Journal
NANO LETTERS
Volume 13, Issue 11, Pages 5627-5634Publisher
AMER CHEMICAL SOC
DOI: 10.1021/nl403270k
Keywords
Transition metal dichalcogenides; indirect band gap; strain; thermal expansion; photoluminescence spectroscopy; exciton
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Funding
- Singapore National Research Foundation under NRF Research Fellowship [NRF-NRFF2011-02]
- Graphene Research Centre
- FEDER through the COMPETE Program
- Portuguese Foundation for Science and Technology (FCT) [PEST-C/FIS/UI607/2011, SFRH/BSAB/1249/2012]
- EC under Graphene Flagship [CNECT-ICT-604391]
- Fundação para a Ciência e a Tecnologia [SFRH/BSAB/1249/2012] Funding Source: FCT
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It has been well-established that single layer MX2 (M = Mo, W and X = S, Se) are direct gap semiconductors with band edges coinciding at the K point in contrast to their indirect gap multilayer counterparts. In few-layer MX2, there are two valleys along the Gamma-K line with similar energy. There is little understanding on which of the two valleys forms the conduction band minimum (CBM) in this thickness regime. We investigate the conduction band valley structure in few-layer MX2 by examining the temperature-dependent shift of indirect exciton photoluminescence peak. Highly anisotropic thermal expansion of the lattice and the corresponding evolution of the band structure result in a distinct peak shift for indirect transitions involving the K and Lambda (midpoint along Gamma-K) valleys. We identify the origin of the indirect emission and concurrently determine the relative energy of these valleys.
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