Strongly-Bound Excitons In Transition Metal Dichalcogenides And Organic Semiconductors

Abstract

Atomically-thin, semiconducting transition metal dichalcogenides (TMDs) and organic semiconductors such as rubrene hold exceptional promise for unique and niche electronic applications which cannot be solved with conventional semiconducting crystalline materials. In particular, the process by which excitons relax in thin TMDs controls device engineering considerations including charge carrier mobility and exciton diffusion length. The decay mechanism and time scales can critically depend on interfaces, method of sample preparation and temperature. Here, I present ultrafast transient reflectivity studies of several chemical vapor deposition (CVD) grown TMD structures, including few-layer 2H MoTe2 on SiO2, MoTe2 1T’-2H homojunctions and monolayer MoS2-WS2 lateral heterojunctions on sapphire. The transient reflectivity of CVD-grown, few-layer (5-10 layers) 2H MoTe2 carried out a both room temperature and cryogenic temperatures demonstrates a temperature and fluence dependence consistent with defect-mediated exciton decay. The optical properties of MoTe2 were additionally found to be stable over the course of 8 months air exposure. The biexponential decay dynamics of monolayer MoS2 and WS2 were shown to be consistent with previous investigations. Both studies of interfaces, including the 2H-1T’ MoTe2 homojunctions and the MoS2-WS2 heterojunction were unable to observe signatures of interfacial charge transfer due to lack of sufficient spatial resolution near the interface crossover. In addition to studies on TMDs, the low-wavenumber Raman modes of both isotopically substituted 13C Rubrene and those of a structural analog to rubrene, fm-rubrene, were measured and compared to native rubrene. The 13C rubrene demonstrated a uniform shift to lower energy intermolecular mode vibrations. The modes of fm-rubrene were characterized for the first time and compared to a predicted computational Raman spectrum showing large (~4%) deviations with theory at low vibrational energies (<200cm-1), suggesting intermolecular coupling becomes influential at this threshold.