Surface-enhanced Raman spectroscopy (SERS) imparts Raman spectroscopy with the capability of detecting analytes at the single-molecule level, but the costs are also manifold, such as a loss of signal reproducibility. Despite remarkable steps having been taken, presently SERS still seems too young to shoulder analytical missions in various practical situations. By the virtue of its unique molecular structure and physical/chemical properties, the rise of graphene opens up a unique platform for SERS studies. In this review, the multi-role of graphene played in SERS is overviewed, including as a Raman probe, as a substrate, as an additive, and as a building block for a flat surface for SERS. Apart from versatile improvements of SERS performance towards applications, graphene-involved SERS studies are also expected to shed light on the fundamental mechanism of the SERS effect.

Graphene: A Platform for Surface‐Enhanced Raman Spectroscopy

In this work, we propose a new configuration of surface plasmon resonance (SPR) sensor that is based on graphene–MoS2 hybrid structures for ultrasensitive detection of molecules. The proposed system displays a phase-sensitivity enhancement factor of more than 500-fold when compared to the SPR sensing scheme without the graphene–MoS2 coating or with only graphene coating. Our hypothesis is that the monolayer MoS2 has a much higher optical absorption efficiency (∼5%) than that of the graphene layer (∼2.3%). Based on our findings, the electron energy loss of MoS2 layer is comparable to that of graphene and this will allow a successful (∼100%) of light energy transfer to the graphene–MoS2 coated sensing substrate. Such process will lead to a significant enhancement of SPR signals. Our simulation shows that a quasi-dark point of the reflected light can be achieved under this condition and this has resulted in a steep phase jump at the resonance angle of our newly proposed SPR system. More importantly, we found that phase interrogation detection approach of the graphene–MoS2 hybrid structures-based sensing system is more sensitive than that of using the regularly angular interrogation method and our theoretical analysis indicates that 45 nm of Au film thickness and 3 coating layers of MoS2 nanosheet are the optimized parameters needed for the proposed SPR system to achieve the highest detection sensitivity range.

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Graphene–MoS2 hybrid nanostructures enhanced surface plasmon resonance biosensors

Hybrid nanostructures composed of graphene or other two-dimensional (2D) nanomaterials and plasmonic metal components have been extensively studied. The unusual properties of 2D materials are associated with their atomically thin thickness and 2D morphology, and many impressive structures enable the metal nanomaterials to establish various interesting hybrid nanostructures with outstanding plasmonic properties. In addition, the hybrid nanostructures display unique optical characteristics that are derived from the close conjunction of plasmonic optical effects and the unique physicochemical properties of 2D materials. More importantly, the hybrid nanostructures show several plasmonic electrical effects including an improved photogeneration rate, efficient carrier transfer, and a plasmon-induced "hot carrier", playing a significant role in enhancing device performance. They have been widely studied for plasmon-enhanced optical signals, photocatalysis, photodetectors (PDs), and solar cells. In this review, the developments in the field of metal/2D hybrid nanostructures are comprehensively described. Preparation of hybrid nanostructures is first presented according to the 2D material type, as well as the metal nanomaterial morphology. The plasmonic properties and the enabled applications of the hybrid nanostructures are then described. Lastly, possible future research in this promising field is discussed.

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Hybrid nanostructures of metal/two-dimensional nanomaterials for plasmon-enhanced applications

Dynamic switching of a plasmonic resonance may find numerous applications in subwavelength optoelectronics, spectroscopy, and sensing. Graphene shows a highly tunable carrier concentration under electrostatic gating, and this could provide an effective route to achieving electrical control of the plasmonic resonance. In this Letter, we demonstrate electrical control of a plasmonic resonance at infrared frequencies using large-area graphene. Plasmonic structures fabricated on graphene enhance the interaction of the incident optical field with the graphene sheet, and the impact of graphene is much stronger at mid-infrared wavelengths. Full-wave simulations, where graphene is modeled as a 1 nm thick effective medium, show excellent agreement with experimental results.

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Electrically Tunable Damping of Plasmonic Resonances with Graphene

Owing to its excellent electrical, mechanical, thermal and optical properties, graphene has attracted great interests since it was successfully exfoliated in 2004. Its two dimensional nature and superior properties meet the need of surface plasmons and greatly enrich the field of plasmonics. Recent progress and applications of graphene plasmonics will be reviewed, including the theoretical mechanisms, experimental observations, and meaningful applications. With relatively low loss, high confinement, flexible feature, and good tunability, graphene can be a promising plasmonic material alternative to the noble metals. Optics transformation, plasmonic metamaterials, light harvesting etc. are realized in graphene based devices, which are useful for applications in electronics, optics, energy storage, THz technology and so on. Moreover, the fine biocompatibility of graphene makes it a very well candidate for applications in biotechnology and medical science.

Plasmons in graphene: Recent progress and applications

Tunability of the surface plasmon resonance wavelength is demonstrated by varying the thickness of Al2O3 spacer layer inserted between the graphene and nanoparticles. By varying the spacer layer thickness from 0.3 to 1.8 nm, the resonance wavelength is shifted from 583 to 566 nm. The shift is due to a change in the electromagnetic field coupling strength between the localized surface plasmons excited in the gold nanoparticles and a single layer graphene film. In contrast, when the graphene film is absent from the system, no noticeable shift in the resonance wavelength is observed upon varying the spacer thickness.

Graphene induced tunability of the surface plasmon resonance

Tunability of the surface plasmon resonance wavelength is demonstrated by varying the thickness of Al2O3 spacer layer inserted between the graphene and nanoparticles. By varying the spacer layer thickness from 0.3 to 1.8 nm, the resonance wavelength is shifted from 583 to 566 nm. The shift is due to a change in the electromagnetic field coupling strength between the localized surface plasmons excited in the gold nanoparticles and a single layer graphene film. In contrast, when the graphene film is absent from the system, no noticeable shift in the resonance wavelength is observed upon varying the spacer thickness.

Due to its high electrical conductivity and excellent transmittance at terahertz frequencies, graphene is a promising candidate as transparent electrodes for terahertz devices. We demonstrate a liquid crystal based terahertz phase shifter with the graphene films as transparent electrodes. The maximum phase shift is 10.8 degree and the saturation voltage is 5 V with a 50 µm liquid crystal cell. The transmittance at terahertz frequencies and electrical conductivity depending on the number of graphene layer are also investigated. The proposed phase shifter provides a continuous tunability, fully electrical controllability, and low DC voltage operation.

Graphene/liquid crystal based terahertz phase shifters

Due to its high electrical conductivity and excellent transmittance at terahertz frequencies, graphene is a promising candidate as transparent electrodes for terahertz devices. We demonstrate a liquid crystal based terahertz phase shifter with the graphene films as transparent electrodes. The maximum phase shift is 10.8 degree and the saturation voltage is 5 V with a 50 um liquid crystal cell. The transmittance at terahertz frequencies and electrical conductivity depending on the number of graphene layer are also investigated. The proposed phase shifter provides a continuous tunability, fully electrical controllability, and low DC voltage operation.

A clear gate-voltage-tunable weak antilocalization and a giant magnetoresistance of \ensuremath{\sim}400$%$ are observed at 1.9 K in single-layer graphene with an out-of-plane field. A large magnetoresistance value of 275$%$ is obtained even at room temperature, implying potential applications of graphene in magnetic sensors. Both the weak antilocalization and giant magnetoresistance persists far away from the charge neutrality point, in contrast to previous reports, and both effects originate from charged impurities. Interestingly, the signatures of Shubnikov--de Haas oscillations and the quantum Hall effect are also observed for the same sample.

Jing Niu

Papers

Graphene induced tunability of the surface plasmon resonance

Graphene induced tunability of the surface plasmon resonance

Graphene/liquid crystal based terahertz phase shifters

Graphene/liquid crystal based terahertz phase shifters

Giant magnetoresistance in single-layer graphene flakes with a gate-voltage-tunable weak antilocalization