Date Published: February 16, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Yu Li, Ziwei Li, Cheng Chi, Hangyong Shan, Liheng Zheng, Zheyu Fang.
Plasmonics has developed for decades in the field of condensed matter physics and optics. Based on the classical Maxwell theory, collective excitations exhibit profound light‐matter interaction properties beyond classical physics in lots of material systems. With the development of nanofabrication and characterization technology, ultra‐thin two‐dimensional (2D) nanomaterials attract tremendous interest and show exceptional plasmonic properties. Here, we elaborate the advanced optical properties of 2D materials especially graphene and monolayer molybdenum disulfide (MoS2), review the plasmonic properties of graphene, and discuss the coupling effect in hybrid 2D nanomaterials. Then, the plasmonic tuning methods of 2D nanomaterials are presented from theoretical models to experimental investigations. Furthermore, we reveal the potential applications in photocatalysis, photovoltaics and photodetections, based on the development of 2D nanomaterials, we make a prospect for the future theoretical physics and practical applications.
Materials present distinct properties as their size reduce into low‐dimensional regime, where atoms and electrons are restricted to limited free degree, involving strong light‐mater interaction, ultrahigh electric conductivity and excellent mechanical flexibility. In the recent years, many kinds of 2D materials are analyzed ranging from graphene,1 layered transition metal dichalcogenides (LTMDCs) to hexagonal boron nitride (hBN),2, 3 their excellent optoelectronic properties make a wide range of optoelectronic applications become possible.4 Semi‐metallic graphene interacts with photons in a large range of energy,5, 6 considering its ultrahigh carrier mobility at room temperature (higher than 104 cm2V−1s−1) and high degree of optical transparency (approximately 97.7%), graphene shows unique capability of supporting and tuning surface plasmons which can be utilized in light detection applications and new light‐involved electronics design.7, 8, 9, 10, 11, 12, 13, 14, 15 MoS2 and WSe2 are two typical 2D semiconductors, which belong to LTMDCs. Compared with graphene, the carrier mobility of few layered semiconductors is relatively low, but it is much higher than that of the bulk semiconductors as gallium arsenide (GaAs) or silicon of the same thickness.16, 17 The optic bandgaps of LTMDCs range from 1.0 to 2.5 eV, making them suitable for light emitters and absorbents in a wide spectrum.18 However, due to the limited light‐harvest ability and manipulating possibility, efficient utilizations of 2D material are far from practical applications.
The atomic thin 2D nanomaterial sheet presents significant light‐matter interaction phenomena, owing to quantum confinement effect, their electronic structures and optical properties are distinctive from their bulk morphology.29 Because of those advanced electronic and optical properties, plasmonic properties of 2D nanomaterials exhibit more attractive characteristics. Metallic graphene support SP mode in the infrared regime while excitonic LTMDCs/plasmonic structures hybrid system exhibits profound coupling effect, which induces more interesting plasmonic properties of 2D nanomaterials.
The properties of plasmons in 2D nanomaterials show strong dependence on the material band structure, which can be influenced by chemical and electric doping effects. Also, constructing 2D nanomaterials into specific patterns and inducing defects can also modulate the plasmonic properties, which come from the boundary condition. More importantly, 2D nanomaterials hybridized with metal nanostructures give us new ideas for the plasmonic modulation methods since the plasmonic tuning techniques of different metal nanostructures have been profoundly researched and show great possibilities, along with advantage of the strong field enhancement effect.
Some 2D nanomaterials with atomic thin thickness show unique optical properties, such as direct band gap light emission, valley polarized PL and distinctive Raman signals. However, the monolayer regime of 2D nanomaterials provides a significant challenge for weak light‐matter interaction, which limits their applications in light‐emitting and optoelectronic devices. 2D nanomaterials interacting with metal nanomaterials constitute abundant heterostructures, which can modulate and enhance the light‐matter interaction of materials via active plasmonic effects. These modulation results in heterostructures can be concluded into two parts depending on optical signals. On the one hand, metallic nanomaterials can enhance or quench the PL intensity of 2D semiconductors. Spectra splitting of shift can also be observed in strong coupling process of exciton and surface plasmon. On the other hand, the heterostructures employed as sensors can significantly enhance the fingerprint Raman spectra of molecules and other biological samples. Due to extraordinary properties of 2D nanomaterials and the flexibility of parameters control as reviewed above, the potential applications are of growing interest in many fields as highly compacted light emitting devices, sensitive optical sensors, and efficient chemical catalyst driven by light power.
In this review, we summarized the plasmonics of 2D nanomaterials, which include 2D materials as graphene, LTMDCs and hybridization with nanoparticles as well as nanostructures. Based on their unique properties and coupling effects, we proposed several research directions for the light‐matter interaction studies, plasmonic tuning methods and potential applications in the future.