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1.	Jago, Roland, 1990, et al. (författare) Current enhancement due to field-induced dark carrier multiplication in graphene 2017 Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 4:2 Tidskriftsartikel (refereegranskat)abstract We present a microscopic study on current generation in graphene in response to an electric field. While scattering is generally considered to reduce the current, we reveal that in graphene Auger processes give rise to a current enhancement via a phenomenon we denote dark carrier multiplication. Based on a microscopic approach, we show that, if other scattering channels are absent, this prevents the carrier distribution to reach a stationary value. Taking into account scattering with phonons a finite current is restored, however its value exceeds the stationary current without scattering.
2.	Karpiak, Bogdan, 1992, et al. (författare) 1D ferromagnetic edge contacts to 2D graphene/h-BN heterostructures 2018 Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 5:1, s. 014001- Tidskriftsartikel (refereegranskat)abstract We report the fabrication of one-dimensional (1D) ferromagnetic edge contacts to two-dimensional (2D) graphene/h-BN heterostructures. While aiming to study spin injection/detection with 1D edge contacts, a spurious magnetoresistance signal was observed, which is found to originate from the local Hall effect in graphene due to fringe fields from ferromagnetic edge contacts and in the presence of charge current spreading in the nonlocal measurement configuration. Such behavior has been confirmed by the absence of a Hanle signal and gate-dependent magnetoresistance measurements that reveal a change in sign of the signal for the electron-and hole-doped regimes, which is in contrast to the expected behavior of the spin signal. Calculations show that the contact-induced fringe fields are typically on the order of hundreds of mT, but can be reduced below 100 mT with careful optimization of the contact geometry. There may be an additional contribution from magnetoresistance effects due to tunneling anisotropy in the contacts, which needs further investigation. These studies are useful for optimization of spin injection and detection in 2D material heterostructures through 1D edge contacts.
3.	Khan, Muhammad Farooq, et al. (författare) High mobility ReSe2 field effect transistors : Schottky-barrier-height-dependent photoresponsivity and broadband light detection with Co decoration 2020 Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 7:1 Tidskriftsartikel (refereegranskat)abstract 2D transition metal dichalcogenides are promising in various electronics and optoelectronics applications and have gained popularity owing to their carrier transport and strong light-matter interactions. To fully realize their potential in field-effect transistors (FETs) and photodetectors, high mobility and high responsivity are imperative. Here, we demonstrate the highest mobility of ∼166 cm2 V-1 s-1 at 200 K for single-layer rhenium diselenide (ReSe2) FETs encapsulated between h-BN flakes at V g = 47 V. The high mobility is attributed to low-resistance contacts of scandium/gold (Sc/Au), with a low Schottky barrier height and reduced charge scattering platform of h-BN. Further, we elucidated the Schottky-barrier-height dependent high photoresponsivity (∼3.2 × 106 A W-1) of few-layer ReSe2 (FL-ReSe2) at 532 nm-wavelength laser light on an h-BN substrate with Sc/Au contacts. Moreover, broadband light detection of undoped and Co-doped few-layer (FL) ReSe2 was performed under different laser wavelengths (400-1100 nm). After the deposition of Co nanoparticles, the photocurrent of FL-ReSe2 increased due to n-doping, as confirmed by the transfer curves of the FL-ReSe2-based undoped and co-doped FETs. Further, the work function decreased from 4.856 to 4.791 eV in FL-ReSe2, as measured by Kelvin probe force microscopy. No light signal was observed at 1100 nm for the undoped ReSe2 (1050 nm < λ cut-off < 1100 nm); however, after doping with Co nanoparticles, the cut-off wavelength exceeded to (λ cut-off > 1100 nm), due to the additional trap states generated in the energy band gap of ReSe2 after Co doping. Further, the transient response of ReSe2 and Co + ReSe2 FETs was estimated so that the rise and decay times are decreased from 1.9 s & 2.7 s to 1.1 s & 1.8 s, respectively. ReSe2 is therefore a promising semiconducting material for electrical and optoelectrical applications.
4.	Kowalczyk, Dorota A., et al. (författare) Local electronic structure of stable monolayers of α-MoO3−x grown on graphite substrate 2021 Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 8:2 Tidskriftsartikel (refereegranskat)abstract We report on van der Waals epitaxy of two-dimensional (2D) molybdenum trioxide (MoO3−x) with monolayer thickness directly grown on highly oriented pyrolytic graphite by thermal evaporation under ultrahigh vacuum. The chemical composition, electronic and crystalline lattice structures of the mono-and few-layer MoO3−x sheets are analysed. Using scanning tunnelling microscopy and spectroscopy, we investigate the electronic properties of MoO3−x as a function of the number of layers and measure the apparent energy gap to be 0.4 eV for the first three layers of MoO3−x on graphite. We carried out density functional theory calculations to shed light on the mechanism underlying the observed narrow bandgap with oxygen deficiency. Moreover, the air exposure effect on monolayer MoO3−x is investigated confirming that the apparent bandgap closes, and additionally we show the reduction of the work function from 5.7 to 4.7 eV. We prove that it is possible to synthesize the 2D, non-stoichiometric, and electrically conductive MoO3−x.
5.	Näslund, Lars-Åke, 1968-, et al. (författare) Chemical bonding of termination species in 2D carbides investigated through valence band UPS/XPS of Ti3C2T xMXene 2021 Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 8:4 Tidskriftsartikel (refereegranskat)abstract MXenes are technologically interesting 2D materials that show potential in numerous applications. The properties of the MXenes depend at large extent on the selection of elements that build the 2D MX-layer. Another key parameter for tuning the attractive material properties is the species that terminate the surfaces of the MX-layers. Although being an important parameter, experimental studies on the bonding between the MX-layers and the termination species are few and thus an interesting subject of investigation. Here we show that the termination species fluorine (F) bonds to the Ti3C2-surface mainly through Ti 3p - F 2p hybridization and that oxygen (O) bonds through Ti 3p - O 2p hybridization with a significant contribution of Ti 3d and Ti 4p. The study further shows that the Ti3C2-surface is not only terminated by F and O on the threefold hollow face-centered-cubic site. A significant amount of O sits on a bridge site bonded to two Ti surface atoms on the Ti3C2-surface. In addition, the results provide no support for hydroxide (OH) termination on the Ti3C2-surface. On the contrary, the comparison of the valence band intensity distribution obtained through ultraviolet- and x-ray photoelectron spectroscopy with computed spectra by density of states, weighed by matrix elements and sensitivity factors, reveals that OH cannot be considered as an inherent termination species in Ti3C2T x . The results from this study have implications for correct modeling of the structure of MXenes and the corresponding materials properties. Especially in applications where surface composition and charge are important, such as supercapacitors, Li-ion batteries, electrocatalysis, and fuel- and solar cells, where intercalation processes are essential.
6.	Shah, Jalil, et al. (författare) Experimental evidence of monolayer arsenene : An exotic 2D semiconducting material 2020 Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 7:2 Tidskriftsartikel (refereegranskat)abstract Group V element analogues of graphene have attracted a lot of attention recently due to their semiconducting band structures and several other interesting properties predicted by theoretical investigations in the literature. In this study, we present atomic and electronic structure data of an arsenic (As) layer on Ag(1 1 1). Low-energy electron diffraction and scanning tunneling microscopy data provide evidence for an ordered layer with a lattice constant of 3.6 Å. This value fits with the theoretical range of 3.54-3.64 Å for buckled arsenene, which is the structure consistently predicted by various theoretical studies. The electronic structure obtained by angle-resolved photoelectron spectroscopy shows the existence of three 2D electron bands within 4 eV below the Fermi level. The number of bands and the agreement between experimental band dispersions and the theoretical band structure provide further evidence for the formation of monolayer buckled arsenene on Ag(1 1 1).
7.	Persson, Ingemar, et al. (författare) On the organization and thermal behavior of functional groups on Ti3C2 MXene surfaces in vacuum 2018 Ingår i: Current Opinion in Chemical Engineering. - : Institute of Physics Publishing (IOPP). - 2211-3398. ; 5:1 Tidskriftsartikel (refereegranskat)abstract The two-dimensional (2D) MXene Ti(3)C(2)Tx is functionalized by surface groups (T-x) that determine its surface properties for, e.g. electrochemical applications. The coordination and thermal properties of these surface groups has, to date, not been investigated at the atomic level, despite strong variations in the MXene properties that are predicted from different coordinations and from the identity of the functional groups. To alleviate this deficiency, and to characterize the functionalized surfaces of single MXene sheets, the present investigation combines atomically resolved in situ heating in a scanning transmission electron microscope (STEM) and STEM simulations with temperature-programmed x-ray photoelectron spectroscopy (TP-XPS) in the room temperature to 750 degrees C range. Using these techniques, we follow the surface group coordination at the atomic level. It is concluded that the F and O atoms compete for the DFT-predicted thermodynamically preferred site and that at room temperature that site is mostly occupied by F. At higher temperatures, F desorbs and is replaced by O. Depending on the O/F ratio, the surface bare MXene is exposed as F desorbs, which enables a route for tailored surface functionalization.
8.	Berghäuser, Gunnar, 1983, et al. (författare) Optical fingerprint of dark 2p-states in transition metal dichalcogenides 2017 Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 4:1 Tidskriftsartikel (refereegranskat)abstract Atomically thin transition metal dichalcogenides exhibit a remarkably strong Coulomb interaction. This results in a fascinating many-particle physics including a variety of bright and dark excitonic states that determine optical and electronic properties of these materials. So far, the impact of dark states has remained literally in the dark to a large extent, since a measurement of these optically forbidden states is very challenging. Here we demonstrate a strategy to measure a direct fingerprint of dark states even in standard linear absorption spectroscopy. We present a microscopic study on bright and dark higher excitonic states in the presence of disorder for the exemplary material of tungsten disulfide (WS2). We show that the geometric phase cancels the degeneration of 2s and 2p states and that a significant disorder-induced coupling of these bright and dark states offers a strategy to circumvent optical selection rules. As a proof, we show a clear fingerprint of dark 2p states in the absorption spectrum of WS2. The predicted softening of optical selection rules through exciton-disorder coupling is of general nature and therefore applicable to related two-dimensional semiconductors.
9.	Forti, S., et al. (författare) Mini-Dirac cones in the band structure of a copper intercalated epitaxial graphene superlattice 2016 Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 3:3 Tidskriftsartikel (refereegranskat)abstract The electronic band structure of an epitaxial graphene superlattice, generated by intercalating a monolayer of Cu atoms, is directly imaged by angle-resolved photoelectron spectroscopy. The 3.2 nm lateral period of the superlattice is induced by a varying registry between the graphene honeycomb and the Cu atoms as imposed by the heteroepitaxial interface Cu/SiC. The carbon atoms experience a lateral potential across the supercell of an estimated value of about 65 meV. The potential leads to strong energy renormalization in the band structure of the graphene layer and the emergence of mini-Dirac cones. The mini-cones' band velocity is reduced to about half of graphene's Fermi velocity. Notably, the ordering of the interfacial Cu atoms can be reversibly blocked by mild annealing. The superlattice indeed disappears at∼220 °C.
10.	Janssen, Tjbm, et al. (författare) Operation of graphene quantum Hall resistance standard in a cryogen-free table-top system 2015 Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 2:3, s. 035015- Tidskriftsartikel (refereegranskat)abstract Wedemonstrate quantum Hall resistance measurements with metrological accuracy in a small cryogen-free system operating at a temperature of around 3.8Kand magnetic fields below 5 T. Operating this system requires little experimental knowledge or laboratory infrastructure, thereby greatly advancing the proliferation of primary quantum standards for precision electrical metrology. This significant advance in technology has come about as a result of the unique properties of epitaxial graphene on SiC.
11.	Mazzola, F., et al. (författare) Graphene coatings for chemotherapy: avoiding silver-mediated degradation 2015 Ingår i: 2d Materials. - : IOP Publishing. - 2053-1583. ; 2:2 Tidskriftsartikel (refereegranskat)abstract Chemotherapy treatment usually involves the delivery of fluorouracil (5-Fu) together with other drugs through central venous catheters. Catheters and their connectors are increasingly treated with silver or argentic alloys/compounds. Complications arising from broken catheters are common, leading to additional suffering for patients and increased medical costs. Here, we uncover a likely cause of such failure through a study of the surface chemistry relevant to chemotherapy drug delivery, i.e. between 5-Fu and silver. We show that silver catalytically decomposes 5-Fu, compromising the efficacy of the chemotherapy treatment. Furthermore, HF is released as a product, which will be damaging to both patient and catheter. We demonstrate that graphene surfaces inhibit this undesirable reaction and would offer superior performance as nanoscale coatings in cancer treatment applications.
12.	Midtvedt, Daniel, 1988, et al. (författare) Strain-displacement relations for strain engineering in single-layer 2d materials 2016 Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 3:1 Tidskriftsartikel (refereegranskat)abstract We investigate the electromechanical coupling in single-layer 2d materials. For non-Bravais lattices, we find important corrections to the standard macroscopic strain-microscopic atomic-displacement theory. We put forward a general and systematic approach to calculate strain-displacement relations for several classes of 2d materials. We apply our findings to graphene as a study case, by combining a tight binding and a valence force-field model to calculate electronic and mechanical properties of graphene nanoribbons under strain. The results show good agreement with the predictions of the Dirac equation coupled to continuum mechanics. For this long wave-limit effective theory, we find that the strain-displacement relations lead to a renormalization correction to the strain-induced pseudo-magnetic fields. A similar renormalization is found for the strain-induced band-gap of black phosphorous. Implications for nanomechanical properties and electromechanical coupling in 2d materials are discussed.
13.	Neumaier, D., et al. (författare) High frequency graphene transistors: Can a beauty become a cash cow? 2015 Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 2:3 Forskningsöversikt (refereegranskat)abstract This is a specially commissioned editorial from the Graphene Flagship Work Package on High Frequency Electronics. This editorial is part of the 2D Materials focus collection on 'Progress on the science and applications of two-dimensional materials', published in association with the Graphene Flagship. It provides an overview of key, recent advances from the 'High Frequency Electronics' work package and is not intended as a comprehensive review of this field.
14.	Panchal, V., et al. (författare) Atmospheric doping effects in epitaxial graphene: correlation of local and global electrical studies 2016 Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 3:1 Tidskriftsartikel (refereegranskat)abstract We directly correlate the local (20 nm scale) and global electronic properties of a device containing mono-, bi- and tri-layer epitaxial graphene (EG) domains on 6H-SiC (0001) by simultaneously performing local surface potential measurements using Kelvin probe force microscopy and global transport measurements. Using well-controlled environmental conditions we investigate the doping effects of N-2, O-2, water vapour and NO2 at concentrations representative of the ambient air. We show that presence of O-2, water vapour and NO2 leads to p-doping of all EG domains. However, the thicker layers of EG are significantly less affected. Furthermore, we demonstrate that the general consensus of O-2 and water vapour present in ambient air providing majority of the p-doping to graphene is a common misconception. We experimentally show that even the combined effect of O-2, water vapour, and NO2 at concentrations higher than typically present in the atmosphere does not fully replicate p-doping from ambient air. Thus, for EG gas sensors it is essential to consider naturally occurring environmental effects and properly separate them from those coming from targeted species.
15.	Quesnel, Etienne, et al. (författare) Graphene-based technologies for energy applications, challenges and perspectives 2015 Ingår i: Current Opinion in Chemical Engineering. - : IOP Publishing. - 2211-3398. ; 2:3, s. 1-16 Tidskriftsartikel (refereegranskat)abstract Here we report on technology developments implemented into the Graphene Flagship European project for the integration of graphene and graphene-related materials (GRMs) into energy application devices. Many of the technologies investigated so far aim at producing composite materials associating graphene or GRMs with either metal or semiconducting nanocrystals or other carbon nanostructures (e.g., CNT, graphite). These composites can be used favourably as hydrogen storage materials or solar cell absorbers. They can also provide better performing electrodes for fuel cells, batteries, or supercapacitors. For photovoltaic (PV) electrodes, where thin layers and interface engineering are required, surface technologies are preferred. We are using conventional vacuum processes to integrate graphene as well as radically new approaches based on laser irradiation strategies. For each application, the potential of implemented technologies is then presented on the basis of selected experimental and modelling results. It is shown in particular how some of these technologies can maximize the benefit taken from GRM integration. The technical challenges still to be addressed are highlighted and perspectives derived from the running works emphasized.
16.	Roche, Stephan, et al. (författare) Graphene spintronics : the European Flagship perspective 2015 Ingår i: Current Opinion in Chemical Engineering. - : Institute of Physics Publishing (IOPP). - 2211-3398. ; 2:3 Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract We review current challenges and perspectives in graphene spintronics, which is one of the most promising directions of innovation, given its room-temperature long-spin lifetimes and the ability of graphene to be easily interfaced with other classes of materials (ferromagnets, magnetic insulators, semiconductors, oxides, etc), allowing proximity effects to be harvested. The general context of spintronics is first discussed together with open issues and recent advances achieved by the Graphene Spintronics Work Package consortium within the Graphene Flagship project. Based on such progress, which establishes the state of the art, several novel opportunities for spin manipulation such as the generation of pure spin current (through spin Hall effect) and the control of magnetization through the spin torque phenomena appear on the horizon. Practical applications are within reach, but will require the demonstration of wafer-scale graphene device integration, and the realization of functional prototypes employed for determined applications such as magnetic sensors or nano-oscillators. This is a specially commissioned editorial from the Graphene Flagship Work Package on Spintronics. This editorial is part of the 2D Materials focus collection on 'Progress on the science and applications of two-dimensional materials,' published in association with the Graphene Flagship. It provides an overview of key recent advances of the spintronics work package as well as the mid-term objectives of the consortium.
17.	Schröder, Ulrike A, et al. (författare) Core level shifts of intercalated graphene 2017 Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 4:1 Tidskriftsartikel (refereegranskat)abstract Through intercalation of metals and gases the Dirac cone of graphene on Ir(111) can be shifted with respect to the Fermi level without becoming destroyed by strong hybridization. Here, we use x-ray photoelectron spectroscopy to measure the C 1s core level shift (CLS) of graphene in contact with a number of structurally well-defined intercalation layers (O, H, Eu, and Cs). By analysis of our own and additional literature data for decoupled graphene, the C 1s CLS is found to be a non-monotonic function of the doping level. For small doping levels the shifts are well described by a rigid band model. However, at larger doping levels, a second effect comes into play which is proportional to the transferred charge and counteracts the rigid band shift. Moreover, not only the position, but also the C 1s peak shape displays a unique evolution as a function of doping level. Our conclusions are supported by intercalation experiments with Li, with which, due to the absence of phase separation, the doping level of graphene can be continuously tuned.
18.	Romagnoli, Priscila, et al. (författare) Making graphene visible in transparent dielectric substrates : Brewster angle imaging 2015 Ingår i: Current Opinion in Chemical Engineering. - : Institute of Physics (IOP). - 2211-3398. ; 2:3 Tidskriftsartikel (refereegranskat)abstract The visibility of graphene is greatly increased by illuminating samples deposited on transparent dielectrics at the substrates’ Brewster angle. Using a commercial ellipsometer, the reflectivity of monolayers of CVD graphene is found to be up to 33 times higher than that of the substrate, i.e., an optical contrast as high as 3200% is obtained, more than 380 times higher than with standard optical microscopy. Also, with a simpler, homemade, experimental setup, a 1400% optical contrast was measured for a monolayer of CVD graphene and linear features as small as ~20 μm were visible in a monolayer, while ~6×17 μm2 trilayers could still be imaged in exfoliated samples. It is also shown that the reflectance/transmittance ratio increases quadratically with the number of graphene layers, which may allow for counting layer numbers and identifying wrinkles and folds in transferred samples.
19.	Del, Sepideh Khandan, et al. (författare) Optimizing the optical and electrical properties of graphene ink thin films by laser-annealing 2015 Ingår i: Current Opinion in Chemical Engineering. - : Institute of Physics (IOP). - 2211-3398. ; 2:1 Tidskriftsartikel (refereegranskat)abstract We demonstrate a facile fabrication technique for graphene-based transparent conductive films. Highly flat and uniform graphene films are obtained through the incorporation of an efficient laser annealing technique with one-time drop casting of high-concentration graphene ink. The resulting thin films are uniform and exhibit a transparency of more than 85% at 550 nm and a sheet resistance of about 30 kΩ/sq. These values constitute an increase of 45% in transparency, a reduction of surface roughness by a factor of four and a decrease of 70% in sheet resistance compared to un-annealed films.
20.	Anzi, L., et al. (författare) Ultra-low contact resistance in graphene devices at the Dirac point 2018 Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 5:2 Tidskriftsartikel (refereegranskat)abstract Contact resistance is one of the main factors limiting performance of short-channel graphene field-effect transistors (GFETs), preventing their use in low-voltage applications. Here we investigated the contact resistance between graphene grown by chemical vapor deposition (CVD) and different metals, and found that etching holes in graphene below the contacts consistently reduced the contact resistance, down to 23 m with Au contacts. This low contact resistance was obtained at the Dirac point of graphene, in contrast to previous studies where the lowest contact resistance was obtained at the highest carrier density in graphene (here 200 m was obtained under such conditions). The 'holey' Au contacts were implemented in GFETs which exhibited an average transconductance of 940 S m−1 at a drain bias of only 0.8 V and gate length of 500 nm, which out-perform GFETs with conventional Au contacts.
21.	Backes, Claudia, et al. (författare) Production and processing of graphene and related materials 2020 Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 7:2 Tidskriftsartikel (refereegranskat)abstract We present an overview of the main techniques for production and processing of graphene and related materials (GRMs), as well as the key characterization procedures. We adopt a 'hands-on' approach, providing practical details and procedures as derived from literature as well as from the authors' experience, in order to enable the reader to reproduce the results. Section I is devoted to 'bottom up' approaches, whereby individual constituents are pieced together into more complex structures. We consider graphene nanoribbons (GNRs) produced either by solution processing or by on-surface synthesis in ultra high vacuum (UHV), as well carbon nanomembranes (CNM). Production of a variety of GNRs with tailored band gaps and edge shapes is now possible. CNMs can be tuned in terms of porosity, crystallinity and electronic behaviour. Section II covers 'top down' techniques. These rely on breaking down of a layered precursor, in the graphene case usually natural crystals like graphite or artificially synthesized materials, such as highly oriented pyrolythic graphite, monolayers or few layers (FL) flakes. The main focus of this section is on various exfoliation techniques in a liquid media, either intercalation or liquid phase exfoliation (LPE). The choice of precursor, exfoliation method, medium as well as the control of parameters such as time or temperature are crucial. A definite choice of parameters and conditions yields a particular material with specific properties that makes it more suitable for a targeted application. We cover protocols for the graphitic precursors to graphene oxide (GO). This is an important material for a range of applications in biomedicine, energy storage, nanocomposites, etc. Hummers' and modified Hummers' methods are used to make GO that subsequently can be reduced to obtain reduced graphene oxide (RGO) with a variety of strategies. GO flakes are also employed to prepare three-dimensional (3d) low density structures, such as sponges, foams, hydro- or aerogels. The assembly of flakes into 3d structures can provide improved mechanical properties. Aerogels with a highly open structure, with interconnected hierarchical pores, can enhance the accessibility to the whole surface area, as relevant for a number of applications, such as energy storage. The main recipes to yield graphite intercalation compounds (GICs) are also discussed. GICs are suitable precursors for covalent functionalization of graphene, but can also be used for the synthesis of uncharged graphene in solution. Degradation of the molecules intercalated in GICs can be triggered by high temperature treatment or microwave irradiation, creating a gas pressure surge in graphite and exfoliation. Electrochemical exfoliation by applying a voltage in an electrolyte to a graphite electrode can be tuned by varying precursors, electrolytes and potential. Graphite electrodes can be either negatively or positively intercalated to obtain GICs that are subsequently exfoliated. We also discuss the materials that can be amenable to exfoliation, by employing a theoretical data-mining approach. The exfoliation of LMs usually results in a heterogeneous dispersion of flakes with different lateral size and thickness. This is a critical bottleneck for applications, and hinders the full exploitation of GRMs produced by solution processing. The establishment of procedures to control the morphological properties of exfoliated GRMs, which also need to be industrially scalable, is one of the key needs. Section III deals with the processing of flakes. (Ultra)centrifugation techniques have thus far been the most investigated to sort GRMs following ultrasonication, shear mixing, ball milling, microfluidization, and wet-jet milling. It allows sorting by size and thickness. Inks formulated from GRM dispersions can be printed using a number of processes, from inkjet to screen printing. Each technique has specific rheological requirements, as well as geometrical constraints. The solvent choice is critical, not only for the GRM stability, but also in terms of optimizing printing on different substrates, such as glass, Si, plastic, paper, etc, all with different surface energies. Chemical modifications of such substrates is also a key step. Sections IV-VII are devoted to the growth of GRMs on various substrates and their processing after growth to place them on the surface of choice for specific applications. The substrate for graphene growth is a key determinant of the nature and quality of the resultant film. The lattice mismatch between graphene and substrate influences the resulting crystallinity. Growth on insulators, such as SiO2, typically results in films with small crystallites, whereas growth on the close-packed surfaces of metals yields highly crystalline films. Section IV outlines the growth of graphene on SiC substrates. This satisfies the requirements for electronic applications, with well-defined graphene-substrate interface, low trapped impurities and no need for transfer. It also allows graphene structures and devices to be measured directly on the growth substrate. The flatness of the substrate results in graphene with minimal strain and ripples on large areas, allowing spectroscopies and surface science to be performed. We also discuss the surface engineering by intercalation of the resulting graphene, its integration with Si-wafers and the production of nanostructures with the desired shape, with no need for patterning. Section V deals with chemical vapour deposition (CVD) onto various transition metals and on insulators. Growth on Ni results in graphitized polycrystalline films. While the thickness of these films can be optimized by controlling the deposition parameters, such as the type of hydrocarbon precursor and temperature, it is difficult to attain single layer graphene (SLG) across large areas, owing to the simultaneous nucleation/growth and solution/precipitation mechanisms. The differing characteristics of polycrystalline Ni films facilitate the growth of graphitic layers at different rates, resulting in regions with differing numbers of graphitic layers. High-quality films can be grown on Cu. Cu is available in a variety of shapes and forms, such as foils, bulks, foams, thin films on other materials and powders, making it attractive for industrial production of large area graphene films. The push to use CVD graphene in applications has also triggered a research line for the direct growth on insulators. The quality of the resulting films is lower than possible to date on metals, but enough, in terms of transmittance and resistivity, for many applications as described in section V. Transfer technologies are the focus of section VI. CVD synthesis of graphene on metals and bottom up molecular approaches require SLG to be transferred to the final target substrates. To have technological impact, the advances in production of high-quality large-area CVD graphene must be commensurate with those on transfer and placement on the final substrates. This is a prerequisite for most applications, such as touch panels, anticorrosion coatings, transparent electrodes and gas sensors etc. New strategies have improved the transferred graphene quality, making CVD graphene a feasible option for CMOS foundries. Methods based on complete etching of the metal substrate in suitable etchants, typically iron chloride, ammonium persulfate, or hydrogen chloride although reliable, are time- and resource-consuming, with damage to graphene and production of metal and etchant residues. Electrochemical delamination in a low-concentration aqueous solution is an alternative. In this case metallic substrates can be reused. Dry transfer is less detrimental for the SLG quality, enabling a deterministic transfer. There is a large range of layered materials (LMs) beyond graphite. Only few of them have been already exfoliated and fully characterized. Section VII deals with the growth of some of these materials. Amongst them, h-BN, transition metal tri- and di-chalcogenides are of paramount importance. The growth of h-BN is at present considered essential for the development of graphene in (opto) electronic applications, as h-BN is ideal as capping layer or substrate. The interesting optical and electronic properties of TMDs also require the development of scalable methods for their production. Large scale growth using chemical/physical vapour deposition or thermal assisted conversion has been thus far limited to a small set, such as h-BN or some TMDs. Heterostructures could also be directly grown. Section VIII discusses advances in GRM functionalization. A broad range of organic molecules can be anchored to the sp(2) basal plane by reductive functionalization. Negatively charged graphene can be prepared in liquid phase (e.g. via intercalation chemistry or electrochemically) and can react with electrophiles. This can be achieved both in dispersion or on substrate. The functional groups of GO can be further derivatized. Graphene can also be noncovalently functionalized, in particular with polycyclic aromatic hydrocarbons that assemble on the sp(2) carbon network by pi-pi stacking. In the liquid phase, this can enhance the colloidal stability of SLG/FLG. Approaches to achieve noncovalent on-substrate functionalization are also discussed, which can chemically dope graphene. Research efforts to derivatize CNMs are also summarized, as well as novel routes to selectively address defect sites. In dispersion, edges are the most dominant defects and can be covalently modified. This enhances colloidal stability without modifying the graphene basal plane. Basal plane point defects can also be modified, passivated and healed in ultra-high vacuum. The decoration of graphene with metal nanoparticles (NPs) has also received considerable attention, as it allows to exploit synergistic effects between NPs and graphene. Decoration can be either achieved chemically or in the gas phase. All LMs,
22.	Bernal, Ivan, 1984, et al. (författare) Exciton broadening and band renormalization due to Dexter-like intervalley coupling 2018 Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 5:2 Tidskriftsartikel (refereegranskat)abstract A remarkable property of atomically thin transition metal dichalcogenides (TMDs) is the possibility to selectively address single valleys by circularly polarized light. In the context of technological applications, it is very important to understand possible intervalley coupling mechanisms. Here, we show how the Dexter-like intervalley coupling mixes A and B states from opposite valleys leading to a significant broadening γB 1s of the B 1s exciton. The effect is much more pronounced in tungsten-based TMDs, where the coupling excitonic states are quasi-resonant. We calculate a ratio γB B 1s /γA B 1s ≈ 4.0, which is in good agreement with the experimentally measured value of 3.9 ± 0.7. In addition to the broadening effect, the Dexter-like intervalley coupling also leads to a considerable energy renormalization resulting in an increased energetic distance between A 1s and B 1s states.
23.	Feierabend, Maja, 1990, et al. (author) Brightening of spin- and momentum-dark excitons in transition metal dichalcogenides 2021 In: 2D Materials. - : IOP Publishing. - 2053-1583. ; 8:1 Journal article (peer-reviewed)abstract Monolayer transition metal dichalcogenides (TMDs) have been in focus of current research, among others due to their remarkable exciton landscape consisting of bright and dark excitonic states. Although dark excitons are not directly visible in optical spectra, they have a large impact on exciton dynamics and hence their understanding is crucial for potential TMD-based applications. Here, we study brightening mechanisms of dark excitons via interaction with phonons and in-plane magnetic fields. We show clear signatures of momentum- and spin-dark excitons in WS2, WSe2 and MoS2, while the photoluminescence of MoSe2 is only determined by the bright exciton. In particular, we reveal the mechanism behind the brightening of states that are both spin- and momentum-dark in MoS2. Our results are in good agreement with recent experiments and contribute to a better microscopic understanding of the exciton landscape in TMDs.
24.	Ferreira, Beatriz, 1996, et al. (author) Signatures of dark excitons in exciton–polariton optics of transition metal dichalcogenides 2023 In: 2D Materials. - : IOP Publishing. - 2053-1583. ; 10 Journal article (peer-reviewed)abstract Integrating 2D materials into high-quality optical microcavities opens the door to fascinating many-particle phenomena including the formation of exciton-polaritons. These are hybrid quasi-particles inheriting properties of both the constituent photons and excitons. In this work, we investigate the so-far overlooked impact of dark excitons on the momentum-resolved absorption spectra of hBN-encapsulated WSe2 and MoSe2 monolayers in the strong-coupling regime. In particular, thanks to the efficient phonon-mediated scattering of polaritons into energetically lower dark exciton states, the absorption of the lower polariton branch in WSe2 is much higher than in MoSe2. It shows unique step-like increases in the momentum-resolved profile indicating opening of specific scattering channels. We study how different externally accessible quantities, such as temperature or mirror reflectance, change the optical response of polaritons. Our study contributes to an improved microscopic understanding of exciton-polaritons and their interaction with phonons, potentially suggesting experiments that could determine the energy of dark exciton states via momentum-resolved polariton absorption.
25.	Fu, Yifeng, 1984, et al. (author) Graphene related materials for thermal management 2020 In: 2D Materials. - : IOP Publishing. - 2053-1583. ; 7:1 Journal article (peer-reviewed)abstract Almost 15 years have gone ever since the discovery of graphene as a single atom layer. Numerous papers have been published to demonstrate its high electron mobility, excellent thermal and mechanical as well as optical properties. We have recently seen more and more applications towards using graphene in commercial products. This paper is an attempt to review and summarize the current status of the research of the thermal properties of graphene and other 2D based materials including the manufacturing and characterization techniques and their applications, especially in electronics and power modules. It is obvious from the review that graphene has penetrated the market and gets more and more applications in commercial electronics thermal management context. In the paper, we also made a critical analysis of how mature the manufacturing processes are; what are the accuracies and challenges with the various characterization techniques and what are the remaining questions and issues left before we see further more applications in this exciting and fascinating field.
26.	Ghosh, Sukanya, et al. (author) Structural distortion and dynamical electron correlation driven enhanced ferromagnetism in Ni-doped two-dimensional Fe5GeTe2 beyond room temperature 2024 In: 2D Materials. - : Institute of Physics Publishing (IOPP). - 2053-1583. ; 11:3 Journal article (peer-reviewed)abstract Achieving beyond room-temperature ferromagnetism in two-dimensional (2D) magnets is immensely desirable for spintronic applications. Fe5GeTe2 is an exceptional van der Waals metallic ferromagnet due to its tunable physical properties and relatively higher Curie temperature (TC) than other 2D magnets. Using density functional theory combined with dynamical electron correlation and Monte Carlo simulations, we find the TC of (Fe1-δNiδ) 5GeTe2 monolayer can increase up to ∼ 400 K at δ ∼ 0.20 (δ: fractional occupation). Two specific Fe sublattices are identified to be the most energetically preferred sites to host Ni. Exchange interactions between particular Fe pairs play a dominating role in controlling TC, influenced by the dopant-induced structural distortions. Dynamical electron correlation induces site- and orbital-specific quasi-particle mass of Fe-d states with varying Ni concentrations. This work provides fundamental insights into 2D magnetism as an interplay of structural and electronic aspects and would guide to tailoring exciting magnetic phenomena in similar systems.
27.	Guo, Sihua, et al. (author) Toward ultrahigh thermal conductivity graphene films 2023 In: 2D Materials. - : IOP Publishing. - 2053-1583. ; 10:1 Journal article (peer-reviewed)abstract With increasing demands of high-performance and functionality, electronics devices generate a great amount of heat. Thus, efficient heat dissipation is crucially needed. Owing to its extremely good thermal conductivity, graphene is an interesting candidate for this purpose. In this paper, a two-step temperature-annealing process to fabricate ultrahigh thermal conductive graphene assembled films (GFs) is proposed. The thermal conductivity of the obtained GFs was as high as 3826 +/- 47 W m(-1) K-1. Extending the time of high-temperature annealing significantly improved the thermal performance of the GF. Structural analyses confirmed that the high thermal conductivity is caused by the large grain size, defect-free stacking, and high flatness, which are beneficial for phonon transmission in the carbon lattice. The turbostratic stacking degree decreased with increasing heat treatment time. However, the increase in the grain size after long heat treatment had a more pronounced effect on the phonon transfer of the GF than that of turbostratic stacking. The developed GFs show great potential for efficient thermal management in electronics devices.
28.	Hagel, Joakim, 1994, et al. (author) Electrical tuning of moiré excitons in MoSe 2 bilayers 2023 In: 2D Materials. - : IOP Publishing. - 2053-1583. ; 10:1 Journal article (peer-reviewed)abstract Recent advances in the field of vertically stacked 2D materials have revealed a rich exciton landscape. In particular, it has been demonstrated that out-of-plane electrical fields can be used to tune the spectral position of spatially separated interlayer excitons. Other studies have shown that there is a strong hybridization of exciton states, resulting from the mixing of electronic states in both layers. However, the connection between the twist-angle dependent hybridization and field-induced energy shifts has remained in the dark. Here, we investigate on a microscopic footing the interplay of electrical and twist-angle tuning of moiré excitons in MoSe2 homobilayers. We reveal distinct energy regions in PL spectra that are clearly dominated by either intralayer or interlayer excitons, or even dark excitons. Consequently, we predict twist-angle-dependent critical electrical fields at which the material is being transformed from a direct into an indirect semiconductor. Our work provides new microscopic insights into experimentally accessible knobs to significantly tune the moiré exciton physics in atomically thin nanomaterials.
29.	Haneef, Tahir, et al. (author) Recent progress in two dimensional Mxenes for photocatalysis : a critical review 2023 In: 2D Materials. - : Institute of Physics (IOP). - 2053-1583. ; 10:1 Research review (peer-reviewed)abstract Transition metal carbides and nitrides, generally known as MXenes have emerged as an alternative to improve photocatalytic performance in renewable energy and environmental remediation applications because of their high surface area, tunable chemistry, and easily adjustable elemental compositions. MXenes have many interlayer groups, surface group operations, and a flexible layer spacing that makes them ideal catalysts. Over 30 different members of the MXenes family have been explored and successfully utilized as catalysts. Particularly, MXenes have achieved success as a photocatalyst for carbon dioxide reduction, nitrogen fixation, hydrogen evolution, and photochemical degradation. The structure of MXenes and the presence of hydrophilic functional groups on the surface results in excellent photocatalytic hydrogen evolution. In addition, MXenes' surface defects provide abundant CO2 adsorption sites. Moreover, their highly efficient catalytic oxidation activity is a result of their excellent two-dimensional nanomaterial structure and high-speed electron transport channels. This article comprehensively discusses the structure, synthesis techniques, photocatalytic applications (i.e. H-2 evolution, N-2 fixation, CO2 reduction, and degradation of pollutants), and recyclability of MXenes. This review also critically evaluates the MXene-based heterostructure and composites photocatalyst synthesis process and their performance for organic pollutant degradation. Finally, a prospect for further research is presented in environmental and energy sciences.
30.	Hofmann, Niklas, et al. (author) Link between interlayer hybridization and ultrafast charge transfer in WS 2 -graphene heterostructures 2023 In: 2D Materials. - 2053-1583. ; 10:3 Journal article (peer-reviewed)abstract Ultrafast charge separation after photoexcitation is a common phenomenon in various van-der-Waals (vdW) heterostructures with great relevance for future applications in light harvesting and detection. Theoretical understanding of this phenomenon converges towards a coherent mechanism through charge transfer states accompanied by energy dissipation into strongly coupled phonons. The detailed microscopic pathways are material specific as they sensitively depend on the band structures of the individual layers, the relative band alignment in the heterostructure, the twist angle between the layers, and interlayer interactions resulting in hybridization. We used time- and angle-resolved photoemission spectroscopy combined with tight binding and density functional theory electronic structure calculations to investigate ultrafast charge separation and recombination in WS2-graphene vdW heterostructures. We identify several avoided crossings in the band structure and discuss their relevance for ultrafast charge transfer. We relate our own observations to existing theoretical models and propose a unified picture for ultrafast charge transfer in vdW heterostructures where band alignment and twist angle emerge as the most important control parameters.
31.	Holt, Ann Julie U., et al. (author) Electronic properties of single-layer CoO2/Au(111) 2021 In: Current Opinion in Chemical Engineering. - : IOP Publishing. - 2211-3398. ; 8:3 Journal article (peer-reviewed)abstract We report direct measurements via angle-resolved photoemission spectroscopy (ARPES) of the electronic dispersion of single-layer (SL) CoO2. The Fermi contour consists of a large hole pocket centered at the (Gamma) over bar point. To interpret the ARPES results, we use density functional theory (DFT) in combination with the multi-orbital Gutzwiller Approximation (DFT+GA), basing our calculations on crystalline structure parameters derived from x-ray photoelectron diffraction and low-energy electron diffraction. Our calculations are in good agreement with the measured dispersion. We conclude that the material is a moderately correlated metal. We also discuss substrate effects, and the influence of hydroxylation on the CoO2 SL electronic structure.
32.	Hu, Yong-Jie, et al. (author) Structural and electronic properties of two-dimensional titanium carbo-oxides 2023 In: 2D Materials. - : IOP Publishing Ltd. - 2053-1583. ; 10:1 Journal article (peer-reviewed)abstract This work was inspired by new experimental findings where we discovered a two-dimensional (2D) material comprised of titanium-oxide-based one-dimensional (1D) sub-nanometer filaments. Preliminary results suggest that the 2D material contains considerable amounts of carbon, C, in addition to titanium, Ti, and oxygen, O. The aim of this study is to investigate the low-energy, stable atomic forms of 2D titanium carbo-oxides as a function of C content. Via a combination of first-principles calculations and an effective structure sampling scheme, the stable configurations of C-substitutions are comprehensively searched by templating different 2D TiO2 polymorphs and considering a two O to one C replacement scheme. Among the searched stable configurations, a structure where the (101) planes of anatase bound the top and bottom surfaces with a chemical formula of TiC1/4O3/2 was of particularly low energy. Furthermore, the variations in the electronic band structure and chemical bonding environments caused by the high-content C substitution are investigated via additional calculations using a hybrid exchange-correlation functional.
33.	Isacsson, Andreas, 1975, et al. (author) Scaling properties of polycrystalline graphene: A review 2017 In: 2D Materials. - : IOP Publishing. - 2053-1583. ; 4:1 Research review (peer-reviewed)abstract We present an overview of the electrical, mechanical, and thermal properties of polycrystalline graphene. Most global properties of this material, such as the charge mobility, thermal conductivity, or Young's modulus, are sensitive to its microstructure, for instance the grain size and the presence of line or point defects. Both the local and global features of polycrystalline graphene have been investigated by a variety of simulations and experimental measurements. In this review, we summarize the properties of polycrystalline graphene, and by establishing a perspective on how the microstructure impacts its large-scale physical properties, we aim to provide guidance for further optimization and improvement of applications based on this material, such as flexible and wearable electronics, and high-frequency or spintronic devices.
34.	Jia, Tao, et al. (author) Epitaxial growth of TiSe2/TiO2 heterostructure 2019 In: Current Opinion in Chemical Engineering. - : Institute of Physics (IOP). - 2211-3398. ; 6:1 Journal article (peer-reviewed)abstract Here we report that TiSe2 thin films can be epitaxially grown on TiO2 substrates despite different lattice symmetry between the two materials. The TiSe2 thin films can be prepared on TiO2 via molecular beam epitaxy (MBE) in two ways: by conventional co-deposition using selenium and titanium sources, and by evaporating just selenium on reconstructed surfaces of TiO2. Both growth methods yield crystalline thin films with similar electronic band structures. TiSe2 films on TiO2 substrates exhibit large electron doping and a lack of charge density wave (CDW) order, which is different from both bulk single crystal TiSe2 and TiSe2 thin films on graphene. These phenomena can be explained by selenium vacancies in the TiSe2 films, which naturally occur when these films are grown on TiO2 substrates. Our successful growth of transition metal dichalcogenide (TMDC) films on a transition metal oxide (TMO) substrate provides a platform to further tune the electrical and optical properties of TMDC thin films.
35.	Jones, Alfred J. H., et al. (author) Visualizing band structure hybridization and superlattice effects in twisted MoS2/WS2 heterobilayers 2022 In: Current Opinion in Chemical Engineering. - : IOP Publishing Ltd. - 2211-3398. ; 9:1 Journal article (peer-reviewed)abstract A mismatch of atomic registries between single-layer transition metal dichalcogenides (TMDs) in a two-dimensional (2D) van der Waals heterostructure produces a moire superlattice with a periodic potential, which can be fine-tuned by introducing a twist angle between the materials. This approach is promising both for controlling the interactions between the TMDs and for engineering their electronic band structures, yet direct observation of the changes to the electronic structure introduced with varying twist angle has so far been missing. Here, we probe heterobilayers comprised of single-layer MoS2 and WS2 with twist angles ranging from 2 degrees to 20 degrees and determine the twist angle-dependent evolution of the electronic band structure using micro-focused angle-resolved photoemission spectroscopy. We find strong interlayer hybridization between MoS2 and WS2 electronic states at the (Gamma) over bar -point of the Brillouin zone, leading to a shift of the valence band maximum in the heterostructure. Replicas of the hybridized states are observed at the center of twist angle-dependent moire mini Brillouin zones. We confirm that these replica features arise from the inherent moire potential by comparing our experimental observations with density functional theory calculations of the superlattice dispersion. Our direct visualization of these features underscores the potential of using twisted heterobilayer semiconductors to engineer hybrid electronic states and superlattices that alter the electronic and optical properties of 2D heterostructures for a wide range of twist angles.
36.	Karpiak, Bogdan, 1992, et al. (author) Magnetic proximity in a van der Waals heterostructure of magnetic insulator and graphene 2020 In: 2D Materials. - : IOP Publishing. - 2053-1583. ; 7:1 Journal article (peer-reviewed)abstract Engineering 2D material heterostructures by combining the best of different materials in one ultimate unit can offer a plethora of opportunities in condensed matter physics. Here, in the van der Waals heterostructures of the ferromagnetic insulator Cr2Ge2Te6 and graphene, our observations indicate an out-of-plane proximity-induced ferromagnetic exchange interaction in graphene. The perpendicular magnetic anisotropy of Cr2Ge2Te6 results in significant modification of the spin transport and precession in graphene, which can be ascribed to the proximity-induced exchange interaction. Furthermore, the observation of a larger lifetime for perpendicular spins in comparison to the in-plane counterpart suggests the creation of a proximity-induced anisotropic spin texture in graphene. Our experimental results and density functional theory calculations open up opportunities for the realization of proximity-induced magnetic interactions and spin filters in 2D material heterostructures and can form the basic building blocks for future spintronic and topological quantum devices.
37.	Kazemi, M., et al. (author) Interaction of excitons with magnetic topological defects in 2D magnetic monolayers : localization and anomalous Hall effect 2023 In: 2D Materials. - : Institute of Physics Publishing (IOPP). - 2053-1583. ; 10:1 Journal article (peer-reviewed)abstract Novel 2D material CrI3 reveals unique combination of 2D ferromagnetism and robust excitonic response. We demonstrate that the possibility of the formation of magnetic topological defects, such as Neel skyrmions, together with large excitonic Zeeman splitting, leads to giant scattering asymmetry, which is the necessary prerequisite for the excitonic anomalous Hall effect. In addition, the diamagnetic effect breaks the inversion symmetry, and in certain cases can result in exciton localization on the skyrmion. This enables the formation of magnetoexcitonic quantum dots with tunable parameters.
38.	Khan, Munis, 1991, et al. (author) High mobility graphene field effect transistors on flexible EVA/PET foils 2024 In: 2D Materials. - 2053-1583. ; 11:3 Journal article (peer-reviewed)abstract Monolayer graphene is a promising material for a wide range of applications, including sensors, optoelectronics, antennas, EMR shielding, flexible electronics, and conducting electrodes. Chemical vapor deposition (CVD) of carbon atoms on a metal catalyst is the most scalable and cost-efficient method for synthesizing high-quality, large-area monolayer graphene. The usual method of transferring the CVD graphene from the catalyst to a target substrate involves a polymer carrier which is dissolved after the transfer process is completed. Due to often unavoidable damage to graphene, as well as contamination and residues, carrier mobilities are typically 1000–3000 cm2(Vs)−1, unless complex and elaborate measures are taken. Here, we report on a simple scalable fabrication method for flexible graphene field-effect transistors that eliminates the polymer interim carrier, by laminating the graphene directly onto office lamination foils, removing the catalyst, and depositing Parylene N as a gate dielectric and encapsulation layer. The fabricated transistors show field- and Hall-effect mobilities of 7000–10 000 cm2(Vs)−1 with a residual charge-carrier density of 2×1011 1 cm−2 at room temperature. We further validate the material quality by terahertz time-domain spectroscopy and observation of the quantum Hall effect at low temperatures in a moderate magnetic field of ∼5 T. The Parylene encapsulation provides long-term stability and protection against additional lithography steps, enabling vertical device integration in multilayer electronics on a flexible platform.
39.	Khatibi, Zahra, 1987, et al. (author) Impact of strain on the excitonic linewidth in transition metal dichalcogenides 2019 In: 2D Materials. - : IOP Publishing. - 2053-1583. ; 6:1 Journal article (peer-reviewed)abstract Monolayer transition metal dichalcogenides (TMDs) are known to be highly sensitive to externally applied tensile or compressive strain. In particular, strain can be exploited as a tool to control the optical response of TMDs. However, the role of excitonic effects under strain has not been fully understood yet. Utilizing the strain-induced modification of electron and phonon dispersion obtained by first principle calculations, we present in this work microscopic insights into the strain-dependent optical response of various TMD materials. We show that the different changes in the excitonic linewidth of diverse TMD monolayers are due to the strain-induced modification of the relative spectral position of bright and dark excitonic states. Our theoretical results explain well the observed partially opposite changes in the excitonic linewidth of different TMDs at room temperature. Furthermore, we predict the linewidth behavior of excitonic resonances in strained TMDs for tensile and compressive strain at low temperatures.
40.	Khorsand Kheirabad, Atefeh, 1991-, et al. (author) MXene/poly(ionic liquid) porous composite membranes for systematized solar-driven interfacial steam generation 2023 In: 2D Materials. - : IOP Publishing. - 2053-1583. ; 10:2 Journal article (peer-reviewed)abstract Herein, we established a synthetic route towards MXene/poly(ionic liquid) (PIL) composite porous membranes as a new platform of solar-thermal conversion materials. These membranes were made by a base-triggered ionic crosslinking process between a cationic PIL and a weak polyacid in solution in the presence of dispersed MXene nanosheets. A three-dimensionally interconnected porous architecture was formed with MXene nanosheets uniformly distributed within it. The unique characteristics of the as-produced composite membranes displays significant light-to-heat conversion and excellent performance for solar-driven water vapor generation. This facile synthetic strategy opens a new avenue for developing composite porous membranes as solar absorbers for the solar-driven water production from natural resources.
41.	Kovtun, Alessandro, et al. (author) Benchmarking of graphene-based materials: Real commercial products versus ideal graphene 2019 In: 2D Materials. - : IOP Publishing. - 2053-1583. ; 6:2 Journal article (peer-reviewed)abstract There are tens of industrial producers claiming to sell graphene and related materials (GRM), mostly as solid powders. Recently the quality of commercial GRM has been questioned, and procedures for GRM quality control were suggested using Raman Spectroscopy or Atomic Force Microscopy. Such techniques require dissolving the sample in solvents, possibly introducing artefacts. A more pragmatic approach is needed, based on fast measurements and not requiring any assumption on GRM solubility. To this aim, we report here an overview of the properties of commercial GRM produced by selected companies in Europe, USA and Asia. We benchmark: (A) size, (B) exfoliation grade and (C) oxidation grade of each GRM versus the ones of 'ideal' graphene and, most importantly, versus what reported by the producer. In contrast to previous works, we report explicitly the names of the GRM producers and we do not re-dissolve the GRM in solvents, but only use techniques compatible with industrial powder metrology. A general common trend is observed: Products having low defectivity (%sp 2 bonds >95%) feature low surface area (<200 m 2 g -1 ), while highly exfoliated GRM show a lower sp 2 content, demonstrating that it is still challenging to exfoliate GRM at industrial level without adding defects.
42.	König, Jonas K., et al. (author) Interlayer exciton polaritons in homobilayers of transition metal dichalcogenides 2023 In: 2D Materials. - : IOP Publishing. - 2053-1583. ; 10:2 Journal article (peer-reviewed)abstract Transition metal dichalcogenides integrated within a high-quality microcavity support well-defined exciton polaritons. While the role of intralayer excitons in 2D polaritonics is well studied, interlayer excitons have been largely ignored due to their weak oscillator strength. Using a microscopic and material-realistic Wannier-Hopfield model, we demonstrate that MoS2 homobilayers in a Fabry-Perot cavity support polaritons that exhibit a large interlayer exciton contribution, while remaining visible in linear optical spectra. Interestingly, with suitable tuning of the cavity length, the hybridization between intra- and interlayer excitons can be ‘unmixed’ due to the interaction with photons. We predict formation of polaritons where > 90 % of the total excitonic contribution is stemming from the interlayer exciton. Furthermore, we explore the conditions on the tunneling strength and exciton energy landscape to push this to 100%. Despite the extremely weak oscillator strength of the underlying interlayer exciton, optical energy can be effectively fed into the polaritons once the critical coupling condition of balanced radiative and scattering decay channels is met. These findings have a wide relevance for fields ranging from nonlinear optoelectronic devices to Bose-Einstein condensation.
43.	Mackenzie, David M. A., et al. (author) Unraveling the electronic properties of graphene with substitutional oxygen 2021 In: Current Opinion in Chemical Engineering. - : IOP PUBLISHING LTD. - 2211-3398. ; 8:4 Journal article (peer-reviewed)abstract We show abrupt changes in the electronic properties of graphene with different types of binding to oxygen. Whereas oxygen bonded to the basal plane in the form of functional groups p-type dopes graphene, we prove that substitutional (i.e. in-plane) oxygen n-type dopes it. Moreover, we determine that impurity scattering potentials introduced by these substitutional atoms are notably larger than those of conventional donors, e.g. nitrogen. Both facts ultimately result in a conduction asymmetry in the system with holes being scattered more strongly than electrons. These findings provide essential insights into the impact of oxygen in carbon nanomaterials such as graphene oxide, oxidized carbon nanotubes or novel two-dimensional pi-conjugated organic frameworks, promising compounds for a wide range of applications including flexible electronics, catalysis, energy storage or biomedicine.
44.	Makarovsky, O., et al. (author) Enhancing optoelectronic properties of SiC-grown graphene by a surface layer of colloidal quantum dots 2017 In: 2D Materials. - : IOP Publishing. - 2053-1583. ; 4:3 Journal article (peer-reviewed)abstract We report a simultaneous increase of carrier concentration, mobility and photoresponsivity when SiC-grown graphene is decorated with a surface layer of colloidal PbS quantum dots, which act as electron donors. The charge on the ionised dots is spatially correlated with defect charges on the SiC-graphene interface, thus enhancing both electron carrier density and mobility. This charge-correlation model is supported by Monte Carlo simulations of electron transport and used to explain the unexpected 3-fold increase of mobility with increasing electron density. The enhanced carrier concentration and mobility give rise to Shubnikov-de Haas oscillations in the magnetoresistance, which provide an estimate of the electron cyclotron mass in graphene at high densities and Fermi energies up to 1.2 x 10(13) cm(-2) and 400 meV, respectively.
45.	Marsden, A. J., et al. (author) Electrical percolation in graphene-polymer composites 2018 In: 2D Materials. - : IOP Publishing. - 2053-1583. ; 5:3 Research review (peer-reviewed)abstract Electrically conductive composites comprising polymers and graphene are extremely versatile and have a wide range of potential applications. The conductivity of these composites depends on the choice of polymer matrix, the type of graphene filler, the processing methodology, and any post-production treatments. In this review, we discuss the progress in graphene-polymer composites for electrical applications. Graphene filler types are reviewed, the progress in modelling these composites is outlined, the current optimal composites are presented, and the example of strain sensors is used to demonstrate their application.
46.	Mouafo, L. D. N., et al. (author) Tuning contact transport mechanisms in bilayer MoSe2 transistors up to Fowler-Nordheim regime 2017 In: 2D Materials. - : IOP Publishing. - 2053-1583. ; 4:1 Journal article (peer-reviewed)abstract Atomically thin molybdenum diselenide (MoSe2) is an emerging two-dimensional (2D) semiconductor with significant potential for electronic, optoelectronic, spintronic applications and a common platform for their possible integration. Tuning interface charge transport between such new 2D materials and metallic electrodes is a key issue in 2D device physics and engineering. Here, we report tunable interface charge transport in bilayer MoSe2 field effect transistors with Ti/Au contacts showing high on/off ratio up to 107 at room temperature. Our experiments reveal a detailed map of transport mechanisms obtained by controlling the interface band bending profile via temperature, gate and source-drain bias voltages. This comprehensive investigation leads to demarcating regimes and tuning in transport mechanisms while controlling the interface barrier profile. The careful analysis allows us to identify thermally activated regime at low carrier density, and Schottky barrier driven mechanisms at higher carrier density demonstrating the transition from low-field direct tunneling/ thermionic emission to high-field Fowler–Nordheim tunneling. Furthermore, we show that the transition voltage Vtrans to Fowler–Nordheim correlates directly to the difference between the chemical potential of the metal electrode and the conduction band minimum in the 2D semiconductor, which opens up opportunities for new theoretical and experimental investigations. Our approach being generic can be extended to other 2D materials, and the possibility of tuning contact transport regimes is promising for designing MoSe2 device applications.
47.	Muhammad, Zahir, et al. (author) Anisotropic phonon and magnon vibration and gate-tunable optoelectronic properties of nickel thiophosphite 2023 In: 2D Materials. - : IOP Publishing. - 2053-1583. ; 10:2 Journal article (peer-reviewed)abstract Transition metal phosphorus trichalcogenides retain spin-charge coupling and lattice vibrations in different layers, which are useful for spintronic and optoelectronic devices. The phonon, magnons and excitonic properties of two-dimensional ternary nickel-phosphorus trisulfides (NiPS3) are investigated using Raman spectroscopy and photoluminescence (PL) study. With magnetic exchange interaction, an exotic phonon scattering degenerates the optical phonons into in-plane Ag and Bg modes. We have observed eight Raman modes with two acoustic anisotropic magnon modes (M1, M2) below the critical temperature for co-(XX), while only M1 at cross (XY) polarizations. The M1 mode is coupled with the phonon Bg mode that can survive after transition temperature. The phonon and magnon modes soften with variations in temperature, which is attributed to anharmonic phonon–phonon coupling and interlayer forces. The polarized Raman shows the two-fold and four-fold symmetry orientations of the phonon and magnon modes, respectively, which exhibit strong in-plane anisotropic phonon/magnon. The PL spectra revealed the existence of bound excitonic features and ensemble emitters in NiPS3. The robust interlayer excitation and structural stability further revealed the optothermal properties. Moreover, the fabricated field-effect transistor on NiPS3 reveals p-type semiconducting nature with an ON/OFF ratio of 5 × 106 and mobility of ∼16.34 cm2 V−1 s−1. In contrast, the rectification ratio indicates their diode characteristics. Similarly, the photocurrent is enhanced by changing the wavelength of light, which shows the potential for optoelectronics. The strong spin-charge interaction provides new insights into these materials’ magneto-optical and thermal properties for memory devices.
48.	Näslund, Lars-Åke, 1968-, et al. (author) The Origin of Ti 1s XANES Main Edge Shifts and EXAFS Oscillations in the Energy Storage Materials Ti2CTx and Ti3C2T x MXenes 2023 In: 2D Materials. - : Institute of Physics Publishing (IOPP). - 2053-1583. ; 10:3 Journal article (peer-reviewed)abstract A potential application of two-dimensional (2D) MXenes, such as Ti2CTx and Ti3C2Tx, is energy storage devices, such as supercapacitors, batteries, and hydride electrochemical cells, where intercalation of ions between the 2D layers is considered as a charge carrier. Electrochemical cycling investigations in combination with Ti 1s x-ray absorption spectroscopy have therefore been performed with the objective to study oxidation state changes during potential variations. In some of these studies Ti3C2Tx has shown main edge shifts in the Ti 1s x-ray absorption near-edge structure. Here we show that these main edge shifts originate from the Ti 4p orbital involvement in the bonding between the surface Ti and the termination species at the fcc-sites. The study further shows that the t2g–eg crystal field splitting (10Dq) observed in the pre-edge absorption region indicate weaker Ti–C bonds in Ti2CTx and Ti3C2Tx compared to TiC and the corresponding MAX phases. The results from this study provide information necessary for improved electronic modeling and subsequently a better description of the materials properties of the MXenes. In general, potential applications, where surface interactions with intercalation elements are important processes, will benefit from the new knowledge presented.
49.	Palekar, Chirag, et al. (author) Anomalous redshift in interlayer exciton emission with increasing twist angle in WSe 2 /MoSe 2 heterostructures 2024 In: 2D Materials. - 2053-1583. ; 11:2 Journal article (peer-reviewed)abstract Van der Waals heterostructures utilizing semiconducting transition metal dichalcogenide (TMDC) monolayers have surfaced as compelling candidates due to their intriguing optical characteristics, which can be effectively controlled by the manipulation of the stacking twist angle. This study investigates the intricate correlation between twist angle, band offset, and interlayer exciton emission within twisted WSe2/MoSe2 heterostructures. Our findings suggest a crucial influence of monolayer stacking order on the band offset and the dipole orientation in twisted heterostructures that leads to either blueshift or redshift in emission energy. Herein, we fabricate heterobilayers with twist angles varying from 1 ∘ to 56 ∘ and observe an anomalous redshift energy of 100 meV in the interlayer exciton emission. Additionally, photoluminescence excitation spectroscopy measurements highlight the systematic twist angle dependence of intralayer exciton resonances, indicating significant angle dependent effects on individual monolayer bandgaps and on the interlayer coupling strength. Our fundamental study of exciton resonances provides comprehensive insights into the nuanced interplay between twist angle, dipole orientation, and dielectric asymmetry, providing a deeper understanding of the factors governing the optical properties of layered TMDC heterostructures.
50.	Perivoliotis, Dimitrios K., et al. (author) Cobalt porphyrin/molybdenum disulfide nanoensembles for light-assisted electrocatalytic water oxidation and selective hydrogen peroxide production 2023 In: Current Opinion in Chemical Engineering. - : Institute of Physics (IOP). - 2211-3398. ; 10:1 Journal article (peer-reviewed)abstract The development of photo/electroactive catalysts sustainably producing hydrogen from water splitting and selectively hydrogen peroxide is of paramount importance to alleviate climate change effects. Herein, an anionic cobalt porphyrin (CoP) derivative is electrostatically interfaced with a positively charged modified molybdenum disulfide (MoS2), forming CoP/MoS2, which is accordingly employed as nonprecious photo/electrocatalyst for water oxidation reaction (WOR) and selective H2O2 production. According to the results, CoP/MoS2 shows remarkable bifunctional photo/electrocatalytic performance for WOR and 2e− pathway O2 reduction reaction (ORR) in alkaline electrolyte. Upon visible light irradiation, electrochemical measurements on a fluorine-doped tin oxide (FTO) coated glass electrode reveal an onset potential of 0.595 mV (ORR) and 1.575 mV (WOR) vs. reversible hydrogen electrode, being improved by approximately 80 mV, in both cases, compared to the dark conditions. Notably, the use of the FTO set-up not only enabled us to evaluate the photo/electrocatalytic activity of the CoP/MoS2 nanoensemble but also mimics the practical conditions in photo/electrochemical devices. The outstanding bifunctional photo/electrocatalytic performance of CoP/MoS2 is attributed to (a) the use of CoP as versatile single-atom molecular catalyst and photosensitizer (b) the strong ion-pair interactions between cationic modified MoS2 and the anionic CoP derivative, which prevent aggregation, ensuring better accessibility of the reactants to cobalt active sites, and (c) the co-existence of 1T and 2H phase at modified MoS2, offering improved electrical conductivity and intrinsic electrocatalytic activity along with enhanced intraensemble electronic interactions upon illumination. This work is expected to inspire the design of advanced and low-cost materials for the sustainable production of renewable fuels.

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