in graphite; this important parameter had previously been surprisingly ill-defined experimentally. Zettl was the first to synthesize boron nitride nanotubes (BNNTs), for which (in sharp contrast to CNTs), the electronic and optical properties are relatively insensitive to wall number, diameter, and chirality. Zettl also found different ways to efficiently synthesize BNNTs, along with related BN-based nanomaterials such as BN nanococoons and BN aerogels. He also developed methods to functionalize the outer surfaces of BNNTs, and fill them with foreign chemical species creating new structures including silocrystals. Zettl showed experimentally that an electric field could be used to modulate the electronic band gap of BNNTs (giant Stark effect). Nanoelectromechanical systems and advances in transmission electron microscopy Zettl developed the transmission electron microscope (TEM) nanomanipulator, which allowed electrical and mechanical stimulation of nanoscale samples while they were being imaged inside the TEM. The nanomanipulator could be configured as a mechanical and/or electrical probe placed with atomic precision, as a scanning tunneling microscope, or as an atomic force microscope with simultaneous force measurement capability. Zettl used the nanomanipulator to prove that multi-wall CNT were composed of nested concentric cylinders rather than scrolls, and he determined the fundamental frictional forces between the cylinders. This led to his invention of the rotational nanomotor that employed nanotube bearings. Other inventions by Zettl that resulted were surface-tension-powered relaxation oscillators, tunable resonators, nanocrystal-powered linear motors, a fully integrated nanoradio receiver, a nanoballoon actuator, and nano-scale electrical and thermal rheostats. Zettl used the nanomanipulator to perform the first electron holography experiments on nanoscale materials, which quantified quantum mechanical field emission from CNTs. Using an architecture similar to that of his nanoradio, Zettl created a nanoelectromechanical “balance” which had single atom mass sensitivity, and with which he observed atomic shot noise for the first time. He developed a suspended graphene membrane that allowed for nearly real-time TEM imaging of individual carbon atom dynamics, and other isolated atomic and molecular species. Zettl’s development of the TEM graphene liquid cell and graphene flow cell brought ultra-high-resolution real-time liquid phase imaging to the TEM world. Zettl also developed nanomechanical biological probes, tailored nanopores, and highly efficient wideband graphene-based mechanical energy transducers. 2D materials Zettl has made key contributions to the synthesis and characterization of a host of 2D materials, including TaS2, MoS2, alloyed NbS2, NbSe2, and 2D quasicrystals. Zettl recently discovered a means to enhance and control quantum light emission in hexagonal-BN heterostructures,