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Membranes that selectively filter for both anions and cations are central to technological applications from clean energy generation to desalination devices. 2D materials have immense potential as these ion-selective membranes due to their thinness, mechanical strength, and tunable surface chemistry; however, currently, only cation-selective membranes have been reported. Here we demonstrate the controllable cation and anion selectivity of both monolayer graphene and hexagonal boron nitride. In particular, we measure the ionic current through membranes grown by chemical vapor deposition containing well-known defects inherent to scalably produced and wet-transferred 2D materials. We observe a striking change from cation selectivity with monovalent ions to anion selectivity by controlling the concentration of multivalent ions and inducing charge inversion on the 2D membrane. Furthermore, we find good agreement between our experimental data and theoretical predictions from the Goldman-Hodgkin-Katz equation and use this model to extract selectivity ratios. These tunable selective membranes conduct up to 500 anions for each cation and thus show potential for osmotic power generation.

The deposition of dielectric materials on graphene is one of the bottlenecks for unlocking the potential of graphene in electronic applications. In this paper we demonstrate the plasma enhanced atomic layer deposition of 10 nm thin high quality Al$_2$O$_3$ on graphene using a monolayer of hBN as protection layer. Raman spectroscopy was performed to analyze possible structural changes of the graphene lattice caused by the plasma deposition. The results show that a monolayer of hBN in combination with an optimized deposition process can effectively protect graphene from damage, while significant damage was observed without an hBN layer. Electrical characterization of double gated graphene field effect devices confirms that the graphene did not degrade during the plasma deposition of Al$_2$O$_3$. The leakage current densities were consistently below 1 nA/mm for electric fields across the insulators of up to 8 MV/cm, with irreversible breakdown happening above. Such breakdown electric fields are typical for Al$_2$O$_3$ and can be seen as an indicator for high quality dielectric films.

Hexagonal boron nitride (hBN) is a promising host material for room-temperature, tunable solid-state quantum emitters. A key technological challenge is deterministic and scalable spatial emitter localization, both laterally and vertically, while maintaining the full advantages of the 2D nature of the material. Here, we demonstrate emitter localization in hBN in all three dimensions via a monolayer (ML) engineering approach. We establish pretreatment processes for hBN MLs to either fully suppress or activate emission, thereby enabling such differently treated MLs to be used as select building blocks to achieve vertical (z) emitter localization at the atomic layer level. We show that emitter bleaching of ML hBN can be suppressed by sandwiching between two protecting hBN MLs, and that such thin stacks retain opportunities for external control of emission. We exploit this to achieve lateral (x-y) emitter localization via the addition of a patterned graphene mask that quenches fluorescence. Such complete emitter site localization is highly versatile, compatible with planar, scalable processing, allowing tailored approaches to addressable emitter array designs for advanced characterization, monolithic device integration, and photonic circuits.

Plasmonic biosensing has emerged as the most sensitive label-free technique to detect various molecular species in solutions and has already proved crucial in drug discovery, food safety and studies of bio-reactions. This technique relies on surface plasmon resonances in ~50 nm metallic films and the possibility to functionalize the surface of the metal in order to achieve selectivity. At the same time, most metals corrode in bio-solutions, which reduces the quality factor and darkness of plasmonic resonances and thus the sensitivity. Furthermore, functionalization itself might have a detrimental effect on the quality of the surface, also reducing sensitivity. Here we demonstrate that the use of graphene and other layered materials for passivation and functionalization broadens the range of metals which can be used for plasmonic biosensing and increases the sensitivity by 3-4 orders of magnitude, as it guarantees stability of a metal in liquid and preserves the plasmonic resonances under biofunctionalization. We use this approach to detect low molecular weight HT-2 toxins (crucial for food safety), achieving phase sensitivity~0.5 fg/mL, three orders of magnitude higher than previously reported. This proves that layered materials provide a new platform for surface plasmon resonance biosensing, paving the way for compact biosensors for point of care testing.

Funder: H2020 Marie Skłodowska-Curie Actions; doi: https://doi.org/10.13039/100010665, Hexagonal boron nitride (h-BN) is well-established as a requisite support, encapsulant and barrier for 2D material technologies, but also recently as an active material for applications ranging from hyperbolic metasurfaces to room temperature single-photon sources. Cost-effective, scalable and high quality growth techniques for h-BN layers are critically required. We utilise widely-available iron foils for the catalytic chemical vapour deposition (CVD) of h BN and report on the significant role of bulk dissolved species in h-BN CVD, and specifically, the balance between dissolved oxygen and carbon. A simple pre-growth conditioning step of the iron foils enables us to tailor an error-tolerant scalable CVD process to give exceptionally large h-BN monolayer domains. We also develop a facile method for the improved transfer of as-grown h-BN away from the iron surface by means of the controlled humidity oxidation and subsequent rapid etching of a thin interfacial iron oxide; thus, avoiding the impurities from the bulk of the foil. We demonstrate wafer-scale (2 inch) production and utilise this h-BN as a protective layer for graphene towards integrated (opto) electronic device fabrication., European Union's Horizon 2020 research and innovation program under Grant Agreement No number 785219. European Union's Horizon 2020 research and innovation program under Grant Agreement No number 796388. the Royal Commission for the Exhibition of 1851. EU Marie Skłodowska-Curie Individual Fellowship (Global) under grant ARTIST (No. 656870). EPSRC (EP/P005152/1, and Doctoral Training Award EP/M508007/1). U.K. Department of Business, Energy and Industrial Strategy (NPL Project Number 121452).

Vertically-aligned crystalline silicon carbide nanowires (VASiCs) (1 mm long and 50–90 nm in diameter) were synthesised in gram scale using SiO 2 -infiltrated vertically-aligned multi-wall carbon nanotubes (VACNTs) and Si powder. In situ residual gas analysis was employed to study their formation and revealed CO to be the main by-product during synthesis. The in situ studies also showed that the formation of VASiCs begins at 1150 °C with the growth rate reaching a maximum at 1350 °C. A possible growth mechanism was established based on both, in situ and ex situ characterisation. The VASiCs have an estimated band gap of 2.15 eV, are photocatalytically active, and show strong light absorbance of up to 577 nm. Under UV–vis light (260–800 nm) as grown VASiCs could remove 90% Rhodamine B (RhB) within 30 min. Over period of 4 h under visible light (400–800 nm) more than 95% RhB was removed demonstrating their potential as visible-light-driven photocatalysts.

Graphene produced by chemical vapor deposition (CVD) is a promising candidate for implementing graphene in a range of technologies. In most device configurations, one side of the graphene is supported by a solid substrate, wheras the other side is in contact with a medium of interest, such as a liquid or other two-dimensional material within a van der Waals stack. In such devices, graphene interacts on both faces via noncovalent interactions and therefore surface energies are key parameters for device fabrication and operation. In this work, we directly measured adhesive forces and surface energies of CVD-grown graphene in dry nitrogen, water, and sodium cholate using a modified surface force balance. For this, we fabricated large (∼1 cm(2)) and clean graphene-coated surfaces with smooth topography at both macro- and nanoscales. By bringing two such surfaces into contact and measuring the force required to separate them, we measured the surface energy of single-layer graphene in dry nitrogen to be 115 ± 4 mJ/m(2), which was similar to that of few-layer graphene (119 ± 3 mJ/m(2)). In water and sodium cholate, we measured interfacial energies of 83 ± 7 and 29 ± 6 mJ/m(2), respectively. Our work provides the first direct measurement of graphene surface energy and is expected to have an impact both on the development of graphene-based devices and contribute to the fundamental understanding of surface interactions.

Single-molecule localization microscopy (SMLM) is a powerful tool that is routinely used for nanoscale optical imaging of biological samples. Recently, this approach has been applied to study optically active defects in two-dimensional (2D) materials. Such defects can not only alter the mechanical and optoelectronic properties of 2D materials but also bring new functionalities, which make them a promising platform for integrated nanophotonics and quantum sensing. Most SMLM approaches, however, provide a field of view limited to similar to 50 x 50 mu m(2), which is not sufficient for high-throughput characterization of 2D materials. Moreover, the 2D materials themselves pose an additional challenge as their nanometer-scale thickness prevents efficient far-field excitation of optically active defects. To overcome these limitations, we present here a waveguide-based platform for large field-of-view imaging of 2D materials via total internal reflection excitation. We use this platform to perform large-scale characterization of point defects in chemical vapor deposition-grown hexagonal boron nitride on an area of up to 100 x 1000 mu m(2) and demonstrate its potential for correlative imaging and high throughput characterization of defects in 2D materials.

We systematically study the effects of residual oxygen in the bulk of Cu foil catalysts on the chemical vapor deposition (CVD) of graphene. While oxidation is widely used to remove impurities in metal catalysts and to control the nucleation density of graphene, we show that minute concentrations of residual bulk oxygen can significantly deteriorate the quality of as-grown graphene highlighted by an increased Raman D/G ratio, increased propensity to postgrowth etching, and increased fraction of multilayer graphene nucleation. Starting from commercial Cu foils, we show that a simple hydrogen annealing step after the initial oxidation allows us to lower the residual oxygen level as measured by time-of-flight secondary ion mass spectrometry to produce graphene of significantly higher quality. This can be effectively combined with a short hydrocarbon exposure time of 10 min to achieve near full monolayer graphene coverage, suitable for emerging industrial applications. We show that residual oxygen can have an equally significant impact on Fe-catalyzed h-BN CVD and discuss the underlying mechanisms with parallels to well-known processes in metallurgy, catalysis, and vacuum science.

Ammonia borane (AB) is among the most promising precursors for the large-scale synthesis of hexagonal boron nitride (h-BN) by chemical vapour deposition (CVD). Its non-toxic and non-flammable properties make AB particularly attractive for industry. AB decomposition under CVD conditions, however, is complex and hence has hindered tailored h-BN production and its exploitation. To overcome this challenge, we report in-depth decomposition studies of AB under industrially safe growth conditions. In situ mass spectrometry revealed a time and temperature-dependent release of a plethora of NxBy-containing species and, as a result, significant changes of the N:B ratio during h-BN synthesis. Such fluctuations strongly influence the formation and morphology of 2D h-BN. By means of in situ gas monitoring and regulating the precursor temperature over time we achieve uniform release of volatile chemical species over many hours for the first time, paving the way towards the controlled, industrially viable production of h-BN.

This thesis focuses on using Chemical Vapour Deposition (CVD) technique to produce various classes of flat 2D materials and also their three-dimensional architectures. Graphene, hexagonal boron nitride (hBN) and tungsten disulfide (WS2) are the main examples presented in the study: a conductor, an insulator and a semiconductor respectively. Such materials are elementary "building blocks" in a variety of applications. Many novel advances to CVD synthesis of 2D materials are identified, developed and discussed in this thesis.

Commercially available Cu foils are leading candidates as substrates employed for the generation of large-area graphene using chemical vapour deposition (CVD) techniques. However, the growth of high-quality graphene on Cu foils is often hindered by contamination particles, which will also be detrimental for many potential applications of graphene. Here we investigate the influence of typical substrate impurities on the formation of CVD graphene using as-received Cu foils of various purities from different suppliers and the same cleaned by popular methods. Analytical characterisation of the Cu foils revealed that contamination particles consist of calcium, aluminium, and silicon oxides. We show that contamination particles are present on foils with purities ranging between 99.8% and 99.9999% and that these particles influence the nucleation density, growth rate, and growth features of graphene domains. Based on our findings we propose new industrially applicable targeted cleaning procedures of immersion in purposely-selected HCl and KOH solutions to chemically dissolve the aforementioned impurities, bringing about improved growth of graphene.

We report a method for transferring graphene, grown by chemical vapor deposition, which produces ultraflat graphene surfaces (root-mean-square roughness of 0.19 nm) free from polymer residues over macroscopic areas (>1 cm(2)). The critical step in preparing such surfaces involves the use of an intermediate mica template, which itself is atomically smooth. We demonstrate the compatibility of these model surfaces with the surface force balance, opening up the possibility of measuring normal and lateral forces, including friction and adhesion, between two graphene sheets either in contact or across a liquid medium. The conductivity of the graphene surfaces allows forces to be measured while controlling the surface potential. This new apparatus, the graphene surface force balance, is expected to be of importance to the future understanding of graphene in applications from lubrication to electrochemical energy storage systems.

Silicon is of interest owing to its high theoretical capacity (4200 mAh/g). However, its enormous volume expansion of silicon during cycling is one of the main reasons for the rapid capacity fading and pulverisation of the anode commonly observed. Here, freestanding multi-wall carbon nanotube (MWCNT)/silicon anodes were prepared using so-called MWCNT buckypapers (BPs) as current collectors instead of the traditional metallic foils. To prepare the anodes, silicon was deposited on these MWCNT BPs to form silicon nanoparticles (SiNPs) and subsequent annealing resulted in the formation of a connecting SiC interface preventing the SiNPs from detaching. An amorphous carbon coating was applied in order to enhance electrical conductivity and overall integrity of these hierarchical anode materials. The morphology and chemical composition of the Si-BP anodes were characterised by means of SEM, TEM, EDX, STEM, XRD, RAMAN, etc. Cycling tests of these binder and additive-free anodes revealed an areal capacity of 0.55 mAh/cm2 after 300 cycles with 99.7% of Columbic efficiency and showed that the presence of the SiC interface and a-C coating played a crucial role in improving their rate capability and stability.

Large-area synthesis of high-quality graphene by chemical vapour deposition on metallic substrates requires polishing or substrate grain enlargement followed by a lengthy growth period. Here we demonstrate a novel substrate processing method for facile synthesis of mm-sized, single-crystal graphene by coating polycrystalline platinum foils with a silicon-containing film. The film reacts with platinum on heating, resulting in the formation of a liquid platinum silicide layer that screens the platinum lattice and fills topographic defects. This reduces the dependence on the surface properties of the catalytic substrate, improving the crystallinity, uniformity and size of graphene domains. At elevated temperatures growth rates of more than an order of magnitude higher (120 μm min−1) than typically reported are achieved, allowing savings in costs for consumable materials, energy and time. This generic technique paves the way for using a whole new range of eutectic substrates for the large-area synthesis of 2D materials., Innovative substrate engineering is necessary to improve the quality of CVD-synthesized graphene. Here the authors demonstrate in situ fabrication of an eutectic Pt-Si alloy that forms a wetting liquid surface on polycrystalline Pt foils, allowing millimetre-sized graphene crystals to grow in minutes.