Frontiers of Physics

Graphene and other Two-Dimensional Materials (Eds. Daria Andreeva, Wencai Ren, Guangcun Shan & Kostya Novoselov)
 
In the past decade, since the discovery of graphene by Geim and Novoselov, a tremendous research effort has been devoted to graphene and other two-dimensional materials due to its fascinating properties. The Nobel Prize in Physics for 2010 was awarded to them “for groundbreaking experiments regarding the two-dimensional material graphene”.  Moreover, in recent years, research on graphene has further aroused large interest in other two-dimensional materials. Among these novel materials beyond graphene, its direct cousins, silicene and germanene (the counterparts of silicon and germanium) have earned special focus. Note that the low-energy theory of these materials is described by Dirac fermions as in graphene, and thus the relatively strong spin-orbit interactions are large enough to make materials such as silicene and germanene to be non-trivial topological insulators. Furthermore, studies on monolayer transition-metal di-chalcogenides and layered double transition-metal carbides (MXenes) are also gaining attention.  The challenges in exploring the physical properties of these 2D materials will be addressed in this special issue. Finally, a variety of potential applications based on these 2D materials will also be reviewed.
 
The scope of this focus issue in Frontiers of Physics would cover all of the aspects from experimental synthesis, experimental characterizations (ARPES, STM, optical absorption etc.), electronic properties, optical properties, and also topological properties, etc. This special issue will present the major recent progress in this field from the best experimental and theoretical teams all over the world. We do hope that the issue will form a broad overview of the current state of this cuttingedge field.
 
Specific materials of interest covered in this issue include
 ●  Graphene
 ●  Silicene
 ●  Germanene
 ●  Transition metal di-chalcogenides, in particular molybdenum disulphide,
 ●  Boron nitride
 ●  Double transition-metal carbides and/or nitrides (MXenes)
 ●  Composite materials
 
We are looking for high profile scientists from China and overseas to contribute Review, Mini-Review, Perspective, or Research Article in the foresaid areas. Please feel free to choose a striking topic that best fits the issue. Co-authorship is welcome. There is no strict length limit for each article, and for each review at least 15 pages length is highly expected. 
 
We set a tentative publishing date of this special issue in August 2018, and we expect all articles to be submitted by July 20, 2018. The sample article (TEX template) can be downloaded via http://journal.hep.com.cn/fop/EN/column/column15258.shtml and the new manuscript can be submitted online through http://mc.manuscriptcentral.com/fop. All PDFs of the special issue will be open accessed, and a copy of the volume will be mailed to all participants.
 
Sincerely,
 
Daria Andreeva, Ulsan National Institute of Science and Technology, South Korea, daria.baeumler@gmail.com, daria@unist.ac.kr
Wencai Ren, Shenyang National Laboratory for Materials Science, Instituteof Metal Research, Chinese Academy of Sciences, wcren@imr.ac.cn
Guangcun Shan, Beihang University, gcshan@buaa.edu.cn
Kostya Novoselov, University of Manchester, UK, Konstantin.Novoselov@manchester.ac.uk

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Two-dimensional materials: Emerging toolkit for construction of ultrathin high-efficiency microwave shield and absorber
Mingjun Hu, Naibo Zhang, Guangcun Shan, Jiefeng Gao, Jinzhang Liu, Robert K. Y. Li
Front. Phys.    2018, 13 (4): 138113.   https://doi.org/10.1007/s11467-018-0809-8
Abstract   PDF (39068KB)

Two-dimensional (2D) materials generally have unusual physical and chemical properties owing to the confined electro-strong interaction in a plane and can exhibit obvious anisotropy and a significant quantum-confinement effect, thus showing great promise in many fields. Some 2D materials, such as graphene and MXenes, have recently exhibited extraordinary electromagnetic-wave shielding and absorbing performance, which is attributed to their special electrical behavior, large specific surface area, and low mass density. Compared with traditional microwave attenuating materials, 2D materials have several obvious inherent advantages. First, similar to other nanomaterials, 2D materials have a very large specific surface area and can provide numerous interfaces for the enhanced interfacial polarization as well as the reflection and scattering of electromagnetic waves. Second, 2D materials have a particular 2D morphology with ultrasmall thickness, which is not only beneficial for the penetration and dissipation of electromagnetic waves through the 2D nanosheets, giving rise to multiple reflections and the dissipation of electromagnetic energy, but is also conducive to the design and fabrication of various well-defined structures, such as layer-by-layer assemblies, core–shell particles, and porous foam, for broadband attenuation of electromagnetic waves. Third, owing to their good processability, 2D materials can be integrated into various multifunctional composites for multimode attenuation of electromagnetic energy. In addition to behaving as microwave reflectors and absorbers, 2D materials can act as impedance regulators and provide structural support for good impedance matching and setup of the optimal structure. Numerous studies indicate that 2D materials are among the most promising microwave attenuation materials. In view of the rapid development and enormous advancement of 2D materials in shielding and absorbing electromagnetic wave, there is a strong need to summarize the recent research results in this field for presenting a comprehensive view and providing helpful suggestions for future development.

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Environmental engineering of transition metal dichalcogenide optoelectronics
Trevor LaMountain, Erik J. Lenferink, Yen-Jung Chen, Teodor K. Stanev, Nathaniel P. Stern
Front. Phys.    2018, 13 (4): 138114.   https://doi.org/10.1007/s11467-018-0795-x
Abstract   PDF (25604KB)

The explosion of interest in two-dimensional van der Waals materials has been in many ways driven by their layered geometry. This feature makes possible numerous avenues for assembling and manipulating the optical and electronic properties of these materials. In the specific case of monolayer transition metal dichalcogenide semiconductors, the direct band gap combined with the flexibility for manipulation of layers has made this class of materials promising for optoelectronics. Here, we review the properties of these layered materials and the various means of engineering these properties for optoelectronics. We summarize approaches for control that modify their structural and chemical environment, and we give particular detail on the integration of these materials into engineered optical fields to control their optical characteristics. This combination of controllability from their layered surface structure and photonic environment provide an expansive landscape for novel optoelectronic phenomena.

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Interfacial charge transfer in WS2 monolayer/CsPbBr3 microplate heterostructure
Zhen-Zhong Yan, Zhao-Han Jiang, Jun-Peng Lu, Zhen-Hua Ni
Front. Phys.    2018, 13 (4): 138115.   https://doi.org/10.1007/s11467-018-0785-z
Abstract   PDF (7127KB)

Integration of heterogenous materials produces compelling physical phenomena and increased performance of optoelectronic devices. In this work, we integrate CsPbBr3 microplate with WS2 monolayer to investigate the interfacial carrier transfer mechanism in the heterojunction. The quenching of photoluminescence (PL) emission from CsPbBr3 and WS2 after heterostructure formation indicates efficient charge transfer in the junction. Low-temperature PL spectra reveal that the decreasing PL of WS2 arises from the vanishing of biexcitons. Photodetection based on the WS2/CsPbBr3 heterostructure is demonstrated. The higher performance from the junction further certifies the occurrence of charge transfer in the heterojunction.

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Vertically aligned γ-AlOOH nanosheets on Al foils as flexible and reusable substrates for NH3 adsorption
Chen Yang, Ying Chen, Dan Liu, Jinfeng Wang, Cheng Chen, Jiemin Wang, Ye Fan, Shaoming Huang, Weiwei Lei
Front. Phys.    2018, 13 (4): 138101.   https://doi.org/10.1007/s11467-018-0747-5
Abstract   PDF (5862KB)

Vertically aligned γ-AlOOH nanosheets (NSs) have been successfully fabricated on flexible Al foils via a solvothermal route without morphology-directing agents. Three different reaction temperature (25, 80, and 120 ?C) and time (30 min, 45 min, and 24 h) are discussed for the growth period, which efficiently tune the density and size of the γ-AlOOH NSs. Meanwhile, the growth speed of the nanosheets confirms that dominant growth stage is seen in the initial 45 min. Furthermore, the interlayer of the γ-AlOOH NSs displays an average height of 140 nm and superhydrophilicity. By dynamic adsorption, the assynthesized γ-AlOOH NSs exhibit an outstanding NH3 adsorption capacity of up to 146 mg/g and stably excellent regeneration for 5 cycles. The mechanism of NH3 adsorption on the in-plane of the γ-AlOOH NSs is explained by the Lewis acid/base theory. The H-bond interactions among the NH3 molecules and the edge groups (-OH) further improve the capture ability of the nanosheets.

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Monolayered semiconducting GeAsSe and SnSbTe with ultrahigh hole mobility
Yu Guo, Nan Gao, Yizhen Bai, Jijun Zhao, Xiao Cheng Zeng
Front. Phys.    2018, 13 (4): 138117.   https://doi.org/10.1007/s11467-018-0810-2
Abstract   PDF (11667KB)

High carrier mobility and a direct semiconducting band gap are two key properties of materials for electronic device applications. Using first-principles calculations, we predict two types of two-dimensional semiconductors, ultrathin GeAsSe and SnSbTe nanosheets, with desirable electronic and optical properties. Both GeAsSe and SnSbTe sheets are energetically favorable, with formation energies of −0.19 and −0.09 eV/atom, respectively, and have excellent dynamical and thermal stability, as determined by phonon dispersion calculations and Born–Oppenheimer molecular dynamics simulations. The relatively weak interlayer binding energies suggest that these monolayer sheets can be easily exfoliated from the bulk crystals. Importantly, monolayer GeAsSe and SnSbTe possess direct band gaps (2.56 and 1.96 eV, respectively) and superior hole mobility (~20 000 cm2·V−1·s−1), and both exhibit notable absorption in the visible region. A comparison of the band edge positions with the redox potentials of water reveals that layered GeAsSe and SnSbTe are potential photocatalysts for water splitting. These exceptional properties make layered GeAsSe and SnSbTe promising candidates for use in future high-speed electronic and optoelectronic devices.

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The rise of two-dimensional MoS2 for catalysis
Jun Mao (毛军), Yong Wang (王勇), Zhilong Zheng (郑智龙), Dehui Deng (邓德会)
Front. Phys.    2018, 13 (4): 138118.   https://doi.org/10.1007/s11467-018-0812-0
Abstract   PDF (56346KB)

Two-dimensional (2D) MoS2 is used as a catalyst or support and has received increased research interest because of its superior structural and electronic properties compared with those of bulk structures. In this article, we illustrate the active sites of 2D MoS2 and various strategies for enhancing its intrinsic catalytic activity. The recent advances in the use of 2D MoS2-based materials for applications such as thermocatalysis, electrocatalysis, and photocatalysis are discussed. We also discuss the future opportunities and challenges for 2D MoS2-based materials, in both fundamental research and industrial applications.

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Graphene based functional devices: A short review
Rong Wang, Xin-Gang Ren, Ze Yan, Li-Jun Jiang, Wei E. I. Sha, Guang-Cun Shan
Front. Phys.    2019, 14 (1): 13603.   https://doi.org/10.1007/s11467-018-0859-y
Abstract   PDF (27911KB)

Graphene is an ideal 2D material system bridging electronic and photonic devices. It also breaks the fundamental speed and size limits by electronics and photonics, respectively. Graphene offers multiple functions of signal transmission, emission, modulation, and detection in a broad band, high speed, compact size, and low loss. Here, we have a brief view of graphene based functional devices at microwave, terahertz, and optical frequencies. Their fundamental physics and computational models were discussed as well.

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The art of designing carbon allotropes
Run-Sen Zhang, Jin-Wu Jiang
Front. Phys.    2019, 14 (1): 13401.   https://doi.org/10.1007/s11467-018-0836-5
Abstract   PDF (7542KB)

Stimulated by the success of graphene and diamond, a variety of carbon allotropes have been discovered in recent years in either two-dimensional or three-dimensional configurations. Although these emerging carbon allotropes share some common features, they have certain different and novel mechanical or physical properties. In this review, we present a comparative survey of some of the major properties of fifteen newly discovered carbon allotropes. By comparing their structural topology, we propose a general route for designing most carbon allotropes from two mother structures, namely, graphene and diamond. Furthermore, we discuss several future prospects as well as current challenges in designing new carbon allotropes.

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Stacking transition in rhombohedral graphite
Tataiana Latychevskaia, Seok-Kyun Son, Yaping Yang, Dale Chancellor, Michael Brown, Servet Ozdemir, Ivan Madan, Gabriele Berruto, Fabrizio Carbone, Artem Mishchenko, Kostya S. Novoselov
Front. Phys.    2019, 14 (1): 13608.   https://doi.org/10.1007/s11467-018-0867-y
Abstract   PDF (11234KB)

Few-layer graphene (FLG) has recently been intensively investigated for its variable electronic properties, which are defined by a local atomic arrangement. While the most natural arrangement of layers in FLG is ABA (Bernal) stacking, a metastable ABC (rhombohedral) stacking, characterized by a relatively high-energy barrier, can also occur. When both types of stacking occur in one FLG device, the arrangement results in an in-plane heterostructure with a domain wall (DW). In this paper, we present two approaches to demonstrate that the ABC stacking in FLG can be controllably and locally turned into the ABA stacking. In the first approach, we introduced Joule heating, and the transition was characterized by 2D peak Raman spectra at a submicron spatial resolution. The transition was initiated in a small region, and then the DW was controllably shifted until the entire device became ABA stacked. In the second approach, the transition was achieved by illuminating the ABC region with a train of 790-nm-wavelength laser pulses, and the transition was visualized by transmission electron microscopy in both diffraction and dark-field imaging modes. Further, using this approach, the DW was visualized at a nanoscale spatial resolution in the dark-field imaging mode.

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Convergent and divergent beam electron holography and reconstruction of adsorbates on free-standing two-dimensional crystals
T. Latychevskaia, C. R. Woods, Yi Bo Wang, M. Holwill, E. Prestat, S. J. Haigh, K. S. Novoselov
Front. Phys.    2019, 14 (1): 13606.   https://doi.org/10.1007/s11467-018-0851-6
Abstract   PDF (20930KB)

Van der Waals heterostructures have been lately intensively studied because they offer a large variety of properties that can be controlled by selecting 2D materials and their sequence in the stack. The exact arrangement of the layers as well as the exact arrangement of the atoms within the layers, both are important for the properties of the resulting device. However, it is very difficult to control and characterize the exact position of the atoms and the layers in such heterostructures, in particular, along the vertical (z) dimension. Recently it has been demonstrated that convergent beam electron diffraction (CBED) allows quantitative three-dimensional mapping of atomic positions in three-dimensional materials from a single CBED pattern. In this study we investigate CBED in more detail by simulating and performing various CBED regimes, with convergent and divergent wavefronts, on a somewhat simplified system: a two-dimensional (2D) monolayer crystal. In CBED, each CBED spot is in fact an in-line hologram of the sample, where in-line holography is known to exhibit high intensity contrast in detection of weak phase objects that are not detectable in conventional in-focus imaging mode. Adsorbates exhibit strong intensity contrast in the zero and higher order CBED spots, whereas lattice deformation such as strain or rippling cause noticeable intensity contrast only in the first and higher order CBED spots. The individual CBED spots can thus be reconstructed as typical in-line holograms, and a resolution of 2.13 Å can in principle be achieved in the reconstructions. We provide simulated and experimental examples of CBED of a 2D monolayer crystal. The simulations show that individual CBED spots can be treated as in-line holograms and sample distributions such as adsorbates, can be reconstructed. Individual atoms can be reconstructed from a single CBED pattern provided the later exhibits high-order CBED spots. The experimental results were obtained in a transmission electron microscope (TEM) at 80 keV on free-standing monolayer hBN containing adsorbates. Examples of reconstructions obtained from experimental CBED patterns at a resolution of 2.7 Å are shown. CBED technique can be potentially useful for imaging individual biological macromolecules, because it provides a relatively high resolution and does not require additional scanning procedure or multiple image acquisitions and therefore allows minimizing the radiation damage.

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Probing interlayer interactions in WSe2-graphene heterostructures by ultralow-frequency Raman spectroscopy
Yue Liu (刘月), Yu Zhou (周煜), Hao Zhang (张昊), Feirong Ran (冉飞荣), Weihao Zhao (赵炜昊), Lin Wang (王琳), Chengjie Pei (裴成杰), Jindong Zhang (张锦东), Xiao Huang (黄晓), Hai Li (李海)
Front. Phys.    2019, 14 (1): 13607.   https://doi.org/10.1007/s11467-018-0854-3
Abstract   PDF (9649KB)

Interlayer interactions at the heterointerfaces of van der Waals heterostructures (vdWHs), which consist of vertically stacked two-dimensional materials, play important roles in determining their properties. The interlayer interactions are tunable from noncoupling to strong coupling by controlling the twist angle between adjacent layers. However, the influence of stacking sequence and individual component thickness on the properties of vdWHs has rarely been explored. In this work, the influence of the stacking sequence of WSe2 and graphene in vdWHs of graphene-on-WSe2 (graphene/WSe2) or WSe2-on-graphene (WSe2/graphene), as well as their thickness, on their interlayer interaction was systematically investigated by ultralow-frequency (ULF) Raman spectroscopy. A series of ULF breathing modes of WSe2 nanosheets in these vdWHs were observed with frequencies highly dependent on graphene thickness. Interestingly, the ULF breathing modes of WSe2 red-shifted in graphene/WSe2 and WSe2/graphene configurations, and the amount of shift in the former was much larger than that in the latter. In contrast, no obvious ULF shift was observed by varying the twist angle between WSe2 and graphene. This indicates that the interlayer interaction is more sensitive to the stacking sequence compared with the twist angle. The results provide alternative approaches to modulate the interlayer interaction of vdWHs and, thus, tune their optical and optoelectronic properties.

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