No abstract

Recent advances in chemical vapour deposition have led to the fabrication of large graphene sheets on metal foils for use in research and development. However, further breakthroughs are required in the way these graphenes are transferred from their growth substrates onto the final substrate. Although various methods have been developed, as yet there is no general way to reliably transfer graphene onto arbitrary surfaces, such as 'soft' ones. Here, we report a method that allows the graphene to be transferred with high fidelity at the desired location on almost all surfaces, including fragile polymer thin films and hydrophobic surfaces. The method relies on a sacrificial 'self-releasing' polymer layer placed between a conventional polydimethylsiloxane elastomer stamp and the graphene that is to be transferred. This self-releasing layer provides a low work of adhesion on the stamp, which facilitates delamination of the graphene and its placement on the new substrate. To demonstrate the generality and reliability of our method, we fabricate high field-strength polymer capacitors using graphene as the top contact over a polymer dielectric thin film. These capacitors show superior dielectric breakdown characteristics compared with those made with evaporated metal top contacts. Furthermore, we fabricate low-operation-voltage organic field-effect transistors using graphene as the gate electrode placed over a thin polymer gate dielectric layer. We finally demonstrate an artificial graphite intercalation compound by stacking alternate monolayers of graphene and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). This compound, which comprises graphene sheets p-doped by partial hole transfer from the F4TCNQ, shows a high and remarkably stable hole conductivity, even when heated in the presence of moisture.

To make high-performance semiconductor devices, a good ohmic contact between the electrode and the semiconductor layer is required to inject the maximum current density across the contact. Achieving ohmic contacts requires electrodes with high and low work functions to inject holes and electrons respectively, where the work function is the minimum energy required to remove an electron from the Fermi level of the electrode to the vacuum level. However, it is challenging to produce electrically conducting films with sufficiently high or low work functions, especially for solution-processed semiconductor devices. Hole-doped polymer organic semiconductors are available in a limited work-function range, but hole-doped materials with ultrahigh work functions and, especially, electron-doped materials with low to ultralow work functions are not yet available. The key challenges are stabilizing the thin films against de-doping and suppressing dopant migration. Here we report a general strategy to overcome these limitations and achieve solution-processed doped films over a wide range of work functions (3.0-5.8 electronvolts), by charge-doping of conjugated polyelectrolytes and then internal ion-exchange to give self-compensated heavily doped polymers. Mobile carriers on the polymer backbone in these materials are compensated by covalently bonded counter-ions. Although our self-compensated doped polymers superficially resemble self-doped polymers, they are generated by separate charge-carrier doping and compensation steps, which enables the use of strong dopants to access extreme work functions. We demonstrate solution-processed ohmic contacts for high-performance organic light-emitting diodes, solar cells, photodiodes and transistors, including ohmic injection of both carrier types into polyfluorene-the benchmark wide-bandgap blue-light-emitting polymer organic semiconductor. We also show that metal electrodes can be transformed into highly efficient hole- and electron-injection contacts via the self-assembly of these doped polyelectrolytes. This consequently allows ambipolar field-effect transistors to be transformed into high-performance p- and n-channel transistors. Our strategy provides a method for producing ohmic contacts not only for organic semiconductors, but potentially for other advanced semiconductors as well, including perovskites, quantum dots, nanotubes and two-dimensional materials.

Graphenes are attracting renewed interests owing to recent advances in micromechanical exfoliation and epitaxial growth methods that make macroscopic 2D sheets of sp 2 -carbon atoms available.[1] A variety of simple yet elegant physics relating to its zero-gap semiconductor character has thus been demonstrated. [2][3][4][5] It would be very desirable to make these materials solution (or more accurately, dispersion) processable by coating or printing, which will open applications for large and/or flexible substrates. Graphite oxide (GO) is a possible candidate for this because it is a precursor to graphene through deoxidation either thermally or by chemical reduction. [6][7][8] Although GO itself has been studied for over a century, [9] its structure and properties remain elusive, and progress has been made only recently to give materials with limited dispersability and electronic quality. [10][11][12][13][14] Here we show that substoichiometric GO nanosheets can be surface-functionalised and purified to show excellent dispersability at the single-sheet level, >15 mg mL À1 in organic solvents, sufficient for spincoating and printing onto a variety of substrates. The films could then be deoxidised to graphene (ca. 80% completion at 300 8C) to give a network of low-dimensional ''graphenite'' tracks and dots on the nanosheets. Though imperfect and disordered, these show well-behaved and trap-free field-effect transistor charge-carrier mobilities for both electrons and holes of the order of 10 cm 2 V À1 s À1 , limited presently by the density of this graphenite network. Devices can be operated continuously in air for both p-and n-channels. The transport activation energies are in the meV region at low temperatures which together with the delocalisation of carriers indicate bandlike transport. The density-of-states at the Fermi level deduced by electrical measurements is higher than in graphite. MNDO-PM3 semiempirical electronic structure calculations relate this to defects in the 1D graphenite network. The fact that charge carriers can still be sufficiently delocalised in such disordered graphenites opens new opportunities for graphenes. It is well-known that chemical oxidation of graphite crystals gives GO which can be exfoliated by rapid-thermal-anneal >1000 8C, [15] or in solvents to give few-layer stacks that aggregate over time. [16,17] Recent work has shown that chemical functionalisation of GO can improve dispersability, particularly in the presence of stabilising polyelectrolytes. [10][11][12][13] However it is crucial to achieve more stable and concentrated dispersions without the added polyelectrolytes or ions, for electronic applications. We show here that substoichiometric (i.e. under-oxidised) GO can be obtained by a modified Staudenmaier oxidation of graphite with potassium chlorate [15] in a concentrated sulphuric-nitric acid mixture to give a material with an empirical formula containing less oxygen than the fully oxidised GO (C 2.0 O 1.0 H x ), [8,9,18] for example, C 2.0 O 0.77 H 0.75 . This material...