The group's research is centered in the general areas of solid-state electronics and photonics with particular attention to apply nanomaterials and micro-/nanofabrication for next-generation electronic, photonic, and optoelectronic devices. The research projects include design, fabrication, and characterization of colloidal quantum dot (CQD) and cellulose nanocrystal (CNC) based electronic and photonic devices, as well as micro-/nanostructured devices for sensing and energy harvesting.

Colloidal quantum dots have been applied for next-generation optoelectronics in the past decade. Various CQD-based devices, such as photodetectors, LEDs and solar cells, show promising results in terms of detection sensitivity, device response time, and other features of merits. While working on various light management strategies (engineering approaches) to enhance the performance of CQD solar cells [1][2], we demonstrated CQD devices for thermophotovoltaics [3], field-effect enhanced triboelectric touch sensors [4], and filter-free narrowband photodetectors [5]. We also carried out experiments to develop highly sensitive, ultrafast photodetectors using CQD materials [6,7].

[1] S. Mahpeykar, Q. Xiong & X. Wang, "Resonance-induced absorption enhancement in colloidal quantum dot solar cells using nanostructured electrodes", Optics Express 22, A1576 (2014).
[2] S. Mahpeykar, Q. Xiong, J. Wei, L. Meng, B. K. Russell, P. Hermansen, A. V. Singhal & X. Wang, "Stretchable hexagonal diffraction gratings as optical diffusers for in-situ tunable broadband photon management", Advanced Optical Materials 4, 1106 (2016).
[3] A. Kiani, H. F. Movahed, S. Hoogland, O. Voznyy, R. Wolowiec, L. Levina, F. P. Garcia de Arguer, P. Pietsch, X. Wang, P. Maraghechi & E. H. Sargent, "Gradient-doped colloidal quantum dot slids enable thermophotovoltaic harvesting of waster heat", ACS Energy Letters 1, 740 (2016).
[4] L. Meng, Q. Xu, S. Fan, C. Dick & X. Wang, "Field-effect enhanced triboelectric colloidal quantum dot flexible sensor", Applied Physics Letters 111, 183103 (2017).
[5] Q. Xiong, F. I. Chowhury & X. Wang, "Filter-free narrowband photodetectors employing colloidal quantum dots", IEEE Journal of Selected Topics in Quantum Electronics 24, 1900406 (2018).
[6] Q. Xu, L. Meng, T. Zeng, K. Sinha, C. Dick & X. Wang, "On-chip colloidal quantum dot devices with a CMOS compatible architecture for near-infrared light sensing", Optics Letters 44, 463 (2019).
[7] Q. Xu, L. Meng, K. Sinha, F. I. Chowdhury, J. Hu & X. Wang, "Ultrafast colloidal quantum dot infrared photodiode", ACS Photonics 7, 1297 (2020).

Micro-/photonics control the behavior of light using metallic and dielectric micro-/nanostructures. In recent years, we have been working on several applications using micro-/nanostructures, including large-area 2D micro-/nanostructures for plasmonics [1], nanostructured optical diffusers for phosphor-converted white LEDs [2], and 3D photonic crystals for fast analysis of hydrocarbon mixtures [3]. We also collaborate with with Prof. Zhang in the Department of Chemical and Materials Engineering at the University of Alberta to investigate surface nanolenses invented by her group [4,5].

[1] Q. Xiong, J. Wei, S. Mahpeykar, L. Meng & X. Wang, "Observation of localized surface plasmons and hybridized surface plasmon polaritons on self-assembled two-dimentional nanocavities", Optics Letters 41, 1506 (2016).
[2] Q. Xu, L. Meng & X. Wang, "Nanocrystal-filled polymer for improving angular color uniformity of phosphor-converted white LEDs", Applied Optics 58, 7649 (2019).
[3] Q. Xu, S. Mahpeykar, I. B. Burgess & X. Wang, "Inverse opal photonic crystals as an optofluidic platform for fast analysis of hydrocarbon mixtures", ACS Applied Materials & Interfaces 10, 20120 (2018).
[4] B. Dyett, Q. Zhang, Q. Xu, X. Wang & X. Zhang, "Extraordinary focusing effect of surface nanolenses in total internal reflection mode", ACS Central Science 4, 1511 (2018).
[5] L. Bao, B. Pinchasik, L. Lei, Q. Xu, H. Hao, X. Wang & X. Zhang, "Controlling of femtoliter liquid on a microlens: a way to flexible dual microlens arrays", ACS Applied Materials & Interfaces 11, 27386 (2019).

Functional electronic devices integrated on flexible substrates are of great interest in both academia and industry for their potential applications in wearable technologies. Recently, we reported a new class of sensors, digital microelectromechanical sensors, based on nano-/microstructure engineering of the widely available plastic – polydimethylsiloxane (PDMS) [1]. These digital sensors can be engineered with various sensitivities for different applications, and have an outstanding durability after long time usage – no degradation after 10,000 times’ bending or pressing in operation. We further improved the response time of these sensors [2], and integrated them for robotic applications [3]. We also demonstrated carbon nanotube based triboelectric devices for flexible touch displays [4].

[1] L. Meng, S. Fan, S. Mahpeykar & X. Wang, "Digital microelectromechanical sensor with an engineered polydimethylsiloxane (PDMS) bridge structure", Nanoscale 9, 1257 (2017).
[2] S. Fan, L. Dan, L. Meng, W. Zheng, A. Elias & X. Wang, “Improved response time of flexible microelectromechanical sensors employing eco-friendly nanomaterials”, Nanoscale 9, 16915 (2017).
[3] S. Fan, L. Meng, L. Dan, W. Zheng & X. Wang, "Polymer microelectromechanical system-integrated flexible sensors for wearable technologies", IEEE Sensors Journal 19, 443 (2019).
[4] L. Meng, Q. Xu, L. Dan & X. Wang, "Single-walled carbon nanotube based triboelectric flexible touch sensors", IEEE/TMS Journal of Electronic Materials 48, 7411 (2019).

Cellulose nanocrystals (CNCs) and lignin, wood-derived eco-friendly materials, recently attract a great attention in academia and industry for their properties of high tensile strength, low density, low thermal expansion, and non-toxicity. Recently, we employed CNC films for transient electronics [1]. We also demonstrated CNC films as host matrix and waveguide [2] for luminescent solar concentrator (LSC) - a solar energy harvesting device that absorbs both direct or diffuse sunlight and is a perfect candidate for window materials in building integrated photovoltaics (BIPV). CNC can be doped into polymers to produce optical diffusers for enhanced light extraction in OLED devices [3] and solar cells [4].

[1] L. Meng, S. Mahpeykar, Q. Xiong, B. Ahvaz & X. Wang, "Strain sensors on water-soluble cellulose nanofibril paper by polydimethylsiloxane (PDMS) stencil lithography", RSC Advances 6, 85427 (2016).
[2] F. I. Chowdhury, C. Dick, L. Meng, S. Mahpeykar, B. Ahvazi & X. Wang, "Cellulose nanocrystals as host matrix and waveguide materials for recyclable luminescent solar concentrators", RSC Advances 7, 32436 (2017).
[3] S. Mahpeykar, Y. Zhao, X. Li, Z. Yang, Q. Xu, Z-H. Lu, E. H. Sargent & X. Wang, "Cellulose nanocrystal:polymer hybrid optical diffusers for index-matching-free light management in optoelectronic devices", Advanced Optical Materials 5, 1700430 (2017).
[4] Q. Xu, L. Meng & X. Wang, "Reducing shadowing losses in silicon solar cells using cellulose nanocrystal:polymer hybrid diffusers", Applied Optics 58, 2505 (2019).