Guest Commentary pubs.acs.org/JPCL
Defects at the Two-Dimensional Limit
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materials present opportunities for highly selective molecular separation. The Perspective by Huang et al. in this issue provides a detailed exploration of graphene for this purpose including nanoporous graphene and graphene oxide membrane structures.16 Alternative 2D materials with inherent anisotropy in their crystal structure (e.g., 2D black phosphorus or “phosphorene”) allow further control over the shape of extended vacancy defects that may prove to be of interest to separation technologies.17 The rapid ascent of phosphorene within the 2D material family is delineated by the Perspective by Kou et al. in this issue.18 In conclusion, defect engineering has emerged as a leading issue in the burgeoning field of 2D materials. Compared to their bulk counterparts, 2D material defects tend to have more significant implications for materials properties. An extreme example is the case of surfaces, which often play a minor role in bulk materials but completely dominate 2D materials where every atom exists on the surface in the atomically thin limit. With the increasing importance of defects, researchers working on 2D materials are either pursuing improved processing methods to minimize defect densities11 or are developing applications that can exploit new phenomena enabled by 2D defect structures.16 A third intriguing possibility is to engineer the structure and properties of defects through chemical functionalization or passivation since defects typically represent points of increased chemical reactivity.19 Indeed, this third option presents great opportunities for chemistry to impact the development of the 2D material field.
uch of the richness of materials science can be attributed to defects. As Ashcroft and Mermin state in their famous treatise on solid state physics:1 “Like human defects, those of crystals come in a seemingly endless variety, many dreary and depressing, and a few fascinating.” In semiconductors, dopant impurities tailor electronic properties and enable modern day electronics. In ionic crystals, vacancies and interstitials provide conductivity and modulate optical properties. In metals, dislocations underlie mechanical strength. In magnetic materials, defects stabilize the orientation of ferromagnetic domains, thus enabling net magnetization in the absence of an applied field. Although the aforementioned examples illustrate the benefits provided by defects, deviations from perfect crystalline structure can also introduce problems. For example, the mobility of charge carriers is compromised by scattering from impurities, grain boundaries, and surfaces. Similarly, pinhole defects in dielectrics lead to leakage current and unwanted power dissipation in field-effect transistors. Dislocations famously quench photoluminescence from nitridebased semiconductors such that the development of growth strategies to minimize these defects led to technologically viable blue light-emitting diodes2 and ultimately the 2014 Nobel Prize in Physics. Understanding and controlling defects thus present significant challenges and opportunities for optimizing the properties of materials. With fewer degrees of freedom, the effects of defects are often accentuated in low-dimensional materials. In particular, with the advent of graphene3 and related atomically thin materials,4 defects in the two-dimensional (2D) limit have come to the forefront in recent years. Although much of the initial fundamental work and prototype device development has occurred on small, exquisitely prepared, micromechanically exfoliated samples where defects are minimized,5 subsequent efforts to scale-up production have suffered from intolerably high defect levels. For example, aggressive chemical exfoliation methods for graphite6 and transition metal dichalcogenides7 often induce unwanted phase changes and extended vacancies that fundamentally change the properties of the resulting 2D materials. To overcome these issues, significant effort has been devoted to alternative solution-based dispersion methods that utilize benign solvents8 and noncovalent surfactant chemistry.9 However, even in these cases, the resulting exfoliated 2D flakes possess relatively small lateral areas, which implies that edge defects limit performance.10 Consequently, scalable growth methods based on chemical vapor deposition (CVD) have been pursued to achieve large-area atomically thin materials with minimal grain boundaries. The Perspective by Hofmann et al. in this issue outlines the latest status of efforts to achieve electronic grade 2D materials via CVD.11 Although many applications for 2D materials benefit from minimal defects,12−14 researchers are also realizing unique opportunities presented by defects at the 2D limit. For example, grain boundaries in atomically thin MoS2 yield gate-tunable memristive phenomena that is of high interest to neuromorphic computing.15 In addition, controlled vacancy defects in 2D © 2015 American Chemical Society
Mark C. Hersam*
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Department of Materials Science and Engineering, Department of Chemistry, and Department of Medicine, Northwestern University, 2220 Campus Drive, Evanston, Illinois 60208-3108, United States
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
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REFERENCES
(1) Ashcroft, N. W.; Mermin, N. D. Solid State Physics; Harcourt College Publishers: Orlando, FL, 1976; p 616. (2) Nakamura, S.; Mukai, T.; Senoh, M. Candela-Class HighBrightness InGaN/AlGaN Double-Heterostructure Blue-Light-Emitting Diodes. Appl. Phys. Lett. 1994, 64, 1687−1689. (3) Banhart, F.; Kotakoski, J.; Krasheninnikov, A. V. Structural Defects in Graphene. ACS Nano 2011, 5, 26−41. (4) Najmaei, S.; Yuan, J. T.; Zhang, J.; Ajayan, P.; Lou, J. Synthesis and Defect Investigation of Two-Dimensional Molybdenum Disulfide Atomic Layers. Acc. Chem. Res. 2015, 48, 31−40. (5) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. Published: July 16, 2015 2738
DOI: 10.1021/acs.jpclett.5b01218 J. Phys. Chem. Lett. 2015, 6, 2738−2739
Guest Commentary
The Journal of Physical Chemistry Letters (6) Gomez-Navarro, C.; Meyer, J. C.; Sundaram, R. S.; Chuvilin, A.; Kurasch, S.; Burghard, M.; Kern, K.; Kaiser, U. Atomic Structure of Reduced Graphene Oxide. Nano Lett. 2010, 10, 1144−1148. (7) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.; Zhang, H. The Chemistry of Two-Dimensional Layered Transition Metal Dichalcogenide Nanosheets. Nat. Chem. 2013, 5, 263−275. (8) Secor, E. B.; Prabhumirashi, P. L.; Puntambekar, K.; Geier, M. L.; Hersam, M. C. Inkjet Printing of High Conductivity, Flexible Graphene Patterns. J. Phys. Chem. Lett. 2013, 4, 1347−1351. (9) Seo, J. W. T.; Green, A. A.; Antaris, A. L.; Hersam, M. C. HighConcentration Aqueous Dispersions of Graphene Using Nonionic, Biocompatible Block Copolymers. J. Phys. Chem. Lett. 2011, 2, 1004− 1008. (10) Green, A. A.; Hersam, M. C. Emerging Methods for Producing Monodisperse Graphene Dispersions. J. Phys. Chem. Lett. 2010, 1, 544−549. (11) Hofmann, S.; Braeuninger-Weimer, P.; Weatherup, R. S. CVDEnabled Graphene Manufacture and Technology. J. Phys. Chem. Lett. 2015, 6, 2714−2721. (12) Liang, Y. T.; Vijayan, B. K.; Gray, K. A.; Hersam, M. C. Minimizing Graphene Defects Enhances Titania NanocompositeBased Photocatalytic Reduction of CO2 for Improved Solar Fuel Production. Nano Lett. 2011, 11, 2865−2870. (13) Liang, Y. T.; Vijayan, B. K.; Lyandres, O.; Gray, K. A.; Hersam, M. C. Effect of Dimensionality on the Photocatalytic Behavior of Carbon-Titania Nanosheet Composites: Charge Transfer at Nanomaterial Interfaces. J. Phys. Chem. Lett. 2012, 3, 1760−1765. (14) Secor, E. B.; Hersam, M. C. Emerging Carbon and Post-Carbon Nanomaterial Inks for Printed Electronics. J. Phys. Chem. Lett. 2015, 6, 620−626. (15) Sangwan, V. K.; Jariwala, D.; Kim, I. S.; Chen, K.-S.; Marks, T. J.; Lauhon, L. J.; Hersam, M. C. Gate-Tunable Memristive Phenomena Mediated by Grain Boundaries in Single-Layer MoS2. Nat. Nanotechnol. 2015, 10, 403−406. (16) Huang, L.; Zhang, M.; Li, C.; Shi, G. Graphene-Based Membranes for Molecular Separation. J. Phys. Chem. Lett. 2015, 6, 10.1021/acs.jpclett.5b00914. (17) Liu, X. L.; Wood, J. D.; Chen, K. S.; Cho, E.; Hersam, M. C. In Situ Thermal Decomposition of Exfoliated Two-Dimensional Black Phosphorus. J. Phys. Chem. Lett. 2015, 6, 773−778. (18) Kou, L.; Chen, C.; Smith, S. C. Phosphorene: Fabrication, Properties, and Applications. J. Phys. Chem. Lett. 2015, 6, 10.1021/ acs.jpclett.5b01094. (19) Hersam, M. C. The Reemergence of Chemistry for PostGraphene Two-Dimensional Nanomaterials. ACS Nano 2015, 9, 4661−4663.
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DOI: 10.1021/acs.jpclett.5b01218 J. Phys. Chem. Lett. 2015, 6, 2738−2739
