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SMP Publications

2021

Laser Speckle Reduction using a Liquid Crystal Diffuser Enhanced with Redox Dopants  

Yihan Jin, David J. Hansford, Steve J. Elston, Stephen M. Morris Adv. Photon. Res. (2021)

In this work, a large reduction in the speckle noise is observed using a thin electro‐responsive film consisting of a chiral nematic liquid crystal (LC) that has been enhanced with the addition of a redox dopant. Two different redox dopants are investigated over a range of concentrations; one being an electron acceptor and the other being an electron donor redox dopant. Results are presented that show that the incorporation of either of these dopants leads to a greater reduction in the speckle contrast than that observed using just the chiral nematic LC host when subjected to electrohydrodynamic instabilities. Furthermore, it is found that the permanent electrochemical reactions typically observed when ionic dopants, such as CTAB, are used are not observed for these devices resulting in a considerable improvement in terms of the operating lifetime of the speckle reducer technology. To conclude we present results that show that the speckle contrast can be reduced to C = 0.11 ± 0.02 at a temperature of 30ºC and demonstrate the improvement of the quality of an image generated using a modified commercial projector fitted with a monochromatic laser source.

Enhancing laser speckle reduction by decreasing the pitch of a chiral nematic liquid crystal diffuser

David J. Hansford, Yihan Jin, Steve J. Elston & Stephen M. Morris Scientific Reports 11, 4818 (2021)

The artefact known as speckle can plague numerous imaging applications where the narrow linewidth of laser light is required, which includes laser projection and medical imaging. Here, we report on the use of thin-flm chiral nematic liquid crystal (LC) devices that can be used to mitigate the infuence of speckle when subjected to an applied electric feld. Results are presented which show that the speckle contrast (a quantitative measure of the presence of speckle) can be signifcantly reduced by decreasing the pitch of the chiral nematic LC from 2700 to 244 nm. Further reduction in the speckle contrast can be observed by operating the difuser technology at a temperature close to the chiral nematic to isotropic transition. At such temperatures, we observe a simultaneous improvement in the transmission of light through the device and a decrease in the electric feld amplitude required for the minimum speckle contrast value. We conclude by presenting a laser projected image of the 1951 USAF target with and without the LC device to demonstrate the visual improvement as a result of the speckle reduction.

Room Temperature Wafer-Scale Synthesis of Highly Transparent, Conductive CuS Nanosheet Films via a Simple Sulfur Adsorption-Corrosion Method

John Hong, Byung-Sung Kim, Bo Hou, Sangyeon Pak, Taehun Kim, A-Rang Jang, Yuljae Cho, Sanghyo Lee, Geon-Hyoung An, Jae Eun Jang, Stephen M. Morris, Jung Inn Sohn, and SeungNam Cha ACS Appl. Mater. Interfaces (2021) 

The development of highly conductive electrodes with robust mechanical durability and clear transmittance in the visible to IR spectral range is of great importance for future wearable/flexible electronic applications. In particular, low resistivity, robust flexibility, and wide spectral transparency have a significant impact on optoelectronic performance. Herein, we introduce a new class of covellite copper monosulfide (CuS) nanosheet films as a promising candidate for soft transparent conductive electrodes (TCEs). An atmospheric sulfur adsorption-corrosion phenomenon represents a key approach in our work for the achievement of wafer-scale CuS nanosheet films through systematic control of the neat Cu layer thickness ranging from 2 to 10 nm multilayers at room temperature. These nanosheet films provide outstanding conductivity (∼25 Ω sq–1) and high transparency (> 80%) in the visible to infrared region as well as distinct flexibility and long stability under air exposure, yielding a high figure-of-merit (∼60) that is comparable to that of conventional rigid metal oxide material-based TCEs. Our unique room temperature synthesis process delivers high quality CuS nanosheets on any arbitrary substrates in a short time (< 1 min) scale, thus guaranteeing the widespread use of highly producible and scalable device fabrication.

2020

Two‐Photon Laser‐Written Photoalignment Layers for Patterning Liquid Crystalline Conjugated Polymer Orientation

Yuping Shi, Patrick S. Salter, Mo Li, Robert A. Taylor, Steve J. Elston, Stephen M. Morris, Donal D. C. Bradley Advanced Functional Materials (2020)

Systematic tuning of chemical and physical structure allows fine control over desired electronic and optical properties, including those of conjugated polymer semiconductors. In the case of physical structure, orientation via liquid crystalline alignment allows access to fundamental optical anisotropies and the associated refractive index modification offers great potential for fabrication of photonic structures. In this paper, photoalignment is used to orient the liquid crystalline conjugated polymer poly(9,9‐dioctylfluorene‐co‐benzothiadiazole) (F8BT), specifically involving two‐photon infrared laser writing of patterns in an azobenzene sulphonic dye (SD1). These patterns are transferred into the overlying film by thermotropic orientation in the nematic melt, then frozen in place by quenching to a room temperature nematic glass. Optimization of laser power and scan speed allows features with linewidths ≤ 1 µm. Photoluminescence (PL) peak anisotropy values reach PLII/PL⊥ = 13 for laser writing, compared with PLII/PL⊥ = 9 for polarized ultraviolet light emitting diode exposure of the same SD1 layer. These two approaches also result in different film microstructures; evidenced by characteristic changes in PL spectra. The anisotropic PL spectra provide information on emissive excited states that complements previous studies on non‐oriented F8BT and related copolymers, also suggesting two emissive states.

A Compact Full 2π Flexoelectro-Optic Liquid Crystal Phase Modulator

Xiuze Wang, Julian A. J. Fells, Yuping Shi, Taimoor Ali, Chris Welch, Georg H. Mehl, Timothy D. Wilkinson, Martin J. Booth, Stephen M. Morris, and Steve J. Elston Adv. Mater. Technol. 2000589 (2020)

Wavefront shaping, which is often achieved using liquid crystal (LC) spatial light modulators, is particularly important for a wide range of applications including laser microfabrication and micromanipulation, microscopy, and quantum optics. In this work, results are presented for the first integrated LC phase modulator that combines a flexoelectro-optic LC layer (that behaves as a switchable λ/2 waveplate) with a polymerized reactive mesogen layer (which acts as a λ/4 waveplate) and a mirrored substrate that creates a double-pass geometry. For a flexoelectro-optic LC layer that exhibits switching angles of ±45° at a voltage of ±85 V a full 2π phase modulation is observed when driven by a 1 kHz waveform. Experimental results are also compared with modeling using Jones calculus of the amplitude and phase variation when the LC and the polymer layer deviate from their desired waveplate conditions. The development and demonstration of an integrated device is particularly significant for applications where size and cost are critical factors such as in LiDAR for the Space and Automotive industries, respectively.

Millisecond optical phase modulation using multipass configurations with liquid crystal devices

Yihan Jin, Steve J. Elston, Julian A. J. Fells, Martin J. Booth, Chris Welch, Georg H. Mehl, and Stephen M. Morris Physical Review Applied 14, 024007 (2020)

We present two configurations for analog 0 to 2π optical phase modulation using liquid crystals (LCs), each of which achieve switching times that are 1 ms or less. One configuration is based on the switching behavior of a so-called nematic pi cell, and the other is based on the flexoelectro-optic effect in chiral nematic LCs when operated in the uniform lying helix geometry. Both configurations exploit a multipass optical arrangement to enhance the available optical phase range, while maintaining a fast switching speed. Moreover, these devices can be operated at or close to room temperature. Experimental data are found to be in good agreement with results predicted from theory for these multipass phase-modulation configurations.

Electrically Tunable Printed Bifocal Liquid Crystal Microlens Arrays

Waqas Kamal, Jia‐De Lin, Steve J. Elston, Taimoor Ali, Alfonso A. Castrejón‐Pita, and Stephen M. Morris Advanced Materials Interfaces 7, 2000578 (2020)

In this communication, the fabrication of electrically tunable bifocal liquid crystal (LC) microlenses using drop‐on‐demand inkjet printing is demonstrated. By treating the glass substrate with a homeotropic alignment layer, the printed droplets are found to form plano‐convex lenses with focal lengths in the range of 220–463 µm, depending upon the number of droplets deposited at each location on the substrate. The precision of the process allows for the microlenses to be deposited in between in‐plane indium tin oxide electrodes. In the presence of a high amplitude electric field, the director within the LC droplets is observed to align with the direction of the applied field, but without any accompanying distortion in the droplet profile. However, these changes in the LC director alignment are found to result in a bifocal behavior rather than a continuous change in the focal length. It is also found that there exists a range of voltages for which two focal planes are observed.

Transmissive flexoelectro-optic liquid crystal optical phase modulator with 2π modulation

Xiuze Wang, Julian A. J. Fells, Taimoor Ali, Jia-De Lin,  Chris Welch, Georg H. Mehl, Timothy D. Wilkinson, Martin J. Booth, Stephen M. Morris, and Steve J. Elston AIP Advances 10, 055011 (2020)
 
In this paper, we demonstrate analog phase modulation in a transmissive configuration using the flexoelectro-optic effect in short-pitch chiral nematic liquid crystal (LC) devices. Two different modes are considered, both of which are shown to generate full 2π phase modulation at 1 kHz switching frequency. The first configuration that is considered consists of a half-wave plate that is placed between two flexoelectro-optic LC devices that are subjected to electric fields that are applied in phase. Second, we demonstrate that a similar phase modulation response can be observed by removing the half-wave plate and subjecting the two flexoelectro-optic LC devices to electric fields whereby the polarities are out of phase. Both configurations demonstrated herein are promising for the development of next-generation LC spatial light modulators, particularly when reflective geometries are challenging or impractical.
 
Electrically-tunable positioning of topological defects in liquid crystals

John J. Sandford O’Neill, Patrick S. Salter, Martin J. Booth, Steve J. Elston, Stephen M. Morris Nature Communications 11, 2203 (2020) 
Topological defects are a consequence of broken symmetry in ordered systems and are important for understanding a wide variety of phenomena in physics. In liquid crystals (LCs), defects exist as points of discontinuous order in the vector field that describes the average orientation of the molecules in space and are crucial for explaining the fundamental behaviour and properties of these mesophases. Recently, LC defects have also been explored from the perspective of technological applications including self-assembly of nanomaterials, optical-vortex generation and in tunable plasmonic metamaterials. Here, we demonstrate the fabrication and stabilisation of electrically-tunable defects in an LC device using two-photon polymerisation and explore the dynamic behaviour of defects when confined by polymer structures laser-written in topologically discontinuous states. We anticipate that our defect fabrication technique will enable the realisation of tunable, 3D, reconfigurable LC templates towards nanoparticle self-assembly, tunable metamaterials and next-generation spatial light modulators for light-shaping. 

A Thin‐Film Flexible Defect‐Mode Laser

Taimoor Ali, Jia‐De Lin, Benjamin Snow, Xiuze Wang, Steve J. Elston, Stephen M. Morris Advanced Optical Materials 8, 1901891 (2020) 

Laser emission from a flexible defect‐mode structure consisting of two photopolymerized liquid crystal thin films separated by a dye‐doped polymethylmethacrylate defect layer is demonstrated. A simple and cost‐effective film transfer technique is used to fabricate the flexible laser and the corresponding laser emission characteristics, which shows single‐mode laser emission at λ = 582 nm, with an excitation threshold of Eth = 12.3 ± 0.5 µJ cm−2 per pulse and a slope efficiency of ηs = 6.0 ± 0.3%, are presented. The polarization state of the laser emission are also presented and are compared with the findings reported in the literature. Finally, laser‐beam steering is demonstrated up to 42° by subjecting the device to a mechanically induced deformation that creates a radius of curvature of 5 mm, which is of potential interest for conformable and wearable technology platforms.

Active Metamaterials with Negative Static Electric Susceptibility

Flynn Castles, Julian A. J. Fells, Dmitry Isakov, Stephen M. Morris, Andrew A. R. Watt, Patrick S. Grant Advanced Materials (2020)

Although well‐established textbook arguments suggest that static electric susceptibility χ(0) must be positive in “all bodies,” it has been pointed out that materials that are not in thermodynamic equilibrium are not necessarily subject to this restriction. Media with inverted populations of atomic and molecular energy levels have been predicted theoretically to exhibit a χ(0) < 0 state, however the systems envisioned require reduced temperature, reduced pressure, and an external pump laser to maintain the population inversion. Further, the existence of χ(0) < 0 has never been confirmed experimentally. Here, a completely different approach is taken to the question of χ(0) < 0 and a design concept to achieve “true” χ(0) < 0 is proposed based on active metamaterials with internal power sources. Two active metamaterial structures are fabricated that, despite still having their power sources implemented externally for reasons of practical convenience, provide evidence in support of the general concept. Effective values are readily achieved at room temperature and pressure and are tunable throughout the range of stability −1 < χ(0) < 0, resulting in experimentally‐determined magnitudes that are over one thousand times greater than those predicted previously. Since χ(0) < 0 is the missing electric analog of diamagnetism, this work opens the door to new technological capabilities such as stable electrostatic levitation.

Plasmonic Effects of Dual-Metal Nanoparticle Layers for High-Performance Quantum Dot Solar Cells

John Hong, Byung-Sung Kim, Bo Hou, Yuljae Cho, Sang Hyo Lee, Sangyeon Pak, Stephen M. Morris, Jung Inn Sohn, and SeungNam Cha Plasmonics 15, 1007–1013 (2020).

To improve quantum dot solar cell performance, it is crucial to make efficient use of the available incident sunlight to ensure that the absorption is maximized. The ability of metal nanoparticles to concentrate incident sunlight via plasmon resonance can enhance the overall absorption of photovoltaic cells due to the strong confinement that results from near-field coupling or far-field scattering plasmonic effects. Therefore, to simultaneously and synergistically utilize both plasmonic effects, the placement of different plasmonic nanostructures at the appropriate locations in the device structure is also critical. Here, we introduce two different plasmonic nanoparticles, Au and Ag, to a colloidal PbS quantum dot heterojunction at the top and bottom interface of the electrodes for further improvement of the absorption in the visible and near-infrared spectral regions. The Ag nanoparticles exhibit strong scattering whereas the Au nanoparticles exhibit an intense optical effect in the wavelength region where the absorption of light of the PbS quantum dot is strongest. It is found that these dual-plasmon layers provide significantly improved short-circuit current and power conversion efficiency without any form of trade-off in terms of the fill factor and open-circuit voltage, which may result from the indirect contact between the plasmonic nanoparticles and colloidal quantum dot films.

 

2019

Direct Epitaxial Synthesis of Selective Two-Dimensional Lateral Heterostructures

Juwon Lee, Sangyeon Pak,Young-Woo Lee,Youngsin Park, A-Rang Jang, John Hong, Yuljae Cho, Bo Hou, Sanghyo Lee, Hu Young Jeong, Hyeon Suk Shin, Stephen M. Morris, SeungNam Cha, Jung Inn Sohn, Jong Min Kim ACS Nano 13, 13047-13055 (2019)

Two-dimensional (2D) heterostructured or alloyed monolayers composed of transition metal dichalcogenides (TMDCs) have recently emerged as promising materials with great potential for atomically thin electronic applications. However, fabrication of such artificial TMDC heterostructures with a sharp interface and a large crystal size still remains a challenge because of the difficulty in controlling various growth parameters simultaneously during the growth process. Here, a facile synthetic protocol designed for the production of the lateral TMDC heterostructured and alloyed monolayers is presented. A chemical vapor deposition approach combined with solution-processed precursor deposition makes it possible to accurately control the sequential introduction time and the supersaturation levels of the vaporized precursors and thus reliably and exclusively produces selective and heterogeneous epitaxial growth of TMDC monolayer crystals. In addition, TMDC core/shell heterostructured (MoS2/alloy, alloy/WS2) or alloyed (Mo1–xWxS2) monolayers are also easily obtained with precisely controlled growth parameters, such as sulfur introduction timing and growth temperature. These results represent a significant step toward the development of various 2D materials with interesting properties.

Electrochemically active binary anion compounds with tailored oxygen vacancy for energy storage system

John Hong, Juwon Lee, Young-Woo Lee, Woon Bae Park, Docheon Ahn, Jong Bae Park, Sangyeon Pak, Jaeyoon Baik, Stephen M. Morris, SeungNam Cha, Kee-Sun Sohn, Jung Inn Sohn, Journal of Power Sources, 444, 227301 (2019)

The search for new materials that exhibit rapid Faradaic energy-storing behavior continues to be ever more important as they offer a promising alternative to battery technology because of their unrivalled ability to deliver large amounts of power along with large amounts of energy. Here, we present a reduced binary anion compound (r-BAC) as a first demonstration of redox-active materials, which are fabricated by a facile and direct activation synthetic method. The r-BAC exhibits excellent energy storage characteristics compared to non-reduced full binary anion compound (f-BAC). Based on the density functional theory (DFT) calculations and the ex-situ chemical study, it is found that the superior electrochemical performance is strongly attributed to not only the Ni cation sites (Ni2+/Ni3+ redox couple) that are energetically more activated by oxygen vacancies, but also to the additive electrochemical activity at the unsaturated sulfur sites (S4+/S6+ redox couple) in a binary anion. Thus, we expect that this study on the binary anion material and the corresponding anion-based charge transfer mechanisms may provide a new strategy for the efficient storage of charge in next-generation energy storage applications.

Dynamic response of large tilt-angle flexoelectro-optic liquid crystal modulators

Julian A. J. Fells, Chris Welch, Wing C. Yip, Steve J. Elston, Martin J. Booth, Georg H. Mehl, Timothy D. Wilkinson, and Stephen M. Morris Opt. Express 27(11), 15184-15193 (2019) 

We present here the first time-resolved tilt-angle and retardance measurements for large-tilt (>45°) flexoelectro-optic liquid crystal modulators. These devices have potential for next generation fast switching (>1 kHz), 0-2π analog phase spatial light modulators (SLMs), with applications in optical beamsteering, microscopy and micromachining. The chiral nematic device used consisted of a mixture of CBC7CB and the chiral dopant R5011 in a nominally 5 µm-thick cell, aligned in the uniform lying helix mode. As the device is dynamically switched over angles of ± 54°, retardance changes of up to 0.17λ are observed. Furthermore, the time-resolved measurements reveal an asymmetry in the tilt in the optic-axis depending on the polarity of the applied electric field. The change in the optic-axis exhibits a pattern dependence, whereby it is determined by both the pulse history and the applied field. This pattern dependence results in tilt-angle errors of up to 8.8°, which could manifest as phase errors as large as 35.2° in potential SLMs. These time domain measurements may allow correction of these deterministic errors, to realize practical devices.

Fast and low loss flexoelectro-optic liquid crystal phase modulator with a chiral nematic reflector

Xiuze Wang, Julian A. J. Fells, Wing C. Yip, Taimoor Ali, Jia-de Lin, Chris Welch, Georg H. Mehl, Martin J. Booth, Timothy D. Wilkinson, Stephen M. Morris & Steve J. Elston Scientific Reports 9, 7016 (2019)

In this paper, we demonstrate a flexoelectro-optic liquid crystal phase-only device that uses a chiral nematic reflector to achieve full 2π phase modulation. This configuration is found to be very tolerant to imperfections in the chiral nematic reflector provided that the flexoelectro-optic LC layer fulfils the half-wave condition. Encouragingly, the modulation in the phase, which operates at kHz frame rates, is also accompanied by low amplitude modulation. The configuration demonstrated herein is particularly promising for the development of next-generation liquid crystal on silicon spatial light modulators.

Robust measurement of flexoelectro-optic switching with different surface alignments

John J. Sandford O’Neill, Julian A. J. Fells, Chris Welch, Georg Mehl, Wing C. Yip, Timothy D. Wilkinson, Martin J. Booth, Steve J. Elston, and Stephen M. Morris  Journal of Applied Physics 125, 093104 (2019)

The alignment of chiral nematic liquid crystals in the so-called uniform lying helix geometry allows for the observation and exploitation of the flexoelectro-optic effect. However, high-quality uniform lying helix alignment is difficult to achieve reliably, and this can potentially impact the accuracy of the measurements made on the flexoelectro-optic switching behaviour. Here, we show that, using an appropriate method, it is possible to make measurements of the flexo-electric coefficients that are not substantially influenced by the alignment quality.

Surface Functionalization-induced Photoresponse Characteristics of Monolayer MoS2 for Fast Flexible Photodetectors

S. Pak, A. Jang, J. Lee, J. Hong, P. Giraud, S. Lee , Y. Cho, G. Ah, Y. Lee, H. S. Shin, S. M. Morris, S. Cha, J. I. Sohn and J. M. Kim, Nanoscale 11, 4726-4734 (2019)

Monolayered, semiconducting molybdenum disulfide (MoS2) is of considerable interest for its potential applications in next-generation flexible, wearable, and transparent photodetectors because it has outstanding physical properties coupled with unique atomically thin dimensions. However, there is still a lack of understanding in terms of the underlying mechanisms responsible for the photoresponse dynamics, which makes it difficult to identify the appropriate device design strategy for achieving a fast photoresponse time in MoS2 photodetectors. In this study, we investigate the importance of surface functionalization on controlling the charge carrier densities in a MoS2 monolayer and in turn the corresponding behavior of the photoresponse in relation to the position of the Fermi-level and the energy band structure. We find that the p-doping and n-doping, which is achieved through the surface functionalization of the MoS2 monolayer, leads to devices with different photoresponse behavior. Specifically, the MoS2 devices with surface functional groups contributing to p-doping exhibited a faster response time as well as higher sensitivity compared to that observed for the MoS2 devices with surface functional groups contributing to n-doping. We attribute this difference to the degree of bending in the energy bands at the metal–semiconductor junction as a result of shifting in the Fermi-level position, which influences the optoelectronic transport properties as well as the recombination dynamics leading to a low dark and thus high detectivity and fast decay time. Based upon these findings, we have also demonstrated the broad applicability of surface functionalization by fabricating a flexible MoS2 photodetector that shows an outstanding decay time of 0.7 s, which is the fastest response time observed in flexible MoS2 detectors ever reported.

Chalcogenide Solution-Mediated Activation Protocol for Scalable and Ultrafast Synthesis of Single-Crystalline 1-D Copper Sulfide for Supercapacitor

John Hong,  Byung-Sung Kim,  Seung-Mo Yang,  A-Rang Jang,  Young-Woo Lee,  Sangyeon Pak, Sanghyo Lee ,  Yuljae Cho,  Dongwoo Kang,  Hyeon Suk Shin,  Jin Pyo Hong,  Stephen M Morris, SeungNam Cha,  Jung Inn Sohn  and  Jong Min Kim Journal of Materials Chemistry A 7, 2529-2535 (2019) 

Traditional synthetic routes for transition metal sulfides typically involve solution and thermal-based processes to exploit their favorable pseudo-capacitive properties. However, there is a practical need to develop alternative processes to fabricate metal sulfide electrodes because of the time-consuming processes (>12 h), additional heat-treatment to active reactants, relatively high post-heat-treatment temperature (200–400 °C) and non-scalable nature of existing synthetic routes. Herein, utilizing a solution-based sulfur precursor, one-dimensional single-crystalline Cu2S nanostructures have been successfully prepared via a solution-based direct synthesis process within 10 min at room temperature without the need for thermal treatment steps. The fabricated electrode exhibits a capacitance of 750 mF cm−2 at a current density of 2 mA cm−2. Moreover, the rate capacitance is maintained at about 82.3% as the current density is increased to 40 mA cm−2, and the capacity retains 90.5% of the initial value after 20 000 cycles. Importantly, as this method involves a solution-based formulation it is compatible with roll-to-roll processes, which is promising for mass and scalable production of the electrodes. The synthetic method ensures a facile and efficient approach to fabricating scalable one-dimensional single crystalline Cu2S nanostructures, highlighting the uniqueness of the solution-based sulfur activation method.

 

2018

Femtosecond fiber Bragg grating fabrication with adaptive optics aberration compensation

Julian Fells, Patrick Salter, Matthew Woolley, Stephen Morris, and Martin Booth Optics Letters 43, 5993-5996 (2018)

We present fiber Bragg gratings (FBGs) fabricated using adaptive optics aberration compensation for the first time to the best of our knowledge. The FBGs are fabricated with a femtosecond laser by the point-by-point method using an air-based objective lens, removing the requirement for immersion oil or ferrules. We demonstrate a general phase correction strategy that can be used for accurate fabrication at any point in the fiber cross-section. We also demonstrate a beam-shaping approach that nullifies the aberration when focused inside a central fiber core. Both strategies give results which are in excellent agreement with coupled-mode theory. An extremely low wavelength polarization sensitivity of 4 pm is reported.

Featured in Top Downloads in Optics Letters December 2018

Consecutive Junction-Induced Efficient Charge Separation Mechanisms for High-Performance MoS2/Quantum Dot Phototransistors

Sangyeon Pak, Yuljae Cho, John Hong, Juwon Lee, Sanghyo Lee, Bo Hou, Geon-Hyoung An, Young-Woo Lee, Jae Eun Jang, Hyunsik Im, Stephen M. Morris, Jung Inn Sohn, SeungNam Cha, and Jong Min Kim ACS Applied Materials & Interfaces  10 (44), 38264–38271 (2018)

Phototransistors that are based on a hybrid vertical heterojunction structure of two-dimensional (2D)/quantum dots (QDs) have recently attracted attention as a promising device architecture for enhancing the quantum efficiency of photodetectors. However, to optimize the device structure to allow for more efficient charge separation and transfer to the electrodes, a better understanding of the photophysical mechanisms that take place in these architectures is required. Here, we employ a novel concept involving the modulation of the built-in potential within the QD layers for creating a new hybrid MoS2/PbS QDs phototransistor with consecutive type II junctions. The effects of the built-in potential across the depletion region near the type II junction interface in the QD layers are found to improve the photoresponse as well as decrease the response times to 950 μs, which is the faster response time (by orders of magnitude) than that recorded for previously reported 2D/QD phototransistors. Also, by implementing an electric-field modulation of the MoS2 channel, our experimental results reveal that the detectivity can be as large as 1 × 1011 jones. This work demonstrates an important pathway toward designing hybrid phototransistors and mixed-dimensional van der Waals heterostructures.

Flexoelectro-optic liquid crystal analog phase-only modulator with 2π range and 1 kHz switching

Julian A.J. Fells, Xiuze Wang, Steve J. Elston, Chris Welch, Georg H. Mehl, Martin J. Booth, and Stephen M. Morris Optics Letters 43, 4362-4365 (2018)

We present a flexoelectro-optic liquid crystal (LC) analog phase modulator with >2π phase range at a 1 kHz switching frequency. The chiral nematic LC mixture consists of the bimesogen CBC7CB with chiral dopant R5011, aligned in the uniform lying helix mode. The mixture exhibits > +/-π∕4 rotation of the optic axis for a drive voltage of +/-21.5 V (E =4.5 V/μm). The rotation of the optic axis is converted into a phase modulation with the aid of a reflective device configuration incorporating a ∼5 μm LC cell, a polarizer, two quarter-wave plates, and a mirror. The residual amplitude modulation is found to be <23%. This flexoelectro-optic phase modulator combination has the potential to enable analog spatial light modulators with very fast frame rates suitable for a range of applications.

Read on Demand Images in Laser‐Written Polymerizable Liquid Crystal Devices

Chloe C. Tartan, John J. Sandford O'Neill, Patrick S. Salter, Jure Aplinc, Martin J. Booth, Miha Ravnik, Stephen M. Morris, Steve J. Elston Advanced Optical Materials 6, 1800515 (2018)

Two‐photon laser writing is a powerful technique for creating intricate, high resolution features in polymerizable materials. Here, using a single‐step process to microfabricate polymer inclusions, the ability to generate read‐on‐demand images and identification codes in a liquid crystal (LC) device is demonstrated. These micrometer‐sized polymer features are encoded directly into LC devices using direct laser writing, which locks‐in the local molecular orientation at the moment of fabrication. By reading the devices with the same voltage amplitude that is used to write the polymer structures, features can be made to disappear as the director profile becomes homogeneous with the surrounding regions, effectively cloaking the structure for both polarized and unpolarized light. It is shown how this process can be used to create micrometer‐scale reconfigurable emoticons and quick‐response codes within a fully assembled LC device, with potential use in authenticity and identification applications.

Highlighted in Nature Photonics, read here.

Characterization of large tilt-angle flexoelectro-optic switching in chiral nematic liquid crystal devices

Xiuze Wang, Julian A. J. Fells, Chris Welch, Maria-Gabriela Tamba, Georg H. Mehl, Stephen M. Morris, Steve J. Elston Liquid Crystals 46, 408-414 (2018)

The ‘flexoelastic ratio’ is commonly used to characterise the electro-optic behaviour of chiral nematic liquid crystal (LC) devices that exhibit flexoelectro-optic switching. There has been renewed interest in this electro-optic effect of late as new LC materials and mixtures have been developed that exhibit large tilt angles, Ø, of the optic axis (Ø ≥ 45°) whilst maintaining a fast response time (< 1 ms). In this study, we compare the different flexoelastic ratios that are obtained for fixed and variable-pitch chiral nematic LCs for materials with a tilt of the optic axis as large as Ø = ± 45°. We show that for large tilt angles of the optic axis the values for the two different flexoelastic ratios measurably diverge. Of the two ratios, we propose that for large tilt-angle mixtures it is more appropriate to use the flexoelastic ratio that is derived for the case when the pitch of the helix is constrained. In addition, a simple way of determining the ‘pitch-constrained’ flexoelastic ratio is presented that involves identifying the electric field amplitude at the point for which the transmission levels are the same for both positive and negative electric field polarities.

Dynamics of pitch change in chiral azobenzene-doped liquid crystals

Cheng-KaiLiua, Min-Cheng TsaiStephen M.Morris, Chian-Yu Chiua, Chii-Chang Chen, Ko-Ting Cheng Journal of Molecular Liquids, 263, 406-412 (2018)

Dynamics of pitch change in chiral azobenzene-doped liquid crystals (CAdLCs) has been investigated. Theoretically, the pitch change in CAdLCs is discontinuous. Completed behavior of the discontinuous pitch jump in a fixed thickness cell is analyzed based on the calibration term, which is a key length parameter of the gradually increasing elastic potential energy with time, according to Hooke's law. At a specific time, the energy reaches maximum value and then releases to provide a compressed/extended force to discontinuously change pitch to minimize the energy of the whole system. At other times, the compressed/extended force disturbs the uniformity of planar texture if the surface anchoring energy is weak. Discontinuous pitch jump can be observed in CAdLCs with a few turns of helix. Thus, a novel method to evaluate the pitch of CAdLCs with only two turns of the helix after UV illumination is feasible. In contrast, discontinuous pitch jump in the previous work is hard to be noticed if CAdLCs has a large number of turns of helix. The behavior of pitch change in CAdLCs on the bases of the number of turns of helix and the cell thickness will be discussed in detail.

Formation of radial aligned and uniform nematic liquid crystal droplets via drop-on-demand inkjet printing into a partially-wet polymer layer

Ellis Parry, Dongjin Kim, Alfonso Castrejon-Pita, Steve J. Elston, and Stephen M. Morris Optical Materials 80, 71–76 (2018)

This paper investigates the drop-on-demand inkjet printing of a nematic liquid crystal (LC) onto a variety of substrates. Achieving both a well-defined droplet boundary and uniformity of the LC director in printed droplets can be challenging when traditional alignment surfaces are employed. Despite the increasing popularity of inkjet printing LCs, the mechanisms that are involved during the deposition process such as drop impact, wetting and spreading have received very little attention, in the way of experiments, as viable routes for promoting alignment of the resultant LC droplets. In this work, radial alignment of the director and uniformity of the droplet boundary are achieved in combination via the use of a partially-wet polymer substrate, which makes use of the forces and flow generated during droplet impact and subsequent wetting process. Our findings could have important consequences for future LC inkjet applications, including the development of smart inks, printable sensors and lasers.

Balancing Charge Carrier Transport in a Quantum Dot P–N Junction toward Hysteresis-Free High-Performance Solar Cells

Yuljae Cho, Bo Hou, Jongchul Lim, Sanghyo Lee, Sangyeon Pak, John Hong, Paul Giraud, A-Rang Jang, Young-Woo Lee, Juwon Lee, Jae Eun Jang, Henry J. Snaith, Stephen M. Morris, Jung Inn Sohn, SeungNam Cha, and Jong Min Kim ACS Energy Lett. 3, 1036–1043 (2018)

In a quantum dot solar cell (QDSC) that has an inverted structure, the QD layers form two different junctions between the electron transport layer (ETL) and the other semiconducting QD layer. Recent work on an inverted-structure QDSC has revealed that the junction between the QD layers is the dominant junction, rather than the junction between the ETL and the QD layers, which is in contrast to the conventional wisdom. However, to date, there have been a lack of systematic studies on the role and importance of the QD heterojunction structure on the behavior of the solar cell and the resulting device performance. In this study, we have systematically controlled the structure of the QD junction to balance charge transport, which demonstrates that the position of the junction has a significant effect on the hysteresis effect, fill factor, and solar cell performance and is attributed to balanced charge transport.

Time-resolved retardance and optic-axis angle measurement system for characterization of flexoelectro-optic liquid crystal and other birefringent devices

Julian A. J. Fells, Steve J. Elston, Martin J. Booth, and Stephen M. Morris Optics Express 26(5), 6126-6142 (2018)

A new polarimeter is presented which gives time-resolved measurements of both the optic-axis angle and the linear phase retardation for modulated birefringent optical devices. It is suitable for characterizing dynamic waveplate devices based on liquid crystal and other materials. It is fully automated and requires no angular alignment of the device under test. The system has an absolute angle error of < ± 0.3° and a retardance error of < ± 0.44°, with considerably better relative accuracy. The method has been tested with a chiral nematic liquid crystal device exhibiting flexoelectro-optic switching at 3 kHz in the uniform lying helix mode. These results represent the first time-resolved tilt-angle and phase retardation measurements for a liquid crystal device operating at fast switching frequencies.

Solvent-induced self-assembly of uniform lying helix alignment of the cholesteric liquid crystal phase for the flexoelectro-optic   effect

S. Bolis, C. C. Tartan, J. Beeckman, P. Kockaert, S. J. Elston and S. M. Morris  Liquid Crystals 45, 774-782 (2018) 

A uniform lying helix (ULH) alignment of cholesteric liquid crystals (LCs) is obtained using a solvent evaporation technique. The solvent evaporation method allows for the spontaneous formation of a virtually defect-free alignment, even in the absence of an external electric field. A small amount of solvent diffuses into the LC and changes its phase into isotropic state where the individual LC molecules are more mobile. As the solvent diffuses out of the LC and consequently evaporates, additional mobility provided by the solvent allows the molecules to reach the lowest energy configuration, dictated by the boundary conditions, the solvent evaporation direction and the elastic forces among the molecules. Compared to a shear-flow-induced alignment, the solvent-induced ULH exhibits a contrast ratio between the bright and dark states that is a factor of 4 times larger, due to the low number of defects in the structure. From measurements of the flexoelectro-optic effect, the difference between the splay and bend flexoelectric coefficients, , for the nematic LC E7 is found to be in agreement with the measured values reported in the literature (12.11.0 pC/m), demonstrating that the solvent self-aligning does not change the electric response of the medium, while improving its optical properties.

Field effect transistors and phototransistors based upon p-type solution-processed PbS nanowires

Paul Giraud, Bo Hou, Sangyeon Pak, Jung Inn Sohn, Stephen Morris, SeungNam Cha and Jong Min Kim Nanotechnology 29, 075202 (2018)

We demonstrate the fabrication of solution processed highly crystalline p-type PbS nanowires via the oriented attachment of nanoparticles. The analysis of single nanowire field effect transistor (FET) devices revealed a hole conduction behaviour with average mobilities greater than 30 cm2/V/s, which is an order of magnitude higher than that reported to date for p-type PbS colloidal nanowires. We have investigated the response of the FETs to near-infrared light excitation and show herein that the nanowires exhibited gate-dependent photo-conductivities, enabling us to tune the device performances. The responsivity was found to be greater than 104 A W−1 together with a detectivity of 1013 Jones, which benefits from a photogating effect occurring at negative gate voltages. These encouraging detection parameters are accompanied by relatively short switching times of 15 ms at positive gate voltages, resulting from a combination of the standard photoconduction and the high crystallinity of the nanowires. Collectively, these results indicate that solution-processed PbS nanowires are promising nanomaterials for infrared photodetectors as well as p-type nanowire FETs.

 

2017

Drop-on-Demand Inkjet Printing of Thermally Tunable Liquid Crystal Microlenses

E. Parry, S. Bolis, S. J. Elston, A. Castrejon-Pita and S. M. Morris, Advanced Engineering Materials, 1700774 (2017)

In this letter, the authors demonstrate Drop‐on‐Demand printing of variable focus, polarization‐independent, liquid crystal (LC) microlenses. By carefully selecting the surface treatment applied to a glass substrate, the authors are able to deposit droplets with a well‐defined curvature and contact angle, which result in micron‐sized lenses with focal lengths on the order of 300–900 µm. Observations with an optical polarizing microscope confirm the homeotopic alignment of the LC director in the droplets, which is in accordance with the polarization independent focal length. Results show that microlenses of different focal lengths can be fabricated by depositing successive droplets onto the same location on the substrate, which can then be used to build up programmable and arbitrary arrays of microlenses of various lens sizes and focal lengths. Finally, the authors utilize the thermal dependency of the order parameter of the LC to demonstrate facile tuning of the focal length. This technique has the potential to offer a low‐cost solution to the production of variable focus, arbitrary, microlens arrays.

Optically Controllable Linear-Polarization Rotator Using Chiral-Azobenzene-Doped Liquid Crystals

C.-K. Liu, C.-Y. Chiu,S. M. Morris, M.-C. Tsai, C.-C. Chen, K.-T. Cheng Materials 10(11), 1299 (2017)

A linear-polarization rotator based on the optically tunable pitch of chiral-azobenzene-doped liquid crystals (CAdLCs) has been investigated. It is shown that the orientation of linearly polarized (LP) light can be optically tuned using CAdLCs and that the transmitted light possesses a good degree of linear polarization (DoLP). Experimental and simulation (4 × 4 Berreman matrix) results show that the rotation angle is dependent on the pitch as well as the number of turns of the cholesteric LC helix. Some causes to affect the DoLP of the output LP lights during photoisomerization are also discussed. Moreover, a calibration term, β(t), is also introduced to elucidate the behavior of the discontinuous change of the CAdLC pitch in a fixed cell thickness. 

Spatial fluctuations of optical solitons due to long-range correlated dielectric perturbations in liquid crystals

S. Bolis, S.-P. Gorza, S. J. Elston, K. Neyts, P. Kockaert, and J. Beeckman Physical Review A 96, 031803(R), (2017)

Charge Transport Modulation of a Flexible Quantum Dot Solar Cell Using a Piezoelectric Effect

Y. Cho, P. Giraud, B. Hou, Y.-W. Lee, J. Hong, S. Lee, S. Pak, J. Lee, J. E. Jang, S. M. Morris, J. I. Sohn, S. Cha, and J. M. Kim Adv. Energy Mater.,1700809 (2017)

Colloidal quantum dots are promising materials for flexible solar cells, as they have a large absorption coefficient at visible and infrared wavelengths, a band gap that can be tuned across the solar spectrum, and compatibility with solution processing. However, the performance of flexible solar cells can be degraded by the loss of charge carriers due to recombination pathways that exist at a junction interface as well as the strained interface of the semiconducting layers. The modulation of the charge carrier transport by the piezoelectric effect is an effective way of resolving and improving the inherent material and structural defects. By inserting a porous piezoelectric poly(vinylidenefluoride‐trifluoroethylene) layer so as to generate a converging electric field, it is possible to modulate the junction properties and consequently enhance the charge carrier behavior at the junction. This study shows that due to a reduction in the recombination and an improvement in the carrier extraction, a 38% increase in the current density along with a concomitant increase of 37% in the power conversion efficiency of flexible quantum dots solar cells can be achieved by modulating the junction properties using the piezoelectric effect.

Strain-Mediated Interlayer Coupling Effects on the Excitonic Behaviors in an Epitaxially Grown MoS2/WS2 van der Waals Heterobilayer

S. Pak, J. Lee, Y.-W. Lee, A-R. Jang, S. Ahn, K. Y. Ma, Y. Cho, J. Hong, S. Lee, H. Y. Jeong, H. Im, H. S. Shin, S. M. Morris, S. Cha, J. Sohn, and J. M. Kim Nano Letters, 17 (9), 5634–5640 (2017)

van der Waals heterostructures composed of two different monolayer crystals have recently attracted attention as a powerful and versatile platform for studying fundamental physics, as well as having great potential in future functional devices because of the diversity in the band alignments and the unique interlayer coupling that occurs at the heterojunction interface. However, despite these attractive features, a fundamental understanding of the underlying physics accounting for the effect of interlayer coupling on the interactions between electrons, photons, and phonons in the stacked heterobilayer is still lacking. Here, we demonstrate a detailed analysis of the strain-dependent excitonic behavior of an epitaxially grown MoS2/WS2 vertical heterostructure under uniaxial tensile and compressive strain that enables the interlayer interactions to be modulated along with the electronic band structure. We find that the strain-modulated interlayer coupling directly affects the characteristic combined vibrational and excitonic properties of each monolayer in the heterobilayer. It is further revealed that the relative photoluminescence intensity ratio of WS2 to MoS2 in our heterobilayer increases monotonically with tensile strain and decreases with compressive strain. We attribute the strain-dependent emission behavior of the heterobilayer to the modulation of the band structure for each monolayer, which is dictated by the alterations in the band gap transitions. These findings present an important pathway toward designing heterostructures and flexible devices.

Highly stable 3D porous heterostructures with hierarchically-coordinated octahedral transition metals for enhanced performance supercapacitors

J. Hong, Y.-W. Lee, D. Ahn, S. Pak, J. Lee, A-R. Jang, S. Lee, Bo Hou, Y. Cho, S. M. Morris, H. S. Shin, S. Cha, J. I. Sohn, J. M. Kim, Nano Energy, 39, 337-345 (2017)

Designing and tailoring the assembly of complex ternary transition metal oxide (TTMO) structures are a key step in the pursuit of high performance pseudo-capacitive materials for the development of next-generation energy storage devices. Here, we present uniquely assembled 3D porous heterostructures with hierarchically-coordinated TTMOs, comprising the multiply interconnected primary nanoporous frameworks of ZnCo2O4/NiMoO4 core-shell structures and the secondary protruding structures of NiMoO4 layered nanosheets. By benefiting from the combination of hierarchically cooperative two TTMOs, the developed 3D ZnCo2O4/NiMoO4 heterostructures with their stable, porous, and conductive features exhibit robust pseudo-capacitive performance with high capacitances of 6.07 F cm–2 and 1480.48 F g–1 at 2 mA cm–2 as well as an excellent cycling stability of 90.6% over 15,000 cycles. Moreover, an asymmetric supercapacitor device can deliver a high energy density of 48.6 Wh kg–1 and a power density of 2820 W kg–1. The superior pseudo-capacitive energy storage characteristics are strongly attributed to the interconnected 3D nanoporous network architectures of the TTMOs along with the secondary layered nanosheets that provide 1) the enlarged surface area with the high conductivity, 2) the facile and multi-access ion paths, and 3) the favorable structural stability. Combined, these results highlight the importance of novel nanostructure design in maximizing the pseudo-capacitive performance and provide a viable way to develop new electrode materials.

Thermodynamically Stable Synthesis of Large-Scale and Highly Crystalline Transition Metal Dichalcogenide Monolayers and their Unipolar n–n Heterojunction Devices

J. Lee, S. Pak, P. Giraud, Y-W. Lee, Y. Cho, J. Hong, A.-R. Jang, H.-S. Chung, W.-K. Hong, H.-Y. Jeong, H. S. Shin, L. G. Occhipinti, S. M. Morris, S. Cha, J. I. Sohn, and J. M. Kim Advanced Materials 29, 1702206 (2017)

Transition metal dichalcogenide (TMDC) monolayers are considered to be potential materials for atomically thin electronics due to their unique electronic and optical properties. However, large‐area and uniform growth of TMDC monolayers with large grain sizes is still a considerable challenge. This report presents a simple but effective approach for large‐scale and highly crystalline molybdenum disulfide monolayers using a solution‐processed precursor deposition. The low supersaturation level, triggered by the evaporation of an extremely thin precursor layer, reduces the nucleation density dramatically under a thermodynamically stable environment, yielding uniform and clean monolayer films and large crystal sizes up to 500 µm. As a result, the photoluminescence exhibits only a small full‐width‐half‐maximum of 48 meV, comparable to that of exfoliated and suspended monolayer crystals. It is confirmed that this growth procedure can be extended to the synthesis of other TMDC monolayers, and robust MoS2/WS2 heterojunction devices are easily prepared using this synthetic procedure due to the large‐sized crystals. The heterojunction device shows a fast response time (≈45 ms) and a significantly high photoresponsivity (≈40 AW−1) because of the built‐in potential and the majority‐carrier transport at the n–n junction. These findings indicate an efficient pathway for the fabrication of high‐performance 2D optoelectronic devices.

Hierarchically assembled tubular shell-core-shell heterostructure of hybrid transition metal chalcogenides for high-performance supercapacitors with ultrahigh cyclability

Y.-W. Lee, B.-S. Kim, J. Hong, H. Choi, H.-S. Jang, B. Hou, S. Pak, J. Lee, S.-H. Lee, S. M. Morris, D. Whang, J. P. Hong, H. S. Shin, S. Cha, J. Sohn, and J. M. Kim  Nano Energy 37, 15–23 (2017)

Pseudo-capacitive transition metal chalcogenides have recently received considerable attention as a promising class of materials for high performance supercapacitors (SCs) due to their superior intrinsic conductivity to circumvent the limitations of corresponding transition metal oxides with relatively poor conductivity. However, the important challenge associated with the utilization of such high-capacitive electrode materials is the development of desirably structured electrode materials, enabling efficient and rapid Faradaic redox reactions and ultra long-term cycling. Here, we propose a hierarchically integrated hybrid transition metal (Cu-Ni) chalcogenide shell-core-shell (HTMC-SCS) tubular heterostructure using a facile bottom-up synthetic approach. The resultant HTMC-SCS electrode exhibits a high volumetric capacitance of 25.9 F cm−3 at a current density of 2 mA cm−2. Furthermore, asymmetric SCs based on an HTMC-SCS heterostructured electrode demonstrate a high power density (770 mW cm−3) and an energy density (2.63 mW h cm−3) as well as an ultrahigh reversible capacity with a capacitance retention of 84% and a long-term cycling stability of over 10,000 cycles. Based on experimental results and density functional theory calculations, these remarkably improved electrochemical features are discussed and explained in terms of the unique combination of the conductive CuS core and active NiS shell materials, hierarchical tubular open geometry with nanoscale inner/outer shell structure, and mechanical stress-mitigating interlayer on shell-core-shell interface, allowing highly reversible and efficient electrochemical redox processes coupled with fast charge transfer kinetics and an electrochemically stable structure.

Monolayer optical memory cells based on artificial trap-mediated charge storage and release

J. Lee, S. Pak, Y.-W. Lee, Y. Cho, J. Hong, P. Giraud, H. Suk Shin, S. M. Morris, J. I. Sohn, S. Cha and J. M. Kim Nature Communications 8, 14734 (2017)

Monolayer transition metal dichalcogenides are considered to be promising candidates for flexible and transparent optoelectronics applications due to their direct bandgap and strong light-matter interactions. Although several monolayer-based photodetectors have been demonstrated, single-layered optical memory devices suitable for high-quality image sensing have received little attention. Here we report a concept for monolayer MoS2 optoelectronic memory devices using artificially-structured charge trap layers through the functionalization of the monolayer/dielectric interfaces, leading to localized electronic states that serve as a basis for electrically-induced charge trapping and optically-mediated charge release. Our devices exhibit excellent photo-responsive memory characteristics with a large linear dynamic range of ∼4,700 (73.4 dB) coupled with a low OFF-state current (<4 pA), and a long storage lifetime of over 104 s. In addition, the multi-level detection of up to 8 optical states is successfully demonstrated. These results represent a significant step toward the development of future monolayer optoelectronic memory devices.

Red green blue emissive lead sulfide quantum dots: heterogeneous synthesis and applications

B. Hou, Y. Cho, B.-S. Kim, D. Ahn, S. Lee, J. B. Park, Y.-W. Lee, J. Hong, H. Im, S. M. Morris, J. Sohn, S. Cha and J.-M. Kim Journal of Materials C 5, 3692-3698 (2017)

Visible emission colloidal quantum dots (QDs) have shown promise in optical and optoelectronic applications. These QDs are typically composed of relatively expensive elements in the form of indium, cadmium, and gallium since alternative candidate materials exhibiting similar properties are yet to be realized. Herein, for the first time, we report red green blue (RGB) photoluminescences with quantum yields of 18% from earth-abundant lead sulfide (PbS) QDs. The visible emissive property is mainly attributed to a high degree of crystallinity even for the extremely small QD sizes (1–3 nm), which is realized by employing a heterogeneous reaction methodology at high growth temperatures (>170 °C). We demonstrate that the proposed heterogeneous synthetic method can be extended to the synthesis of other metal chalcogenide QDs, such as zinc sulfide and zinc selenide, which are promising for future industrial applications. More importantly, benefiting from the enlarged band gaps, the as-prepared PbS solar cells show an impressive open circuit voltage (∼0.8 V) beyond that reported to date.

Generation of 3-dimensional polymer structures in liquid crystalline devices using direct laser writing

C. C. Tartan, P. S. Salter, T. D. Wilkinson, M. J. Booth, S. M. Morris, and S. J. Elston RSC Advances 7, 507-511 (2017)  

Direct laser writing is a powerful nonlinear fabrication technique that provides high intensities in the focal plane of a sample to engineer multidimensional structures with submicron feature sizes. Dielectrically and optically anisotropic soft matter is of particular interest when considering a host medium in which exotic topological characteristics may be generated. In this manuscript, we adopt a novel approach for direct laser writing of polymeric structures, whereby the photo-sensitive resin is liquid crystalline (LC) and aligned within electrically addressable LC devices. We show that the laser written polymer structures retain the optical properties of the liquid crystal resin at the point of laser exposure. Thus, birefringent polymer structures can be written, with the orientation of the optic axis tuned during fabrication through switching the liquid crystal with an applied electric field. This gives the potential to create complex spatial variations of the polymer refractive index on a micron scale. Furthermore, we present a range of structures for controlling the liquid crystal alignment in devices, including two-dimensional arrays of polymer pillars, a polymer checkerboard that creates a controllable disclination network, and 3-dimensional helical polymer ribbons and knots. This work introduces a new degree of freedom for the direct laser writing of advanced photonic materials as well as offering significant advances for the control of soft matter.

 

2016

Solubility-Dependent NiMoO4 Nanoarchitectures: Direct Correlation between Rationally Designed Structure and Electrochemical Pseudokinetics

John Hong, Young-Woo Lee, Bo Hou, Wonbae Ko, Juwon Lee, Sangyeon Pak, JinPyo Hong, Stephen M. Morris, SeungNam Cha, Jung Inn Sohn, and Jong Min Kim ACS Appl. Mater. Interfaces 8 (51), 35227–35234 (2016)

Tailoring the binary metal oxide along with developing new synthetic methods for controlling resultant nanostructures in a predictive way is an essential requirement for achieving the further improved electrochemical performance of pseudocapacitors. Here, through a rational design of the supersaturation-mediated driving force for hydrothermal nucleation and crystal growth, we successfully obtain one-dimensional (1-D) nickel molybdenum oxide (NiMoO4) nanostructures with controlled aspect ratios. The morphology of the 1-D NiMoO4 nanostructures can be tuned from a low to a high aspect ratio (over a range of diameter sizes from 80 to 800 nm). Such a controllable structure provides a platform for understanding the electrochemical relationships in terms of fast relaxation times and improved ion-diffusion coefficients. We show that the 1-D NiMoO4 electrode with a high aspect ratio (HAR) exhibits a much higher specific capacitance of 1335 F g–1 at a current density of 1 A g–1 compared to the other electrodes with a relatively low aspect ratio, which is due to the unique physical and chemical structure being suitable for electrochemical kinetics. We further demonstrate that an asymmetric supercapacitor consisting of the tailored HAR-NiMoO4 electrode can achieve an energy density of 40.7 Wh kg–1 and a power density of 16 kW kg–1.

Speckle contrast reduction of laser light using a chiral nematic liquid crystal diffuser

David J. Hansford, Julian A. J. Fells, Steve J. Elston, and Stephen M. Morris Applied Physics Letters 109, 261104 (2016)

High coherence in laser light causes spatially distributed interference called speckle. In applications such as holographic projection, this undesirable side effect degrades image clarity. The current methods of speckle reduction, such as a rotating ground-glass diffuser, require additional bulky moving parts. Here, we present an alternative technology based upon a compact, electrohydrodynamic chiral nematic liquid crystal device. A spatially random phase modulation of the incident light is achieved through the electrohydrodynamic instabilities that are induced by an alternating electric field. Using a chiral nematic liquid crystal device that is doped with an ionic compound, we find that the speckle contrast can be reduced by as much as 80%.

Wavelength Tuning of the Photonic Band Gap of an Achiral Nematic Liquid Crystal Filled into a Chiral Polymer Scaffold

S. M. Wood, J. A. J. Fells, S. J. Elston, and S. M. Morris Macromolecules 29, 8643-8652 (2016)

We demonstrate electric-field-induced wavelength tuning of the entire photonic band gap of an achiral nematic liquid crystal (LC) filled into a chiral polymer scaffold. This chiral polymer scaffold has been formed by creating a template of a chiral nematic LC phase, which remarkably does not compromise the optical finesse of the band gap when compared to that of a conventional, polymer-stabilized chiral nematic LC. We present results on the spectral shift and temporal evolution of the photonic band gap in the presence of an external ac electric field. It is shown that initially there is a rapid (τ ≈ 1 ms) blue-shift of the long-wavelength band-edge followed by a considerably slower blue-shift (τ ≈ 6.5 s) of the entire band gap. We compare the results with those obtained for a polymer-stabilized chiral nematic LC where only a blue-shift of the long-wavelength band-edge is observed. Consequently, we find that for the templated sample the tuning range is more than a factor of 2 greater than that observed for the polymer-stabilized chiral nematic LC for the same range of electric field amplitudes. It is also found that there is little in the way of hysteresis upon increasing and decreasing the applied electric field magnitude. Finally, we present experimental evidence that suggests that the blue-shift of the entire band gap is due to an additional tuning mechanism present only for the case of the templated samples. This is believed to be caused by a contraction of the pitch that results from a translational motion of the polymer network. The greater tuning range observed in these templated samples is potentially important for the development of tunable 1-dimensional photonic band gaps and LC lasers. Furthermore, it avoids the use of dc electric fields that can lead to long-term issues regarding stability.

Enhanced charge carrier transport properties in colloidal quantum dot solar cells via organic and inorganic hybrid surface passivation

John Hong, Bo Hou, Jongchul Lim, Sangyeon Pak, Byung-Sung Kim, Yuljae Cho, Juwon Lee, Young-Woo Lee, Paul Giraud, Sanghyo Lee, Jong Bae Park, Stephen M. Morris, Henry J. Snaith, Jung Inn Sohn, SeungNam Cha and Jong Min Kim Journal of Materials Chemistry A, 4, 18769-18775 (2016)

Colloidal quantum dots (CQDs) are extremely promising as photovoltaic materials. In particular, the tunability of their electronic band gap and cost effective synthetic procedures allow for the versatile fabrication of solar energy harvesting cells, resulting in optimal device performance. However, one of the main challenges in developing high performance quantum dot solar cells (QDSCs) is the improvement of the photo-generated charge transport and collection, which is mainly hindered by imperfect surface functionalization, such as the presence of surface electronic trap sites and the initial bulky surface ligands. Therefore, for these reasons, finding effective methods to efficiently decorate the surface of the as-prepared CQDs with new short molecular length chemical structures so as to enhance the performance of QDSCs is highly desirable. Here, we suggest employing hybrid halide ions along with the shortest heterocyclic molecule as a robust passivation structure to eliminate surface trap sites while decreasing the charge trapping dynamics and increasing the charge extraction efficiency in CQD active layers. This hybrid ligand treatment shows a better coordination with Pb atoms within the crystal, resulting in low trap sites and a near perfect removal of the pristine initial bulky ligands, thereby achieving better conductivity and film structure. Compared to halide ion-only treated cells, solar cells fabricated through this hybrid passivation method show an increase in the power conversion efficiency from 5.3% for the halide ion-treated cells to 6.8% for the hybrid-treated solar cells.            

Wavelength-tuneable laser emission from a dye-doped achiral nematic liquid crystal dispersed into a chiral polymer scaffold

Simon J. Wood, Steve J. Elston, and Stephen M. Morris Molecular Crystals and Liquid Crystals 632, 89-96 (2016)

We compare the emission characteristics of a thin-film liquid crystal (LC) laser created using a polymer-stabilized, dye-doped chiral nematic LC to that of an LC laser that was fabricated using an achiral, dye-doped nematic refilled into a chiral polymer scaffold that was templated from the same chiral nematic host. Both lasers exhibit wavelength tuning upon the application of an external electric field. However, for the templated sample, tuning is found to occur across a broader wavelength-range for the same electric field amplitude. We discuss the benefits of the templated approach and how it can be used to circumvent dye bleaching that may occur during photo-polymerisation.

Enhanced Ferroelectric Property of P(VDF-TrFE-CTFE) Film Using Room-Temperature Crystallization for High-Performance Ferroelectric Device Applications

Yuljae Cho, Docheon Ahn, Jong Bae Park, Sangyeon Pak, Sanghyo Lee, Byoung Ok Jun, John Hong, Su Yong Lee, Jae Eun Jang, Jinpyo Hong, Stephen M. Morris, Jung Inn Sohn, Seung Nam Cha, Jong Min Kim Advanced Electronic Materials 2, 1600225 (2016)

The ferroelectric β‐phase in a poly(vinylidenefluoride‐trifluoroethylene‐chlorotrifluoroethylene) film is successfully formed using a room temperature solvent‐annealing process and shows enhanced ferroelectricity compared to a thermally annealed film. Applications in electronics and energy are demonstrated with significantly enhanced device performances for a solvent annealing (SA) film employed devices compared to a thermal annealing film employed due to enhanced ferroelectricity of the SA film.

Porous nematic microfluidics for generation of umbilic defects and umbilic defect lattices

Jure Aplinc, Stephen Morris, and Miha Ravnik Physical Review Fluids 1, 023303 (2016)

We demonstrate that porous nematic microfluidics is a potential route for the generation of nematic umbilic defects and regular umbilic defect lattices. By using numerical modeling we show that the mutual (backflow) coupling between the flow velocity and the orientation director field of the nematic liquid crystal leads to the formation of positive umbilic defects at local peaks and to the formation of negative umbilic defects at the local saddles in the flow profile. The number of flow peaks and the index of the flow saddles (i.e., the number of the valleys) are shown to be directly related to the strength of the umbilic defect, effectively relating the two fields at the geometrical level. The regular arrangement of the barriers in the porous channels is demonstrated to lead to the formation of regular lattices of umbilic defects, including square, triangular, and even kagome lattices. Experimental realization of such systems is discussed, with particular focus on microfluidic-tunable birefringent photonic band structures and lattices.

Localised polymer networks in chiral nematic liquid crystals for high speed photonic switching

Chloe C. Tartan, Patrick. S Salter, Martin J. Booth, Stephen M. Morris & Steve J. Elston Journal of Applied Physics 119, 183106 (2016)

Self-assembled periodic structures based upon chiral liquid crystalline materials have significant potential in the field of photonics ranging from fast-switching optoelectronic devices to low-threshold lasers. The flexoelectro-optic effect, which is observed in chiral nematic liquid crystals (LCs) when an electric field is applied perpendicular to the helical axis, has significant potential as it exhibits analogue switching in 10–100 μs. However, the major technological barrier that prohibits the commercial realisation of this electro-optic effect is the requirement of a uniform, in-plane alignment of the helix axis between glass substrates. Here, it is shown that periodic polymer structures engineered in the nematic phase of a chiral nematic LC device using direct laser writing can result in the spontaneous formation of the necessary uniform lying helix (ULH) state. Specifically, two-photon polymerization is used in conjunction with a spatial light modulator so as to correct for aberrations introduced by the bounding glass substrates enabling the polymer structures to be fabricated directly into the device. The ULH state appears to be stable in the absence of an externally applied electric field, and the optimum contrast between the bright and dark states is obtained using polymer structures that have periodicities of the order of the device thickness.

Wavelength-tuneable laser emission from stretchable chiral nematic liquid crystal gels via in situ photopolymerization

S. M. Wood, F. Castles, S. J. Elston, and S. M. Morris, RSC Advances, 6, 31919-31924 (2016)

Stretchable liquid crystal laser gels are free-standing films with emission wavelengths that may be tuned by applying a mechanical strain. These laser gels offer potential for industrial applications such as micro-actuators and pressure sensors due to their small dimensions and suitability for solution processing. Here, we demonstrate examples of such laser gels that comprise multiple regions emitting at different wavelengths, each of which may be reversibly and selectively tuned without measurable hysteresis. These gels are created from a combination of reactive and non-reactive mesogenic molecules using an in situ photo-polymerization technique that is compatible with a variety of commercially available materials and is therefore more versatile than previous methodologies for creating elastomeric liquid crystal lasers. In this paper, we vary the concentration of reactive mesogen and study the corresponding change in the mechanochromic properties of the resulting films. By doping the gels with a fluorescent dye, laser emission that can tuned continuously (by ∼40 nm) upon mechanical extension along a direction that is perpendicular to the helicoidal axis of the chiral nematic liquid crystal phase is observed. Moreover, tuning of the wavelength is found to be reversible and does not exhibit any measurable hysteresis, thereby allowing repeatable selection of a desired laser wavelength by controlling the film elongation. By virtue of the versatility of the technique, it is possible to photo-polymerise different areas of the thin-films at different temperatures to pattern the gels in such a way that different regions of the gel emit at different laser wavelengths.

Stabilizing the uniform lying helix alignment in chiral nematic liquid crystals using direct laser writing

Chloe C. Tartan, Patrick. S Salter, Martin J. Booth, Stephen M. Morris & Steve J. Elston, Ferroelectrics, 495, 167-173 (2016)

We investigate a novel approach for controlling the alignment of a fast-switching chiral nematic liquid crystal state using a high resolution two-photon absorption laser scanning lithography technique with aberration correction that permits the engineering of photonic structures in-situ. Walls of polymer network are engineered parallel and perpendicular to the helical axis of a uniform lying helix in chiral nematic liquid crystals in order to stabilize the alignment in the absence of an electric field and enhance the flexo-electro-optic response.

Structured Organic–Inorganic Perovskite toward a Distributed Feedback Laser

M. Saliba, S. M. Wood, J. B. Patel, P. K. Nayak, J. Huang, J. A. Alexander-Webber, B. Wenger, S. D. Stranks, M. T. Horantner, J. Tse-Wei Wang, R. J. Nicholas, L. M. Herz, M. B. Johnston, S. M. Morris, H. J. Snaith, and M. K. Riede Advanced Materials, 28, 923-929 (2016)

A general strategy for the in‐plane structuring of organic–inorganic perovskite films is presented. The method is used to fabricate an industrially relevant distributed feedback (DFB) cavity, which is a critical step toward all‐electrially pumped injection laser diodes. This approach opens the prospects of perovskite materials for much improved optical control in LEDs, solar cells, and also toward applications as optical devices.

2015

Asymmetric director structures and flexoelectricity in nematic pi-cell devices

C. C. Tartan and S. J. Elston, Applied Physics Letters 107, 081902 (2015)

The sum of the flexoelectric coefficients in a liquid crystal material has been measured in nematic pi-cell devices, based on a method that exploits the asymmetry in the director configurations of the different states in a pi-cell, the uniform surface alignment polarities, and the influence of ions. A value of |e1 + e3| = 10 pC m−1 was measured from data-theory comparisons in the standard commercial eutectic E7 nematic liquid crystal mixture.

Polarized Phosphorescence of Isotropic and Metal-Based Clustomesogens Dispersed into Chiral Nematic Liquid Crystalline Films

S. M. Wood, M. Prévôt, M. Amela-Cortes, S. Cordier, S. J. Elston, Y. Molard and S. M. Morris Advanced Optical Materials, 3, 1368-1372, (2015)

Active devices combining a light source, color generation, and polarization control are of interest for applications from displays to quantum cryptography. In this paper, results on the photoluminescence properties of transition metal clustomesogens that are dispersed in chiral liquid crystal materials are presented to demonstrate their potential as next‐generation polarized light sources.

Asymmetric Director Structures and Ions in the Measurement of the Flexoelectric Sum (e1 + e3)

C. C. Tartan and S. J. Elston, Molecular Crystals and Liquid Crystals, 610, 77-88 (2015)

Hybrid Aligned Nematic (HAN) devices filled with highly ionic liquid crystal materials screen out the internal bias due to surface polarization, making it possible to determine the flexoelectric sum, e1 + e3. A transient change is observed in the device response due to differing polarities of an external offset applied to an AC signal, with a time constant similar to the characteristic ion relaxation time in the material. Assuming ions screen out the internal bias, data-theory fits in the low frequency, low voltage perturbative regime gives a value of /verte1 + e3/vert = 10 pCm−1 for the standard commercial eutectic E70 nematic liquid crystal mixture.

Enhanced Amplified Spontaneous Emission in Perovskites using a Flexible Cholesteric Liquid Crystal Reflector

S. D Stranks, S. M Wood, K. Wojciechowski, F. Deschler, M. Saliba, H. Khandelwal, J. B Patel, S. Elston, L. M. Herz, M. B Johnston, A. P. H. J. Schenning, M. G. Debije, M. Riede, S. M Morris, and H. J. Snaith, Nano Letters 15, 4935-4941 (2015)

Organic–inorganic perovskites are highly promising solar cell materials with laboratory-based power conversion efficiencies already matching those of established thin film technologies. Their exceptional photovoltaic performance is in part attributed to the presence of efficient radiative recombination pathways, thereby opening up the possibility of efficient light-emitting devices. Here, we demonstrate optically pumped amplified spontaneous emission (ASE) at 780 nm from a 50 nm-thick film of CH3NH3PbI3 perovskite that is sandwiched within a cavity composed of a thin-film (∼7 μm) cholesteric liquid crystal (CLC) reflector and a metal back-reflector. The threshold fluence for ASE in the perovskite film is reduced by at least two orders of magnitude in the presence of the CLC reflector, which results in a factor of two reduction in threshold fluence compared to previous reports. We consider this to be due to improved coupling of the oblique and out-of-plane modes that are reflected into the bulk in addition to any contributions from cavity modes. Furthermore, we also demonstrate enhanced ASE on flexible reflectors and discuss how improvements in the quality factor and reflectivity of the CLC layers could lead to single-mode lasing using CLC reflectors. Our work opens up the possibility of fabricating widely wavelength-tunable “mirror-less” single-mode lasers on flexible substrates, which could find use in applications such as flexible displays and friend or foe identification.

Improving the stability of organosiloxane smectic A liquid crystal random lasers using redox dopants

A. A. Khan, S. M. Morris, D. J. Gardiner, M. M. Qasim, T. D. Wilkinson, and H. J. Coles, Optical Materials, 42, 441-448 (2015)

In this paper, we focus on the development of liquid crystal (LC) visible-light scattering devices for random lasers. These light-scattering devices are based upon binary mixtures that consist of an organosiloxane smectic A LC and a wide temperature range nematogen LC. Both the temperature range of the smectic A phase and the dielectric anisotropy of the binary mixture are increased compared with that of the neat organosiloxane compound. In the latter case, the increase in the dielectric anisotropy results in a reduction of the magnitude of the electric field required to induce a clear state. Furthermore, it is found that the electric field threshold continues to decrease with increasing concentration of the nematic compound. For the random laser devices, the Pyrromethene 597 laser dye was added to a mixture that was optimized for scattering and it was found that the absorption properties of the dye becomes unstable in the presence of the electro-hydrodynamic instabilities that are required to generate scattering in the LC cells. This is believed to be due to electro-chemical reactions that occur at the electrodes. To avoid dye degradation and ensure repeatable electro-optic behaviour, a reduction–oxidation (redox) couple is dispersed within the dye-doped binary mixture. It is shown that the addition of redox dopants helps to stabilize the dye in the scattering mixtures, and also increases the long-term repeatability of the scattering behaviour. Finally, we conclude by characterizing the random laser emission of the dye-doped binary mixture and demonstrate improved stability.

Hybrid aligned nematic based measurement of the sum (e 1+e 3) of the flexoelectric coefficients

C. C. Tartan and S. J. Elston, Journal of Applied Physics 117, 064107 (2015)

A new method has been established for the measurement of the sum of the flexoelectric coefficients e1+e3 in liquid crystals by exploiting the properties of highly ionic materials in order to screen out the internal bias due to the different surface alignment polarities in a Hybrid Aligned Nematic (HAN) liquid crystal device. It has been shown that responses to pulses are independent of the external offset of a signal applied to a HAN device filled with a highly ionic material. Driving the device with step changes in the offset leads to either a transient increase or transient decrease in the response, depending on the polarity of the offset, while the equilibrium response remains the same. The time constant of the transient effect is consistent with the relaxation time of the ions present in the material. Assuming these ions screen out the internal bias completely, the remaining response can be used as a measure of the flexoelectric effect. Based on this approach, a value of (10 ± 2) pC m−1 was found for the modulus of the flexoelectric sum in the standard commercial eutectic E70 nematic liquid crystal mixture.

Optically Switchable Smart Windows with Integrated Photovoltaic Devices

H.-K. Kwon, K.-T. Lee, K. Hur, S. H. Moon, M. M. Qasim, T. D. Wilkinson, J.-Y. Han, H. Ko, I.-K. Han, B. Park, B. K. Min, B.-K. Ju, S. M. Morris, R. H. Friend, and D. H, Ko,Advanced Energy Materials, 5, 1401437 (2015)

An optically controlled energy‐harvesting smart window that incorporates a semitransparent solar cell with a photo­sensitive liquid crystal (LC) layer is demonstrated. The LC layer can switch between a transparent (day mode) and an opaque (night mode) state depending upon the incident solar radiation. Combined with a photovoltaic cell, this window provides a template for future smart window systems.

2014

Polarization-independent phase modulation using a blue-phase liquid crystal over silicon device

R. Hyman, A. Lorenz, S. M. Morris, and T. D. Wilkinson, Applied Optics, 53, 6925 (2014)

Liquid crystal over silicon (LCoS) spatial light modulator technology has become dominant in industries such as pico-projection, which require high-quality reflective microdisplays for intensity modulation of light. They are, however, restricted from being used in wider optical applications, such as computer-generated holography, adaptive optics, and optical correlation, due to their phase modulation ability. The main drawback of these devices is that their modulation is based on simple planar or twisted nematic liquid crystals, which are inherently slow mechanisms due to their viscoelastic properties. Their use is also limited due to fact that the phase modulation is dependent on the state of polarization of the illumination. In this paper, we demonstrate that a polymer-stabilized blue-phase liquid crystal can offer both phase modulation and high speed switching in a silicon backplane device which is independent of the input polarization state. The LCoS device shows continuous phase modulation of light with a submillisecond switching time and insensitivity to the input light polarization direction. This type of phase modulation opens up a whole new class of applications for LCoS technology.

Stretchable liquid-crystal blue-phase gels

F Castles, S. M. Morris, J. M. Hung, M. M. Qasim, A. D. Wright, S. Nosheen, S. S. Choi, B. I Outram, S. J. Elston, C. Burgess, L. Hill, T. D. Wilkinson, H. J. Coles, Nature Materials, 13, 817-821(2014)

Liquid-crystalline polymers are materials of considerable scientific interest and technological value1,2,3. An important subset of these materials exhibit rubber-like elasticity, combining the optical properties of liquid crystals with the mechanical properties of rubber. Moreover, they exhibit behaviour not seen in either type of material independently2, and many of their properties depend crucially on the particular mesophase employed. Such stretchable liquid-crystalline polymers have previously been demonstrated in the nematic, chiral-nematic, and smectic mesophases2,4. Here, we report the fabrication of a stretchable gel of blue phase I, which forms a self-assembled, three-dimensional photonic crystal that remains electro-optically switchable under a moderate applied voltage, and whose optical properties can be manipulated by an applied strain. We also find that, unlike its undistorted counterpart, a mechanically deformed blue phase exhibits a Pockels electro-optic effect, which sets out new theoretical challenges and possibilities for low-voltage electro-optic devices.

Electrical addressing of polymer-stabilized hyper-twisted chiral nematic liquid crystals with interdigitated electrodes: Experiment and model

A. Lorenz, D. J. Gardiner, S. M. Morris, F. Castles, M. M. Qasim, S. S. Choi, W. S. Kim, H. J. Coles, and T. D. Wilkinson, Applied Physics Letters 104, 071102 (2014)

Electro-optic switching in short-pitch polymer stabilized chiral nematic liquid crystals was studied and the relative contributions of flexoelectric and dielectric coupling were investigated: polymer stabilization was found to effectively suppress unwanted textural transitions of the chiral nematic liquid crystal and thereby enhance the electro-optical performance (high optical contrast for visible light, a near ideal optical hysteresis, fast electro-optic response). Test cells were studied that possessed interdigitated electrodes to electrically address the liquid crystal. Based on simulations, a well-fitted phenomenological description of the electro-optic response was derived considering both flexoelectro-optic and Kerr-effect based electro-optic response.