Nano News: 08/07/15

New 2D transistor material made using precision lasers

Molybdenum ditelluride (MoTe2) is a crystalline compound that if pure enough can be used as a transistor. Its molecular structure is an atomic sandwich made up of one molybdenum atom for every two tellurium atoms[HY1] . It was first made in the 1960’s via several different fabrication methods, but until last year it had never been made in a pure enough form to be suitable for electronics. Last year a multi-discipline research team led by South Korea’s Institute for Basic Science (IBS) Center for Integrated Nanostructure Physics at Sungkyunkwan University (SKKU) director Young Hee Lee devised a fabrication method for the creation of pure MoTe2. Not only did they succeed in making MoTe2 in pure form, they were able to make two types of it — a semiconducting variety called 2H-MoTe2 (2H because of its hexagonal shape) and a metallic variety called 1T'-MoTe2 (1T’ because it has an octahedral shape) — which are both stable at room temperature. Making MoTe2 in a pure form was very difficult and it was seen by some as a black sheep of the transition metal dichalcogenides (TMD) family and purposefully ignored. TMDs are molecules that can be made exceedingly thin, only several atomic layers, and have an electrical property called a band gap, which makes them ideal for making electrical components, especially transistors. MoTe2 crystal

Figure 1. Top, 2H-MoTe2 Bottom, 1T’-MoTe2. A TMD crystal follows an MX2 format: there is one transition metal, represented by M (M can be Tungsten, Molybdenum, etc.) and two chalcogenides, the X2 (Sulfur, Selenium, or Tellurium). These atoms form a thin, molecular sandwich with the one metal and two chalcogenides, and depending on their fabrication method can exist in several slightly different shaped atomic arrangements. The overwhelming majority of microchips that exist in electronics now are made from silicon, and they work extremely well. However, as devices get smaller there is an increasing demand to shrink the size of the logic chips that make those devices work. As the chips approach single or several atom thickness, (commonly referred to as 2-dimensional), silicon no longer works as well as it does in a larger, 3-dimensional (3D) scale. As the scale approaches 2 dimensions (2D), the band gap of silicon changes (higher band gap than that of its 3D form) and the contact points with metal connections on silicon are no longer smooth enough to be used efficiently in electrical circuits.

Figure 2. A simulation of the process of converting the 2H-MoTe2 into 1T'-MoTe2 with laser-irradiation. This is the perfect opportunity to employ new, exotic TMD materials. The IBS research team was able to exploit the two versions of MoTe2 and make one 2D crystal that was composed of the semiconducting 2H-MoTe2 and the metallic 1T'-MoTe2. This configuration is superior to using silicon as well as other 2D semiconductor because the boundary where the semiconducting (2H) and metallic (1T') MoTe2 meet to have what’s called am ohmic homojunction. This is a connection that forms at the boundary between two different structural phases in a single material. Despite one MoTe2 state being a semiconductor and one being metallic, the team was able to create an ohmic homojunction between them, making an extremely efficient connection. To do this, the team started with a piece of their pure 2H-MoTe2 which was several atoms thick. They directed a 1 µm wide laser (a human hair is 17 to 181 µm) at the 2H-MoTe2 which locally heated the sample and changed the affected area into 1T'-MoTe2. With this method, the team was able to create a 2D transistor that utilized an amalgamation of both the semiconducting properties of the 2H-MoTe2 material as well as the high conductivity of the 1T'-MoTe2 ("Phase patterning for ohmic homojunction contact in MoTe2").

Figure 3. the 2H-MoTe2 and 1T'-MoTe2 transition line and metal electrodes attached to the 1T'-MoTe2. This is a clever solution to several problems that have hindered scientists and engineers in the past. By using only one material in the device channel and the metal-semiconductor junction, it is more energy efficient since the joints between the two phases of the MoTe2 are fused seamlessly realizing an ohmic contact at the joints. Because 1T’-MoTe2 is such a good conductor, metal electrodes can be applied to it directly, saving any additional work of finding a way to attach metal leads. This new fabrication technique is a hyper-efficient way of utilizing the available MoTe2 without any wasted or extraneous parts. When asked about its potential for future use, Professor Heejun Yang of SKKU said, “There are many candidates for 2D semiconductors, but MoTe2 has a band gap of around 1 eV which is similar to silicon’s band gap and it allows an ohmic homojunction at the semiconductor-metal junctions.” This means that MoTe2 can replace silicon without much change in the current voltage configurations used with today’s silicon technologies. The dual-phase MoTe2 transistor looks promising for use in new electronic devices as demand for components increases for materials that are small, light and extremely energy efficient.

Forcing a molecular light switch

A chance observation led to RIKEN researchers discovering an organic compound whose fluorescence wavelength varies greatly when it is subjected to a mechanical force ("Reversible near-infrared/blue mechanofluorochromism of aminobenzopyranoxanthene"). This property makes it an attractive material for various applications in security as well as medical imaging and therapy.

Figure 1: RIKEN logo created by selectively grinding cis-ABPX01. This gives rise to regions with different fluorescence properties. (Image: Shinichiro Kamino, RIKEN Center for Life Science Technologies) Shinichiro Kamino and Shuichi Enomoto of the RIKEN Center for Life Science Technologies, along with Atsuya Muranaka and colleagues from the RIKEN Center for Sustainable Resource Science and other scientists based in Japan, were studying some dyes when they noticed that a solid-state dye, cis-ABPX01, fluoresced at both near-infrared and blue frequencies. Subsequent spectroscopic and x-ray diffraction analysis indicated that this dual fluorescence stemmed from the two different crystal structures that the dye could adopt. The blue fluorescence came from a crystal arrangement of single molecules, whereas the near-infrared fluorescence originated from a configuration in which the repeating units consisted of two molecules joined together. “This relationship between fluorescence and molecular structure inspired us to think that simple mechanical grinding might reduce the near-infrared fluorescence and enhance the blue fluorescence,” Kamino explains. Sure enough, the researchers observed this predicted change in fluorescence when they ground the mixture of structures in a mortar. By selectively grinding certain regions, they could produce areas with different fluorescence properties (Fig. 1). The change also proved to be readily reversible: exposing the ground material to dichloromethane vapor restored the near-infrared fluorescence while reducing the blue fluorescence. Furthermore, this reversible switching could be repeated several times. While other molecules have been discovered whose fluorescence properties vary on applying a mechanical force, the shift in fluorescence wavelength between near-infrared and blue light observed for cis-ABPX01 is considerably larger than that for other compounds. This remarkable dual fluorescence raises the possibility that cis-ABPX01 could be used as a component in signaling systems for a wide range of industrial, biological and medical applications. “We think that these molecules could be used to sense mechanical forces in cells and tissues,” says Kamino. He explains that there is growing interest in the field known as mechanobiology, which looks at the role of mechanical forces in altering cellular activity and tissue behavior in biological systems. The researchers plan to investigate using cis-ABPX01 and related molecules to detect medically significant changes in living systems. They are also interested exploring applications such as security tags whose optical properties change when they are subjected to mechanical tampering and other applications in which it is important to detect mechanical forces.

Spins in artificial atoms resemble those in natural ones

By extending the study of coupled quantum dots to five-electron systems, RIKEN researchers have confirmed that the spin-based electron-filling rules for natural atoms apply to artificial molecules ("Vanishing current hysteresis under competing nuclear spin pumping processes in a quadruplet spin-blockaded double quantum dot").

Figure 1: The Pauli exclusion principle prohibits two electrons with the same spin orientation from occupying the same orbital. This prevents a spin ‘up’ electron from moving from the left quantum dot to the right one for the case shown here. (Image: Shinichi Amaha, RIKEN Center for Emergent Matter Science) Systems consisting of electrons and semiconductor quantum dots—nanostructures that exhibit quantum properties—are highly intriguing artificial structures that in many ways mimic naturally occurring atoms. For example, electrons occupy the energy levels of quantum dots according to the same rules that determine how electrons fill atomic shells. Such systems are of both fundamental interest, for investigating phenomena related to nuclear spin, and applied interest, for manipulating spin in future quantum computers. The Pauli exclusion principle, which prohibits any two electrons in an atom from having identical sets of quantum numbers, gives rise to a phenomenon known as the Pauli spin blockade in quantum-dot systems. This effect prevents electrons from following certain energetically favorable paths through a quantum-dot system since two electrons with the same spin cannot occupy the same energy level. The Pauli spin blockade has been well studied in artificial molecules consisting of two quantum dots and two electrons. Shinichi Amaha and Seigo Tarucha from RIKEN’s Center for Emergent Matter Science, in collaboration with researchers in Japan and Canada, have extended the study of spin blockade to multilevel quantum-dot systems that have more than two electrons. This requires accessing high-spin states, which is difficult to achieve in practice. Using a two-quantum-dot system with three effective levels, the researchers have achieved spin blockade by exploiting Hund’s first rule, which dictates that electrons in an atom will first fill unoccupied orbitals of a subshell with greater total spin state. They used this principle to prepare the high-spin states needed for spin blockade (Fig. 1). The team discovered that the current of the device varied unexpectedly with the applied magnetic field. In most devices with spin effects, the current lags behind changes to the magnetic field, a phenomenon known as hysteresis. The researchers found that the hysteresis of their system follows the expected spin states based on a consideration of Hund’s rule and that in certain magnetic field regions two hysteresis effects cancelled each other out—clear evidence that competing ‘up’ and ‘down’ nuclear spin pumping processes influence the current. These findings are expected to open the way to use arrays of such quantum dots as simulators for spin filling in real molecules. “Using an array of quantum dots as artificial atoms could assist investigations of novel spin-related phenomena in real molecules,” says Amaha.