This project has produced a range of new 2D materials in the form of inks including TiS2, Co(OH)2, Zn(OH)2, GeS, talc, BiOCl, FeS2, SnP3, RedP, Fe2O3 and described the nanosheet exfoliation mechanism. We developed new methods to characterize the nanosheets and measure their size.
We demonstrated a number of methods to print the resultant inks including: inkjet printing, spray coating, aerosol jet printing, vacuum filtration, slurry coating and Langmuir-Schaeffer deposition. Detailed film characterization (SEM, AFM, optical spectroscopy, XRD) was done on these films. We also developed a new characterization method using SEM and FIB to generate 3D images of nanosheet networks allowing a quantitative analysis of morphological factors such as porosity, tortuosity and nanosheet alignment.
We learned to print complex materials such as polymer/nanosheet composites and nano:nano composites which we showed to yield very high performance printed electrochemical devices e.g. battery electrodes.
We performed comprehensive electrical testing on our networks, for example measuring both conductivity and carrier mobility and how conduction depends on network morphology. We developed a theoretical model describing the relationship between nanosheet size and network conductivity and which allows the junction resistance to be extracted from data. This allowed us to quantify junction resistances for networks of graphene, WS2, WSe2, MoS2, obtaining values from 10 kOhm for graphene to 1 GOhm for the semiconducting nanosheets.
We also made printed devices consisting of only graphene nanosheet networks. These were predominantly piezo resistive sensing devices. we were able to optimize printing methods, produce very high sensitivity sensors and to quantify the sensing mechanism.
In addition, silver nanoplatelet inks were deposited via aerosol jet to yield highly conductive patterns which perform extremely well as EMI shields, transparent conductors and electrodes.
We showed that vertically stacked printed networks of conductive/insulating/conductive materials could be used as printed capacitors. We investigated a number of materials in the conductor/insulator/conductor structure using silver nanoplatelets as conductors and boron nitride and BiOCl nanosheets as insulators. We developed this theme of stacked films to produce hetero stacks including semiconducting 2D materials. We first produced heterostacks of ITO, WS2 and carbon nanotubes. We showed that these structures displayed ohmic or diode behaviour depending on the doping state of the WS2. We developed this work further printing more stacks consisting of ITO/PEDOT:PSS/ WSe2/ ZnO/Aluminium. These stacks were found to act as high performance diodes with vet high rectification ratio >10,000 and sensitive light detection performance.
Most importantly, we used printed stacks as electronic devices. Using electrolytic gating, we demonstrated all-printed, vertically stacked transistors with graphene source, drain, and gate electrodes, channels fabricated from MoS2, Ws2, MoSe2 or WSe2, and a boron nitride (BN) separator, all formed from printed networks of nanosheet. The porous interior of the BN network was filled with an ionic liquid that allows electrolytic gating in a solid-like structure. Nanosheet network transistors displayed on/off ratios of up to 10000, and mobilities of >0.1 square centimeters per volt per second. Collaborating with researchers in France, we developed novel chemical functionalization strategies which reduced junction resistance by a factor of 10, leading to a proportional mobility enhancement.
While this work was the first demonstration of printed nanosheet transistors, and a major breakthrough, the achieved mobilities were low. We used the work described above to quantify junction resistances to produce better nanosheet networks with lower junction resistance and higher mobility. This involved developing electrochemical exfoliation methods to produce high aspect ratio nanosheets which tend to form low resistance junctions as well as perfecting Langmuir Schaefer deposition methods which tend to yield highly aligned networks with overlapping nanosheets. We also developed a new method, based on impedance spectroscopy, to directly measure junction resistance. we found that the combination of high aspect ratio nanosheets and aligned films gave junction resistances of ~MOhm, much lower then the GOhm values described above. this allowed us to produce transistors based on MoS2 and WS2 networks, with mobilities above 10 cm2/Vs, close to the global state-of-the-art for printed transistors.
This project has yielded 31 papers in international journals, including high impact journals such as science and nature energy. Are paper on printed nanosheet transistors which appeared in science has been cited over 300 times. We have reported the results obtained in this project are many many conferences, with over 30 invited talks given on this topic. We have submitted 2 patent applications on printed device formation and the production of piezoresistive sensors. 1 of my postdocs has set up a spin out company to develop these printed sensors.
