Research

Selected Publications


Controlled Scalable Synthesis of 2D Materials

Two-dimensional (2D) transition metal dichalcogenide (TMDC) materials like MoS2 and WS2 in forms of a monolayer or fewlayer of atoms bear a tantalizing potential to revolutionize the semiconductor industry.

Unlike graphene, which is gapless by nature or shows a trivial bandgap (< 0.25 eV) by engineering, 2D TMDC materials may exhibit a bandgap in amplitude comparable to those of the conventional semiconductors used in the semiconductor industry, like group IV (Si, Ge) or group III-V (such as GaAs, GaP, InP) semiconductors. For instance, monolayer MoS2 has a direct optical bandgap of around 1.9 eV3. Additionally, unlike the conventional semiconductor materials, whose electrical and optical functionalities would dramatically degrade due to the presence of surface defects if the dimension being scaled down to the atomic level, 2D TMDC materials may show excellent electrical and optical properties because its surface is perfectly passivated with no dangling bonds. Other than that, 2D TMDC materials often exhibit good chemical and thermal stability and high mechanical strength.In essence, 2D TMDC materials like monolayer MoS2 and WS2 provide an attractive semiconductor option at the atomic level and promise to open up a new age of atomic-scale semiconductor technology, in which the electronic and optical devices like integrated circuits, lasers, LEDs, solar cells, photodetectors would be scaled down to a truly atomic level. This atomic-scale miniaturization would not only dramatically improve the performance of semiconductor devices but also would enable new functionalities that cannot be obtained with the existing semiconductor devices.However, one major challenge for exploring 2D TMDC materials to develop atomic-scale semiconductor technology lies in the lack of capabilities to manufacture these materials in scalable and controlled ways. Additionally, the manufacturing process should be reasonably cost-effective or could be potentially made cost-effective.We have developed a unique self-limiting CVD process that can be used to grow centimeter-scale uniform atomically thin MoS2 films with controlled layer numbers. MoCl5 and sulfur were used as precursors. The synthesized MoS2 film shows excellent uniformity, crystalline quality, and good optical/electrical properties comparable to the MoS2 exfoliated from bulk materials .

Optical images of 2D TMDC films with size up to 2-inch wafer scale.

Optical images of 2D TMDC films with size up to 2-inch wafer scale.

Selected Publications

(35) Controlled Scalable Synthesis of Uniform, High-Quality Monolayer and Few-layer MoS2 Films

Yifei Yu, Chun Li, Yi Liu, Liqin Su, Yong Zhang, Linyou Cao
Scientific Reports 3, 1866, (2013). View.

(41) Surface Energy-Assisted Transfer of Centimeter-scale Monolayer and Fewlayer MoS2 Films onto Arbitrary Substrates

Alper Gurarslan, Yifei Yu, Liqin Su, Yiling Yu, Francisco Suarez, Shanshan Yao, Yong Zhu, Mehmet Ozturk, Yong Zhang, Linyou Cao
ACS Nano 8, 11522-11528, (2014). View.


2D Materials Photonics

2D TMDC materials provide a remarkable excitonic system with exciton binding energy more than one order of magnitude higher than conventional semiconductors. Therefore, they may enable the development of novel atomic-scale photonic devices like lasers or LEDs that would feature extreme flexibility, high efficiency, and low cost. However, despite the great potential, the development of novel photonic devices with 2D TMDC materials has been delayed by the lack of fundamental understanding. We are interested in pursuing better understanding for the light-matter interaction and excitonic properties of 2D TMDC materials.

Knowledge on these issues may provide useful guidance for the rational design of photonic devices. Some of our recent breakthroughs include:

1. Demonstrated that the dielectric function of atomically thin MoS2 materials is dominated by excitonic effect, rather than the effect of band strucutures.2. The excitonic effect dominated dielectric function can be dramatically tuned using electrical ways.3. Demonstrated that the luminescence efficiency of 2D TMDC materials can be lowered by orders of magnitude due to the doping effect of substrates and/or the effect of substrates to facilitate defect-assisted non-radiative exciton recombination.4. Designed optical superabsorbers (absorption efficiency > 75%) with atomically thin MoS2 films for either narrowband or broadband (bandwidth > 300 nm) incidence.

Light-matter interaction in 2D TMDC materials

Light-matter interaction in 2D TMDC materials

Selected Publications

(51) Atomically Thin MoS2 Narrowband and Broadband Light Superabsorbers

Lujun Huang, Guoqing Li, Alper Gurarslan, Yiling Yu, Ronny Kirste, Wei Guo, Junjie Zhao, Ramon Collazo, Zlatko Sitar, Gregory N. Parsons, Michael Kudenov, and Linyou Cao
ACS Nano DOI: 10.1021/acsnano.6b02195, , (2016). View.

(53) Giant Gating Tunability in Refractive Index of Transition Metal Dichalcogenide Monolayers

Yiling Yu, Yifei Yu, Lujun Huang, Haowei Peng, and Linyou Cao
Submitted , , (2016). View.

(50) Engineering Substrate Interactions for High Luminescence Efficiency of Transition Metal Dichalcogenide Monolayers

Yifei Yu, Yiling Yu, Chao Xu, Yong-Qing Cai, Liqin Su, Yong Zhang, Yong-Wei Zhang, Kenan Gundogdu, Linyou Cao
Advanced Functional Materials , DOI: 10.1002/adfm.201600418, (2016). View.

(49) Fundamental Limits of Exciton-Exciton Annihilation for Light Emission in Transition Metal Dichalcogenide Monolayers

Yiling Yu, Yifei Yu, Chao Xu, Andy Barrette, Kenan Gundogdu, Linyou Cao
Phys. Rev. B Rapid Communication 93, 20111(R), (2016). View.

(44) Exciton-dominated Dielectric Functions of Atomically Thin MoS2 Films

Yiling Yu, Yifei Yu, Yongqing Cai, Wei Li, Alper Gurarslan, Hartwin Peelaers, David E. Aspnes, Chris G. Van de Walle, Nhan V. Nguyen, Yong-Wei Zhang, Linyou Cao
Scientific Reports 5 , 16996, (2015). View.

(46) Two-Dimensional Transition Metal Dichalcogenide Materials: Toward An Age of Atomic-Scale Photonics

Linyou Cao
MRS Bulletin 40, 592-599, (2015). View.

(43) Efficient Interlayer Relaxation and Transition of Excitons in Epitaxial and Non-epitaxial MoS2/WS2 Heterostructures

Yifei Yu, Shi Hu, Liqin Su, Lujun Huang, Yi Liu, Zhenghe Jin, Alexander A. Purezky, David B. Geohegan, Ki Wook Kim, Yong Zhang, Linyou Cao
Nano Letters 15, 486-491, (2015). View.

(36) Many Body Effects in Valleytronics: Direct Measurement of Valley Lifetimes in Single Layer MoS2

Cong Mai,Andrew Barrette, Yifei Yu, Yuriy Semenov, Ki Wook Kim, Linyou Cao, Kenan Gundogdu
Nano Letters 14, 202-206, (2014). View.


2D Materials Catalysis

Transition metal dichalcogenide materials like molybdenum disulfide (MoS2) have been widely considered to be a promising low-cost catalyst for hydrogen evolution in water. These material bears particular implications for the storage of solar energy due to its capability to efficiently absorb solar radiation4,5 and fast interfacial charge transfer. However, the catalytic activity of MoS2 is way inferior to that of Pt, the golden standard for hydrogen evolution catalysts. One key challenge for improving the catalytic activity of MoS2 is the lack of unambiguous understanding for its catalytically active sites.

The atomically thin MoS2 film provides new opportunities to study the fundamentals of the catalytic behavior. More specifically, it allows one to student the catalytic activities as a function of physical features controlled in atomic precision, which may be very difficult in previous studies. For instance, by taking the advantage of the well-defined physical features, we demonstrate that the catalytic activities of MoS2 films show clear dependence on the layer number, the exchange current density exponentially decreasing with the layer number increasing. Additionally, we have unambiguously demonstrated that, except the edge sites, sulfur vacancies may provide excellent catalytic activities, while grain boundaries may only show very weak activities. Except providing an unprecedented platform for the fundamental studies, the atomically thin film may also provide opportunities to optimize the catalytic performance by engineering the catalyst at the truly atomic level.

Excellent Catalytic Performance of Monolayer MoS2 Films with Little Edge Sites

Excellent Catalytic Performance of Monolayer MoS2 Films with Little Edge Sites

Selected Publications

(52) All The Catalytic Sites of MoS2 for Hydrogen Evolution

Guoqing Li, Du Zhang, Qiao Qiao, Yifei Yu, David Peterson, Abdullah Zafar, Raj Kumar, Stefano Curtarolo, Frank Hunte, Steve Shannon, Yimei Zhu, Weitao Yang, and Linyou Cao
Journal of the American Chemical Society 138, 16632-16638, (2016). View.

(42) Engineering the Composition and Crystallinity of Molybdenum Sulfide for High-performance Electrocatalytic Hydrogen Evolution

Yanpeng Li, Yifei Yu, Sheng-Yang Huang, Yufeng Huang, Robert A. Nielsen, Williams A. Goddard III, Yao Li, Linyou Cao
ACS Catalysis 5, 448-451, (2015). View.

(37) Layer-dependent Electrocatalysis of MoS2 for Hydrogen Evolution

Yifei Yu, Shengyang Huang, Yanpeng Li, Stephan Steinmann, Weitao Yang, Linyou Cao
Nano Letters 14, 553-558, (2014). View.