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...
Heterostructures are central to the efficient manipulation of charge carriers, excitons and photons for high-performance semiconductor devices. Although these can be formed by stepwise evaporation of molecular semiconductors, they are a considerable challenge for polymers owing to re-dissolution of the underlying layers. Here we demonstrate a simple and versatile photocrosslinking methodology based on sterically hindered bis(fluorophenyl azide)s. The photocrosslinking efficiency is high and dominated by alkyl side-chain insertion reactions, which do not degrade semiconductor properties. We demonstrate two new back-infiltrated and contiguous interpenetrating donor-acceptor heterostructures for photovoltaic applications that inherently overcome internal recombination losses by ensuring path continuity to give high carrier-collection efficiency. This provides the appropriate morphology for high-efficiency polymer-based photovoltaics. We also demonstrate photopatternable polymer-based field-effect transistors and light-emitting diodes, and highly efficient separate-confinement-heterostructure light-emitting diodes. These results open the way to the general development of high-performance polymer semiconductor heterostructures that have not previously been thought possible.
Although organic semiconductors have received the most attention, the development of compatible passive elements, such as interconnects and electrodes, is also central to plastic electronics. For this, ligand-protected metal-cluster films have been shown to anneal at low temperatures below 250 degrees C to highly conductive metal films, but they suffer from cracking and inadequate substrate adhesion. Here, we report printable metal-cluster-polymer nanocomposites that anneal to a controlled-percolation nanostructure without complete sintering of the metal clusters. This overcomes the previous challenges while still retaining the desired low transformation temperatures. Highly water- and alcohol-soluble gold clusters (75 mg ml-1) were synthesized and homogeneously dispersed into poly(3,4-ethylenedioxythiophene) to give a material with annealed d.c. conductivity tuneable between 10(-4) and 10(5) S cm-1. These composites can inject holes efficiently into all-printed polymer organic transistors. The insulator-metal transformation can also be electrically induced at 1 MV cm-1, suggesting possible memory applications.
Electronically-conducting polymers are a remarkable class of materials that can be used as electrode, interconnect and active layers in organic electronics, [1] such as thin-film transistors (OTFTs), light-emitting diodes (OLEDs), electrochromic devices, [2,3] super-capacitors, [4] actuators [5] and biosensors. [6] A particularly important member is p-type poly(3,4-ethylenedioxythiophene) (PEDT) doped in poly(styrenesulfonic acid) (PSSH), which has unprecedented thermal and electrochemical stability. [1,3,[7][8][9] We provide direct evidence here, however, that it undergoes injection de-doping by an electric current when stress-biased beyond a few tens of kV cm -1 between gold and other electrodes. Charge modulation spectroscopy shows that the oxidation state of PEDT decreases as holes are extracted marginally faster than they are injected. The decrease becomes locked-in when coupled to a compensating electrochemical oxidation of the counter-ion. In PEDT:PSSH this leads to a sharp fall in the doping level across the electrode gap, particularly at the negative contact, as evidenced by micro-Raman spectroscopy. Impedance spectroscopy gives the de-doped insulating width as of the order of tens of nm, which appears to be self-limited. This mechanism is the origin of the deep conductor-to-insulator transformation in PEDT:PSSH [10] and perhaps also other conducting polymers.[11] By substituting the acidic H + in the counter polyelectrolyte with the neutral and larger tetramethylammonium (TMA) ion, it is possible to shut down the electrochemistry and raise the electric field threshold for permanent de-doping by one order of magnitude.PEDT:PSSH has proven to be a particularly versatile conducting polymer with a DC conductivity (r DC ) tune-able over several orders of magnitude (10 1 to 10 -6 S cm -1 ) by matrix dilution and/or solvent treatment [12] to suit particular applications. It is obtained by persulfate oxidation of 3,4-ethylenedioxythiophene in the presence of PSSH, [1] to give a PSSH-rich inter-polyelectrolyte complex that is water-soluble and solution-processable.[13] PEDT:PSSH is strongly acidic on account of the excess PSSH, and often contaminated with ions leftover from the oxidation reaction. This excess PSSH may be undesirable in some circumstances. H + can be replaced readily, however, by other M + with a change in workfunction, but not processibility, the PEDT doping level, or its electronic conductivity. [14] Therefore it is appropriate to denote this material PEDT:PSSM, wherein PEDT is "doped" into the PSSM matrix. Figure 1a shows the chemical structure of materials (M = H, TMA) used in this work. We rigorously purified them by dialysis to remove low molecular-weight oligomers and ionic impurities (see Experimental) to be sure that the effects observed are intrinsic and not impurity-related. A further notation: We used "PEDT" here to refer to the material in general, PEDT + to refer to its p-doped positively-charged state, and PEDT 0 to refer to the undoped uncharged state.The PEDT doping level is typ...
We have extended the well known bisfluorinated(phenyl azide) (bisFPA) methodology to develop an ionic bisFPA process suitable for photo‐crosslinking a wide variety of polyelectrolyte thin films. The crosslinking efficiencies (0.1–1.0 crosslink per photo‐reaction) are sufficiently high for the gel fraction to exceed 80 % for crosslinker concentrations of only a few weight %. This method is based on the photo‐induced formation of singlet nitrenes from FPAs and their insertion into unactivated C–H or other bonds, which thus general and not dependent on the presence of specific chemical functional groups. By derivatizing with ionic charge groups, we obtained ionic bisFPAs that can be properly dispersed into polyelectrolyte thin films. The sorbed moisture always present in these films however severely limits the photo‐crosslinking efficiency, apparently through nitrene protonation and intersystem crossing. This can be avoided by dehydration of the films, in some cases, to 130 °C for 10 min in nitrogen before photo‐exposure. We found that efficient photo‐crosslinking can then be achieved for polyelectrolytes even when they have nucleophilic groups. These include poly(styrenesulfonic acid) and their salts, poly(acrylic acid) and their salts, poly(dimethyldiallylammonium salts), as well as the electrically‐conducting poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonic acid) complex (PEDT:PSSH). We further demonstrate using this ionic bisFPA methodology both photo‐patterning and post‐deposition chemical modifications of polyelectrolyte thin films. This opens broad new possibilities in membrane, sensor and actuator technologies, as well as for organic semiconductor plastic electronics (such as field‐effect transistors) and polyelectrolyte‐based devices.
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