed for manipulating a bit to one attojoule (10-18),” says Ramesh. To achieve that kind of per-bit energy consumption, we need to take advantage of all possible pathways. The negative capacitance of ferroelectrics is going to be a very important one,” he says. This work was enabled by access to CITRIS’s Marvell Nanofabrication Laboratory, a research facility on the UC Berkeley campus that specifically encourages the exploration of new materials and processes. One of the most advanced academic nanofabrication labs of its type in the world, the NanoLab is the birthplace of other game-changing technologies, such as the three-dimensional FinFET transistor that has led the way to scaling far beyond the limits of ordinary transistors. “Today,” says professor Ming Wu, Marvell NanoLab Faculty Director, “every single transistor built for next-generation microprocessors or computers is FinFET.” “CITRIS’s Marvell NanoLab has state-of-the-art equipment for making semiconductor devices and integrated circuits,” says Wu. “But we take these tools and capabilities and apply them to materials that are so new that industry fabrication labs would not touch them. New materials like these negative capacitance ferroelectrics are not only welcome here, they are actively encouraged.” “The next step,” says Salahuddin, “is to try to make actual transistors such that they can exploit the new phenomenon, We need to make sure they are compatible with silicon processing, that they are manufacturable, and that the measurement techniques we’ve now proven in principle are practical and scalable.”
Improved photocatalyst microparticles containing gold nanoparticles can be used to purify water. (Image: A*STAR Institute of Materials Research and Engineering) Photocatalytic materials harness sunlight to create electrical charges, which provide the energy needed to drive chemical reactions in molecules attached to the catalyst’s surface. In addition to decomposing harmful molecules in water, photocatalysts are used to split water into its components of oxygen and hydrogen; hydrogen can then be employed as a green energy source. Hor and his team set out to improve an existing catalyst. Oxygen-based compounds such as strontium titanate (SrTiO3) look promising, as they are robust and stable materials and are suitable for use in water. One of the team’s innovations was to enhance its catalytic activity by adding small quantities of the metal lanthanum, which provides additional usable electrical charges. Catalysts also need to capture a sufficient amount of sunlight to catalyze chemical reactions. So to enable the photocatalyst to harvest more light, the scientists attached gold nanoparticles to the lanthanum-doped SrTiO3 microspheres (see image). These gold nanoparticles are enriched with electrons and hence act as antennas, concentrating light to accelerate the catalytic reaction. The porous structure of the microspheres results in a large surface area, as it provides more binding space for organic molecules to dock to. A single gram of the material has a surface area of about 100 square meters. “The large surface area plays a critical role in achieving a good photocatalytic activity,” comments Luo. To demonstrate the efficiency of these catalysts, the researchers studied how they decomposed the dye rhodamine B in water. Within four hours of exposure to visible light 92 per cent of the dye was gone, which is much faster than conventional catalysts that lack gold nanoparticles. These microparticles can also be used for water splitting, says Luo. The team showed that the microparticles with gold nanoparticles performed better in water-splitting experiments than those without, further highlighting the versatility and effectiveness of these microspheres.
The figure shows how much light (PLE, photoluminescence excitation) the individual pyramid-shaped antenna emits at various wavelengths (colours) of light in comparison to the antennas in the array. The individual antenna peaks at a wavelength of about 650 nanometer, whereas the antennas in the array peak at about 580 nanometer. The micrograph (top right corner) was made with an electron microscope. The colour of the arrow corresponds to the colour used in the other figure. Individual antennas A straight nanoscale antenna will mainly respond to the electric field of light. This means that the effects of the magnetic field of light, which holds half of the energy of light, are disregarded. For a long time this was not considered as an issue that could be solved, because most of the metals used to fabricate antennas do not respond to the magnetic field of light anyway. This changed recently, due to the rapid developments in metamaterial research. What seemed to be impossible in the past – making antennas that respond strongly to the magnetic field of light — can now be done by structuring metals on the nanoscale. With these ideas in mind, the AMOLF and Philips researchers built the pyramid shaped antenna. By carefully designing the height and inclination of the antenna’s side walls, the researchers found that the response to the magnetic field of light is almost as strong as the response to the electric field of light. Antennas in an array After witnessing the described effects in individual nanoscale antennas, the researchers took it one step further and placed multiple pyramid-shaped antennas in an array. The effect that the antennas have on each other turns out to be quite striking. At certain wavelengths (colours) of light, the antennas can couple to each other via the light that is scattered on the surface of the array. This makes the group of antennas more effective in beaming light than the sum of the individual antennas. In addition, the antenna array may operate collectively at one wavelength, while at the same time the antennas operate individually at a different wavelength. Thus, the same array of pyramid-shaped antennas may beam light of a certain colour upward, and of a different colour downward. Applications The array of nanoscale pyramid-shaped antennas has great potential for the improvement of LEDs. Currently, many LEDs are designed to emit light in one direction, for instance only ‘upward’. Such LEDs are used for example in automotive lighting or spotlight illumination. Unfortunately, the light emitting material inside a LED emits light with equal intensities both upward and downward. Since only the ‘upward’ emission is useful, the downward moving light needs to be recycled by adding several optical elements, such as mirrors, to the LED. These elements make the LED bulky and less efficient, since some light is inevitably lost during the recycling process. Integrating the pyramid shaped antennas in the LED has great potential for overcoming these disadvantages. The pyramid-shaped antennas are able to selectively beam one colour of light upward. If an undesired colour is present, this can be beamed downward. This development could greatly enhance the efficiency of single LEDs and improve the integration of LEDs in combined light systems.