Direct observation of resistive heating at graphene wrinkles and grain boundaries

Abstract

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 ...

Review of flash sintering: materials, mechanisms and modelling

“…On a grain-boundary scale, experimental evidence of this differential heating driven by crystallographic imperfections has been given by Grosse et al [132]. They investigated the grain boundary overheating in a pure graphene sheet at its grain boundaries.…”

Scaling properties of polycrystalline graphene: a review

“…For example, Joule expansion microscopy measurements have revealed strong Joule heating localized at graphene GBs, which can be many times larger than Joule heating in the grains. This has important implications for the reliability of graphene devices, as localized device failure could occur without a significant increase in the average device temperature [23]. Other measurements have shown that electrical noise is greatly enhanced by graphene GBs, which is detrimental for low-noise devices but may be useful for sensor applications [27].…”

“…GB [18][19][20][21][23][24][25][26][27]. The origin of this enhanced resistance has been probed with scanning tunneling spectroscopy (STS), and these measurements have revealed that GBs tend to be n-doped compared to the surrounding grains, such that they act as an electrical potential barrier to charge transport [40,41].…”

“…Therefore, because of its promise for large-area electronic applications, a detailed understanding of the electrical transport properties of polycrystalline graphene is crucial. To this end, a great deal of experimental [13,[18][19][20][21][22][23][24][25][26][27] and theoretical [7,[28][29][30][31][32][33][34][35][36][37][38] effort has has been devoted to studying charge transport across individual graphene GBs, and several reviews have already discussed this topic in great detail [5,26,39]. Therefore, here we briefly summarize the main features of electrical transport across individual graphene GBs before shifting our focus to a more global perspective of charge transport in polycrystalline graphene.…”

“…The GB resistivity thus provides an intrinsic measure of the charge transport properties across the GB, independent of the device or measurement technique. Beyond four-terminal electrical measurements, the resistivity of individual GBs has been measured using a.c. electron force microscopy (AC-EFM) [13], four-point scanning tunneling potentiometry (STP) [21], and Joule expansion microscopy [23]. The average r GB of a series of polycrystalline samples can also be extracted by measuring the sheet resistance of each sample as a function of the average grain size [44][45][46][47][48][49] and then fitting to a simple ohmic scaling law,…”