A nanowire based triboelectric nanogenerator for harvesting water wave energy and its applications APL Materials 5, 074104 (2017) Tribolectric charging results from contact between surfaces, but precisely what is meant by each is not defined or understood, as they relate to charging. The recent microscopic evidence that contact charging can result from material transfer provides incentive to examine how contact charging is affected by these two factors. It is suggested that vigorous rubbing or pressing of two polymers results in transfer of deeper layers than would result from light contacts. Different layers can have substantially different compositions because polymers are typically not homogeneous as a function of depth, so contact and surface are related in this way. This could account for charge transfer between identical polymers, especially in asymmetric contacts in which the frictional force on one polymer differs from that on the other, so that material from different depths is transferred. This review outlines the roles of physics, chemistry and surface analysis in sufficient detail to focus on these subjects. It also makes suggestions how these concepts could be applied to some of the current leading edge research in this area.

EXTENDED ABSTRACTThe presence of electrical charges resulting from surface contact of particles has many implications both in nature and industry. The mechanism(s) whereby these charges occur have been the subject of a number of studies that have been driven primarily by the need to accurately control these charges in modern applications such as electrophotography, powder coating, and materials separation as well as in industrial hazards. This presentation is an attempt to summarize current understanding of the factors affecting contact charging among small particles and to highlight where further work may be beneficial.A review of the literature shows that there is much variability in reported results. Many of the inconsistencies in this published data can be attributed to such things as surface and experimental variability, the nature of the contact, the charge species involved and the effect of charge back-flow [1]. However, notwithstanding these limitations there is general consensus that for conductors and semiconductors, contact charging may be fully explained by the theory proposed by Harper [2]. This states that when contact between surfaces occurs, thermodynamic equilibrium is established and this implies that free electrons are shared across the interface in proportion to the difference in the work functions of the two materials. The residual charge after separation was shown by Lowell [3] to be related to the product of the contact potential difference and the capacitance defined at a separation distance of about 1nm, where charge relaxation due to tunneling ceases. In practice contact charging is generally ignored when conductors are involved since the systems are usually well grounded. However, we have recently pointed out that this phenomenon can give rise to unexpected problems if contact and separation of small metal parts occurs in a situation where the parts may be collected in isolation from ground [4]. For the cases of insulator-insulator or insulator-conductor contact the issue is less clear since neither of these situations can give rise to thermodynamic equilibrium in the time scale observed for charge transfer. As a result it has been suggested that a number of electrons with high energy states can exist within the forbidden gap of an insulator but localized at the surface of the material [5][6]. This leads to the concept of an "effective" work function that may be considered as a surface rather than a bulk property of the ma-

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