(7do) Ubiquitous Energy Harvesting through Chemically Engineered 2D Materials

The Internet of Everything (IoE) promises to build a “nervous system” for our society and fuse the digital and physical worlds, by bringing pervasive and ambient intelligence to daily objects. Especially, IoE provides the hardware foundation for smart control and green manufacturing for the next-generation “intelligent chemical plant”. In order to make this vision a reality, a flexible, efficient and always-on energy harvester that can be seamlessly integrated with arbitrarily-shaped objects is a must, but still missing. Among the various possible energy sources, Wi-Fi wireless charging seems to be an ideal choice thanks to the ever-increasing Wi-Fi coverage. However, the high frequencies used for Wi-Fi communications (2.4 GHz and 5.9 GHz) have remain elusive to all the RF harvesters (i.e. rectennas) made of flexible semiconductors. In this regard, two-dimensional (2D) materials open up intriguing possibilities owing to their unique electronic properties, excellent mechanical flexibility and robustness. Moreover, chemically engineered 2D materials provides a new degree-of-freedom to further tailor the properties of 2D materials for energy applications. During my PhD, I have successfully demonstrated a unique MoS₂ metallic-semiconducting phase-junction through a self-aligned chemical approach. Such a chemical phase-junction of MoS₂ enables RF energy harvesting up to 10 GHz: one order of magnitude improvement in speed compared to the state-of-the-art flexible rectifiers. It is the first flexible rectifier operating up to the X-band and it fully covers the Wi-Fi channels, as well as the global satellite positioning (GPS), the cellular communication 4G band, the Bluetooth, and even the next-generation 5G radio system. By integrating with an antenna, our MoS₂-enabled rectenna (rectifying antenna) successfully demonstrates direct energy harvesting of electromagnetic radiation in the Wi-Fi band to power a commercial LED. Importantly, the operation of our device requires zero external bias and therefore eliminates the need of a battery. This work opens up exciting opportunities for a new generation of large-area distributed electronics based on 2D materials and paves the way towards using the existing Wi-Fi infrastructure as an energy hotspot for wireless charging. Moreover, our MoS₂rectifier also realizes successful frequency conversion as a flexible mixer (beyond 10 GHz) which is a critical component for any wireless communication systems.

In addition to smart chemical manufacturing, I’m also dedicated to the chemical engineering of 2D material itself. We developed a highly nonintrusive doping technology for graphene based on chlorination, which can introduce strong hole doping and maintain the 2D nature and long-range periodicity of the electronic states of graphene intact. The all-surface nature of 2D materials provides us with a unique tool to tailor the electronic properties through chemical functionalization. We have used a suite of X-ray techniques to investigate the effect of surface functionalizing dopants on the electronic and chemical states of 2D materials, with special focuses on their surface binding energy, bonding configuration and work function shift.

Research Interests:

1. System-level Integration of Self-powered Sensor Network for Intelligent Chemical Plants and Healthcare

The future Internet will not only connect people but also almost every object and machine in the world. It is predicted that the number of sensors will grow from billions in 2012 to trillions by 2025. It becomes extremely challenging to provide power and connectivity to sensor nodes as their number grows exponentially. We believe that 2D materials can play a significant role in this perspective. Our previous work on chemically engineered MoS₂ RF energy harvester and mixer provide key enabling technologies for this application. By integrating them with sensors, we aim to demonstrate a system-level largely distributed ultralight sensor network for ubiquitous sensing in next generation of intelligent chemical plants and healthcare. In particular, such a system will allow (1) ubiquitous and wireless energy harvesting and (2) flexible radios that can be wirelessly read out by a cell phone or a Wi-Fi station for real-time data-collection and control.

2. “Smart Skin” for Soft-Robotics and Bio-medical Applications

The ultralight, flexible and mechanically robust 2D materials represent an ideal platform to build a “Smart Skin” that can be integrated with robots as an interactive surface with perceptions of environmental stimuli. By putting together the building blocks of energy harvester, sensing, communication and logic circuits that I have previously worked upon separately, it is exciting to build a self-powered and transparent “smart skin” for applications such as soft-robots, human-machine interfaces and assistive medical devices, etc.

3. Adaptive "Synthetic Cells and Neuro Synapses"

The biological cell represents one of the most amazing micro-machines with unprecedented performance-to-volume ratio (i.e. performance density) and adaptability. By using the unique properties of 2D materials, especially their outstanding mechanical flexibility, and electrical and chemical properties, we will develop a foldable and adaptable "synthetic cell" with autonomous behaviors. This unique family of van-der-Waals materials can enable foldings into various geometric surfaces of specific funtionalities such as antennas, sensors, transducers and energy harvesters, and eventually into a super-structure that mimic the behaviors of a biological cell. Moreover, The chemically engineered 2D materials provide a full spectrum of electronic materials from metal, semiconducters and insulators. The stacking of different 2D van-der-Waals layers provides novel voltage-tunable junctions to mimic the different states of the synaptic connections and achieve the dynamic reconfigurability between different post-synaptic effects. In our biological neuro cells, both excitatory and inhibitory neurotransmitters can be co-released from axon terminals which are fundametally important to learning and memory processes. In our 2D materials-based synthetic cells and synapses, the highly tunable charge transfer mechnism and junction formation will be exploited to resemble the various biological synaptic activities. The 2D materials-based artificial neural network would be an ideal architecture which is highly parallel, extremely compact and power-efficient.

Teaching Interests:

I have worked as a teaching assistant for MIT graduate level course “Applied Quantum and Statistical Physics” (Prof. Peter Hagelstein, 2015), and have been highly rated by the students (6.9/7.0, the highest evaluation score in the last five years). In 2016, I was also invited as a guest lecturer for the same class for one week. During my PhD, I have also mentored 1 PhD student, 1 master student, and 2 undergraduate students.

I would like to teach various physics-related and device-related chemical engineering classes, such as Thermodynamics and Statistical Mechanics, Transport Phenomena, Micro/Nano Processing Technology and NanoElectronics. With a bachelor degree in condensed matter physics and a PhD degree in electrical engineering, these classes are also core courses in my departments. I would be very excited to bring my physics insights and device perspectives into these traditional chemical engineering classes. I would also be happy to teach new classes, such as Fundamentals of Energy Harvesting, Physics of Low-dimensional Materials, Nanoscale Fabrication, etc.

Grant Proposal Writings:

[1] “Ubiquitous 2-Dimensional Electronics (U2DE) Initiative”, pre-proposal for NSF program under the supervision of Professor Tomas Palacios, 2013

[2] “Foldable and Adaptive Two-Dimensional Electronics”, full proposal for MURI Program (AFOSR) under the supervision of Professor Tomas Palacios. 3+2 year financial support awarded beginning in September 2015.

[3] “Graphene-Based High Efficiency CMOS-integrated Long Wavelength Infrared Detection”, full proposal for DAPAR program under the supervision of Professor Tomas Palacios, 2015

[4] “Providing Power and Connectivity for Next Generation of Ubiquitous Sensing Systems”, white paper for MIT.nano Center of Excellence, under the supervision of Professor Tomas Palacios, 2017 (pending)

Selected Publications:

[1] Zhang, X.; Schiros, T.; Nordlund, D.; Shin, Y. C.; Kong, J.; Dresselhaus, M.; Palacios, T. X-Ray Spectroscopic Investigation of Chlorinated Graphene: Surface Structure and Electronic Effects. Adv. Funct. Mater. 2015, 25, 4163–4169.

[2] Zhang, X.; Hsu, A.; Wang, H.; Song, Y.; Kong, J.; Dresselhaus, M. S.; Palacios, T. Impact of Chlorine Functionalization on High-Mobility Chemical Vapor Deposition Grown Graphene. ACS Nano 2013, 7, 7262–7270.

[3] Zhang, X.; Grajal, J.; Ujwal, R.; Wang, X.; Chern, W.; Zhou, L.; Lin, Y.; Shen, P.; Ji, X.; Ling, X.; Zubair, A.; Zhang, Y.; Wang, H.; Dubey, M.; Kong, J.; Dresselhaus, M. S.; Palacios, T. Two-dimensional MoS₂-enabled Rectenna for Ubiquitous Energy Harvesting in the Wi-Fi Band. (In submission)

[4] Zhang, X.; Grajal, J. ; Wang, X.; Ujwal, R.; Xiang, J.; Shen, P.; Zhang, Y.; Kong, J.; Dresselhaus, M. S.; Palacios, T. Large-scale Chemical Vapor Deposition Grown MoS₂ Schottky diode for Frequency mixing and Wireless Charging. (In submission)

[5] Wang, X.; Zhang, X.; Sun, L.; Lee, D.; Lee, S; Yang, S.; Dinca, M.; Palacios, T.; Gleason, K.; Ultrahigh Electrical Conductivity of oCVD PEDOT Thin Films and the Wafer Scale Fabrication of the 13.6MHz Rectifiers based on the PEDOT-Si Diode. (In submission)