We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

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Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

Black phosphorus (BP), an emerging narrow direct band-gap two-dimensional (2D) layered material that can fill the gap between the semi-metallic graphene and the wide-bandgap transition metal dichalcogenides (TMDs), had been experimentally found to exhibit the saturation of optical absorption if under strong light illumination. By taking advantage of this saturable absorption property, we could fabricate a new type of optical saturable absorber (SA) based on mechanically exfoliated BPs, and further demonstrate the applications for ultra-fast laser photonics. Based on the balanced synchronous twin-detector measurement method, we have characterized the saturable absorption property of the fabricated BP-SAs at the telecommunication band. By incorporating the BP-based SAs device into the all-fiber Erbium-doped fiber laser cavities, we are able to obtain either the passive Q-switching (with maximum pulse energy of 94.3 nJ) or the passive mode-locking operation (with pulse duration down to 946 fs). Our results show that BP could also be developed as an effective SA for pulsed fiber or solid-state lasers.

Mechanically exfoliated black phosphorus as a new saturable absorber for both Q-switching and Mode-locking laser operation

A decade after the first report, the family of two-dimensional (2D) carbides and nitrides (MXenes) includes structures with three, five, seven, or nine layers of atoms in an ordered or solid solution form. Dozens of MXene compositions have been produced, resulting in MXenes with mixed surface terminations. MXenes have shown useful and tunable electronic, optical, mechanical, and electrochemical properties, leading to applications ranging from optoelectronics, electromagnetic interference shielding, and wireless antennas to energy storage, catalysis, sensing, and medicine. Here we present a forward-looking review of the field of MXenes. We discuss the challenges to be addressed and outline research directions that will deepen the fundamental understanding of the properties of MXenes and enable their hybridization with other 2D materials in various emerging technologies.

The world of two-dimensional carbides and nitrides (MXenes)

2D transition metal carbides, carbonitrides, and nitrides, known as MXenes, are a rapidly growing family of 2D materials with close to 30 members experimentally synthesized, and dozens more studied theoretically. They exhibit outstanding electronic, optical, mechanical, and thermal properties with versatile transition metal and surface chemistries. They have shown promise in many applications, such as energy storage, electromagnetic interference shielding, transparent electrodes, sensors, catalysis, photothermal therapy, etc. The high electronic conductivity and wide range of optical absorption properties of MXenes are the key to their success in the aforementioned applications. However, relatively little is currently known about their fundamental electronic and optical properties, limiting their use to their full potential. Here, MXenes' electronic and optical properties from both theoretical and experimental perspectives, as well as applications related to those properties, are discussed, providing a guide for researchers who are exploring those properties of MXenes.

Electronic and Optical Properties of 2D Transition Metal Carbides and Nitrides (MXenes).

MXenes are an emerging family of highly-conductive 2D materials which have demonstrated state-of-the-art performance in electromagnetic interference shielding, chemical sensing, and energy storage. To further improve performance, there is a need to increase MXenes' electronic conductivity. Tailoring the MXene surface chemistry could achieve this goal, as density functional theory predicts that surface terminations strongly influence MXenes' Fermi level density of states and thereby MXenes' electronic conductivity. Here, we directly correlate MXene surface de-functionalization with increased electronic conductivity through in situ vacuum annealing, electrical biasing, and spectroscopic analysis within the transmission electron microscope. Furthermore, we show that intercalation can induce transitions between metallic and semiconductor-like transport (transitions from a positive to negative temperature-dependence of resistance) through inter-flake effects. These findings lay the groundwork for intercalation- and termination-engineered MXenes, which promise improved electronic conductivity and could lead to the realization of semiconducting, magnetic, and topologically insulating MXenes.

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Control of MXenes' electronic properties through termination and intercalation.

2D transition metal carbides, nitrides, and carbonitides called MXenes have attracted much attention due to their outstanding properties. However, MXene's potential in laser technology is not explored. It is demonstrated here that Ti3 CN, one of MXene compounds, can serve as an excellent mode-locker that can produce femtosecond laser pulses from fiber cavities. Stable laser pulses with a duration as short as 660 fs are readily obtained at a repetition rate of 15.4 MHz and a wavelength of 1557 nm. Density functional theory calculations show that Ti3 CN is metallic, in contrast to other 2D saturable absorber materials reported so far to be operative for mode-locking. 2D structural and electronic characteristics are well conserved in their stacked form, possibly due to the unique interlayer coupling formed by MXene surface termination groups. Noticeably, the calculations suggest a promise of MXenes in broadband saturable absorber applications due to metallic characteristics, which agrees well with the experiments of passively Q-switched lasers using Ti3 CN at wavelengths of 1558 and 1875 nm. This study provides a valuable strategy and intuition for the development of nanomaterial-based saturable absorbers opening new avenues toward advanced photonic devices based on MXenes.

Metallic MXene Saturable Absorber for Femtosecond Mode-Locked Lasers.

We experimentally demonstrate a femtosecond mode-locked, all-fiberized laser that operates in the 2 μm region and that incorporates a saturable absorber based on a bulk-structured bismuth telluride (Bi(2)Te(3)) topological insulator (TI). Our fiberized saturable absorber was prepared by depositing a mechanically exfoliated, ~30 μm-thick Bi(2)Te(3) TI layer on a side-polished optical fiber platform. The bulk crystalline structure of the prepared Bi(2)Te(3) layer was confirmed by Raman and X-ray photoelectron spectroscopy measurements. The modulation depth of the prepared saturable absorber was measured to be ~20.6%. Using the saturable absorber, it is shown that stable, ultrafast pulses with a temporal width of ~795 fs could readily be generated at a wavelength of 1935 nm from a thulium/holmium co-doped fiber ring cavity. This experimental demonstration confirms that bulk structured, TI-based saturable absorbers can readily be used as an ultra-fast mode-locker for 2 μm lasers.

A femtosecond pulse fiber laser at 1935 nm using a bulk-structured Bi 2 Te 3 topological insulator

We experimentally demonstrate the use of a bulk-structured Bi2Te3 topological insulator (TI) as an ultrafast mode-locker to generate femtosecond pulses from an all-fiberized cavity. Using a saturable absorber based on a mechanically exfoliated layer about 15 μm thick deposited onto a side-polished fiber, we show that stable soliton pulses with a temporal width of ~600 fs can readily be produced at 1547 nm from an erbium fiber ring cavity. Unlike previous TI-based mode-locked laser demonstrations, in which high-quality nanosheet-based TIs were used for saturable absorption, we chose to use a bulk-structured Bi2Te3 layer because it is easy to fabricate. We found that the bulk-structured Bi2Te3 layer can readily provide sufficient nonlinear saturable absorption for femtosecond mode-locking even if its modulation depth of ~15.7% is much lower than previously demonstrated nanosheet-structured TI-based saturable absorbers. This experimental demonstration indicates that high-crystalline-quality atomic-layered films of TI, which demand complicated and expensive material processing facilities, are not essential for ultrafast laser mode-locking applications.

A femtosecond pulse erbium fiber laser incorporating a saturable absorber based on bulk-structured Bi2Te3 topological insulator.

We demonstrate the use of an all-fiberized, mode-locked 1.94 μm laser with a saturable absorption device based on a tungsten disulfide (WS2)-deposited side-polished fiber. The WS2 particles were prepared via liquid phase exfoliation (LPE) without centrifugation. A series of measurements including Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM) revealed that the prepared particles had thick nanostructures of more than 5 layers. The prepared saturable absorption device used the evanescent field interaction mechanism between the oscillating beam and WS2 particles and its modulation depth was measured to be ~10.9% at a wavelength of 1925 nm. Incorporating the WS2-based saturable absorption device into a thulium-holmium co-doped fiber ring cavity, stable mode-locked pulses with a temporal width of ~1.3 ps at a repetition rate of 34.8 MHz were readily obtained at a wavelength of 1941 nm. The results of this experiment confirm that WS2 can be used as an effective broadband saturable absorption material that is suitable to passively generate pulses at 2 μm wavelengths.

Mode-locked, 1.94-μm, all-fiberized laser using WS₂ based evanescent field interaction.

Mono- and few-layer transition metal dichalcogenides (TMDCs) have been widely used as saturable absorbers for ultrashort laser pulse generation, but their preparation is complicated and requires much expertise. The possible use of bulk-structured TMDCs as saturable absorbers is therefore a very intriguing and technically important issue in laser technology. Here, for the first time, it is demonstrated that defective, bulk-structured WTe2 microflakes can serve as a base saturable absorption material for fast mode-lockers that can produce femtosecond pulses from fiber laser cavities. They have a modulation depth of 2.85%, from which stable laser pulses with a duration of 770 fs are readily obtained at a repetition rate of 13.98 MHz and a wavelength of 1556.2 nm, which is comparable to the performance achieved using mono- and few-layer TMDCs. Density functional theory calculations show that the oxidative and defective surfaces of WTe2 microflakes do not degrade their saturable absorption performance in the near-infrared range, allowing for a broad range of operative bandwidth. This study suggests that saturable absorption is an intrinsic property of TMDCs without relying on their structural dimensionality, providing a new direction for the development of TMDC-based saturable absorbers.

Joonhoi Koo

Papers

Metallic MXene Saturable Absorber for Femtosecond Mode-Locked Lasers.

A femtosecond pulse fiber laser at 1935 nm using a bulk-structured Bi 2 Te 3 topological insulator

A femtosecond pulse erbium fiber laser incorporating a saturable absorber based on bulk-structured Bi2Te3 topological insulator.

Mode-locked, 1.94-μm, all-fiberized laser using WS₂ based evanescent field interaction.

Near-Infrared Saturable Absorption of Defective Bulk-Structured WTe2 for Femtosecond Laser Mode-Locking