Honors & Awards


  • The Carbon Journal Prize 2020, Carbon (Scientific journal) (2020)
  • TomKat Center Postdoctoral Fellowship in Sustainable Energy, TomKat Center for Sustainable Energy, Stanford University (2020)
  • Graduate Student Travel Award, Graduate College, University of Illinois at Urbana-Champaign (2019)
  • MRS Graduate Student Award (GSA) Gold Award, Materials Research Society (MRS) (2019)
  • Materials Research Society Arthur Nowick Graduate Student Award, Materials Research Society (MRS) (2019)
  • Graduate Teaching Fellowship (for course instructor), Mechanical Science and Engineering Department, University of Illinois at Urbana-Champaign (2018)

Professional Education


  • Ph.D., University of Illinois at Urbana-Champaign, Mechanical Engineering (2020)
  • M.S., KAIST (Korea Advanced Institute of Science and Technology), Mechanical Engineering (2013)
  • B.S., KAIST (Korea Advanced Institute of Science and Technology), Mechanical Engineering (2011)

Stanford Advisors


All Publications


  • Curved neuromorphic image sensor array using a MoS2-organic heterostructure inspired by the human visual recognition system NATURE COMMUNICATIONS Choi, C., Leem, J., Kim, M., Taqieddin, A., Cho, C., Cho, K., Lee, G., Seung, H., Jong, H., Song, Y., Hyeon, T., Aluru, N. R., Nam, S., Kim, D. 2020; 11 (1): 5934

    Abstract

    Conventional imaging and recognition systems require an extensive amount of data storage, pre-processing, and chip-to-chip communications as well as aberration-proof light focusing with multiple lenses for recognizing an object from massive optical inputs. This is because separate chips (i.e., flat image sensor array, memory device, and CPU) in conjunction with complicated optics should capture, store, and process massive image information independently. In contrast, human vision employs a highly efficient imaging and recognition process. Here, inspired by the human visual recognition system, we present a novel imaging device for efficient image acquisition and data pre-processing by conferring the neuromorphic data processing function on a curved image sensor array. The curved neuromorphic image sensor array is based on a heterostructure of MoS2 and poly(1,3,5-trimethyl-1,3,5-trivinyl cyclotrisiloxane). The curved neuromorphic image sensor array features photon-triggered synaptic plasticity owing to its quasi-linear time-dependent photocurrent generation and prolonged photocurrent decay, originated from charge trapping in the MoS2-organic vertical stack. The curved neuromorphic image sensor array integrated with a plano-convex lens derives a pre-processed image from a set of noisy optical inputs without redundant data storage, processing, and communications as well as without complex optics. The proposed imaging device can substantially improve efficiency of the image acquisition and recognition process, a step forward to the next generation machine vision.

    View details for DOI 10.1038/s41467-020-19806-6

    View details for Web of Science ID 000595871500012

    View details for PubMedID 33230113

    View details for PubMedCentralID PMC7683533

  • Kirigami-inspired strain-insensitive sensors based on atomically-thin materials MATERIALS TODAY Yong, K., De, S., Hsieh, E. Y., Leem, J., Aluru, N. R., Nam, S. 2020; 34: 58–65
  • Ultrasensitive detection of nucleic acids using deformed graphene channel field effect biosensors NATURE COMMUNICATIONS Hwang, M., Heiranian, M., Kim, Y., You, S., Leem, J., Taqieddin, A., Faramarzi, V., Jing, Y., Park, I., van der Zande, A. M., Nam, S., Aluru, N. R., Bashir, R. 2020; 11 (1): 1543

    Abstract

    Field-effect transistor (FET)-based biosensors allow label-free detection of biomolecules by measuring their intrinsic charges. The detection limit of these sensors is determined by the Debye screening of the charges from counter ions in solutions. Here, we use FETs with a deformed monolayer graphene channel for the detection of nucleic acids. These devices with even millimeter scale channels show an ultra-high sensitivity detection in buffer and human serum sample down to 600 zM and 20 aM, respectively, which are ∼18 and ∼600 nucleic acid molecules. Computational simulations reveal that the nanoscale deformations can form 'electrical hot spots' in the sensing channel which reduce the charge screening at the concave regions. Moreover, the deformed graphene could exhibit a band-gap, allowing an exponential change in the source-drain current from small numbers of charges. Collectively, these phenomena allow for ultrasensitive electronic biomolecular detection in millimeter scale structures.

    View details for DOI 10.1038/s41467-020-15330-9

    View details for Web of Science ID 000530024800002

    View details for PubMedID 32210235

    View details for PubMedCentralID PMC7093535

  • Interaction of 2D materials with liquids: wettability, electrochemical properties, friction, and emerging directions NPG ASIA MATERIALS Snapp, P., Kim, J., Cho, C., Leem, J., Haque, M., Nam, S. 2020; 12 (1)
  • A snapshot review on exciton engineering in deformed 2D materials MRS ADVANCES Leem, J. 2020; 5 (64): 3491–3506
  • Uniaxially crumpled graphene as a platform for guided myotube formation MICROSYSTEMS & NANOENGINEERING Kim, J., Leem, J., Kim, H., Kang, P., Choi, J., Haque, M., Kang, D., Nam, S. 2019; 5: 53

    Abstract

    Graphene, owing to its inherent chemical inertness, biocompatibility, and mechanical flexibility, has great potential in guiding cell behaviors such as adhesion and differentiation. However, due to the two-dimensional (2D) nature of graphene, the microfabrication of graphene into micro/nanoscale patterns has been widely adopted for guiding cellular assembly. In this study, we report crumpled graphene, i.e., monolithically defined graphene with a nanoscale wavy surface texture, as a tissue engineering platform that can efficiently promote aligned C2C12 mouse myoblast cell differentiation. We imparted out-of-plane, nanoscale crumpled morphologies to flat graphene via compressive strain-induced deformation. When C2C12 mouse myoblast cells were seeded on the uniaxially crumpled graphene, not only were the alignment and elongation promoted at a single-cell level but also the differentiation and maturation of myotubes were enhanced compared to that on flat graphene. These results demonstrate the utility of the crumpled graphene platform for tissue engineering and regenerative medicine for skeletal muscle tissues.

    View details for DOI 10.1038/s41378-019-0098-6

    View details for Web of Science ID 000494277800001

    View details for PubMedID 31700672

    View details for PubMedCentralID PMC6826050

  • Crack-assisted, localized deformation of van der Waals materials for enhanced strain confinement 2D MATERIALS Leem, J., Lee, Y., Wang, M., Kim, J., Mun, J., Haque, M., Kang, S., Nam, S. 2019; 6 (4)
  • Colloidal Photonic Crystal Strain Sensor Integrated with Deformable Graphene Phototransducer ADVANCED FUNCTIONAL MATERIALS Snapp, P., Kang, P., Leem, J., Nam, S. 2019; 29 (33)
  • Ultraviolet to Mid-Infrared Emissivity Control by Mechanically Reconfigurable Graphene NANO LETTERS Krishna, A., Kim, J., Leem, J., Wang, M., Nam, S., Lee, J. 2019; 19 (8): 5086–92

    Abstract

    Spectral emissivity control is critical for optical and thermal management in the ambient environment because solar irradiance and atmospheric transmissions occur at distinct wavelength regions. For instance, selective emitters with low emissivity in the solar spectrum but high emissivity in the mid-infrared can lead to significant radiative cooling. Ambient variations require not only spectral control but also a mechanism to adjust the emissivity. However, most selective emitters are fixed to specific wavelength ranges and lack dynamic control mechanisms. Here we show ultraviolet to mid-infrared emissivity control by mechanically reconfiguring graphene, in which stretching and releasing induce dynamic topographic changes. We fabricate crumpled graphene with pitches ranging from 40 nm to 10 μm using deformable substrates. Our measurements and computations show that 140 nm-pitch crumpled graphene offers ultraviolet emissivity control in 200-300 nm wavelengths whereas 10 μm-pitch crumpled graphene offers mid-infrared emissivity control in 7-19 μm wavelengths. Significant emissivity changes arise from interference induced by the periodic topography and selective transmissivity reductions. Dynamic stretching and releasing of 140 nm and 10 μm pitch crumpled graphene show reversible emissivity peak changes at 250 nm and at 9.9 μm wavelengths, respectively. This work demonstrates the unique potential of crumpled graphene as a reconfigurable optical and thermal management platform.

    View details for DOI 10.1021/acs.nanolett.9b01358

    View details for Web of Science ID 000481563800036

    View details for PubMedID 31251631

  • Photonic crystallization of two-dimensional MoS2 for stretchable photodetectors NANOSCALE Kim, R., Leem, J., Muratore, C., Nam, S., Rao, R., Jawaid, A., Durstock, M., McConney, M., Drummy, L., Rai, R., Voevodin, A., Glavin, N. 2019; 11 (28): 13260–68

    Abstract

    Low temperature synthesis of high quality two-dimensional (2D) materials directly on flexible substrates remains a fundamental limitation towards scalable realization of robust flexible electronics possessing the unique physical properties of atomically thin structures. Herein, we describe room temperature sputtering of uniform, stoichiometric amorphous MoS2 and subsequent large area (>6.25 cm2) photonic crystallization of 5 nm 2H-MoS2 films in air to enable direct, scalable fabrication of ultrathin 2D photodetectors on stretchable polydimethylsiloxane (PDMS) substrates. The lateral photodetector devices demonstrate an average responsivity of 2.52 μW A-1 and a minimum response time of 120 ms under 515.6 nm illumination. Additionally, the surface wrinkled, or buckled, PDMS substrate with conformal MoS2 retained the photoconductive behavior at tensile strains as high as 5.72% and over 1000 stretching cycles. The results indicate that the photonic crystallization method provides a significant advancement in incorporating high quality semiconducting 2D materials applied directly on polymer substrates for wearable and flexible electronic systems.

    View details for DOI 10.1039/c9nr02173f

    View details for Web of Science ID 000476564300049

    View details for PubMedID 31197304

  • High-Mobility MoS2 Directly Grown on Polymer Substrate with Kinetics-Controlled Metal-Organic Chemical Vapor Deposition ACS APPLIED ELECTRONIC MATERIALS Mun, J., Park, H., Park, J., Joung, D., Lee, S., Leem, J., Myoung, J., Park, J., Jeong, S., Chegal, W. C., Nam, S., Kang, S. 2019; 1 (4): 608–16
  • Mechanical instability driven self-assembly and architecturing of 2D materials 2D MATERIALS Wang, M., Leem, J., Kang, P., Choi, J., Knapp, P., Yong, K., Nam, S. 2017; 4 (2)
  • A stretchable crumpled graphene photodetector with plasmonically enhanced photoresponsivity NANOSCALE Kim, M., Kang, P., Leem, J., Nam, S. 2017; 9 (12): 4058–65

    Abstract

    Graphene has been widely explored for flexible, high-performance photodetectors due to its exceptional mechanical strength, broadband absorption, and high carrier mobility. However, the low stretchability and limited photoabsorption of graphene have restricted its applications in flexible and highly sensitive photodetection systems. Various hybrid systems based on photonic or plasmonic nanostructures have been introduced to improve the limited photoresponsivity of graphene photodetectors. In most cases, the hybrid systems succeeded in the enhancement of photoresponsivity, but showed limited mechanical stretchability. Here, we demonstrate a stretchable photodetector based on a crumpled graphene-gold nanoparticle (AuNP) hybrid structure with ∼1200% enhanced photoresponsivity, compared to a conventional flat graphene-only photodetector, and exceptional mechanical stretchability up to a 200% tensile strain. We achieve plasmonically enhanced photoresponsivity by integrating AuNPs with graphene. By crumpling the hybrid structure, we realize mechanical stretchability and further enhancement of the optical absorption by densification. We also demonstrate that our highly stretchable photodetector with enhanced photoresponsivity can be integrated on a contact lens and a spring structure. We believe that our stretchable, high performance graphene photodetector can find broad applications for conformable and flexible optical sensors and dynamic mechanical strain sensors.

    View details for DOI 10.1039/c6nr09338h

    View details for Web of Science ID 000397966400003

    View details for PubMedID 28116377

  • Mechanically Self-Assembled, Three-Dimensional Graphene-Gold Hybrid Nanostructures for Advanced Nanoplasmonic Sensors NANO LETTERS Leem, J., Wang, M., Kang, P., Nam, S. 2015; 15 (11): 7684–90

    Abstract

    Hybrid structures of graphene and metal nanoparticles (NPs) have been actively investigated as higher quality surface enhanced Raman spectroscopy (SERS) substrates. Compared with SERS substrates, which only contain metal NPs, the additional graphene layer provides structural, chemical, and optical advantages. However, the two-dimensional (2D) nature of graphene limits the fabrication of the hybrid structure of graphene and NPs to 2D. Introducing three-dimensionality to the hybrid structure would allow higher detection sensitivity of target analytes by utilizing the three-dimensional (3D) focal volume. Here, we report a mechanical self-assembly strategy to enable a new class of 3D crumpled graphene-gold (Au) NPs hybrid nanoplasmonic structures for SERS applications. We achieve a 3D crumpled graphene-Au NPs hybrid structure by the delamination and buckling of graphene on a thermally activated, shrinking polymer substrate. We also show the precise control and optimization of the size and spacing of integrated Au NPs on crumpled graphene and demonstrate the optimized NPs' size and spacing for higher SERS enhancement. The 3D crumpled graphene-Au NPs exhibits at least 1 order of magnitude higher SERS detection sensitivity than that of conventional, flat graphene-Au NPs. The hybrid structure is further adapted to arbitrary curvilinear structures for advanced, in situ, nonconventional, nanoplasmonic sensing applications. We believe that our approach shows a promising material platform for universally adaptable SERS substrate with high sensitivity.

    View details for DOI 10.1021/acs.nanolett.5b03672

    View details for Web of Science ID 000364725400077

    View details for PubMedID 26501429

  • Three-Dimensional Integration of Graphene via Swelling, Shrinking, and Adaptation NANO LETTERS Choi, J., Kim, H., Wang, M., Leem, J., King, W. P., Nam, S. 2015; 15 (7): 4525–31

    Abstract

    The transfer of graphene from its growth substrate to a target substrate has been widely investigated for its decisive role in subsequent device integration and performance. Thus far, various reported methods of graphene transfer have been mostly limited to planar or curvilinear surfaces due to the challenges associated with fractures from local stress during transfer onto three-dimensional (3D) microstructured surfaces. Here, we report a robust approach to integrate graphene onto 3D microstructured surfaces while maintaining the structural integrity of graphene, where the out-of-plane dimensions of the 3D features vary from 3.5 to 50 μm. We utilized three sequential steps: (1) substrate swelling, (2) shrinking, and (3) adaptation, in order to achieve damage-free, large area integration of graphene on 3D microstructures. Detailed scanning electron microscopy, atomic force microscopy, Raman spectroscopy, and electrical resistance measurement studies show that the amount of substrate swelling as well as the flexural rigidities of the transfer film affect the integration yield and quality of the integrated graphene. We also demonstrate the versatility of our approach by extension to a variety of 3D microstructured geometries. Lastly, we show the integration of hybrid structures of graphene decorated with gold nanoparticles onto 3D microstructure substrates, demonstrating the compatibility of our integration method with other hybrid nanomaterials. We believe that the versatile, damage-free integration method based on swelling, shrinking, and adaptation will pave the way for 3D integration of two-dimensional (2D) materials and expand potential applications of graphene and 2D materials in the future.

    View details for DOI 10.1021/acs.nanolett.5b01036

    View details for Web of Science ID 000357964100043

    View details for PubMedID 26086170

  • Photoinduced synthesis of Ag nanoparticles on ZnO nanowires for real-time SERS systems RSC ADVANCES Kang, H., Leem, J., Sung, H. 2015; 5 (1): 51–57

    View details for DOI 10.1039/c4ra11296b

    View details for Web of Science ID 000345924600008

  • Controllable Ag nanostructure patterning in a microfluidic channel for real-time SERS systems NANOSCALE Leem, J., Kang, H., Ko, S., Sung, H. 2014; 6 (5): 2895–2901

    Abstract

    We present a microfluidic patterning system for fabricating nanostructured Ag thin films via a polyol method. The fabricated Ag thin films can be used immediately in a real-time SERS sensing system. The Ag thin films are formed on the inner surfaces of a microfluidic channel so that a Ag-patterned Si wafer and a Ag-patterned PDMS channel are produced by the fabrication. The optimum sensing region and fabrication duration for effective SERS detection were determined. As SERS active substrates, the patterned Ag thin films exhibit an enhancement factor (EF) of 4.25 × 10(10). The Ag-patterned polymer channel was attached to a glass substrate and used as a microfluidic sensing system for the real-time monitoring of biomolecule concentrations. This microfluidic patterning system provides a low-cost process for the fabrication of materials that are useful in medical and pharmaceutical detection and can be employed in mass production.

    View details for DOI 10.1039/c3nr04829b

    View details for Web of Science ID 000332127200051

    View details for PubMedID 24477564

  • Continuous synthesis of zinc oxide nanoparticles in a microfluidic system for photovoltaic application NANOSCALE Kang, H., Leem, J., Yoon, S., Sung, H. 2014; 6 (5): 2840–46

    Abstract

    This study describes the synthesis of zinc oxide nanoparticles (ZnO NPs) using a microfluidic system. A continuous and efficient synthetic process was developed based on a microfluidic reactor in which was implemented a time pulsed mixing method that had been optimized using numerical simulations and experimental methods. Numerical simulations revealed that efficient mixing conditions could be obtained over the frequency range 5-15 Hz. This system used ethanol solutions containing 30 mM sodium hydroxide (NaOH) or 10 mM dehydrated zinc acetate (Zn(OAc)2) under 5 Hz pulsed conditions, which provided the optimal mixing performance conditions. The ZnO NPs prepared using the microfluidic synthetic system or batch-processed system were validated by several analytical methods, including transmission electron microscopy (TEM), energy dispersive X-ray spectrometry (EDS), X-ray diffraction (XRD), UV/VIS NIR and zeta (ζ) potential analysis. Bulk-heterojunction organic photovoltaic cells were fabricated with the synthesized ZnO NPs to investigate the practicability and compared with batch-process synthesized ZnO NPs. The results showed that microfluidic synthesized ZnO NPs had good preservability and stability in working solution and the synthetic microfluidic system provided a low-cost, environmentally friendly approach to the continuous production of ZnO NPs.

    View details for DOI 10.1039/c3nr06141h

    View details for Web of Science ID 000332127200044

    View details for PubMedID 24469327

  • Vacuum-assisted microcontact printing (mu CP) for aligned patterning of nano and biochemical materials JOURNAL OF MATERIALS CHEMISTRY C Kang, H., Leem, J., Ko, S., Yoon, S., Sung, H. 2013; 1 (2): 268–74

    View details for DOI 10.1039/c2tc00288d

    View details for Web of Science ID 000314801200014