Nano Explorations

A student webinar series from MIT.nano

As a way to keep our nano community connected during the COVID-19 pandemic, MIT.nano launched a virtual seminar series—Nano Explorations—in April 2020. After a brief hiatus in August and September, the series is now returning with talks twice a month! Join us for presentations by MIT students on their work in nanoscience, nanotechnology, and other advanced research fields.

How to attend:
Each 45-minute seminar will consist of a 20-30 minute research talk followed by Q&A. Attendees can join and participate in the series via Zoom. Meeting ID#: 860 986 455.

Upcoming Talks

Tuesday, October 27 at 11 a.m. EDT
Strategies for high-performance solid-state photon upconversion based on triplet exciton annihilation

Ting-An Lin, PhD candidate
Electrical Engineering & Computer Science (EECS)

Photon upconversion, a non-linear optical process to convert low-energy photons into higher energies, has various applications such as photovoltaics, infrared sensing, and bio-imaging. Particularly, upconversion based on triplet exciton annihilation is one of the most promising approaches to achieve high efficiency at low excitation intensity for practical applications. However, the reported performance in solid-state is limited due to energy back transfer, material aggregation, and weak optical absorption, which complicates the integration with solid-state applications.

In this talk, Lin will discuss the research group's proposed strategies to improve the performance in solid-state. In a green-to-blue upconverter consisting of a bilayer of an absorbing and an upconverting material, they reduced energy back transfer by inserting a blocking layer in between and mitigate aggregation by doping the absorber into a host material. The upconversion efficiency had a 7-fold enhancement with the excitation intensity reduced by 9 times. To improve optical absorption, they investigated an infrared-to-visible upconverter and integrate the up-converting layers into a Fabry-Pérot microcavity. At the resonant wavelength, absorption increases 74-fold and the threshold excitation intensity is reduced by two orders of magnitude to a sub-solar flux. Their work demonstrates the importance of device structure engineering to improve the performance of solid-state photon upconversion, and offers a path toward practical applications.


Tuesday, November 10 at 11 a.m. EST
Nano-scale mechanical switches with squeezable molecular springs—Squitches

Jinchi Han, PhD candidate
Electrical Engineering & Computer Science (EECS)

Nanoelectromechanical (NEM) switches are a candidate technology for beyond-CMOS energy-efficient computing. They can exhibit near-zero static leakage, large on-off current ratio, steep subthreshold slope, and high robustness in harsh environments. NEMs are, however, challenged by significant van der Waals interaction at the nanoscale between their contacting electrodes, which can result in compromised performance in terms of turn-on voltage and switching speed, critical characteristics for good device reliability.

A way to address the NEM electrode stiction challenge will be presented in this talk, which will explore an approach of fabricating an electrostatically-controlled nanogap using self-assembled molecular spacer layer sandwiched between atomically-smooth conductive nanostructures. The molecular layer acts like a spring between the two sandwiching electrodes, compressing as needed under the electrostatically-applied “squeeze” to modify the tunneling current. Hence, we referred to this NEM structure as the squeezable-switch or the “squitch”. The operating squitch structures show a sharp electrical switching behavior with several-orders-of-magnitude on-off current ratio, as the tunneling gap is modified by only ~1 nm in distance. This unique working principle allows squitches to simultaneously achieve low turn-on voltages and low time delays, while surmounting the challenge of NEM device electrode stiction.

Past Talks

Thursday, July 30
Integrating object form and electronic function in rapid prototyping and personal fabrication

Junyi Zhu, PhD candidate
Electrical Engineering & Computer Science (EECS)

Rapid prototyping is a key technique that enables users to quickly realize their digital designs, therefore it has been widely used in early-stage prototyping and small-scale customized fabrication. A long-term vision in Human-Computer Interaction is to create interactive objects for which all functions are directly integrated with the form and fabricated in one-go. So far, rapid prototyping has mainly focused on fabricating passive objects for which the form of an object can be freely designed, but recently we have also moved towards digital specification and fabrication of object functions for interactive design. These advances offer the promise that eventually in rapid function prototyping, the interactive object form and function would be under the same design consideration, therefore the object form could follow its designated function, and function adapt upon its physical form, and vice versa.

In this talk, Zhu presents two projects in this domain: MorphSensor and CurveBoards. MorphSensor is a 3D electronics design tool for designing electronic function in the context of a prototype’s three-dimensional shape. MorphSensor unifies electronic and physical object design in one 3D workspace as one complete workflow, which leads to better form and function integration. CurveBoards are 3D breadboards directly integrated into the surface of physical prototypes. CurveBoards better preserve the object’s look and feel while maintaining high circuit fluidity, which enables designers to prototype and iterate function in the context of form.


Tuesday, July 28
Solid-state platform for Boston quantum network

Michael Walsh, PhD
Electrical Engineering & Computer Science (EECS)

Quantum emitters, such as color centers (e.g., nitrogen-vacancy color centers in diamond), have a wide range of applications in quantum information processing, bio-imaging, and quantum-sensing. Such quantum emitters are typically addressed optically and store their quantum state as an electron spin that can subsequently be read out optically. For this process to work effectively, an efficient light-matter interaction must be achieved, which is difficult given the small interaction cross-section of an atomic memory with the optical field.

In this talk, Walsh addresses two problems that relate to the engineering of a device that demonstrates a quantum advantage. The first problem centers on the fact that most quantum emitters are randomly positioned throughout their host lattice making it difficult to lithographically pattern structures intended to increase the light-matter interaction. While there is a non-zero chance that a small number of randomly aligned structures will coincide with randomly positioned emitters, when efforts to scale such a system are made the yield drops exponentially. The second problem has to do with scaling. As systems scale up to larger sets of interacting qubits, it becomes increasingly necessary to produce quantum emitters with narrow optical transitions and long spin coherence times.


Thursday, July 23
Fast and energy-efficient monocular depth estimation on embedded systems

Diana Wofk, MEng '20
Electrical Engineering & Computer Science (EECS)

Depth sensing is a critical function for many robotic tasks such as localization, mapping and obstacle detection. There has been a significant and growing interest in performing depth estimation from monocular RGB camera images, due to the relatively low cost, size, weight and power of cameras. However, state-of-the-art depth estimation algorithms are based on fairly large deep neural networks, which have high computational complexity and energy consumption. This poses a significant challenge when performing real-time depth estimation on an embedded system, for instance, a mobile phone or a platform mounted on a Micro Aerial Vehicle (MAV).

Our work addresses this problem of fast and energy-efficient depth estimation on embedded platforms. Our proposed network, FastDepth, runs at 178 fps on the Jetson TX2 embedded GPU, with active power consumption of 8.8 W. We seek to further improve energy efficiency by deploying onto a low-power embedded FPGA. Using an algorithm-hardware co-design approach, we develop a dataflow design and an accelerator architecture that minimizes off-chip memory accesses and offers dedicated support for depthwise separable convolutional layers. This talk will give an overview of our approach and the strategies we take in accelerating learning-based depth estimation on embedded systems.


Tuesday, July 21
2D-Material enabled colloidal electronics

Albert Liu, PhD candidate
Chemical Engineering

Graphene and other 2D materials possess desirable mechanical and functional properties for incorporation into or onto novel colloidal particles, potentially granting them unique electronic and optical functions. However, this application has not yet been realized because conventional top-down lithography scales poorly for the production of colloidal solutions.

Liu describes an “autoperforation” technique providing a means of spontaneous assembly for colloidal microparticles comprised of 2D molecular surfaces at scale. Such particles demonstrate remarkable chemical, mechanical and thermal stability. They can function as aerosolizable memristor arrays capable of storing digital information, as well as dispersible and recoverable probes for large-scale collection of chemical information in water and soil.


Thursday, July 16
Building neuromorphic computing units with battery materials

Juan Carlos Gonzalez Rosillo, Postdoctoral Associate
Materials Science and Engineering (DMSE)

Specialized hardware for neural networks requires materials with tunable symmetry, retention and speed at low power consumption. Advances over the last years on understanding and implementing memristor technology had positioned them as a major candidate to overcome bottlenecks in current electronic-based transistors in terms of downscaling capabilities and energy consumption. The vast majority of memristive devices are based on two types of ions: either oxygen vacancy migration, in the so-called Valence Change Memories (VCM), or a metal cation, usually Ag+ and Cu2+, in the so-called Electrochemical Metallization Cells (ECM). Despite their excellent performance, their widespread implementation in today’s integrated circuits is delayed due to the need to address cycle-to-cycle and device-to-device variabilities while circumventing electroforming and asymmetry, which are inherent issues associated to the filamentary nature of the switching mechanism.

Recently, Li-ion is emerging as an alternative, given the higher diffusivity of Li+ when compared to oxygen, and the ability of Li-oxides solid state conductors to accumulate and deplete lithium at the interfaces and bulk. We have proposed lithium titanates, originally developed as Li-ion battery anode materials, as promising candidates for memristive-based neuromorphic computing hardware.

In this seminar, Gonzalez Rosillo discusses the non-volatile, non-filamentary bipolar resistive switching characteristics of lithium titanates compounds, Li4+3xTi5O12, as a function of the lithiation degree. We have employed a recently proposed strategy to overcome lithium loss during thin film deposition and finely control the final lithiation degree of the films to create stoichiometrically lithiated Li4Ti5O12 spinel phase and a highly lithiated Li7Ti5O12 rock- salt phase memristive devices. By using ex- and in-operando spectroscopy to monitor the Lithium filling and emptying of structural positions during electrochemical measurements, we investigate the controlled formation of a metallic phase (Li7Ti5O12) percolating through an insulating medium (Li4Ti5O12) with no volume changes under voltage bias, thereby controlling the spatially averaged conductivity of the film device.

We present a theoretical model to explain the observed hysteretic switching behavior based on electrochemical nonequilibrium thermodynamics, in which the metal-insulator transition results from electrically driven phase separation of Li4Ti5O12 and Li7Ti5O12. Permittivity enhancement drives lithium ions to regions of high electric field intensity, which become metallic filaments above a critical applied bias, and the ions relax back to their low-conductivity initial state at lower voltages. One of the most striking outcomes is that the metal-insulator transition of llithium titanate can be uniquely modulated for neuromorphic computing purposes, such as control of the neural pulse train symmetry in conductance and the resistance on-to-off ratio, simply by adjusting the lithium stoichiometry and phase pattern of the films. We report ability of highly lithiated phase of Li7Ti5O12 for Deep Neural Network applications, given the large retentions and symmetry, and opportunity for the low lithiated phase of Li4Ti5O12 towards Spiking Neural Network applications, due to the shorter retention and large resistance changes. Our findings pave the way for lithium oxides to enable thin-film memristive devices with adjustable symmetry and retention.


Tuesday, July 14
New frontiers in THz quantum cascade lasers

Ali Khalatpour, PhD
Electrical Engineering & Computer Science

Terahertz (THz) frequencies remain among the least utilized in the electromagnetic spectrum, largely due to the lack of powerful and compact sources. The invention of THz quantum cascade lasers (QCLs) was a major breakthrough to bridge the so-called “THz gap” between semiconductor electronic and photonic sources. However, their demanding cooling requirement has confined the technology in a laboratory environment. A portable and high-power THz laser system will have a qualitative impact on applications in medical imaging, communications, quality control, security, and biochemistry.

Here, by adopting a novel design strategy to achieve a clean 3-level system, we have developed THz QCLs (at ~4 THz) with a maximum operating temperature of 250 K, far exceeding the existing records. The new record is the major breakthrough in the THz QCL field since its invention in 2001. The high operating temperature enables portable THz systems to perform real-time imaging with a room-temperature THz camera, as well as fast spectral measurements with a room-temperature detector.


Thursday, July 9
Liquid-crystal-based integrated optical phased arrays for augmented reality

Milica Notaros, PhD candidate
Electrical Engineering & Computer Science

Augmented reality (AR) head-mounted displays that project information directly in the user’s field of view have many wide-reaching applications in defense, medicine, engineering, gaming, etc. However, current commercial head-mounted displays are bulky, heavy, and indiscreet. Moreover, these current displays are not capable of producing holographic images with full depth cues; this lack of depth information results in users experiencing eyestrain and headaches that limit long-term and widespread use of these displays (an effect known as the vergence-accommodation conflict).

In this talk, recent advances in the development of Visible Integrated Photonics Enhanced Reality (VIPER), a novel integrated-photonics-based holographic display, are reviewed. The VIPER display consists of a single transparent chip with integrated liquid crystal that sits directly in front of the user’s eye and projects visible-light 3D holograms that only the user can see. It presents a highly-discreet and fully-holographic solution for the next generation of AR displays.


Tuesday, July 7
Solid-state spin-integrated circuits for quantum sensing and control

Christopher Foy, PhD
Electrical Engineering & Computer Science

Spin systems are an increasingly important quantum-sensing platform. In particular, atomic defect centers in diamond called nitrogen-vacancy (NV) centers offer impressive room temperature imaging capabilities for both magnetic fields and temperature. NV-based sensing platforms have found utility in solid-state physics, biological systems, and vector magnetometry. These applications highlight the immense promise of NV quantum sensors. Despite this promise, the use of NV centers within commercial devices remains limited to date, with many impediments to transitioning this platform from the laboratory.

This talk describes the development of solid-state spin-integrated circuits (S3IC) for quantum sensing and control with the overarching goal of creating scalable NV platforms. We present two major experiments that develop S3IC. These expand the application space of NV centers and improve device functionality. The first application was to develop an NV spin microscope capable of wide-field temperature and magnetic field imaging to elucidate functional device behavior at the microscopic scale. The second experiment was integrating the essential components of an NV spin microscope, spin control and detection, with integrated electronics. In this manner, S3IC combines the exceptional sensitivity of NV centers with the robustness and scalability of modern electronic chip-scale platforms.

This co-integration of spin systems into integrated electronics shows a potential path for migrating previous proof-of-principal sensing demonstrations into affordable packages that demonstrate both much greater system integration and custom electronic architectures. In short, this work demonstrates advances in NV-ensemble quantum sensing platforms and establishes a foundation for future integration efforts, perhaps inspiring innovations in both application space and the development of new quantum devices.


Thursday, July 2
Dynamically programmable surfaces for high-speed optical modulation

Cheng Peng, PhD
Electrical Engineering & Computer Science

Dynamically programmable surfaces for spatiotemporal control of light are crucial to many optoelectronic technologies including high-speed optical communication, display and projection, autonomous driving, optical information processing, imaging, and optical control in quantum computation. Currently available electro-optic spatial light modulators (SLMs) are often bulky, inefficient, and have limited operation speeds.

This talk describes the development of a compact, high-speed, electro-optic SLM architecture based on a two-dimensional array of tunable microcavities. Optimized microcavity designs can enable high-speed, high diffraction efficiency SLMs with standard-CMOS-compatible driving voltages. High-speed electro-optic material options are also discussed.


Tuesday, June 30
Programming a quantum computer with quantum instructions

Morten Kjaergaard, Postdoctoral Associate
Electrical Engineering & Computer Science

The use of quantum bits to construct quantum computers opens the door to dramatic computational speedups for certain problems. The maturity of modern quantum computers has moved the field from being predominantly a quantum device-focused research area to also include practical quantum-computing application focused research.

In this talk, Kjaergaard discusses a new experimental result on a foundational aspect of how to program quantum computers. A central principle of classical computer programming is the equivalence between data and instructions about what to do with that data. In quantum computers this equivalence is broken: Classical hardware is used to generate the sequence of operations to be executed on the quantum data stored in the quantum computer. Our experiment shows for the first time how the instruction-data symmetry can be restored to quantum computers. We use superconducting qubits as a platform to implement high-fidelity quantum operations enabling the so-called Density Matrix Exponentiation algorithm, to generate these quantum instructions. This algorithm provides large quantum speedups for a family of other quantum algorithms, which Kjaergaard briefly discusses.


Thursday, June 25
Miniaturizing power electronics through piezoelectric energy storage

Jessica Boles, PhD candidate
Electrical Engineering & Computer Science

Power electronics play a vital role in the technological advancement of transportation, energy systems, manufacturing, healthcare, information technology, and many other major industries. Demand for power electronics with smaller volume, lighter weight, and lower cost often motivates designs that better utilize a converter's energy storage components (ie. magnetics). However, further progress in converter miniaturization will eventually require new energy storage technologies with fundamentally higher energy density and efficiency capabilities. This prompts investigation into piezoelectric energy storage for power conversion; piezoelectrics have comparatively superior volume scaling properties. 

This talk explores the realm of practical, low-loss dc-dc converter implementations that leverage piezoelectric resonators (PRs) as their only energy storage components for high power density. We find auspicious converter implementations through (1) identifying topologies and switching sequences that best utilize the PR and (2) constraining their operation for high-efficiency behaviors. Effective use of the PR's resonant cycle enables these implementations to achieve strong experimental performance, suggesting that these PR-based converters are promising alternatives to those based on traditional energy storage. With further development, PR-based converters may pave the way for high-performance miniaturization of power electronics.


Tuesday, June 23
Bandwidth-scalable integrated-fluxgate magnetometers for contactless current sensing

Preet Garcha, PhD
Electrical Engineering & Computer Science

Contactless current sensing has many applications, including power management, motor health monitoring, and electric vehicle battery management. Prof. Lang's group recently demonstrated the use of an array of integrated fluxgate (IFG) magnetometers to replace traditional Hall sensors with field concentrators, for a low-cost, low-area, and easy-to-install current sensing solution. However, IFG sensors burn current in a feedback loop to balance out high magnetic fields in the core for linearity, making them power hungry. Moreover, previous implementations can not be duty cycled efficiently to save power, because of the inherent non-linearity of IFG and the long convergence time needed to settle to a new value. Prior works in IFG also have limited bandwidth, which is insufficient for fault detection.

In this talk, Garcha presented a bandwidth-scalable IFG magnetic-to-digital converter for energy-efficient contactless current sensing. The system uses a mixed signal front-end design to enable efficient duty cycling by waking up from the last converged point, along with employing quick convergence techniques, leading to significant reduction in power consumption at low bandwidths of 1 kHz for power monitoring. It also employs fast read-out circuits to achieve a high sampling rate and a bandwidth > 100 kHz for motor health diagnosis. The digital integrator enables > 2 mT measured range of magnetic fields, for indirect current measurement of over +/50 A at 0.5 cm distance from the wire.


Thursday, June 18
Electronic cells: Autonomous micromachines from 2D materials

Volodymyr Koman, Postdoctoral Associate
Chemical Engineering

Electronic cells are micromachines encompassing autonomous on-board functions, such as sensing, computation, communication, locomotion, and power management. Akin to their biological counterparts, electronic cells bring specialized capabilities to previously inaccessible locations. Here, we present the design and fabrication of the first in its kind electronic cell composed of the nanoelectronic circuit on top of a SU-8 particle. Powered by a 2D material-based photodiode, the on-board circuit connects a chemiresistor element and a memristor element, enabling on-board detection and storage capabilities.

Koman demonstrates how the research group's cells sense and record information about the presence of ammonia and dispersed soot when aerosolized in the enclosed tubes, dispersed in a hydrodynamic flow of pipelines, or sprayed over large surfaces. Electronic cells may find widespread application as probes in confined environments, such as the human digestive tract, oil and gas conduits, chemical and biosynthetic reactors, and autonomous environmental sensors. Ref: Nature Nano 13, 819–827 (2018).


Tuesday, June 16
Low-frequency energy harvesting at the MEMS scale

Haluk Akay, PhD candidate
Mechanical Engineering

Vibrational energy harvesting devices seek to deliver useable electric power in remote or mobile applications by drawing energy from ambient sources of vibration. Due to the spectrum of such ambient vibrations occurring at a very low frequency (below 100Hz), major design challenges must be overcome when developing a piezoelectric energy harvesting device to function in these conditions, namely generating strain at the micro-scale and operating over a wide bandwidth of low input frequencies.

The culmination of three generations of this MEMS design effort by our research group is a bi-stable buckled beam energy harvester that relies on non-linear oscillations to translate input vibrations to axial strain of piezoelectric elements to produce electric energy and achieve state-of-the art energy harvesting operation among MEMS harvesters of 50% bandwidth below 70Hz at 0.5g. This talk will focus on the device’s design and fabrication, as well as characterization of dynamic performance to identify opportunities of continued device design optimization.


Thursday, June 4
Nanoscale membranes for electromechanical systems

Apoorva Murarka, Postdoctoral Associate
Electrical Engineering & Computer Science

Micro- and nano-electromechanical systems (MEMS/NEMS) are a technology field that branched out of semiconductor integrated circuit (IC) manufacturing about four decades ago, and one that forms the backbone of the Internet of Things era. However, as MEMS devices have become ubiquitous, they have also been limited by the narrow platform of IC material sets and design parameters, which significantly constrain prevalent MEMS functions and applications. In order to expand the application space of MEMS/NEMS, it is imperative that novel material platforms and manufacturing methods are considered.

This talk will explore an approach that simplifies fabrication of mechanically-active nanostructured elements over relatively large areas, and yields electromechanical systems with low operating voltages and high energy efficiency. Specifically, a suspended membrane of nanoscale thickness (or "nanomembrane") is first separately fabricated, and then additively donated via contact-transfer printing to complete a nanostructured variable-capacitance device. These purely metallic, suspended nanomembranes exhibit ideal spring-like behavior at human auditory frequencies, and the resulting variable-capacitance NEMS are utilized as electrostatic speakers. The NEMS speakers demonstrate superior acoustic performance in terms of acoustic pressure frequency response uniformity in both free-field and pressure-field radiation, below 10 Volts actuation.


Tuesday, June 2
Nanophotonic designs for wide field-of-view chip-scale LiDAR

Josuè Jacob Lopez, PhD candidate
Electrical Engineering & Computer Science

Optical beam steering has numerous applications including light detection and ranging (LiDAR) for autonomous navigation and free-space optical communication. Ideal solutions need to be low in size, weight, power consumption, and cost (SWaP-C) while maintaining long distance ranging, high resolution, and a large field-of-view (FOV). Although there has been significant progress and investment, there is still no long-term solution for long range LiDAR applications. 

Recently, the Soljačić Group has proposed a planar lens-based solution that overcomes challenges for on-chip optical beam steering. We discuss the recent developments of this approach including the second-generation Luneburg lens inspired design that leverages both nanophotonic design and wafer-scale fabrication. The nanophotonic lens has a proposed in-plane FOV of 160° with near diffraction-limited resolution and no off-axis aberrations. This approach opens a path towards chip-scale optical beam steering with low SWaP-C.


Thursday, May 28 at 11 a.m. EST
Perovskite quantum dots as potentially scalable quantum light emitters

Hendrik Utzat, Postdoctoral Associate

Chemically prepared colloidal semiconductor quantum dots have long been proposed as scalable and color-tunable single emitters in quantum optics, but they have typically suffered from prohibitively incoherent emission. Using advanced photon-correlation spectroscopy, Utzat will demonstrate that individual colloidal lead halide perovskite quantum dots (PQDs)—unlike any other colloidal quantum dot material—display highly efficient single-photon emission with optical coherence times as long as 80 ps, an appreciable fraction of their 210ps radiative lifetimes.

These measurements suggest that PQDs should be explored as building blocks in sources of indistinguishable single photons and entangled photon pairs for optical communication applications. Utzat will demonstrate chemical tunability of the single-emitter photo-physics of PQDs as an avenue to rationally design perovskite-based quantum emitters that will benefit from the straightforward hybrid-integration with nano-photonic components that has been demonstrated for colloidal materials.


Tuesday, May 26
Engineering myelination in vitro

Daniela Espinosa-Hoyos, PhD candidate
Chemical Engineering

Oligodendrocyte progenitor cells (OPCs) are a class of multipotent cells that, when differentiated properly, engage and enclose neuronal axons with a myelin sheath. Poor remyelination, due to hindered OPC migration, axon engagement, or differentiation, is associated with poor nervous system function in diseases such as multiple sclerosis. Understanding causes and potential treatments of disorders characterized by incomplete myelin production or myelin degeneration are particularly challenging due to a lack of preclinical, in vitro tools that replicate key aspects of the OPC- and oligodendrocyte-neuron interactions, including the physical and mechanical properties of this biological niche.

Espinosa-Hoyos discusses the development hybrid polymers that can be microfabricated using light-based additive manufacturing with enhanced biocompatibility and mechanical tunability. The research group used these polymers to fabricate three-dimensional arrays of polymeric microfibers representing key geometric, mechanical, and surface chemistry components of biological axons, which enable the study of OPC engagement and subsequent myelination in vitro. Using these artificial axons, they mimicked features of demyelinating lesions and demonstrated that murine oligodendrocyte production and wrapping of myelin-like membranes depend on physical and biochemical properties of these fibers. Furthermore, they demonstrated cell-material interactions with human oligodendrocytes that can now facilitate assays for the discovery and development of new therapeutics.


Thursday, May 21
Pushing the efficiency limit of lead halide perovskite solar cells

Jason Yoo, PhD candidate

Lead halide perovskite solar cells are an emerging technology that can be solution processed to yield low-cost, light weight, and flexible photovoltaics. Much of the early work has been focused on developing device structures and processing techniques to improve light absorption and eliminate detrimental traps within the bulk of the perovskite active layer. As a result, the device efficiency of perovskite solar cells has improved from ~3% up to ~20% in less than a decade. However, the device efficiency of perovskite solar cells still needs to be improved to compete with traditional photovoltaic technologies, such as Silicon and GaAs, and to ultimately realize the theoretically determined Shockley-Queisser efficiency limit. 

In this talk, Yoo presents work on further improving the device efficiency by developing a novel interface passivation strategy called selective precursor dissolution (SPD) strategy. The post treatment of the bulk perovskite thin film with 2D perovskites via SPD strategy prevented formation of a detrimental non-perovskite phase at the interface and resulted in much improved thin film quality with reduced detrimental interface recombination. As a result, a certified power conversion efficiency of 22.6% is achieved from a quasi steady-state measurement along with an electroluminescence efficiency up to ~9%. In addition, Yoo discusses the challenges that need to be tackled in order for perovskite solar cells to become a successful photovoltaic technology at a commercial level.


Tuesday, May 19
External field effects on defects in functional oxides: Experiments and simulations

Yen-Ting Chi, PhD candidate
Materials Science & Engineering

Functional oxides have been widely used in important applications such as solid oxide fuel cells, batteries, and memristive devices. In most cases, functional oxides operate under harsh environments including high temperature, strain, or electric field. In this talk, Chi will illustrate the effect of elastic strain on electronic defect type switching, and ultra-high electric field effect assisted defect formation using SrTiO3 as perovskite model material.

Sub-nano scale defects play important roles in determining functional oxides properties. With device dimension scaling down to tens or few nano meters, high strain can be induced or applied to alter defect properties. We assessed the effects of biaxial strain on the stability of electronic defects computationally, consistent with prior experimental observations with epitaxial thin films. We also demonstrated an experimental technique capable of applying dynamic strain and measuring the transport properties of the same functional oxide thin film at any operation conditions (temperatures, oxygen partial pressure) in-situ.

Thin film devices such as memristor operate under very high electric field. We developed a computational model that accurately reflects both macroscopic and microscopic material properties under electric field simultaneously. We identified a direct relationship between point defect polarizability and lattice constant, which can be generalized to other materials with similar crystal structure. Together, this model and identification of driving factors in defect displacement under electric field contribute generalizable approaches to study and optimization of materials that exhibit memristive switching.


Thursday, May 14
Nanoscale phenomena during evaporation of saline drops: Salt patterns & crystal ejection 

Samantha McBride, MechE PhD '19
Lecturer, Materials Science & Engineering

Evaporation of a single drop on a surface is a surprisingly complex phenomena with applications ranging across fields of self-assembly and nanotechnology. Particles or solutes within an evaporating drop will arrange into different patterns due to competing processes of evaporative flow, recirculation, and microscale energetic interactions.

In this talk, McBride presents on two novel phenomena that arise during evaporation of a saline drop as a result of nanoscale interactions. First, she shows how crystallizing salt from an evaporating film on a highly hydrophilic material leaves a patterned record of thin film instabilities that arise from microscopic forces. The crystallized material creates a number of extraordinarily ordered nano- and micro-structures including hexagonal lattices, lines, branches, and triangular sawtooth structures. This simple method can be used for inexpensive preparation of nano/micro-scale patterns and textures.

Next, McBride discusses a curious phenomenon in which salt globes grown from evaporating drops on heated superhydrophobic surfaces proceed to self-eject via growth of crystalline legs. The unusual ejecting effect is caused by the specific texture of the nano-structured superhydrophobic surface, which prevents crystal intrusion/spreading and confines evaporation to limited points at the surface. The striking effect could find application in fouling-resistant materials exposed to saline waters.


Tuesday, May 12
Hybridized magnons in van der Waals antiferromagnets and circuit quantum electrodynamics

Justin Tony Hou, PhD candidate
Electrical Engineering & Computer Science

Magnons are collective excitations of spin waves in magnetic materials. In hybridized magnon systems, magnons can be coherently converted to other excitations, such as photons and qubits, with potential applications to hybrid quantum systems and quantum information processing. 

In this talk, Justin Hou introduces two novel hybridized magnon systems: layered van der Waals antiferromagnets CrCl3 for strong magnon-magnon coupling, and on-chip superconducting resonator systems for scalable magnon-photon coupling. The results on CrCl3 established it as a convenient platform for studying antiferromagnetic dynamics in GHz and demonstrated magnon-magnon coupling within a single material. The results on superconducting resonator systems demonstrated a circuit quantum electrodynamics architecture for magnon-photon coupling, which opens up opportunities to study spintronic effects in quantum limit and applications of spintronic effects to quantum information processing.


Thursday, May 7
2D-material-enabled multifunctional mid-IR optoelectronics

Skylar Deckoff-Jones, PhD candidate
Materials Science & Engineering

Layered van der Waals (2D) materials have demonstrated huge potential for photonic devices with their varied and tunable optical properties. They can be easily integrated into planar photonic structures on virtually any substrate due to their van der Waals bonding, thereby enhancing light matter interaction. Recently, our group has developed the integration of 2D materials with chalcogenide glasses to realize high performance photonic devices with unique architectures. This platform offers a versatile method to prototype devices that can fully utilize the properties of 2D materials. In this presentation, Deckoff-Jones shows how 2D materials such as graphene, black phosphorus, and tellurene can be employed to realize high performance optoelectronics devices in the mid-infrared.


Tuesday, May 5
Development of an artificial spiking neuron using superconducting nanowires

Emily Toomey, PhD '20
Electrical Engineering & Computer Science

In light of the growing need for faster, more energy-efficient computation, researchers are rapidly developing architectures inspired by the parallelism and performance of the human brain. Spiking neural networks are perhaps the most bio-realistic approach, mimicking the unique spiking dynamics of neurons to attain superior energy efficiency with the additional benefit of temporal information.

In this talk, Toomey presents a power-efficient artificial neuron made from superconducting nanowires, which naturally generates spiking based on the nonlinear transition between the superconducting and resistive states. Simulations are used to evaluate designs of different neuron components, including a synapse that uses nanowires as a tunable inductor to allow for fan-out with adjustable connectivity. Experimental results of a soma fabricated in 25-nm-thick niobium nitride will also be presented.


Tuesday, April 28
Commercializing a new nanofiltration technology: Pressure, concentration, and rejection

Brendan Smith, Postdoctoral Associate
Materials Science & Engineering

Approximately 10% of total global energy consumption is spent on industrial separations, processes which trade energy for entropy to transform complex mixtures into their individual components. Despite filling an essential and often central role across today’s mega-industries, the majority of separations are still carried out via antiquated and inefficient methods such as thermal distillation. Membrane filtration is an attractive alternative with the potential to achieve identical outcomes while using as little as one-tenth of the energy and reducing capital and operational costs; however, its implementation has been limited by the lack of ultra-durable and sufficiently selective membrane technologies.

This talk explores the development of a new type of silicon-based filtration membrane that aims to fill this void, and shares experiences from the parallel market research journey, which has inspired the inventors to apply the technology in the industrial world through a startup venture.


Thursday, April 23
Seeing superlattices: Imaging hidden moiré periods at the nanoisland-2D material interface using 4D scanning transmission electron microscopy

Kate Reidy, PhD candidate
Materials Science & Engineering

Opportunities are emerging to combine van der Waals (2D) materials with (3D) metals/semiconductors to explore fundamental charge-transport phenomena at their interfaces, and exploit them for devices. Recent advances in scanning transmission electron microscopy (STEM) allow detailed analysis of atomic structure, properties, and ordering at these interfaces.

In this talk, Reidy describes the use of 4D STEM to directly image hidden moiré periodicities arising from epitaxial growth of nanoislands on 2D materials in ultra-high vacuum (UHV). Reidy will highlight the role of emerging microscopy techniques in unveiling the alignment and ordering of moiré superlattices, and discuss the implications of moiré periodicities on the properties of 2D/3D junctions.


Tuesday, April 21
Sensing presence in virtual reality

Yuwei Li, PhD candidate
Device realization laboratory, Mechanical Engineering

Marwa AlAlawi, Undergraduate
Mechanical Engineering

What is presence? How can we measure and enhance presence in VR? In this talk, Yuwei and Marwa introduce their research on presence-inducing VR experiences and the corresponding physiological and behavioral responses from the VR users.

This talk was originally scheduled as part of Talk SENSE, a monthly series powered by SENSE.nano that focuses on topics related to sensors, sensing systems, and sensing techniques.


Thursday, April 16
Nanoscale insights into the mechanisms of cellular growth and proliferation

Kacper Rogala, Postdoctoral Associate
Whitehead Institute for Biomedical Research

Growth and proliferation of human cells is controlled by a large molecular machine called mTORC1 that acts as a molecular equivalent of an AND logic gate. mTORC1 integrates multiple environmental signals, such as nutrients and growth factors, and orders the cell to either grow and divide in times of plenty, or stand-by and recycle when nutrients are scarce. Using electron cryomicroscopy we were able to reveal how mTORC1 recognizes nutrient signals, which provided a nanoscale-precision blueprint for the design of therapies aimed at deregulated mTORC1 in diseases of cellular growth, such as cancer.


Tuesday, April 14
Reconfigurable meta-optics with chalcogenide alloys

Mikhail Shalaginov, Postdoctoral Associate
Materials Science & Engineering

Recent progress in nanophotonics has enabled planar-interface systems, termed as metasurfaces, which hold a potential to extend the functionalities of light manipulation and provide size, weight, power, and cost (SWaP-C) benefits. Significant efforts nowadays are geared toward building active metasurfaces, whose properties can be varied post-fabrication. Numerous switchable meta-devices have been demonstrated; however, almost all of them either have a miniscule tuning range or suffer from excessive optical losses.

In this seminar, Mikhail Shalaginov shares his team’s approach to implement high-performance reconfigurable metasurfaces made of low-loss optical phase-change materials. More specifically, the team has developed a new non-volatile chalcogenide alloy GeSbSeTe exhibiting high index contrast and broadband transparency in both amorphous and crystalline states. Based on this material platform, they demonstrated a mid-infrared varifocal metalens that features diffraction-limited performance, focusing efficiencies above 20% in both states, and a record-high switching contrast ratio of 30dB. Their work demonstrates that non-mechanical active metasurfaces can achieve optical quality on par with conventional precision bulk optics involving mechanical moving parts, thereby pointing to a cohort of exciting applications fully unleashing the SWaP-C benefits of active metasurface optics in imaging, sensing, display, and optical ranging.


Thursday, April 9
Dance-inspired investigation of human movement

Praneeth Namburi, Postdoctoral Associate
Research Laboratory of Electronics (RLE), Electrical Engineering & Computer Science 

In this talk, Namburi focuses on his group's efforts to formalize a dancer’s approach to movement. Their overarching hypothesis is that dancers stabilize their joints through stretches – which is observed during common activities such as walking and running. However, most untrained individuals are only able to apply this form of stabilization during activities such as walking, that seemingly ‘just happen’, much like how we ‘see’. In contrast, the best dancers and athletes are able to generalize this stretch-based joint stabilization beyond walking to their art form.

To understand how dancers organize movement through stretches, we use motion tracking and electromyography. This talk focuses on our hypotheses, preliminary findings, and how our work can potentially benefit several fields, including soft robotics, neuroscience, and AI.

This talk was originally scheduled as part of Talk SENSE, a monthly series powered by SENSE.nano that focuses on topics related to sensors, sensing systems, and sensing techniques.


Tuesday, April 7
Magnetism in the ultrathin limit

Dahlia Klein, PhD candidate

A primary question in the emerging field of two-dimensional van der Waals magnetic materials is how exfoliating crystals to the few-layer limit influences their magnetism. Studies on CrI3 have shown a different magnetic ground state for ultrathin exfoliated films, but the origin is not yet understood. We use electron tunneling through few-layer crystals of the layered antiferromagnetic insulator CrCl3 to probe its magnetic order, finding a ten-fold enhancement in the antiferromagnetic interlayer exchange compared to bulk crystals. Moreover, polarization-dependent Raman spectroscopy reveals that exfoliated thin films of CrCl3 possess a different low temperature stacking order than bulk crystals. 

Temperature-dependent Raman spectra further attribute this difference in stacking to the absence of a stacking phase transition in these thin films, even though it is well established in bulk CrCl3. We hypothesize that this difference in stacking is the origin of the unexpected magnetic ground states in the ultrathin chromium trihalides. Our work provides new insight into the connection between stacking order and interlayer interactions in novel two-dimensional magnets, which may be relevant for correlating stacking faults and mechanical deformations with the magnetic ground states of other more exotic layered magnets.


Thursday, April 2
On-chip BioPhotonic particle and gas sensors: Ensuring a safe environment indoors and outdoors

Robin Singh, PhD candidate
Mechanical Engineering

There are a number of unintended situations for potential exposure to bioaerosols such as viruses, bacteria, and fungi. For instance, the current pandemic scenario of COVID-19 occurring across the world. It is crucial to develop ultra-sensitive, low cost, and scalable methods to sense and detect such micro/nanoparticles and gas molecules in the air.

In this talk, Singh discusses current research work on developing an on-chip photonic particle and gas sensor operating in near-IR and mid-IR ranges to perform IR spectroscopy. These sensors enable the in-situ physio-chemical characterization of aerosol particles without compromising their functionality and sensitivity.


Tuesday, March 31
Manufacturing large-area perovskite thin films: The good, the bad, and the ugly

Richard Swartwout, PhD candidate
Electrical Engineering & Computer Science

Lead halide perovskites have gained considerable interest due to desirable optoelectronic properties that make them useful for next generation photovoltaics. However, despite impressive gains in solar power conversion efficiency ( > 25%) with small scale devices there are still challenges with scaling perovskites to a competitive commercial scale.

Although easy to form, these materials are also easy to break down, requiring highly toxic solvent systems for processing. Lead salts, which give perovskites their unique defect tolerance is also highly regulated and toxic. In this talk, Swartwout will discuss the current manufacturing challenges for perovskite thin films and how we have approached solutions.