During October 2019, some of our CDT students hosted a one-day student conference at the University of Exeter. This was co-organised with some PhD students from the Centre for Photonics and Photonic Materials (CPPM) from University of Bath- the “Exeter-Bath Knowledge Transfer”. The aim of this event was to bring together researchers from both institutions and share the research that is being carried out in optics and photonics. In total, 23 students attended this event (9 from Bath, 14 from Exeter) and had the opportunity to present their work and learn about each other’s research projects. This event was useful for interchanging ideas and to begin establishing a network between the CPPM and CDT.
The program of the conference consisted of several oral presentations, some white board explanations and a poster session. At the end of the conference, our students gave a tour around some of the laboratories and the nanofabrication clean room facilities. Finally, a social was held at the Imperial to end a successful day.
The Exeter-Bath Knowledge Transfer event was organised thanks to seed funding that Iago Rodriguez Diez, Harry Penketh, Ben Hogan and students from the CPPM obtained from the collaborative discussion-led incubator workshop “Light@Bath”. This workshop took place in May at University of Bath for the International Day of Light and was supported by the EPSRC-funded project Reimagining Recruitment. The purpose of the event was to bring together researchers who work in photonics in order to exchange ideas, propose problems that are yet to be solved and generate collaborations. The outcome of this workshop was the organization of the one-day conference “Exeter-Bath Knowledge Transfer”.
Iago, one of our CDT students who co-ordinated the event, reported:
“This event was a very nice opportunity for getting to know what another research centre working in photonics does with light. As the main Exeter co-organizer of the event, this was a very good practice for me to challenge my organization and time management skills in terms of personal development. Thanks to everyone involved in the event!”
This paper describes the first observation of a liquid crystal phase for dispersions of two-dimensional tungsten disulfide particles in organic solvents. We detail the synthesis methods used to obtain the liquid crystals. We then characterise them, observing interesting and unexpected dichroism properties some of which can be controlled by the application of a magnetic field. We then demonstrate the first applications of the liquid crystals by producing highly uniform thin films of tungsten disulfide, which are then shown to be useful for developing future terahertz modulation devices. This paper represents the culmination of two and a half years of hard work, and involved collaborations with ITMO University (Russia) and Massachusetts Institute of Technology (USA)
The first observation of liquid crystalline dispersions of liquid phase-exfoliated tungsten disulfide flakes is reported in a range of organic solvents. The liquid crystals demonstrate significant birefringence as observed in the linear and circular dichroism measurements respectively. In particular, linear dichroism is observed throughout the visible range while broad-band circular dichroism can be observed in the range from 500–800 nm. Under an applied magnetic field of ±1.5 T the circular dichroism can be switched ON/OFF, while the wavelength range for switching can be tuned from large to narrow range by the proper selection of the host solvent. In combination with photoluminescence capabilities of WS2, this opens a pathway to a wide variety of applications, such as deposition of highly uniform films over large areas for photovoltaic and terahertz devices.
This work looks at the properties of the tiny diamonds formed during the detonation of explosives. Particularly, we look at how light can be emitted from the diamonds due to the defects that occur within them as a result of their violent creation. The diamonds have applications in imaging, as the emission is both very bright and confined to a small area, amongst others. Such diamonds have, for example, previously been used to image inside cells as their small sizes mean they can easily move within biological samples. The key results from this paper are that the method used to produce the diamonds gives a high concentration of defects, and therefore brighter emission, and the demonstration that quantum effects allow us to switch the emission on or off by using a magnetic field.
The content of nitrogen-vacancy (NV−) colour centres in the nanodiamonds (DNDs) produced during the detonation of nitrogen-containing explosives was found to be 1.1 ± 0.3 ppm. This value is impressive for nanodiamonds of size < 10 nm with intentionally created NV− centres. The concentration was estimated from the electron paramagnetic resonance as determined from the integrated intensity of the g = 4.27 line. This line is related with “forbidden” ∆ms = 2 transitions between the Zeeman levels of a NV− centre’s ground triplet state. Confocal fluorescence microscopy enables detection of the red photoluminescence (PL) of the NV− colour centres in nanoscale DND aggregates formed from the 5-nm nanoparticles. Subwavelength emitters consisting of NV− with sizes a few times smaller than the diffraction-limited spot are clearly distinguished. We have further observed an abrupt drop in the PL intensity when mixing and anti-crossing of the ground and excited states spin levels in NV− occurs under an applied external magnetic field. This effect is a unique quantum feature of NV− centres, which cannot be observed for other visible domain light-emitting colour centres in a diamond lattice.
By using new and unique methods developed at the University of Exeter, we have produced thin films of different two-dimensional (2D) materials. These thin films can then be transferred easily to almost any other surface. In this work we have then investigated the properties of the different 2D materials in the terahertz frequency range. The results show that the tested 2D materials are suitable for applications in terahertz modulation devices. Ultimately, this work, with further improvement, could lead to cheaper and improved terahertz systems with applications ranging from non-destructive materials testing, to biomedical imaging, and security and communications technologies.
Time-resolved terahertz spectroscopy has become a common method both for fundamental and applied studies focused on improving the quality of human life. However, the issue of finding materials applicable in these systems is still relevant. One of the appropriate solution is 2D materials. Here, we demonstrate the transmission properties of unique graphene-based structures with iron trichloride FeCl3 dopant on glass, sapphire and Kapton polyimide film substrates that previously were not investigated in the framework of the above-described problems in near infrared and THz ranges. We also show properties of a thin tungsten disulfide WS2 film fabricated from liquid crystal solutions transferred to a polyimide and polyethylene terephthalate substrates. The introduction of impurities, the selection of structural dimensions and the use of an appropriate substrate for modified 2D layered materials allow to control the transmission of samples for both the terahertz and infrared ranges, which can be used for creation of effective modulators and components for THz spectroscopy systems.
Ben Hogan, fourth year CDT PGR, has been awarded a SPIE IDL Micro Grant for $2000- one of only two UK universities to receive it this year.
The UNESCO International Day of Light is a global initiative highlighting to the citizens of the world the importance of light and light-based technologies in their lives, for their futures, and for the development of society. SPIE IDL Micro Grants support local community events and activities that highlight the critical role that light plays in our daily lives. The grants are awarded globally on a competitive basis, with 14 awarded in 2019.
Ben will use the $2000 to run three concurrent events in April and May, to promote the critical role that light plays in our daily lives to the general public.
The events that will be run are as follows:
• A photo competition for the local community, themed around light.
• A poster competition for researchers, with the aim being to convey cutting-edge research in simple terms for non-specialists.
• Lighting up RAMM – On Saturday 18th May, PGRs will provide hands-on demonstrations, activities and displays in the RAMM museum in Exeter.
• Exeter EnLIGHTens – PGRs are inviting schoolchildren of all ages (and their teachers) to pose us their questions about light. PGRs will then answer as many of their questions as possible in the form of video demonstrations of practical experiments, which will be made freely available online.
Lung cancer is one of the most common and aggressive cancers, with mortality rates of about 1.4 million per year, worldwide. The lack of clinical symptoms of early-stage lung cancer is a critical global challenge which leads to late-stage diagnosis and hence inability to cure patients. One potential solution is to monitor the makeup of people’s breath, in order to detect changes occurring due to the presence of cancer in the lungs. This paper shows that patterned multilayer graphene is a suitable electrode for the specific and selective analysis of breath samples in future devices.
Highly selective, fast detection of specific lung-cancer biomarkers (CMs) in exhaled human breath is vital to the development of enhanced sensing devices. Today, e-nose is a promising approach for the diagnosis of lung cancer. Nevertheless, considerable challenges to early-stage disease diagnostics still remain: e.g. decrease in sensor sensitivities in the presence of water vapor, sensor drift leading to the inability to calibrate exactly, relatively short sensor lifetimes, and difficulty discriminating between multiple diseases.
However, there is a wide scope for breath diagnostics techniques, and all advanced electrodes applicable to e-nose devices will benefit them. Here, we present the promising sensing capabilities of bare multi-layer graphene (MLG) as a proof of concept for advanced e-nose devices and demonstrate its utility for biomolecule discrimination of the most common lung CMs (ethanol, isopropanol, and acetone). We report on a comparative study involving exposure of the three CM solutions on flat MLG (f-MLG) and patterned MLG (p-MLG) electrodes, where the electrical conductivity of p-MLG is significantly increased while applying acetone. Based on sensitivity tests, we demonstrate the ability to monitor the electrical response of graphene electrodes employing graphene of various wettabilities. Specifically, the f-MLG electrode displays almost 2 times higher sheet resistance (30 Ω sq−1) compared to the hydrophilic p-MLG (12 Ω sq−1). We show significant sensitivity to selected specific molecules of pristine f-MLG and p-MLG while applying CM solutions with a 1.4 × 105 ppm concentration.
Finally, we show the selectivity of f-MLG and p-MLG-based sensors when exposed to 2.0 × 105 ppm solutions containing different CM combinations. Both sensors were selective in particular to acetone, since the presence of acetone leads to a sheet resistance increase. We demonstrate that an advanced e-nose approach integrated with MLG electrodes has significant potential as a design concept for utilization of molecular detection at variable concentrations such as in early-stage disease diagnosis. This early-stage approach will provide convenient and reusable complex monitoring of CMs compared to typical contact sensors which require target analysis and are limited by disposable measuring. Moreover, further integration of the Internet of Things will introduce advanced e-nose devices as a biotechnological innovation for disease resilience with the potential for commercialization.