Stanford Advisors

  • Wendy Gu, Postdoctoral Faculty Sponsor

All Publications

  • Fatigue Behavior of Polymer Encapsulated Graphene to Mitigate Interfacial Fatigue Damage ADVANCED ENGINEERING MATERIALS Islam, M., Kumral, B., Wang, G., Cui, T., Hou, Y., Pan, P., Liu, X., Filleter, T. 2023
  • Defect Engineering of Graphene for Dynamic Reliability. Small (Weinheim an der Bergstrasse, Germany) Kumral, B., Demingos, P. G., Cui, T., Serles, P., Barri, N., Singh, C. V., Filleter, T. 2023: e2302145


    The interface between two-dimensional (2D) materials and soft, stretchable polymeric substrates is a governing criterion in proposed 2D materials-based flexible devices. This interface is dominated by weak van der Waals forces and there is a large mismatch in elastic constants between the contact materials. Under dynamic loading, slippage, and decoupling of the 2D material is observed, which then leads to extensive damage propagation in the 2D lattice. Herein, graphene is functionalized through mild and controlled defect engineering for a fivefold increase in adhesion at the graphene-polymer interface. Adhesion is characterized experimentally using buckling-based metrology, while molecular dynamics simulations reveal the role of individual defects in the context of adhesion. Under in situ cyclic loading, the increased adhesion inhibits damage initiation and interfacial fatigue propagation within graphene. This work offers insight into achieving dynamically reliable and robust 2D material-polymer contacts, which can facilitate the development of 2D materials-based flexible devices.

    View details for DOI 10.1002/smll.202302145

    View details for PubMedID 37291948

  • Author Correction: Mechanical regulation of lithium intrusion probability in garnet solid electrolytes [Jan, 10.1038/s41560-022-01186-4, 2023] NATURE ENERGY McConohy, G., Xu, X., Cui, T., Barks, E., Wang, S., Kaeli, E., Melamed, C., Gu, X., Chueh, W. C. 2023
  • Mechanical regulation of lithium intrusion probability in garnet solid electrolytes NATURE ENERGY McConohy, G., Xu, X., Cui, T., Barks, E., Wang, S., Kaeli, E., Melamed, C., Gu, X., Chueh, W. C. 2023
  • Dipole Engineering through the Orientation of Interface Molecules for Efficient InP Quantum Dot Light-Emitting Diodes. Journal of the American Chemical Society Lee, S., Park, S. M., Jung, E. D., Zhu, T., Pina, J. M., Anwar, H., Wu, F. Y., Chen, G. L., Dong, Y., Cui, T., Wei, M., Bertens, K., Wang, Y. K., Chen, B., Filleter, T., Hung, S. F., Won, Y. H., Kim, K. H., Hoogland, S., Sargent, E. H. 2022


    InP-based quantum dot (QD) light-emitting diodes (QLEDs) provide a heavy-metal-free route to size-tuned LEDs having high efficiency. The stability of QLEDs may be enhanced by replacing organic hole-injection layers (HILs) with inorganic layers. However, inorganic HILs reported to date suffer from inefficient hole injection, the result of their shallow work functions. Here, we investigate the tuning of the work function of nickel oxide (NiOx) HILs using self-assembled molecules (SAMs). Density functional theory simulations and near-edge X-ray absorption fine structure put a particular focus onto the molecular orientation of the SAMs in tuning the work function of the NiOx HIL. We find that orientation plays an even stronger role than does the underlying molecular dipole itself: SAMs having the strongest electron-withdrawing nitro group (NO2), despite having a high intrinsic dipole, show limited work function tuning, something we assign to their orientation parallel to the NiOx surface. We further find that the NO2 group─which delocalizes electrons over the molecule by resonance─induces a deep lowest unoccupied molecular orbital level that accepts electrons from QDs, producing luminescence quenching. In contrast, SAMs containing a trifluoromethyl group exhibit an angled orientation relative to the NiOx surface, better activating hole injection into the active layer without inducing luminescence quenching. We report an external quantum efficiency (EQE) of 18.8%─the highest EQE among inorganic HIL-based QLEDs (including Cd-based QDs)─in InP QLEDs employing inorganic HILs.

    View details for DOI 10.1021/jacs.2c09705

    View details for PubMedID 36327099

  • Mechanical reliability of monolayer MoS2 and WSe2 MATTER Cui, T., Mukherjee, S., Onodera, M., Wang, G., Kumral, B., Islam, A., Shayegannia, M., Krishnan, G., Barri, N., Serles, P., Zhang, X., Sassi, L. M., Tam, J., Bassim, N., Kherani, N. P., Ajayan, P. M., Machida, T., Singh, C., Sun, Y., Filleter, T. 2022; 5 (9): 2975-2989
  • Molecular design and structural optimization of nanocellulose-based films fabricated via regioselective functionalization for flexible electronics CHEMICAL ENGINEERING JOURNAL Dias, O., Konar, S., Pakharenko, V., Graziano, A., Leao, A., Tjong, J., Jaffer, S., Cui, T., Filleter, T., Sain, M. 2022; 440
  • A Carbon-Based Biosensing Platform for Simultaneously Measuring the Contraction and Electrophysiology of iPSC-Cardiomyocyte Monolayers ACS NANO Dou, W., Malhi, M., Cui, T., Wang, M., Wang, T., Shan, G., Law, J., Gong, Z., Plakhotnik, J., Filleter, T., Li, R., Simmons, C. A., Maynes, J. T., Sun, Y. 2022


    Heart beating is triggered by the generation and propagation of action potentials through the myocardium, resulting in the synchronous contraction of cardiomyocytes. This process highlights the importance of electrical and mechanical coordination in organ function. Investigating the pathogenesis of heart diseases and potential therapeutic actions in vitro requires biosensing technologies which allow for long-term and simultaneous measurement of the contractility and electrophysiology of cardiomyocytes. However, the adoption of current biosensing approaches for functional measurement of in vitro cardiac models is hampered by low sensitivity, difficulties in achieving multifunctional detection, and costly manufacturing processes. Leveraging carbon-based nanomaterials, we developed a biosensing platform that is capable of performing on-chip and simultaneous measurement of contractility and electrophysiology of human induced pluripotent stem-cell-derived cardiomyocyte (iPSC-CM) monolayers. This platform integrates with a flexible thin-film cantilever embedded with a carbon black (CB)-PDMS strain sensor for high-sensitivity contraction measurement and four pure carbon nanotube (CNT) electrodes for the detection of extracellular field potentials with low electrode impedance. Cardiac functional properties including contractile stress, beating rate, beating rhythm, and extracellular field potential were evaluated to quantify iPSC-CM responses to common cardiotropic agents. In addition, an in vitro model of drug-induced cardiac arrhythmia was established to further validate the platform for disease modeling and drug testing.

    View details for DOI 10.1021/acsnano.2c04676

    View details for Web of Science ID 000819589200001

    View details for PubMedID 35715006

  • Quantum-size-tuned heterostructures enable efficient and stable inverted perovskite solar cells NATURE PHOTONICS Chen, H., Teale, S., Chen, B., Hou, Y., Grater, L., Zhu, T., Bertens, K., Park, S., Atapattu, H. R., Gao, Y., Wei, M., Johnston, A. K., Zhou, Q., Xu, K., Yu, D., Han, C., Cui, T., Jung, E., Zhou, C., Zhou, W., Proppe, A. H., Hoogland, S., Laquai, F., Filleter, T., Graham, K. R., Ning, Z., Sargent, E. H. 2022
  • Mechanical Size Effect of Freestanding Nanoconfined Polymer Films MACROMOLECULES Wang, G., Najafi, F., Ho, K., Hamidinejad, M., Cui, T., Walker, G. C., Singh, C., Filleter, T. 2022; 55 (4): 1248-1259
  • Friction of magnetene, a non-van der Waals 2D material SCIENCE ADVANCES Serles, P., Arif, T., Puthirath, A. B., Yadav, S., Wang, G., Cui, T., Balan, A., Yadav, T., Thibeorchews, P., Chakingal, N., Costin, G., Singh, C., Ajayan, P. M., Filleter, T. 2021; 7 (47): eabk2041


    [Figure: see text].

    View details for DOI 10.1126/sciadv.abk2041

    View details for Web of Science ID 000720347400021

    View details for PubMedID 34788102

    View details for PubMedCentralID PMC8597991

  • Fatigue resistance of atomically thin graphene oxide CARBON Najafi, F., Wang, G., Cui, T., Anand, A., Mukherjee, S., Filleter, T., Sain, M., Singh, C. 2021; 183: 780-788
  • Multication perovskite 2D/3D interfaces form via progressive dimensional reduction NATURE COMMUNICATIONS Proppe, A. H., Johnston, A., Teale, S., Mahata, A., Quintero-Bermudez, R., Jung, E., Grater, L., Cui, T., Filleter, T., Kim, C., Kelley, S. O., De Angelis, F., Sargent, E. H. 2021; 12 (1): 3472


    Many of the best-performing perovskite photovoltaic devices make use of 2D/3D interfaces, which improve efficiency and stability - but it remains unclear how the conversion of 3D-to-2D perovskite occurs and how these interfaces are assembled. Here, we use in situ Grazing-Incidence Wide-Angle X-Ray Scattering to resolve 2D/3D interface formation during spin-coating. We observe progressive dimensional reduction from 3D to n = 3 → 2 → 1 when we expose (MAPbBr3)0.05(FAPbI3)0.95 perovskites to vinylbenzylammonium ligand cations. Density functional theory simulations suggest ligands incorporate sequentially into the 3D lattice, driven by phenyl ring stacking, progressively bisecting the 3D perovskite into lower-dimensional fragments to form stable interfaces. Slowing the 2D/3D transformation with higher concentrations of antisolvent yields thinner 2D layers formed conformally onto 3D grains, improving carrier extraction and device efficiency (20% 3D-only, 22% 2D/3D). Controlling this progressive dimensional reduction has potential to further improve the performance of 2D/3D perovskite photovoltaics.

    View details for DOI 10.1038/s41467-021-23616-9

    View details for Web of Science ID 000664874700003

    View details for PubMedID 34108463

    View details for PubMedCentralID PMC8190276

  • Clean manufacturing of nanocellulose-reinforced hydrophobic fl exible substrates JOURNAL OF CLEANER PRODUCTION Dias, O., Konar, S., Leao, A., Yang, W., Tjong, J., Jaffer, S., Cui, T., Filleter, T., Sain, M. 2021; 293
  • Fracture and Fatigue of Al2O3-Graphene Nanolayers NANO LETTERS Amirmaleki, M., Cui, T., Zhao, Y., Tam, J., Goel, A., Sun, Y., Sun, X., Filleter, T. 2021; 21 (1): 437-444


    Al2O3-graphene nanolayers are widely used within integrated micro/nanoelectronic systems; however, their lifetimes are largely limited by fracture both statically and dynamically. Here, we present a static and fatigue study of thin (1-11 nm) free-standing Al2O3-graphene nanolayers. A remarkable fatigue life of greater than one billion cycles was obtained for films <2.2 nm thick under large mean stress levels, which was up to 3 orders of magnitude longer than that of its thicker (11 nm) counterpart. A similar thickness dependency was also identified for the elastic and static fracture behavior, where the enhancement effect of graphene is prominent only within a thickness of ∼3.3 nm. Moreover, plastic deformation, manifested by viscous creep, was observed and appeared to be more substantial for thicker films. This study provides mechanistic insights on both the static and dynamic reliability of Al2O3-graphene nanolayers and can potentially guide the design of graphene-based devices.

    View details for DOI 10.1021/acs.nanolett.0c03868

    View details for Web of Science ID 000611082000059

    View details for PubMedID 33373247

  • Graphene fatigue through van der Waals interactions SCIENCE ADVANCES Cui, T., Yip, K., Hassan, A., Wang, G., Liu, X., Sun, Y., Filleter, T. 2020; 6 (42)


    Graphene is often in contact with other materials through weak van der Waals (vdW) interactions. Of particular interest is the graphene-polymer interface, which is constantly subjected to dynamic loading in applications, including flexible electronics and multifunctional coatings. Through in situ cyclic loading, we directly observed interfacial fatigue propagation at the graphene-polymer interface, which was revealed to satisfy a modified Paris' law. Furthermore, cyclic loading through vdW contact was able to cause fatigue fracture of even pristine graphene through a combined in-plane shear and out-of-plane tear mechanism. Shear fracture was found to mainly initiate at the fold junctions induced by cyclic loading and propagate parallel to the loading direction. Fracture mechanics analysis was conducted to explain the kinetics of an exotic self-tearing behavior of graphene during cyclic loading. This work offers mechanistic insights into the dynamic reliability of graphene and graphene-polymer interface, which could facilitate the durable design of graphene-based structures.

    View details for DOI 10.1126/sciadv.abb1335

    View details for Web of Science ID 000579164600012

    View details for PubMedID 33055156

    View details for PubMedCentralID PMC7556834

  • Toughening of graphene-based polymer nanocomposites via tuning chemical functionalization COMPOSITES SCIENCE AND TECHNOLOGY Najafi, F., Wang, G., Mukherjee, S., Cui, T., Filleter, T., Singh, C. 2020; 194
  • Tailoring the Mechanical and Electrochemical Properties of an Artificial Interphase for High-Performance Metallic Lithium Anode ADVANCED ENERGY MATERIALS Sun, Y., Amirmaleki, M., Zhao, Y., Zhao, C., Liang, J., Wang, C., Adair, K. R., Li, J., Cui, T., Wang, G., Li, R., Filleter, T., Cai, M., Sham, T., Sun, X. 2020; 10 (28)
  • Enhanced sensitivity of nanoscale subsurface imaging by photothermal excitation in atomic force microscopy REVIEW OF SCIENTIFIC INSTRUMENTS Yip, K., Cui, T., Filleter, T. 2020; 91 (6): 063703


    Photothermal excitation of the cantilever for use in subsurface imaging with atomic force microscopy was compared against traditional piezoelectric excitation. Photothermal excitation alleviates issues commonly found in traditional piezoelectrics such as spurious resonances by producing clean resonance peaks through direct cantilever excitation. A calibration specimen consisting of a 3 × 3 array of holes ranging from 200 to 30 nm etched into silicon and covered by graphite was used to compare these two drive mechanisms. Photothermal excitation exhibited a signal-to-noise ratio as high as four times when compared to piezoelectric excitation, utilizing higher eigenmodes for subsurface imaging. The cleaner and sharper resonance peaks obtained using photothermal excitation revealed all subsurface holes down to 30 nm through 135 nm of graphite. In addition, we demonstrated the ability of using photothermal excitation to detect the contact quality variation and evolution at graphite-polymer interfaces, which is critical in graphene-based nanocomposites, flexible electronics, and functional coatings.

    View details for DOI 10.1063/5.0004628

    View details for Web of Science ID 000543545500001

    View details for PubMedID 32611036

  • Fatigue of graphene NATURE MATERIALS Cui, T., Mukherjee, S., Sudeep, P. M., Colas, G., Najafi, F., Tam, J., Ajayan, P. M., Singh, C., Sun, Y., Filleter, T. 2020; 19 (4): 405-+


    Materials can suffer mechanical fatigue when subjected to cyclic loading at stress levels much lower than the ultimate tensile strength, and understanding this behaviour is critical to evaluating long-term dynamic reliability. The fatigue life and damage mechanisms of two-dimensional (2D) materials, of interest for mechanical and electronic applications, are currently unknown. Here, we present a fatigue study of freestanding 2D materials, specifically graphene and graphene oxide (GO). Using atomic force microscopy, monolayer and few-layer graphene were found to exhibit a fatigue life of more than 109 cycles at a mean stress of 71 GPa and a stress range of 5.6 GPa, higher than any material reported so far. Fatigue failure in monolayer graphene is global and catastrophic without progressive damage, while molecular dynamics simulations reveal this is preceded by stress-mediated bond reconfigurations near defective sites. Conversely, functional groups in GO impart a local and progressive fatigue damage mechanism. This study not only provides fundamental insights into the fatigue enhancement behaviour of graphene-embedded nanocomposites, but also serves as a starting point for the dynamic reliability evaluation of other 2D materials.

    View details for DOI 10.1038/s41563-019-0586-y

    View details for Web of Science ID 000508325000003

    View details for PubMedID 31959950

  • Nanomechanical elasticity and fracture studies of lithium phosphate (LPO) and lithium tantalate (LTO) solid-state electrolytes NANOSCALE Amirmaleki, M., Cao, C., Wang, B., Zhao, Y., Cui, T., Tam, J., Sun, X., Sun, Y., Filleter, T. 2019; 11 (40): 18730-18738


    All-solid-state batteries (ASSBs) have attracted much attention due to their enhanced energy density and safety as compared to traditional liquid-based batteries. However, cyclic performance depreciates due to microcrack formation and propagation at the interface of the solid-state electrolytes (SSEs) and electrodes. Herein, we studied the elastic and fracture behavior of atomic layer deposition (ALD) synthesized glassy lithium phosphate (LPO) and lithium tantalate (LTO) thin films as promising candidates for SSEs. The mechanical behavior of ALD prepared SSE thin films with a thickness range of 5 nm to 30 nm over suspended single-layer graphene was studied using an atomic force microscope (AFM) film deflection technique. Scanning transmission electron microscopy (STEM) coupled with AFM was used for microstructural analysis. LTO films exhibited higher stiffness and higher fracture forces as compared to LPO films. Fracture in LTO films occurred directly under the indenter in a brittle fashion, while LPO films failed by a more complex fracture mechanism including significant plastic deformation prior to the onset of complete fracture. The results and methodology described in this work open a new window to identify the potential influence of SSEs mechanical performance on their operation in flexible ASSBs.

    View details for DOI 10.1039/c9nr02176k

    View details for Web of Science ID 000490991700019

    View details for PubMedID 31591615

  • Investigating the detection limit of subsurface holes under graphite with atomic force acoustic microscopy NANOSCALE Yip, K., Cui, T., Sun, Y., Filleter, T. 2019; 11 (22): 10961-10967


    The subsurface imaging capabilities of atomic force acoustic microscopy (AFAM) was investigated by imaging graphite flakes suspended over holes in a silicon dioxide substrate. The graphite thickness and the hole size were varied to determine the detection limit on the maximum graphite thickness and the smallest detectable hole size. Parameters including operating frequency, eigenmode, contact force, and cantilever stiffness were investigated for their influence of defect detection. AFAM was reliably able to detect 2.5 μm diameter holes through a maximum graphite thickness of 570 nm and sub 100 nm holes through 140 nm of graphite. The smallest detectable defect size was a 50 nm hole covered by an 80 nm thick graphite flake. Increasing the graphite thickness and decreasing the hole size both resulted in a decrease in subsurface contrast. However, the non-linear trend observed from increasing the graphite thickness indicates thickness has a greater effect on subsurface defect detection than variations in defect size. Through investigating various parameters, we have found certain cases to increase the observed contrast of the embedded subsurface holes, however the smallest detectable defect size remained the same. This technique's ability to reveal sub 100 nm defects buried under graphite has previously only been demonstrated in much softer polymer systems.

    View details for DOI 10.1039/c9nr03730f

    View details for Web of Science ID 000470756000038

    View details for PubMedID 31140525

  • Effect of lattice stacking orientation and local thickness variation on the mechanical behavior of few layer graphene oxide CARBON Cui, T., Mukherjee, S., Cao, C., Sudeep, P. M., Tam, J., Ajayan, P. M., Singh, C., Sun, Y., Filleter, T. 2018; 136: 168-175
  • Characterization and atomistic modeling of the effect of water absorption on the mechanical properties of thermoset polymers ACTA MECHANICA Cui, T., Verberne, P., Meguid, S. A. 2018; 229 (2): 745-761