Research
Research Summary
My research activities focus on three areas: (1) new synthesis and characterization tools for nanoscience and nanomaterial research; (2) emerging multi-physics phenomena at the nanoscale and their electronic, optoelectronic, and nano-electro-mechanical system (NEMS) applications; (3) integrated multifunctional nanosystems.
Area 1: New Tools for Nanoscience and Nanomaterial Research
Project 1.A: Deep Learning Enabled Optical Characterization of 2D Materials
Characterization of nanomaterial morphologies with advanced microscopy and/or spectroscopy tools plays an indispensable role in nanoscience and nanotechnology research. However, the interpretation of imaging data heavily relies on the “intuition” of experienced researchers. As a result, many of the deep graphical features are often unused because of difficulties in processing the data and finding the correlations. Such difficulties can be well addressed by deep learning. In this work, we use the optical characterization of two-dimensional (2D) materials as a case study, and demonstrate a neural network based algorithm for the material and thickness identification of exfoliated 2D materials with high prediction accuracy and real-time processing capability. Further analysis shows that the trained network can be used to predict physical properties of the materials. Finally, a transfer learning technique is applied to adapt the pretrained network to more optical characterization applications such as identifying layer numbers of chemically synthesized graphene domains.
Project 1.B: Atomic Layer Substitution for Janus Monolayer Semiconductors and Their Heterostructures
Manipulating materials with atomic-scale precision is essential for the development of next-generation material design toolbox. The family of 2D materials provides an ideal platform to realize atomic-level material architectures. We developed a novel atomic-scale material design tool that selectively breaks and forms chemical bonds of 2D materials at room temperature, called atomic-layer substitution (ALS), through which we can substitute the top layer chalcogen atoms within the 3-atom-thick transition-metal dichalcogenides using arbitrary patterns. Flipping the layer via transfer allows us to perform the same procedure on the other side, yielding programmable in-plane multi-heterostructures with different out-of-plane crystal symmetry and electric polarization. This electric dipole can be selectively patterned to be zero (MoS2, MoSe2), positive (MoSSe), and negative (MoSeS) on such a 2D material “canvas”, which would enable many novel nanostructures and devices with intriguing electrical and optoelectronic properties.
Project 1.C: Synthesis of 2D Materials and Their Lateral Heterostructures
Diverse parallel stitched 2D heterostructures, including metal–semiconductor, semiconductor–semiconductor, and insulator–semiconductor, are synthesized directly through selective “sowing” of aromatic molecules as the seeds in the chemical vapor deposition (CVD) method. The methodology also enables the large‐scale fabrication of lateral heterostructures.
Area 2: Nanoscale Multi-Physics Phenomena and Their Device Applications
Project 2.A: High-Performance Monolayer Transistors with Semimetallic Ohmic Contact
Atomically thin two-dimensional (2D) semiconductors have great potential for realizing ultimately scaled high-performance electronic devices. However, because of metal-induced gap states (MIGS), energy barriers at the metal-semiconductor interface, which fundamentally lead to high contact resistances and poor current-delivery capabilities, have restrained the advancement of 2D semiconductor transistors to date. In this project, we study a novel ohmic contact technology between semimetallic bismuth and semiconducting monolayer transition metal dichalcogenides (TMDs) where MIGS is sufficiently suppressed and degenerate states in the TMD are spontaneously formed in contact with bismuth. Through this approach, we achieve zero Schottky barrier height, a record-low contact resistance (RC ) of 123 Ω μm, and a record-high on-state current density (ION) of 1135 µA/µm on monolayer MoS2. We also demonstrate that excellent ohmic contacts can be formed on various monolayer semiconductors, including MoS2, WS2, and WSe2. Our reported RC values are a significant improvement for 2D semiconductors, and approaching the quantum limit. This technology unveils the full potential of high-performance monolayer transistors that are on par with the state-of-the-art 3D semiconductors, enabling further device down-scaling and extending Moore’s Law.
Publication:
P.-C. Shen*, C. Su*, Y. Lin*, A.-S. Chou*, et al. "Ultralow contact resistance between semimetal and monolayer semiconductors", Nature 593, 211-217 (2021). Link
Media Coverage:
MIT: Advance may enable “2D” transistors for tinier microchip components
UC Berkeley College of Engineering: 2D transistor technology overcomes barrier to ever-shrinking computer chips
UC Berkeley EECS Department: 2-D semiconductor contact resistances approach the quantum limit
National Taiwan University: 挑戰物理極限 延續摩爾定律 臺大攜手台積電、美國麻省理工學院跨國研究登Nature
Project 2.B: An Ultrasensitive Thermo-Mechanical Mid-Infrared Detector
With nanoengineering, it is possible to fabricate nanometer-sized quantum tunneling barriers that can be tuned mechanically. Such a tremendous mechanical tunability can be harnessed for mechanical sensors and many other types of sensors with extremely high sensitivity. Here we demonstrate two nanostructures that implement such a mechanically tunable tunneling barrier and use them for either a mechanical/strain sensor or a mid-infrared bolometric detector. The first nanostructure is the self-assembled graphene nanoflake network composed of a resistance network of sub-micron graphene flakes that connect with each other with <100 nm overlap. The second nanostructure is a metal nano-gap with the gap defined by self-assembled monolayers (SAMs) . The proposed structures show high gauge factors and/or improved linear dynamic range as a strain sensor. Such mechanical sensors can also be integrated with a thermal actuator to realize a highly sensitive, uncooled bolometer-type mid-infrared detector . The measured temperature coefficient of resistance (TCR) can be as high as 5 K-1, which is more than one order of magnitude better than the state of the art.
Project 2.C: Light-Matter Interactions in 2D Lateral Heterostructures
A variety of unique light-matter interaction phenomena have been discovered in graphene, which promise many novel optoelectronic applications. Most of the effects are only accessible by breaking the spatial symmetry. The recent development of direct synthesis of lateral heterostructures offers new opportunities to achieve the desired asymmetry. As a proof of concept, we study the photothermoelectric effect in an asymmetric lateral heterojunction between the Dirac semimetallic monolayer graphene and the parabolic semiconducting monolayer MoS2. Very different hot-carrier cooling mechanisms on the graphene and the MoS2 sides allow us to resolve the asymmetric thermalization pathways of photoinduced hot carriers spatially with electrostatic gate tunability. We also demonstrate the potential of graphene-2D semiconductor lateral heterojunctions as broadband infrared photodetectors. The proposed structure shows an extreme in-plane asymmetry and provides a new platform to study light-matter interactions in low-dimensional systems.
Publication:
Y. Lin*, Q. Ma*, P.-C. Shen*, et al. "Asymmetric hot-carrier thermalization and broadband photoresponse in graphene-2D semiconductor lateral heterojunctions", Sci. Adv. 5, eaav1493 (2019). Link
Project 2.D: Integrated Circuits Based on Graphene-MoS2-Graphene Lateral Heterostructures
We developed a large-scale synthetic approach to construct lateral heterostructures between graphene and 2D semiconductors such as MoS2. Temperature transport measurements have shown that such a lateral structure has a very small Schottky barrier height, which is ideal for the 1D contact to 2D semiconductor field effect transistors. Based on this idea, we implemented a variety of building blocks for logic integrated circuits.
Publication:
X. Ling*, Y. Lin*, et al. "Parallel stitching of 2D materials", Adv. Mater. 28, 2322-2329 (2016). Link
Project 2.E: Excitons and Defects in 2D Semiconductors
Because of the strong quantum confinement, excitons, a combinational state of electron-hole pairs, have shown to be stable and dominate the luminescence properties of semiconducting 2D transition metal dichalcogenides (TMDs). We studied several important aspects about the excitonic photoluminescence of monolayer TMDs, including the effects of dielectric screening, electrostatic doping, and the special defect-bound excitonic optical transitions.
Publication:
Y. Lin, et al. "Dielectric screening of excitons and trions in single-layer MoS2", Nano Lett. 14, 5569-5576 (2014). Link
P.-C. Shen, Y. Lin, et al. "Unraveling the effect of multiple defect states in synthetic monolayer MoS2 through electronic and optical probes", APS March Meeting 2018, Los Angeles, CA, March 2018.
Y. Lin, et al. "Optical properties of monolayer molybdenum disulfide at high carrier density", MRS 2013 Fall Meeting, Boston, MA, December 2013.
Project 2.F: High-Efficiency Graphene-Semiconductor Schottky Junction Solar Cells
Graphene-based photovoltaic devices have attracted great interest since the recent implementation of graphene/semiconductor heterojunction solar cells, due to the fascinating electronic and optical characteristics of graphene. We optimized the device structure both experimentally and theoretically to improve the energy conversion efficiency. Simulation results demonstrate that a maximal conversion efficiency of ∼9.2% can be achieved. To verify, Schottky junction solar cells based on acid-modified graphene films and silicon pillar arrays are fabricated, delivering enhanced cell performance with efficiencies of up to 7.7%, which is a new record of graphene based solar cells.
Publication:
Y. Lin*, X. Li*, et al. ”Graphene/semiconductor heterojunction solar cells with modulated antireflection and graphene work function", Energy Environ. Sci. 6, 108-115 (2013). Link
Area 3: Integrated Multifunctional Nanosystems
Project 3.A: Graphene-CMOS Monolithic Integratoin for Thermal Imaging Systems
The low dimensionality of 2D materials and the easy and universal transfer process make it possible to decouple the low-temperature fabrication process from the high-temperature growth of high-quality 2D materials and realize the monolithic integration between 2D material based technologies and conventional silicon CMOS technologies. As a proof-of-concept, we developed a back-end-of-line process to fabricate a graphene thermopile mid-infrared detector array directly onto a specifically designed silicon CMOS integrated circuit chip. Such a monolithically integrated graphene-CMOS system can perform real-time thermal imaging that can potentially be used in next generation night vision goggles, surveillance cameras as well self-driving systems.
Publication:
A. Nourbakhsh, L. Yu, Y. Lin, et al. "Heterogeneous integration of 2D materials and devices on a Si platform", in: Topaloglu R., Wong HS. (eds) Beyond-CMOS Technologies for Next Generation Computer, Springer, Charm (2019).