SHI ET AL. VOL. 7 ’ NO. 2 ’ 1072–1080 ’ 2013 www.acsnano.org 1072 December 30, 2012 C 2012 American Chemical Society Exciton Dynamics in Suspended Monolayer and Few-Layer MoS 2 2D Crystals Hongyan Shi, †,‡ Rusen Yan, § Simone Bertolazzi, ^ Jacopo Brivio, ^ Bo Gao, ‡ Andras Kis, ^ Debdeep Jena, § Huili Grace Xing, § and Libai Huang †, * † Radiation Laboratory and § Department of Electrical Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States, ‡ Department of Physics, Harbin Institute of Technology, Harbin, China, and ^ Electrical Engineering Institute, Ecole Polytechnique Federale de Lausanne(EPFL), CH-1015 Lausanne, Switzerland T ransition metal dichalcogenides are layered materials where the adjacent planes are held together by van der Waals interactions. 1,2 These weak interlayer interactions allow for micromechanical ex- foliation of single layers from bulk crystal- line samples similar to the fabrication of graphene from graphite. 2 Single layers of transition metal dichalcogenides can be either semiconducting or metallic depend- ing on the structure. 1,2 In particular, MoS 2 has recently emerged as a promising candi- date for the semiconducting analogue of graphene. 2À6 Field-effect transistors have been successfully fabricated from mono- layer and few-layer MoS 2 samples with per- formance which rivals that of those from graphene ribbons. 3 Quantum confinement effects are ex- pected to be important for monolayer and few-layer MoS 2 structures because the thickness becomes smaller than the exciton Bohr radius. 2,7À10 A recent computational work predicted the Bohr radius for mono- layer MoS 2 to be 0.93 nm with a large exciton bing energy of ∼0.9 eV. 9 Unlike that of a conventional bulk semiconductor, interlayer interaction in layered structures modifies excitonic properties in a complex manor. 9 Indeed, a dramatic enhancement of photoluminescence quantum efficiency in monolayer MoS 2 has been observed in photoluminescence (PL) experiments and was attributed to indirectÀdirect band gap crossover in the monolayer due to such quantum confinement effects. 11,12 More re- cently, long-lived (>1 ns) valley polarization was observed in monolayer MoS 2 . 13,14 While the steady-state optical spec- troscopy of MoS 2 2D crystals has been reported, 1,15,16 very limited work has been done on ultrafast time-resolved spectro- scopy of this system. 17À19 Recent tempera- ture-dependent PL lifetime measurements provided important information on exciton lifetimes in monolayers. 17 Transient absorption * Address correspondence to lhuang2@nd.edu. Received for review August 29, 2012 and accepted December 29, 2012. Published online 10.1021/nn303973r ABSTRACT Femtosecond transient absorption spectroscopy and microscopy were employed to study exciton dynamics in suspended and Si 3 N 4 substrate-supported monolayer and few-layer MoS 2 2D crystals. Exciton dynamics for the monolayer and few-layer struc- tures were found to be remarkably different from those of thick crystals when probed at energies near that of the lowest energy direct exciton (A exciton). The intraband relaxation rate was enhanced by more than 40 fold in the monolayer in comparison to that observed in the thick crystals, which we attributed to defect assisted scattering. Faster electronÀhole recombination was found in monolayer and few-layer structures due to quantum confinement effects that lead to an indirectÀdirect band gap crossover. Nonradiative rather than radiative relaxation pathways dominate the dynamics in the monolayer and few-layer MoS 2 . Fast trapping of excitons by surface trap states was observed in monolayer and few-layer structures, pointing to the importance of controlling surface properties in atomically thin crystals such as MoS 2 along with controlling their dimensions. KEYWORDS: MoS 2 . atomically thin 2D crystal . exciton dynamics . transient absorption spectroscopy . transient absorption microscopy . quantum confinement . surface defects ARTICLE