The new (old?) materials zoo
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Ah, life in the 21st century. Isn’t it glorious? Every day I wake up, it seems there is a new type of material, or a new use for an old material, or a new materials process that promises to make our lives more enjoyable. We live in a wonderful world of materials, with engineered materials at the nanoscale and atomic scale, including heterostructures, quantum wells, quantum wires/nanowires, quantum dots/nanocrystals, 2D materials, and a variety of new compounds and alloys (ternary, quaternary, and quinternary materials). Some of these, such as heterostructures and quantum wells, have been around for 30 years or more. Others, such as graphene, have been known for much longer, but have recently taken on new life. Work on a broad class of so-called 2D materials emerged in 2004 with the identification and measurement of the properties of single atomic layers of carbon in a hexagonal lattice.1 The earliest work on record for this material involved discussions of the energy-band structure of a single hexagonal layer of graphite.2 The more recent work, which started through exfoliation of graphite from pencil lead, has been followed by suggestions of potential applications in microelectronics, optoelectronics, flexible electronics and sensors, among others. The reemergence of graphene was followed quickly by reports of silicene, germanene, and a variety of 2D layers of transition-metal dichalcogenides (e.g., MoS2, MoSe2, MoTe2, WS2, WSe2).3 Developments around these include nanoribbons, nanowires, heterostructures, and quantum wells. Other fascinating new areas of materials science include materials for spintronics4,5 and topological insulators.6 Spintronic materials and devices are based upon ideas of spin transport in solid-state materials. Most fundamental semiconductor transistors work on the basis of transport of charge carriers (electrons and/ or holes). Channels for electrical transport are formed and then controlled by electrical biases. The movement of charge from one region of a device to another is the foundation of modern integrated circuits, including the microprocessors that drive our computers, tablets, eReaders, phones, and automobiles. As the need for higher processing power has evolved, transistors have become significantly smaller in size. Transistors long ago reached the point where some or all of their dimensions were comparable to the wavelength of the charge carriers, which means that quantum mechanical effects have to be taken into consideration in
discussing their performance. At some point, this trend in downsizing (or, as one of my friends calls it, “smallifying”) has to end, as fundamental limits exist below which transistors (at least those based upon the common semiconductor-device materials) cannot maintain their performance. Intel is currently producing microprocessors with transistors with some dimensions on the scale of 14 nm. Microprocessors and other integrated circuits with transistors scaled to the 5-nm node seem to be just around the corner. The transistors in these devices may be na
