We directly measure the nanometer-scale temperature rise at wrinkles and grain boundaries (GBs) in functioning graphene devices by scanning Joule expansion microscopy with $50 nm spatial and $0.2 K temperature resolution. We observe a small temperature increase at select wrinkles and a large ($100 K) temperature increase at GBs between coalesced hexagonal grains. Comparisons of measurements with device simulations estimate the GB resistivity (8-150 X lm) among the lowest reported for graphene grown by chemical vapor deposition. An analytical model is developed, showing that GBs can experience highly localized resistive heating and temperature rise, most likely affecting the reliability of graphene devices. Our studies provide an unprecedented view of thermal effects surrounding nanoscale defects in nanomaterials such as graphene. V C 2014 AIP Publishing LLC. [http://dx.Graphene, a monolayer of hexagonally arranged carbon atoms, has been the subject of intense research due to its thinness ($3.4 Å ), unique linear band structure, 1 and quasiballistic electrical and thermal transport up to micron length scales at room temperature. 2,3 Graphene applications typically rely on material growth by chemical vapor deposition (CVD) on metal substrates. 4 This process can produce graphene up to meter dimensions, 5 but typically of a polycrystalline nature, with the sheet being made up of a patchwork of grains connected by grain boundaries (GBs). 6 In addition, various transfer processes from the metallic growth substrate onto other substrates (e.g., SiO 2 , BN, and plastics) can lead to wrinkling of the monolayer material. 7 Not surprisingly, GBs and wrinkles are expected to degrade the thermal, 8 electrical, 9,10 and mechanical 11 properties of graphene. Recent work has measured the electrical resistance of graphene GBs, 9-14 which is important as they limit the overall electrical performance of graphene devices grown by CVD. 6 However, the associated temperature rise resulting from nanometer-scale resistive heating of GBs is currently unknown. Understanding this aspect is important both from a graphene device perspective (e.g. reliability) and also as a unique platform directly connecting the technology of nanoscale thermometry tools with the science of atomic-scale heat generation at defects within realistic devices.In this study, we measured the nanometer-scale temperature rise in CVD grown hexagonal graphene grains using scanning Joule expansion microscopy (SJEM), 15-18 a thermometry technique based on atomic force microscopy (AFM). We specifically study the resistive heating at graphene wrinkles and GBs, giving insight into the coupled electrical and thermal properties of such nanoscale defects. We observe a small temperature rise at wrinkles and a larger temperature rise at GBs (150%-300% greater than the surrounding graphene) due to the finite GB resistivity and to non-uniform current flow across GBs, visualized here with nanometer-scale resolution. Figure 1(a) shows the optical image of a typical GB device used in ...