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Preview Shining Light on Triboelectric Phenomena Daniel J. Lacks1,* and R. Mohan Sankaran1,* In this issue of Matter, two independent papers (Liu et al. and Hao et al.) describe tunable materials interfaces that convert mechanical and light energy to electricity via a novel ‘‘photo-tribo-electric’’ effect. film in contact with a perovskite film, illumination increases the current out- put by three times compared to the dark. While the precise mechanism un- derlying the complex interactions of triboelectricity and photoexcitation on electric power generation remains to be understood, the authors suggest that electron-hole charge carriers are created and separated more effectively when both light and mechanical stimu- lation are applied. The 18th-century scientists’ explanation of the scientific basis of triboelectric generators was, of course, primitive. It was believed that electrical charge was due to an electrical fluid that exists in pores of materials. Benjamin Franklin theorized that charge transfer occurs because a material ‘‘when rubbed, has its rubbed surface somewhat stretched, or its solid parts drawn a little farther asunder, so that the vacancies in which the electrical fluid resides, become larger, affording room for more of that fluid.’’7 Science has progressed incredibly since the 18th century. But, surpris- ingly, our fundamental understanding of triboelectric charging is not much more advanced than Benjamin Frank- lin’s description long ago. Tribo- electric charging is a deceptively com- plex process because it is a surface phenomenon—material surfaces are imperfect to begin with and are further altered by their rubbing. As we described in our review articles,8,9 ex- periments by many researchers have revealed the importance of reactive radicals created by the mechanical breaking of bonds, and of material 1Department of Chemical and Biomolecular Engineering, Case Western Reserve University, Cleveland, OH 44106, USA *Correspondence: daniel.lacks@case.edu (D.J.L.), mohan@case.edu (R.M.S.) https://doi.org/10.1016/j.matt.2019.08.002 Exactly 300 years ago, Francis Hauks- bee described his invention of an ‘‘electrical machine’’ in his book, Physico-Mechanical Experiments on Various Subjects,1 was published.2 The basic idea was that a person turns a handle to spin one material (often glass) that was in contact with another softer material (e.g., wool); the rub- bing of the two materials leads to electrostatic charge, which can be car- ried away by a conductor of electrical current. Over the next few decades, others improved on the design; these electrical machines became common and provided the first sources of electrical power. In fact, in 1775, Jo- seph Priestley remarked ‘‘No philoso- pher, for instance, can now be satis- fied, if he be not able to supply a conductor from the clouds, as well as from the friction of his glass globes or tubes.’’3 These electrical machines, where the mechanical energy is converted to electricity by rubbing together two sur- faces, would later be called triboelec- tric generators. In the 18th century, electricity was just a curiosity. Today, our lifestyles depend on it. Most of our electricity is produced by converting mechanical energy to electricity, as was done in the electrical machines of Hauksbee and his contemporaries. However, the technology that currently dominates electricity production is fundamentally different—rather than relying on tribo- electric charging, today’s generators create electricity by the motion of magnetic fields through conductors, which is a more efficient energy con- version process. Recently, there has been a resurgent interest in triboelectric generators for niche situations. In the modern world, very small and localized electrical power is sometimes needed—for ex- ample, to power a sensor or small LED. In such cases, even ambient mo- tion that is otherwise wasted—such as a person walking or wave motion on a sea—may provide enough mechanical energy to meet the need. Generators needed in these cases face very different design constraints than grid- scale ones—size, weight, cost, and simplicity are more important here. Triboelectric ‘‘nanogenerators’’ are well suited for these applications and have been the focus of much recent research.4 Now, Liu et al.5 and Hao et al.6 have shown that combining light with tribo- electric nanogenerators can lead to synergistic effects. Using a Si-based Schottky solar cell, Liu et al.5 find that sliding a stainless-steel tip increases the current output of the device by $50 times and the overall power output by $28 times in comparison to photoexcitation alone. Similarly, Hao et al.6 demonstrate that for a dynamic Schottky junction composed of an Al 552 Matter 1, 550–564, September 4, 2019 a 2019 Elsevier Inc.PDF Image | Shining Light on Triboelectric Phenomena
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