Table of contents

As the only non-carbon elemental layered allotrope, few-layer black phosphorus or phosphorene has emerged as a novel two-dimensional (2D) semiconductor with both high bulk mobility and a band gap. Here we report fabrication and transport measurements of phosphorene-hexagonal BN (hBN) heterostructures with one-dimensional edge contacts. These transistors are stable in ambient conditions for >300 h, and display ambipolar behavior, a gate-dependent metal–insulator transition, and mobility up to 4000 cm² V⁻¹ s⁻¹. At low temperatures, we observe gate-tunable Shubnikov de Haas magneto-oscillations and Zeeman splitting in magnetic field with an estimated g-factor ∼2. The cyclotron mass of few-layer phosphorene (FLP) holes is determined to increase from 0.25 to 0.31 m_e as the Fermi level moves towards the valence band edge. Our results underscore the potential of FLP as both a platform for novel 2D physics and an electronic material for semiconductor applications.

We study the environmental instability of mechanically exfoliated few-layer black phosphorus (BP). From continuous measurements of flake topography over several days, we observe an increase of over 200% in volume due to the condensation of moisture from air. We find that long term exposure to ambient conditions results in a layer-by-layer etching process of BP flakes. Interestingly, flakes can be etched down to single layer (phosphorene) thicknesses. BPʼs strong affinity for water greatly modifies the performance of fabricated field-effect transistors (FETs) measured in ambient conditions. Upon exposure to air, we differentiate between two timescales for changes in BP FET transfer characteristics: a short timescale (minutes) in which a shift in the threshold voltage occurs due to physisorbed oxygen and nitrogen, and a long timescale (hours) in which strong p-type doping occurs from water absorption. Continuous measurements of BP FETs in air reveal eventual degradation and break-down of the channel material after several days due to the layer-by-layer etching process.

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/□. 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.

The growth of Fe clusters up to nine atoms over graphene/Cu(111) is investigated within the density functional theory. Graphene is weakly physisorbed on Cu(111) through van der Waals force. The structures of Fe clusters over graphene/Cu(111) grow differently compared to gas-phase Fe clusters where Fe clusters are predicted to form towards a pyramid-like structure on graphene/Cu(111). The graphene is negatively charged upon the adsorption of Fe clusters as a result of charge transfer from Fe to graphene. Despite the fact that the electronic structure of graphene is affected by Fe clusters, magnetic moment of Fe clusters over graphene/Cu(111) remains relatively high. This suggests that graphene can be a potential substrate for supporting Fe clusters towards applications in magnetism and catalysis.

Successful fabrication of one monolayer FeSe on SrTiO₃ represented a real breakthrough in searching for high-T_c Fe-based superconductors ([1]). Motivated by this important discovery, we studied the effects of tensile strain on one monolayer and bulk iron-chalcogenide superconductors (FeSe and FeTe), showing that it produces important magnetic and electronic changes in the systems. We found that the magnetic ground state of bulk and monolayer FeSe is the block-checkerboard phase, which turns into the collinear stripe phase under in-plane tensile strain. FeTe, in both bulk and monolayer phases, shows two magnetic transitions upon increasing the tensile strain: from bicollinear in the ground state to block-checkerboard ending up to the collinear antiferromagnetic phase which could bring it in the superconducting state. Finally, the study of the mechanical properties of both FeSe and FeTe monolayers reveals their enormous tensile strain limits and opens the possibility to grow them on different substrates.

First-principles density functional theory calculations are performed in one-dimensional single-layer WS₂ nanoribbons with zigzag- and armchair-edges. Magnetic ordering, optical response, and chemical reactivity are investigated. Our results demonstrated that WS₂ zigzag nanoribbons exhibit a ferromagnetic-metallic behavior that is attributed to the edges; the resulting magnetic moments are mainly localized in S and W edge atoms. Furthermore, the magnetic ordering along the edges depends on the zigzag nanoribbon's width. Armchair nanoribbons exhibit semiconducting behavior. Optical response results demonstrated that there exists a strong optical polarization anisotropy enhancing a well defined absorption intensity peak, with polarization along the nanoribbons axis. Regarding chemical reactivity, ribbons are exposed to water (H₂O), thiophene (C₄H₄S), and carbon monoxide (CO) molecules. Results reveal that H₂O can be covalently joined to the edges via the W-atoms in the ribbons with zigzag-edges, whereas in ribbons with armchair edges, H₂O is dissociated in OH and H, and these species are joined to W and S atoms respectively. Results for thiophene on zigzag nanoribbons demonstrated that C₄H₄S molecules are absorbed by W-terminated edges, whereas in armchair ribbons, the C₄H₄S is linked to the edges by binding to the sulfur. Interestingly, CO molecules give rise to half-metallicity and surprising ferromagnetism in zigzag and armchair nanoribbons, respectively. The results discussed here could help to understand the physical and chemical properties of edges in transition metal dichalcogenides materials.

We report improvement of hole injection efficiency of a graphene anode by tuning its work function (WF) via surface fluorination. We used chemical vapor deposition to synthesize high-quality graphene sheets and then treated them with CHF₃ plasma to induce fluorination. We used x-ray photoelectron spectroscopy to examine the fluorine coverage and the kind of chemical bonds in fluorinated graphene (FG). Also, we used ultraviolet photoelectron spectroscopy to systematically study the changes in the WF and sheet resistance of the FG sheets with varying plasma exposure time (0, 10, 30, 60, 90 s) to find an optimum fluorination condition for hole injection. The WF of graphene sheets was increased by up to 0.74 eV, as a result of the formation of carbon-fluorine bonds that function as negative surface dipoles. We fabricated hole-only devices and conducted dark injection space-charge-limited-current transient measurement; the fluorination greatly increased the hole injection efficiency of graphene anodes (from 0.237 to 0.652). The enhanced hole injection efficiency of FG anodes in our study provides wide opportunities for applications in graphene-based flexible/stretchable organic optoelectronics.

Few-layer tungsten diselenide (WSe₂) is attractive as a next-generation electronic material as it exhibits modest carrier mobilities and energy band gap in the visible spectra, making it appealing for photovoltaic and low-powered electronic applications. Here we demonstrate the scalable synthesis of large-area, few-layer WSe₂ via replacement of oxygen in hexagonally stabilized tungsten oxide films using dimethyl selenium. Cross-sectional transmission electron microscopy reveals successful control of the final WSe₂ film thickness through control of initial tungsten oxide thickness, as well as development of layered films with grain sizes up to several hundred nanometers. Raman spectroscopy and atomic force microscopy confirms high crystal uniformity of the converted WSe₂, and time domain thermo-reflectance provide evidence that near record low thermal conductivity is achievable in ultra-thin WSe₂ using this method.

The surface chemistry of MoS₂, WSe₂ and MoSe₂ upon ultraviolet (UV)–O₃ exposure was studied in situ by x-ray photoelectron spectroscopy (XPS). Differences in reactivity of these transition metal dichalcogenides (TMDs) towards oxidation during UV–O₃ were observed and correlated with density functional theory calculations. Also, sequential HfO₂ depositions were performed by atomic layer deposition (ALD) while the interfacial reactions were monitored by XPS. It is found that the surface oxides generated on MoSe₂ and WSe₂ during UV–O₃ exposure were reduced by the ALD process ('self-cleaning effect'). The effectiveness of the oxide reduction on these TMDs is discussed and correlated with the HfO₂ film uniformity.

SnO₂/graphene composite has been regarded as the alternative anode material for next generation high-performance lithium-ion batteries (LIBs). Here we report an efficient and facile one-pot strategy for the synthesis of SnO₂/graphene composite through a surfactant-assisted redox process. The presence of surfactant can provide homogeneous nucleation sites for SnO₂ nanoparticles formation, thus ensuring the generated SnO₂ nanoparticles have a tiny size of ∼5 nm and are uniformly distributed on the graphene sheets. Simultaneously, the random aggregation of graphene sheets leads to the in-situ encapsulation of SnO₂ nanoparticles into graphene layers, forming a mechanically robust composite structure. These unique structural features are not only favorable for fast electrons transport and Li ions diffusion, but also capable of effectively buffering the volume changes of SnO₂ nanoparticles. As a consequence, the SnO₂/graphene composite exhibits superior Li storage performance in terms of large reversible capacity, good cycling stability and excellent rate capability.

In this work we present a simple pathway to obtain large single-crystal graphene on copper (Cu) foils with high growth rates using a commercially available cold-wall chemical vapour deposition (CVD) reactor. We show that graphene nucleation density is drastically reduced and crystal growth is accelerated when: (i) using ex situ oxidized foils; (ii) performing annealing in an inert atmosphere prior to growth; (iii) enclosing the foils to lower the precursor impingement flux during growth. Growth rates as high as 14.7 and 17.5 μm min⁻¹ are obtained on flat and folded foils, respectively. Thus, single-crystal grains with lateral size of about 1 mm can be obtained in just 1 h. The samples are characterized by optical microscopy, scanning electron microscopy, x-ray photoelectron spectroscopy, Raman spectroscopy as well as selected area electron diffraction and low-energy electron diffraction, which confirm the high quality and homogeneity of the films. The development of a process for the quick production of large grain graphene in a commonly used commercial CVD reactor is a significant step towards an increased accessibility to millimetre-sized graphene crystals.

In this work, the consequence of the high band-edge density of states on the carrier statistics and quantum capacitance in transition metal dichalcogenide two-dimensional semiconductor devices is explored. The study questions the validity of commonly used expressions for extracting carrier densities and field-effect mobilities from the transfer characteristics of transistors with such channel materials. By comparison to experimental data, a new method for the accurate extraction of carrier densities and mobilities is outlined. The work thus highlights a fundamental difference between these materials and traditional semiconductors that must be considered in future experimental measurements.

In this research, we have analyzed the electrochemical characteristics of the different rGO/Co₃O₄ composites prepared by controlling the rGO surface characteristics and its relationship between the growth of Co₃O₄ nanoparticles and the performance of the pseudocapacitor. Reduced graphene oxide/cobalt oxide (rGO/Co₃O₄) nanocomposites of different morphologies were prepared through the simple hydrothermal method. First, different kinds of graphite precursors, crumpled and planar with different properties, were used to determine the most suitable substrate to grow Co₃O₄ nanoparticles. As a result, rGO/Co₃O₄ composite synthesized from planar graphite shows a higher specific capacitance of 207.2, 170.1, and 141.5 Fg⁻¹ at 1, 2, and 5 Ag⁻¹ than the one prepared from crumpled graphite. In the second part, planar graphite, confirmed to be the most suitable substrate from the previous part, was oxidized under various oxidation conditions to increase the oxygen functional groups attached on the GO surfaces and observed to see how it affects the growth of Co₃O₄ nanoparticles and its influence on the electrochemical performance of the rGO/Co₃O₄ pseudocapacitor. As a result, the one with the largest amount of functional groups had the Co₃O₄ nanoparticles well dispersed and grown on the rGO substrate in small nanoparticle sizes, as small as 5.9 nm, leading to an improved electrochemical performance. Thus, the specific capacitance with the least amount of oxygen functional groups are 207.2, 170.1, and 141.5 Fg⁻¹ and for the largest amount of functional groups are 411.5, 371.4, and 292.7 Fg⁻¹, at 1, 2, and 5 Ag⁻¹ respectively. This approach could become a guideline for the ideal fabrication of rGO/metal oxide composite for further research involving rGO based pseudocapacitors.

We report experimental and theoretical investigations of a new nanomaterial: monolayer and few-layer talc. We show, through atomic force microscopy (AFM) measurements, that natural talc mineral can be mechanically exfoliated down to monolayer flakes. Our AFM-based mechanical characterization also shows that single- and few-layer talc flakes, of several square-microns, present properties similar to those of graphene, BN and MoS₂, including the existence of folds and the recently reported negative dynamic compressibility. From first principles calculations, we also predict the mechanical properties of monolayer talc. We obtain theoretical values of monolayer talc breaking strength that are near graphene's record values and its 2D elastic modulus. We also predict that the flexural rigidity of monolayer talc should be more than thirty times larger than that of graphene, but that it could still be bent to very small curvatures without fracturing.

We study theoretically 'graphene-like' plasmonic metamaterials constituted by two-dimensional arrays of metallic nanoparticles, including perfect honeycomb structures with and without inversion symmetry, as well as generic bipartite lattices. The dipolar interactions between localized surface plasmons (LSPs) in different nanoparticles gives rise to collective plasmons (CPs) that extend over the whole lattice. We study the band structure of CPs and unveil its tunability with the orientation of the dipole moments associated with the LSPs. Depending on the dipole orientation, we identify a phase diagram of gapless or gapped phases in the CP dispersion. We show that the gapless phases in the phase diagram are characterized by CPs behaving as massless chiral Dirac particles, in analogy with electrons in graphene. When the inversion symmetry of the honeycomb structure is broken, CPs are described as gapped chiral Dirac modes with an energy-dependent Berry phase. We further relax the geometric symmetry of the honeycomb structure by analysing generic bipartite hexagonal lattices. In this case we study the evolution of the phase diagram and unveil the emergence of a sequence of topological phase transitions when one hexagonal sublattice is progressively shifted with respect to the other.

Graphene sustains transverse out-of-plane mechanical vibrations (flexural phonons). At the nanometer scale, these appear as traveling ripples, or cavities, if excited in counter-phase in alternate multilayers. In this work we explore by means of classical molecular dynamics simulations the possibility of using these moving nano-cavities to actively transport hydrogen. We find that the gas can be efficiently transported for hundreds of nanometers in the wave propagation direction, before the phonons damp down. Therefore, this effect could be used to move and pump gases through multilayers graphene based frameworks.

A residual disorder in the gate system is the main problem on the way to create artificial graphene based on two-dimensional electron gas. The disorder can be significantly screened/reduced due to the many-body effects. To analyse the screening/disorder problem we consider AlGaAs/GaAs/AlGaAs heterostructure with two metallic gates. We demonstrate that the design least susceptible to the disorder corresponds to the weak coupling regime (opposite to tight binding) which is realised via a system of quantum anti-dots. The most relevant type of disorder is the area disorder which is a random variation of areas of quantum anti-dots. The area disorder results in the formation of puddles. Other types of disorder, the position disorder and the shape disorder, are practically irrelevant. The formation/importance of puddles dramatically depends on the parameters of the nanopatterned heterostructure. A variation of the parameters by 20–30% can change the relative amplitude of puddles by orders of magnitude. Based on this analysis we formulate criteria for the acceptable design of the heterostructure aimed at the creation of the artificial graphene.

We have decoupled the intrinsic electrostatic effects arising in monolayer and few-layer MoS₂ from those influenced by the flake-substrate interaction. Using ultrasonic force microscopy nanomechanical mapping, we identify the change from supported to suspended flake regions on a trenched substrate. These regions are correlated with the surface potential as measured by scanning Kelvin probe microscopy. Relative to the supported region, we observe an increase in surface potential contrast due to suppressed charge transfer for the suspended monolayer. Using Raman spectroscopy we observe a red shift of the E¹_2g mode for monolayer MoS₂ deposited on Si, consistent with a more strained MoS₂ on the Si substrate compared to the Au substrate.

A lithium-sulfur battery with a very high theoretical energy density (2600 Wh kg⁻¹) is one of the most promising candidates for next-generation energy storage devices. However, there are still many problems impeding the practical use of lithium-sulfur batteries, including the 'shuttle effect' and irreversible loss of active materials. Enhancing the interfacial interaction between the carbon hosts and the sulfur-containing guests by rational nitrogen doping is an effective route. First principle calculations were performed to illustrate the adsorption behavior between sulfur/lithium (poly)sulfides and pristine/nitrogen-doped graphene nanoribbons with different edge structures. N-dopants on doped graphene nanoribbon in pyrrolic and pyridinic forms donated extra binding energies of 1.12 ∼ 1.41 eV and 0.55 ∼ 1.07 eV, respectively. Quaternary nitrogen enriched on the edge can benefit from the adsorption of active materials. Compared with pristine graphene nanoribbon, nitrogen-doped graphene nanoribbons exhibited strong-couple interactions for anchoring sulfur-containing species, achieving high stability and reversibility, which was consistent with experimental findings. These results shed light on the cathode design of lithium-sulfur batteries and on the potential to understand host--guest interactions in other energy storage systems.

Single-layer MoS₂ is a direct-gap semiconductor whose electronic band structure strongly depends on the strain applied to its crystal lattice. While uniaxial strain can be easily applied in a controlled way, e.g., by bending of a flexible substrate with the atomically thin MoS₂ layer on top, experimental realization of biaxial strain is more challenging. Here, we exploit the large mismatch between the thermal expansion coefficients of MoS₂ and a silicone-based substrate to apply a controllable biaxial tensile strain by heating the substrate with a focused laser. The effect of this biaxial strain is directly observable in optical spectroscopy as a redshift of the MoS₂ photoluminescence. We also demonstrate the potential of this method to engineer more complex strain patterns by employing highly absorptive features on the substrate to achieve non-uniform heat profiles. By comparison of the observed redshift to strain-dependent band structure calculations, we estimate the biaxial strain applied by the silicone-based substrate to be up to 0.2%, corresponding to a band gap modulation of 105 meV per percentage of biaxial tensile strain.