LEI ET AL. VOL. 8 ’ NO. 2 ’ 1263–1272 ’ 2014 www.acsnano.org 1263 January 06, 2014 C 2014 American Chemical Society Evolution of the Electronic Band Structure and Efficient Photo-Detection in Atomic Layers of InSe Sidong Lei, † Liehui Ge, † Sina Najmaei, † Antony George, † Rajesh Kappera, ‡ Jun Lou, † Manish Chhowalla, ‡ Hisato Yamaguchi, § Gautam Gupta, § Robert Vajtai, † Aditya D. Mohite, §, * and Pulickel M. Ajayan †, * † Department of Materials Science and NanoEngineering, Rice University, Houston, Texas 77005, United States, ‡ Department of Materials Science, Rutgers, the State University of New Jersey, Piscataway, New Jersey 08854, United States, and § MPA-11 Materials Synthesis and Integrated Devices, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States T ransformation of a material from bulk to two-dimensional (2D) results in the realization of new physical phenom- ena. The resulting properties form the basis for futuristic thin film technologies. Since the discovery of graphene from bulk gra- phite in 2004, 1 it has been viewed as an ideal material for next generation applica- tions in photonics, nonlinear optics, THz electronics, flexible transparent electrodes, sensors, conductive composites, and gas separation membranes. 2À10 Despite exten- sive studies, 11À16 graphene-based FETs still cannot compete with traditional silicon- based electronic devices as it suffers from an intrinsic bottleneck for its use in optoe- lectronic applications. Graphene is a zero band gap semiconductor in which the con- duction and valence bands meet at the Fermi energy. This implies that there are no electronic states in graphene that allow photoexcited carriers to be generated and have long enough lifetimes 17,18 to develop optoelectronic devices such as photodetec- tors and photovoltaics. Although some re- ports showed graphene has photoresponse and responsivity is high, the dark current is usually very high due the absent of band gap. 19À21 Despite these shortcomings, gra- phene research has served as a catalyst for the birth of a new field, beyond graphene, in the form of novel 2D layered semiconduct- ing materials also known as transition-metal dichalcogenides (TMDC) that exhibit novel electrical and optical properties. 22À32 Re- cent studies have shown that single-layer MoS 2 has a photoresponsivity of 0.42 mA/W and a band gap of 1.8 eV. 33 Lopez-Sanchez et al. showed that the photoresponsivity of MoS 2 can be significantly enhanced by * Address correspondence to amohite@lanl.gov, ajayan@rice.edu. Received for review August 29, 2013 and accepted January 6, 2014. Published online 10.1021/nn405036u ABSTRACT Atomic layers of two-dimensional (2D) materials have recently been the focus of extensive research. This follows from the footsteps of graphene, which has shown great potential for ultrathin optoelectronic devices. In this paper, we present a comprehensive study on the synthesis, characterization, and thin film photodetector application of atomic layers of InSe. Correlation between resonance Raman spectroscopy and photoconductivity measurements allows us to systematically track the evolution of the electronic band structure of 2D InSe as its thickness approaches few atomic layers. Analysis of photoconductivity spectra suggests that few-layered InSe has an indirect band gap of 1.4 eV, which is 200 meV higher than bulk InSe due to the suppressed interlayer electron orbital coupling. Temperature-dependent photocurrent measurements reveal that the suppressed interlayer interaction also results in more localized p z -like orbitals, and these orbitals couple strongly with the in-plane E 0 and E 00 phonons. Finally, we measured a strong photoresponse of 34.7 mA/W and fast response time of 488 μs for a few layered InSe, suggesting that it is a good material for thin film optoelectronic applications. KEYWORDS: photodetector . 2D layered materials . resonance Raman scattering . InSe . photoconductivity ARTICLE