Identification of Ubiquitously Present Polymeric Adlayers on 2D Transition Metal Dichalcogenides.

The interest in 2D materials continues to grow across numerous scientific disciplines as compounds with unique electrical, optical, chemical, and thermal characteristics are being discovered. All these properties are governed by an all-surface nature and nanoscale confinement, which can easily be altered by extrinsic influences, such as defects, dopants or strain, adsorbed molecules, and contaminants. Here, we report on the ubiquitous presence of polymeric adlayers on top of layered transition metal dichalcogenides (TMDs). The atomically thin layers, not evident from common analytic methods, such as Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), or scanning electron microscopy (SEM), could be identified with highly resolved time-of-flight secondary ion mass spectrometry (TOF-SIMS). The layers consist of hydrocarbons, which preferentially adsorb to the hydrophobic van der Waals surfaces of TMDs, derived from the most common methods. Fingerprint fragmentation patterns enable us to identify certain polymers and link them to those used during preparation and storage of the TMDs. The ubiquitous presence of polymeric films on 2D materials has wide reaching implications for their investigation, processing, and applications. In this regard, we reveal the nature of polymeric residues after commonly used transfer procedures on MoS2 films and investigate several annealing procedures for their removal.

Functionalisation of Graphene Sensor Surfaces for the Specific Detection of Biomarkers.

We report on a controllable and specific functionalisation route for graphene field-effect transistors (GFETs) for the recognition of small physiologically active molecules. Key element is the noncovalent functionalisation of the graphene surface with perylene bisimide (PBI) molecules directly on the growth substrate. This Functional Layer Transfer enables the homogeneous self-assembly of PBI molecules on graphene, onto which antibodies are subsequently immobilised. The sensor surface was characterised by atomic force microscopy, Raman spectroscopy and electrical measurements, showing superior performance over conventional functionalisation after transfer. Specific sensing of small molecules was realised by monitoring the electrical property changes of functionalised GFET devices upon the application of methamphetamine and cortisol. The concentration dependent electrical response of our sensors was determined down to a concentration of 300 ng ml-1 for methamphetamine.