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Semiconductors have been fundamental to various devices that are typically operated with electric field, such as transistors, memories, sensors, and resistive switches. There is growing interest in the development of novel inorganic materials for use in transistors and semiconductor switches, which can be operated with a temperature gradient. Here, we show that a crystalline semiconducting noble metal sulfide, AgCuS, exhibits a sharp temperature dependent reversible p-n-p type conduction switching, along with a colossal change in the thermopower (ΔS of ~1757 μV K(-1)) at the superionic phase transition (T of ~364 K). In addition, its thermal conductivity is ultralow in 300-550 K range giving AgCuS the ability to maintain temperature gradients. We have developed fundamental understanding of the phase transition and p-n-p type conduction switching in AgCuS through temperature dependent synchrotron powder X-ray diffraction, heat capacity, Raman spectroscopy, and positron annihilation spectroscopy measurements. Using first-principles calculations, we show that this rare combination of properties originates from an effective decoupling of electrical conduction and phonon transport associated with electronic states of the rigid sulfur sublattice and soft vibrations of the disordered cation sublattices, respectively. Temperature dependent p-n-p type conduction switching makes AgCuS an ideal material for diode or transistor devices that operate reversibly on temperature or voltage changes near room temperature.

Defects play a pivotal role in the device performance of a photocatalytic, light-emitting, or photovoltaic system. Herein, graphitic carbon nitride (g-C 3 N 4 ) nanosheets are prepared at different calcination temperatures, and the evolution of defects in the system is studied by positron annihilation spectroscopy (PAS) and photoluminescence (PL) spectroscopy. Steady-state PL spectra show that free and defect-bound excitonic emission peaked at 2.78, 2.58, and 2.38 eV are dominant with above-band-gap excitation. Timeresolved PL studies reveal a significant enhancement of excitonic lifetime from 17.4 ns for free exciton to 27.4 ns in case of defect-bound exciton. We provide a direct correlation between the defects observed by PAS and those of the excitonic lifetime found from PL studies. Below-band-gap excitation activates defect emission, and it is characterized by a short carrier lifetime (∼0.14 ns). An excitation power-dependent PL study with 405 nm laser shows a progressive red shift and narrowing of the emission line. We have interpreted the different PL features with defect band filling of exciton, interplanar, intraplanar, interchain exciton migration, etc. These results are significant for tuning the optoelectronic properties of g-C 3 N 4 nanosheets and exploiting their applications in various emerging areas.

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