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RADIO-FREQUENCY ELECTRONICS Downscaling with Spiral inductors that are made from intercalated multilayer graphene, and exploit the kinetic of the material, could help deliver miniaturized radio-frequency integrated circuits. Wen Huang and Xiuling Li

nductors are used in almost all radio- The researchers — who are based at factor (a measure of its efficiency) for the frequency (RF) integrated circuits for the University of California Santa Barbara, same inductance density. Isignal filtering and impedance tuning. Shibaura Institute of Technology and Kinetic inductance is distinct from In order to be compatible with Shanghai Jiao Tong University — fabricated traditional ‘magnetic inductance’ and is semiconductor processing technologies, their bromine-intercalated graphene spirals the result of the kinetic energy of mobile the design of on-chip RF inductors is by first transferring millimetre-sized highly charge carries. In low-dimensional carbon typically based on planar spiral structures. oriented pyrolytic graphite slices onto allotropes, the kinetic inductance can To construct the spiral paths, which current quartz substrates. Bromine gas was then potentially be significantly larger than in has to flow through, copper and aluminium diffused into the gaps between the graphene conventional metals, due to their larger are commonly used. In these metals, layers. Finally, the flakes were patterned momentum relaxation times and small inductance is only determined by Faraday’s into spirals using photolithography and number of conducting channels2,3. law of induction, which is a function of the a polymer dielectric layer was added to Notably, Banerjee and colleagues show induced magnetic field and the structural isolate the spirals from overlapping metal that an analytical estimation of the kinetic design of the . Such spiral structures contacts (Fig. 1a,b). Although the resulting inductance of intercalated multilayer have, however, intrinsic drawbacks, inductor spirals have a conductivity that graphene exhibits a 1∕ n2D dependence, including weak magnetic coupling between is only around one sixth of that of bulk compared with a 1∕n2D dependency for turns and difficulties integrating a magnetic copper, it is five times higher than that of multilayer graphene without intercalation, core, which results in a low inductance undoped multilayer graphene. Moreover, where n2D represents the areal carrier density and a large on-chip footprint. These compared to a copper-based inductor on density. This implies that the heavier limitations are set to become increasingly a quartz substrate with the same layout, bromine-intercalated multilayer graphene significant as the Internet of Things (IoT) the intercalated graphene inductors will have less of a cancelling effect on develops, as this will require a tremendous exhibit a maximum inductance that is 1.5 the kinetic inductance, and therefore an number of miniaturized RF integrated times higher, which is attributed to the increase in bromine concentration will lead circuits. Writing in Nature Electronics, contribution of the kinetic inductance of the to a proportional increase in the quality Kaustav Banerjee and colleagues now show material (Fig. 1c). In other words, compared factor. The researchers also indicated that that on-chip RF spiral inductors that are to conventional copper inductors, this an increase in the number of layers in the made from intercalated multilayer graphene technology could have an on-chip footprint multilayer graphene leads to a decrease in can offer high inductance density or reduced that is reduced by a third for the same the kinetic inductance, but an appropriate size dimensions1. inductance, or could have a higher quality number of layers in the multilayer

abc Intercalated MLG 1.8 10

200 m Metal contact G μ G I-MLG SU-8 dielectric Cu S S 1.6 8

25 μm G G 6 Q -factor 1.4

LM+LK 4 Inductance (nH) 1.2

LM 2 1.0

SiO2 substrate 0 0102030405060 Frequency (GHz)

Fig. 1 | Spiral inductors made from bromine-intercalated multilayer graphene. a, Schematic of the spiral inductor design. b, Optical image of a fabricated chip containing an array of the spiral inductors. Inset: optical image of one spiral inductor from the array. G, ground; S, signal. c, Measured inductance and quality factor (Q-factor) versus frequency for the inductor design shown in (b), compared to a copper inductor. The intercalated multilayer graphene (I-MLG) 1 inductor is shown in blue, the copper inductor in red. LM, magnetic inductance; LK, kinetic inductance. Figure adapted from ref. , Macmillan Publishers Ltd.

6 Nature Electronics | VOL 1 | JANUARY 2018 | 6–7 | www.nature.com/natureelectronics © 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. © 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. news & views

graphene is necessary to lower the contact working mechanism. Moreover, kinetic the growth, transfer and intercalation of resistance. Previous work has indicated that inductance is independent of substrate graphene in terms of quality and scale, as higher bromine concentrations4, or other geometry and could offer superb flexibility well as circuit level integration. ❐ intercalation guests3,5, can lead to increased for wearable devices. conductivity in graphene, which suggests Beyond these device possibilities, it Wen Huang and Xiuling Li* that further down-scaling of the size of the might also be valuable to explore ways to Department of Electrical and Computer Engineering, intercalated graphene inductors could also improve the scalability of on-chip inductors Micro and Nanotechnology Laboratory, Materials be possible. even further, through both material and Research Laboratory, University of Illinois at Urbana- The work of Banerjee and colleagues structural innovations, with a goal of Champaign, Urbana, IL, USA. also suggests that kinetic induction eliminating all issues associated with current *e-mail: [email protected] could be utilized in a variety of planar inductors. In terms of structural different applications in areas such as innovations, techniques that can create self- Published online: 8 January 2018 communication, sensing and energy rolled-up membranes6–8 could, for example, https://doi.org/10.1038/s41928-017-0015-7 storage and transfer. To start, other be applied to fabricate three-dimensional promising passive components can be intercalated graphene coils that stand References 1. Kang, J. et al. Nat. . https://doi.org/10.1038/s41928-017- envisioned with the current platform. above a substrate. An evolution of on-chip 0010-z (2018). For example, more compact and efficient electronics can be envisioned in which 2. Ashcrof, N. W. & Mermin, N. D. Solid State Physics (Saunders on-chip RF transformers could be created both two-dimensional materials and three- College, Philadelphia, PA, 1976). 3. Jiang, J. et al. Nano Lett. 17, 1482–1488 (2017). by simply configuring magnetically dimensional structural design is exploited 4. Tongay, S. et al. Phys. Rev. B 81, 115428 (2010). coupled coils made of such intercalated to deliver novel passive devices and sensors 5. Dresselhaus, M. S. Adv. Phys 51, 1–186 (2002). multilayer graphene. Alternatively, with improved performance in areas such 6. Huang, W. et al. Nano Lett. 12, 6283–6288 (2012). loop antennas, for use in near-field as inductance density, power density and 7. Yu, X. et al. Sci. Rep. 5, 9661 (2015). 8. Huang, W. et al. In Proc. IEEE MTT-S Int. Microwave communication tags, could be significantly sensitivity. In all cases, delivering practical Symp. 1645–1648 (IEEE, 2017). https://doi.org/10.1109/ miniaturized using the kinetic induction utility will require strategies for optimizing MWSYM.2017.8058953

Nature Electronics | VOL 1 | JANUARY 2018 | 6–7 | www.nature.com/natureelectronics 7 © 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. © 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.