Tag Archives: Stanford

Diamondoids Make Three Atoms Wide Wires

At the SLAC National Accelerator Laboratory of the Department of Energy, and the Stanford University, scientists have discovered a new method of using diamondoids. These extremely tiny bits of diamonds, these diamondoids can be used to assemble atoms into the thinnest possible electrical wires—only three atoms wide.

The diamondoids do this by grabbing different types of atoms and bringing them together as is done in LEGO units. The scientists are of the opinion this new technique has the potential of creating tiny wires suitable for a wide range of applications. This could include fabrics for generating electricity, superconducting materials for conducting electricity without any loss, and optoelectronic devices employing both light and electricity. The scientists have reported their findings in Nature Materials.

According to Hao Yan, a lead author of the paper and a postdoctoral researcher at Stanford, the process self-assembles tiny, conductive wires of the smallest possible size. The process involves simply dumping the ingredients together, with the results coming in only half an hour.

The researchers have made an animation to show the molecular building blocks joining the tip of the growing nanowire. In each block, there is a diamondoid, attached to sulfur and copper atoms. Just as LEGO blocks do, the diamondoids only fit together in specific ways that their shape and size dictate. While the insulating diamondoids form an outer shell, the sulfur and copper atoms make up a conductive wire in the center.

Although several methods exist for self-assembly of materials, the method with diamondoids is the first one to make a nanowire with a solid, crystalline core. According to a co-author of the study Nicholas Melosh, the core also has good electronic properties.

The semiconducting core of the needle-like wires—a combination of copper and sulfur, known as chalcogenide—is surrounded by an insulating shell formed by diamondoids.

According to Melosh, this miniscule size is very important. In reality, the material exists in only one or two dimensions—as wires or sheets of atomic-scale dots. At these dimensions, the material has very different properties, extraordinarily different compared to those of the same material when made in bulk. With the new method, researchers were able to assemble the materials with atomic precision and control.

The scientists used the diamondoids as assembly tools, as these are tiny, with interlocking cages of carbon and hydrogen. The SLAC laboratory extracted and separated the diamondoids by size and geometry from petroleum fluids, where the diamondoids occur naturally. Melosh and professor Zhi-Xun Shen from SLAC/Stanford are leading a SIMES research program over the past decade. They have found several potential uses for the diamondoids. This ranges from making tiny electronic gadgets to improving electron microscope images.

The research team found that tiny diamonds attract each other strongly—through van der Waals forces—a fact they exploited. Because of this attraction, the microscopic diamondoids can clump together much the same way as sugar crystals do, this being the only reason they are visible to the naked eye. The scientists started with the smallest possible dimensions of the diamondoids. They used single cages containing just 10 carbon atoms, to each of which they attached a sulfur atom. When the sulfur atom bonded with a single copper ion, it created the basic building block for a nanowire.

The Energy Efficient RRAMs

Engineers at Stanford are making 3-D memory chips that can offer faster and more energy efficient solutions for computer memory. These are the Resistive random Access Memory or RRAMs, which are based on a new semiconductor material. It stores data based on temperature and voltage. However, the actual workings of RRAMs continued to be a mystery until a team at Stanford used a new tool for their investigations. They found the optimal temperature range to be lower than they had expected. This could lead to memory that is more efficient.

Conventional computer chips operate on a two dimensional plane. Typically, the CPU and memory communicate with each other through the data bus. While both the CPU and memory components have advanced technically, the data bus has lagged, leading to a slowdown of the entire system when crunching large amounts of data.

The special semiconductor RRAMs can be stacked one on top of the other, creating a 3-D structure. This brings the memory and its logic components closer together. As conventional silicon devices cannot replicate this, the 3-D high-rise chips can work at much higher speeds and be more energy efficient. Not only is this a better solution for tacking the challenges of Big Data, it can also extend the battery life of mobile devices.

The RRAMs work more like a switch. As explained by the Stanford engineers, in their natural state, the RRAM materials behave just as insulators do—resist the flow of electrons. However, when zapped with an electric field, a filament-like path opens up in the material, and electrons can flow through it. A second jolt closes the filament, and the material returns to being the insulator it was. Alternating between the two states generates a binary code with no signal transfer representing a zero and the passage of electrons representing a one.

The temperature rise of the material when subjected to the electric field causes the filament to form, allowing electrons to pass through. So far, the engineers were unable to estimate the exact temperature of the material that caused the switch. They needed much more precise information about the fundamental behavior of the RRAM material before they could hope to produce reliable devices.

As the engineers had no way of measuring the heat produced by a jolt of electricity, they heated the RRAM chips using a hot plate, while not applying any voltage. They then monitored the flow of electrons as filaments began to form. This allowed the team to measure the exact temperature band necessary for the materials to form the filaments. The engineers found the filaments formed between 26.7 and 126.7°C. Therefore, future RRAM devices will require less electricity for generating these temperatures, and that would make them more energy efficient.

Although at this moment, RRAMs are not yet ready to be incorporated into consumer devices, the researchers are confident that the discovery of the temperature range will speed up development work.

According to Ziwen Wang, a member of the team, the voltage and temperature discovered can be the predictive design inputs for enabling the design of a better memory device. The researchers will be presenting their find at the IEEE International Electron Devices Meeting in San Francisco.