Physics

Dr. Deep Jariwala
University of Pennsylvania, U.S.A.

Abstract: 

The isolation of a growing number of two-dimensional (2D) materials has inspired worldwide efforts to integrate distinct 2D materials into van der Waals (vdW) heterostructures. While a tremendous amount of research activity has occurred in assembling disparate 2D materials into “all-2D” van der Waals heterostructures,^{1, 2} this concept is not limited to 2D materials alone. Given that any passivated, dangling bond-free surface will interact with another via vdW forces, the vdW heterostructure concept can be extended to include the integration of 2D materials with non-2D materials that adhere primarily through noncovalent interactions.³ In the first part of this talk I will present our work on emerging mixed-dimensional (2D + nD, where n is 0, 1 or 3) heterostructure devices. I will present two distinct examples of gate-tunable p-n heterojunctions.^4-6I will show that when a single layer n-type molybdenum disulfide (MoS₂) (2D) is combined with p-type semiconducting single walled carbon nanotubes (1D), the resulting p-n junction is gate-tunable and shows a tunable diode behavior with rectification as a function of gate voltage and a unique anti-ambipolar transfer behavior.⁴ The same concept when extended to p-type organic small molecule semiconductor (pentacene) (0D) and n-type 2D MoS₂ leads to a tunable p-n junction with a photovoltaic effect and an asymmetric anti-ambipolar transfer response.⁶ I will present the underlying charge transport and photocurrent responses in both the above systems using a variety of scanning probe microscopy techniques as well as computational methods. Finally, I will show that the anti-ambipolar field effect observed in the above systems can be generalized to other semiconducting heterojunction systems and extended over large areas with practical applications in wireless communication circuits.⁵  

       The second part of talk will discuss my more recent work on photovoltaic devices from 2D semiconductors such as transition metal dichalcogenides (TMDCs). High efficiency inorganic photovoltaic materials (e.g., Si, GaAs and GaInP) can achieve maximum above-bandgap absorption as well as carrier-selective charge collection at the cell operating point. I will show experimental demonstration of light confinement in ultrathin (< 15 nm) Van der Waals semiconductors (MoS₂, WS₂ and WSe₂) leading to nearly perfect absorption.⁷ I will further present the fabrication and performance of our, broadband absorbing, heterostructure photovoltaic devices using sub-15 nm TMDCs as the active layers, with record high quantum efficiencies.^{7, 8} I will then present ongoing work on addressing the key remaining  challenges⁹ for application of 2D materials and their heterostructures in high efficiency photovoltaics which entails engineering of interfaces and open-circuit voltage as well as on going work on novel band-bending heterojunctions as well as probing of buried metal/semiconductor interfaces¹⁰ with sub 50 nm resolutions. I will conclude by giving a broad perspective of future work on 2D materials from fundamental science to applications.

References:

1.         Grigorieva, I. V.; Geim, A. K. Nature 2013, 499, 419-425.

2.         Jariwala, D.; Sangwan, V. K.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C. ACS Nano2014, 8, 1102-20.

3.         Jariwala, D.; Marks, T. J.; Hersam, M. C. Nat. Mater. 2017, 16, 170-181.

4.         Jariwala, D.; Sangwan, V. K.; et al. Proc. Nat. Acad. Sci. USA 2013, 110, 18076–18080.

5.         Jariwala, D.; Sangwan, V. K.;  et al. Nano Lett. 2015, 15, 416-421.

6.         Jariwala, D.; Howell, S. L.; et al. Nano Lett. 2016, 16, 497–503.

7.         Jariwala, D.; Davoyan, A. R.; et al. Nano Lett. 2016, 16, 5482-5487.

8.         Wong, J.; Jariwala, D.; et al. ACS Nano 2017, 11, 7230–7240.

9.          Jariwala, D.; Davoyan, A; et al. ACS Photonics 2017, 4, 4962-4970

10.       Jariwala, D.; Krayev, A; et al. 2D Mater. 2018, 5, 035003