•  Developed a portfolio of wearable sensors that are capable of continuously analyzing the composition and flow rate of sweat for non-invasive medical and diagnostic devices (Nature 2016; PNAS 2017; Nature Electronics 2018; Science Advances 2019; Science Advances 2020; Nature Comm 2021).
•   The sensors can operate even when the subjects are at near rest, where sweat rates are low, without the use of any induction mechanism (Nature Comm, 2021).
•  Able to detect cystic fibrosis patients through Cl- and Na+ concentration measurements using wearable bands (PNAS 2017). We observed strong regulation of ions in sweat, including pH, Cl- and Na+ (Nature 2016; Science Advances 2019). We observed strong correlation of vitamin C (Advanced Materials, 2021), nicotine (ACS Sensors, 2020), caffeine (Advanced Materials, 2018) and levodopa levels (Nano Letters, 2019) in sweat to intake, and were able to measure the absorption and metabolism rates. We showed a strong correlation between local sweat rate measurements and whole body weight loss (Science Advances 2019), and have observed a strong dependence of sweat rate on mental stress (Nature Comm, 2021).

•   Demonstrated near-unity photoluminescence quantum yield in TMDC monolayers at all exciton concentrations. Specifically, neutral exciton recombination at low exciton concentrations is entirely radiative (Science 2019, Science 2015) as observed by moving the Fermi level near mid-gap. At high exciton concentrations, nonradiative exciton-exciton annihilation is eliminated by applying a small mechanical strain to favorably tune the band structure (Science 2021).
•   Reported molecular monolayers with near-unity photoluminescence quantum yield with a lifetime of 10’s of psec. This material could result in the development of the fastest LEDs (Nature Comm, 2019; Matter 2020).
•   Developed actively variable mid-IR LEDs and photodetectors based on black phosphorous. A single device can be actively tuned to operate between 0.22 to 0.53 eV by reversible application of strain, taking advantage of the extraordinary sensitivity of black phosphorous bandgap to strain. (Nature 2021).
•   Measured the quantum unit of absorbance in 2D semiconductors (PNAS, 2013).  The magnitude of absorption associated with each interband transition in a 2D semiconductor (non-excitonic system) is ~1.6%, independent of material thickness or other fundamental material properties. The total absorption is thus dictated by the total number of allowed interband transitions.
•   First report of spatially indirect emission (i.e., spatially indirect excitons) in van der Waals heterobilayers (PNAS 2014).

•    Demonstrated 1nm physical gate length transistors with near-ideal switching performance (Science 2016). The heavier effective mass and lower dielectric constant combined with atomically thin and uniform thickness, and naturally passivated surfaces of MoS2 provided advantages over silicon in FET scaling. The work highlights the key material and device properties that need to be carefully considered in the design of future nanoscale transistors.

•    Demonstrated highly sensitive gas sensors based on bulk Si MOSFETs that are surface modified (Science Advances, 2017; ACS Nano, 2018, ACS Sensors 2019, Advanced Materials 2020). The gate electrode is replaced with a chemical sensing layer whose work function changes selectively with gas exposure. The result is a current modulation in the underlying Si MOSFET. Integrated micro-heaters, operating slightly above the Dew point, mitigate ambient humidity effects which was one of the fundamental challenges in the field. With proper doping and electrostatics, ultrathin inversion layers are generated in bulk MOSFETs, resulting in high sensitivity.

•    Developed ultrathin body III-V on insulator (XOI) device concept as a platform for integrating high mobility III-V nanosheets (down to 3 nm in thickness) on Si for low power electronics (Nature, 2010).  Reported p- and n-type III-V FETs with high mobilities and subthreshold swing as low as ~70 mV/decade.

•    Developed a templated liquid phase growth process for direct synthesis of single-crystalline III-V structures with user defined geometry and topography on virtually any substrate (Nature Comm 2016), without the need for lattice matching constraints. The growth can be performed at low substrate temperatures, down to 180C, making it compatible with plastics, glass and Si CMOS (PNAS 2020).

•    Developed monolayer doping (MLD) for ultrashallow junctions. The process utilizes self-assembled monolayers of dopant containing species on semiconductor surfaces (including Si, Ge and III-V’s), followed by formation of a silicon dioxide cap and subsequent diffusion by thermal annealing (Nature Materials, 2008; Nano Letters, 2009).  The process yielded the shallowest junctions reported at the time in silicon, down to ~3 nm in thickness. It demonstrated that surface diffusion can be tuned to achieve ultrashallow junctions without sacrificing dopant dose uniformity. The dopant containing monolayers can be formed using a half-cycle of an ALD process (ACS Applied Materials & Interfaces, 2017). A polymer-based counterpart of the process was also developed, which, for example, could be used for printing dopant speciess for Si PVs to eliminate expensive photolithography steps (The Journal of Physical Chemistry Letters, 2013).

•    Demonstrated a dopant-free Si solar cell concept with all fabrication processes performed at near room temperature (Nature Energy 2016; Advanced Energy Materials 2019; ACS Energy Letters 2018; Nature Energy 2019). We utilized extremely low work function (e.g., LiF) and high work function (e.g., MoOx) materials as electron and hole contacts, respectively without the need for heavily doped regions. Solar cells with efficiencies >23% have now been reported based on this device architecture.