Symposium X—Frontiers of Materials Research

Symposium X_Thursday 1_800 wideAditya D. Mohite, Rice University

The Rise of 2D Halide Perovskites

Written by Rahul Rao

When you think of the materials that make solar cells, chances are that you’re thinking of silicon. But silicon has challengers: a class of materials known as halide perovskites, which have achieved gains in efficiency in the last decade that took silicon some thirty years. Still, they have some way to go before they can match silicon's impressive durability.

At Wednesday’s Symposium X, titled “The Rise of 2D Halide Perovskites,” Aditya Mohite of Rice University spoke about his group’s recent work with one avenue of trying to boost that hardiness. His approach: making perovskites that are, as the title suggests, flattened in a two-dimensional plane.

It's a subject with which Mohite is intimately familiar. In 2016, while he was at Los Alamos National Laboratory, Mohite and his colleagues showed that 2D perovskites endured better than their 3D counterparts, even under trying conditions of relatively high humidity and constant light exposure. Mohite says it was a landmark achievement that let flat perovskites shine in a then-3D-dominated field. But to build them into practical solar cells, researchers needed to zoom in on several challenges.

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One issue arises from the beginning. Making 2D halide perovskites might mean dissolving a crystalline powder in a solution, then depositing it on a surface, letting it form a film. Impurities often seep in during that process. As a result, even if you have a powder with properties you desire, they might vary in the final film, and the same impurities can drag down the solar cell’s efficiency. 

So, Mohite and his colleagues developed a new, phase-selective method of depositing perovskites. They found that this new method reduced many of the impurities, and drastically improved both the durability and efficiency of the material.

After you make the 2D material, its properties might change under light — a rather crucial consideration for a material that’s meant to be out in the Sun for the entirety of its lifetime. Mohite and his colleagues studied these effects by taking their perovskites to a synchrotron beamline. Most notably, they found that the material seemed to contract in the light. If they put the material back in the dark, the effect reversed.

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It turned out that the culprits were iodine atoms, situated in the perovskites’ structure such that they faced each other. If those atoms were placed closer, the material experienced more contraction. In fact, Mohite and his colleagues found an unexpected boon: contraction made it easier for electrical charge to traverse the material, boosting its conductivity and, again, its efficiency.

Two-dimensional perovskites don’t need to be separate from their 3D counterparts; indeed, they seem to work best together. Mohite’s group has explored taking, for instance, a sheet of glass, stacking a layer of 3D perovskite atop it, then topping it all with a 2D film. 

Through exploring such stacks, Mohite’s group has found the best stability yet. One such system could continue operating for hundreds and thousands of hours, even in the raw heat and humidity of Rice University’s Houston climate. If progress like this can continue, Mohite said, then halide perovskites — which can already compete with silicon’s efficiency — may soon approach its durability, too.

Symposium X—Frontiers of Materials Research features lectures aimed at a broad audience to provide meeting attendees with an overview of leading-edge topics.

Symposium SB03—Robotic Materials for Advanced Machine Intelligence

Binbin Ying, Massachusetts Institute of Technology

An Anti-Freezing, Ambient-Stable and Highly Stretchable Ionic Skin with Strong Surface Adhesion for Wearable Sensing and Soft Robotics

Written by Corrisa Heyes

iSkin, a hydrogel-base ionic skin, is a flexible, stretchable, tough, strain sensor able to operate at temperatures down to -95°C and in humidity conditions ranging from 15-90%. iSkin has strong surface adhesion capabilities, as demonstrated by applications on human skin, textiles, and soft robots. Examples of these applications showed wearable gait-measurement sensing, smart winter coat strain sensing, and a stretchable electronic glove for human-machine interfacing. Future work on improving fabrication methods will allow for more complex sensing capabilities and possibly the development of microsensor arrays. Ultimately, such systems might be used to develop robots with sensor capabilities approaching, or even surpassing, human touch senses.

Symposium EQ04—Advanced Soft Materials and Processing Concepts for Flexible Printed Optoelectronic Devices and Sensors

Written by Corrisa Heyes


Unyong Jeong, Pohang University of Science and Technology

Skin-Inspired Deformable Devices for Artificial Skins and Health Care

Unyong Jeong discussed advances in stretchable nanomaterials able to provide tactile functionality with pressure sensing capabilities for applications in bio-interfaces and gentle-touch robotics. These ‘synthetic skins’ can be utilized in place of conventional rigid devices, even under deformation conditions where more general materials would either fail or under-perform. Additionally, the associated groundwork for integration into more complex systems was addressed with a primary focus on providing a deeper understanding of the hardware aspects of this emerging technology. Next steps include a study of manufacturing constraints and scalability. 


Georg Gramlich, Karlsruhe Institute of Technology

Aerosol Jet Printing Process Considerations for Radio Frequency Packaging Applications

Georg Gramlich presents the challenges related to aerosol jet (AJ) printing. As an example of high precision, contactless deposition technique, AJ offers the capability to print in non-planar conditions and couples well with the emergence of microwave integrated circuits (MMICs) for packaging MMICs into radio frequency (RF) substrates. This technique provides shorter, well-matched connections when compared to traditionally used bond wires. Such processes are not without their fair share of initial roadblocks and design challenges. Methods of mitigating process development setbacks such as dry sintering, print speed, and print path irregularities were presented. Future work needs to be done to increase feature density and resolution.


 Elliot Strand, University of Colorado

Wearable Active-Matrix Pressure Sensor Arrays for Spatiotemporal Measurement of Human Vital Signs

Wearable active-matrix pressure sensor arrays address the limitations of traditional single-point sensors (e.g. pulse oximeters and blood pressure cuffs) which are large, difficult to operate correctly, and not appropriate for long-term use. The active-matrix addresses the issue of finding an arterial pulse by presenting a larger sensing area compared to a single-point sensor, low power requirements allow for the development of unwired wearable configurations, and the optimized inkjet printing design allows for ultrathin, comfortable semi-long-term wear. The current design uses an impermeable substrate that would become irritating in longer-term applications; however, future work will involve collaboration to utilize a breathable substrate currently in development.

Symposium NM03—2D MXenes—Synthesis, Properties and Applications

Written by Mohit Saraf


Ekaterina Pomerantseva, Drexel University

MXene-Derived Oxides as Electrodes for Energy Storage

The prime focus of Ekaterina Pomerantseva’s talk was on using MXenes as precursors for synthesis of oxide materials. The versatile chemistry and two-dimensional morphology of MXenes make them unique precursors for the synthesis of other classes of materials. She first talked about α-V2O5 derived δ-V2O5.H2O (BVO) material, where the synthesis takes longer, involves an aging step, and requires higher temperatures. In a recent study, her group replaced α-V2O5 with V2CTx MXene that was oxidized with the help of H2O2, and the synthesis can be completed without the aging step at lower temperatures. A flower-like morphology was observed for the prepared materials. The resultant V2CTx-derived δ-V2O5.nH2O structure was intercalated with various ions such as Li and Mg and was used for electrochemical energy storage applications. Pomerantseva also discussed recent results of MXene derived K-intercalated V2O5 (KVO) showing promising results for battery applications. Pomerantseva said that this synthesis route of using MXenes as precursors can be extended onto other MXenes to produce oxides with different chemistries or mixed oxides.


Vadym Mochalin, Missouri University of Science and Technology

Chemistry of 2D Transition Metal Carbides and Carbonitrides (MXenes)

Vadym Mochalin highlighted the importance of understanding the MXenes chemistry including their synthesis and stability. MXenes have been considered as unstable in aqueous colloids due to oxidation and therefore the challenge is improving their chemical and temperature stability. However, this oxidation process can be used to prepare composites. Mochalin gave an example of MXene-titania composites that can be produced by a simple, inexpensive, and environmentally benign process of delaminating and storing MXenes under ambient environment which leads to the partial oxidation of MXene to prepare a composite. This composite demonstrates attractive properties for photoresistors with memory effect and sensitivity to the environment, as well as many other photo- and environment-sensing applications. He also showed experiments explaining that it is the hydrolysis, not oxidation, which is primarily responsible for MXene instability. According to Mochalin, acidic environment catalyzes the hydrolysis of MXenes and basic environment inhibits MXene hydrolysis. Also, high pH suppresses MXene hydrolysis and antioxidants suppress MXene oxidation. The synergistic actions of high pH and antioxidants is most beneficial for the shelf life of aqueous MXene colloids at room temperature. He also provided some examples illustrating connections between understanding MXene chemistry and potential applications. Overall, he emphasized understanding the chemistry of MXenes.


Majid Beidaghi, Auburn University 

Assembling MXenes Heterostructures and 3D Printing of MXenes for Energy Storage 

Majid Beidaghi says that despite having unique physicochemical properties, the application of MXenes in energy storage depends on their assembly into electrode structures. His group is developing assembly of MXenes flakes into such electrode structures that exhibit high power and energy. He also discussed the effects of MXene synthesis conditions and importance of controlling flake size on the electrochemical performance. He also demonstrated several videos of three-dimensional (3D) printing of the MXene electrode where Beidaghi’s research group successfully used MXene inks for printing on different substrates. Moreover, he emphasized on developing heterostructured MXenes for superior chemical and electrochemical stability and energy storage capability.


Michael Naguib, Tulane University 

Nanoengineering MXenes Interlayer Spacing for High Performance Electrochemical Energy Storage Electrodes 

Michael Naguib first introduced the MXenes and their properties such as electrical conductivity and capability of hosting multivalent ions, which make them promising candidates for electrochemical energy storage. He talked about the structural and computational diversity and tunability of MXenes and highlighted the importance of intercalation in MXenes to escalate their properties to be used in a variety of applications. He provided several examples of intercalation of cations into the MXene structure which were successfully used in supercapacitors, Na-ion capacitors, and other applications. His group also tested MXenes in several new electrolytes such as room temperature ionic liquid (RTIL) which showed great promise in electrochemical applications. He mentioned that tuning the d-spacing of MXene structures is a viable approach to unlock new applications. The prime focus of his talk was that a small change in the structure can affect the performance significantly and therefore the intercalation could be a promising strategy to enrich the MXene family. He also mentioned briefly about the recent discovery of transition metal carbochalcogenides (TMCC).


Teng Zhang, Drexel University

Electrochemical Performance of Vanadium Containing MXenes in Aqueous Electrolytes

Teng Zhang talked about the electrochemistry of vanadium-containing MXenes in aqueous electrolytes. He emphasized that since the discovery of MXenes in 2011, the primary candidate of study has been Ti3C2, but many MXenes with different compositions and structures have been synthesized but not widely explored. According to Zhang, vanadium-containing MXenes are also fascinating candidates for redox energy storage because of the presence of vanadium transition metal exhibiting multiple oxidation states. He showed the electrochemical results on two different vanadium-containing MXenes, V2C and V4C3, which were used for supercapacitors utilizing various aqueous electrolytes such as KOH and H2SO4. He mentioned that the number of layers in MXenes greatly affects the charge storage mechanism and electrochemistry. He believes that the ongoing work will clarify the structural chemistry-electrochemistry relations in MXenes; however, a proper understanding of the charge storage mechanism in difference electrolytes is important for advancing the field of MXene electrochemistry.

Symposium SB03—Robotic Materials for Advanced Machine Intelligence

Jordan Raney, University of Pennsylvania

Electronics-Free Soft Robot with Multi-Stimuli Responsive Control

Written by Corrisa Heyes

Jordan Raney presents a nematic soft robot that can respond to environmental conditions autonomously. The robot moves mechanically and turns in response to the presence of light, heat, and/or solvents. This autonomous movement is achieved by incorporating responsive material features to the robot soft body. Furthermore, simple logic gate computation is achieved by including additional features. This is an impressive first step toward fully autonomous robotics with sensing and response capabilities without electronics. Applications for this technology encompass adverse environmental conditions where a traditional electronic robot cannot perform (e.g., electromagnetic interference) or long-term missions (e.g., send to Mars) where power generation is prohibitively expensive. 

Symposium NM01—Beyond Graphene 2D Materials—Synthesis, Properties and Device Applications

Young Hee Lee, Sungkyunkwan University, and IBS Center for Integrated Nanostructure Physics

van der Waals Layered Magnetic Semiconductors

Written by Mohit Saraf 

Young Hee Lee introduced two-dimensional (2D) van der Waals (vdW) transition metal dichalcogenides (TMDs) and explained their rich materials library and physics. He explained the properties of these 2D vdW such as strong coulombic interaction, reduced charge screening, and strong spin-orbit coupling. He also discussed about the history of diluted magnetic semiconductors. He said the key research target in this field is to realize the long-range order ferromagnetism and goals are gate modulation, long range magnetic ordering and Tc over room temperature. He discussed their chemical vapor deposition (CVD) work for V-doped WSe2 monolayer and explained some phenomenon such as V-site, their possible semiconductor nature, V-doping level, spin-selective exciton hole coupling, long-range magnetic ordering, spin modulation by gate-bias with magnetic force microscopy (MFM), defect induced magnetism, selenium (Se)-vacancies, transport with light, and trapping/de-trapping in Se vacancy states via grating. He concluded that Se vacancy itself provokes magnetic order due to strong spin-orbit coupling.

He also discussed about ferromagnetism via enriched Se-vacancy in V-WSe2, annealing temperature dependent magnetic ordering, atomic structure of annealed 0.5% V-doped WSe2, theoretical analyses, enhancement of magnetic moment via Se-vacancy, magnetic interaction of V. He discussed both theoretical and experimental work and said that V-dopant coupled with Se-vacancies escalate magnetic order and escalate Tc. The highlight of his talk was room-temperature ferromagnetism in monolayer TMDs and bulk materials.

Symposium NM06—Nanoscale Mass Transport Through 2D and 1D Nanomaterials

Mehrnaz Mojtabavi, Northeastern University 

Nanopores in Self-Assembled Monolayer-to-Multilayer MXene Films—From Fabrication to Application 

Written by Mohit Saraf 

Mehrnaz Mojtabavi highlighted the importance of nanopore technology that has gained wide attention in single-molecule studies of biomolecules. This technology provides an opportunity for sensing, manipulation, and sequencing of biopolymers such as DNA, RNA, and proteins. She discussed the importance of two-dimensional (2D) materials such as graphene, MoS2 in this emerging technology and introduced the promising candidature of MXenes in this area. She said that despite the robustness of this emerging technology, the sensing resolution is usually limited by pore thickness and access resistance. These issues can be addressed using MXenes-based technology as they are hydrophilic and possess excellent electrical conductivity. She used two MXenes, Ti3C2 and Ti2C, in her work and achieved promising results. She also introduced four MXenes-based approaches for nanopore technologies, that is, (i) single molecule sensing with MXene nanopores, (ii) water-scale MXene film fabrication and transfer, (iii) ionically active MXene nanopore actuators, and (iv) ion-fountain nanopore reader. She concluded by saying that 2D materials, particularly MXenes, are promising candidates for nanopore-based single molecule sensing, and development in this field will open new possibilities in the future.

Symposium EQ04—Advanced Soft Materials and Processing Concepts for Flexible Printed Optoelectronic Devices and Sensors

Elliot Strand, University of Colorado

Environmental Monitoring with Additively Manufactured Tattoo-Based Bioelectronics

Written by Corrisa Heyes

Traditional Internet of Things (IoT) environmental sensing devices have increased our ability to sustainably and effectively increase crop yield in an ever-shrinking agricultural environment. However, these sensors are prohibitively expensive to use at the desired scale. The answer: tattoos for plants! This work presents sub-micron thick bioelectronic sensors based on carbon capacitors printed on tattoo transfer paper in order to give our food source some stylish new art that can act to monitor environmental conditions in real-time and tap into the existing IoT networks for data reporting. Lab testing with simulated weather conditions gives the current integration of the sensor a lifespan measured in weeks, but the true test will be seeing how the new ink responds in the field. If they perform well, we could see a future where cornfields have sensing coverage measured in meters and cash crops can be monitored individually.

Symposium SF06—Recent Advances in Structural Materials from Bulk to Nanoscale

Heung Nam Han, Seoul National University

Neural Networks Approach to Correlate Plastic Properties with Indentation Data in Anisotropic Metals

Written by Corrisa Heyes

Heung Nam Han presents a combined finite element, neural network (FE-NN) method for modeling plasticity properties of anisotropic metals based on spherical and Knoop indentation. Tensile and plasticity testing is both high-cost and destructive of the sample. Conversely while instrumented indentation testing (IIT) has a reliability issue and numerical inverse analysis are generally computationally expensive, the marriage of the two has shown good agreement with uniaxial plastic curves from traditional tensile testing. This FE-NN method provides a robust modeling capability for IIT data. Knoop indentation is noted to be more effective than spherical due to the variability of measurements based on the associated indentation axis. While this work is currently focused specifically on single phase, anisotropic alloys, future work aims to expand to include a broader range of materials.