Measuring Photoexcited Free Charge Carriers in Mono- to Few-Layer Transition-Metal Dichalcogenides with Steady-State Microwave Conductivity

Abstract

Photoinduced generation of mobile charge carriers is the fundamental process underlying many applications, such as solar energy harvesting, solar fuel production, and efficient photodetectors. Monolayer transition-metal dichalcogenides (TMDCs) are an attractive model system for studying photoinduced carrier generation mechanisms in low-dimensional materials because they possess strong direct band gap absorption, large exciton binding energies, and are only a few atoms thick. While a number of studies have observed charge generation in neat TMDCs for photoexcitation at, above, or even below the optical band gap, the role of nonlinear processes (resulting from high photon fluences), defect states, excess charges, and layer interactions remains unclear. In this study, we introduce steady-state microwave conductivity (SSMC) spectroscopy for measuring charge generation action spectra in a model WS2 mono- to few-layer TMDC system at fluences that coincide with the terrestrial solar flux. Despite utilizing photon fluences well below those used in previous pump–probe measurements, the SSMC technique is sensitive enough to easily resolve the photoconductivity spectrum arising in mono- to few-layer WS2. By correlating SSMC with other spectroscopy and microscopy experiments, we find that photoconductivity is observed predominantly for excitation wavelengths resonant with the excitonic transition of the multilayer portions of the sample, the density of which can be controlled by the synthesis conditions. These results highlight the potential of layer engineering as a route toward achieving high yields of photoinduced charge carriers in neat TMDCs, with implications for a broad range of optoelectronic applications.

Spatially Resolved Persistent Photoconductivity in MoS2–WS2 Lateral Heterostructures

“…45,46 However, the band alignment may not be the same for lateral and vertical TMD heterostructures, in general, 47 and for MoS 2 −WS 2 , in particular. 48 Furthermore, in part because the bandgaps differ by only ∼200 meV with only a small band offset, 44 the band alignment depends sensitively on strain and layer thickness 49 and may be further complicated by the presence of interfacial traps 50 and alloying. The nature of the interface can also be complicated by the possible presence of an interfacial exciton that can lead to increased recombination rates.…”

Spatially Resolved Persistent Photoconductivity in MoS$_2$-WS$_2$ Lateral Heterostructures

“…36,37 However, the band alignment may not be the same for lateral and vertical TMD heterostructures in general, 38 and for MoS 2 -WS 2 in particular. 39 Furthermore, in part because the bandgaps differ by only ≈ 200 meV with only a small band offset, 35 the band alignment depends sensitively on strain and layer thickness, 40 and may be further complicated by the presence of interfacial traps 41 and alloying. The nature of the interface can also be complicated by the possible presence of an interfacial exciton that can lead to increased recombination rates.…”

“…57 And although the mechanism remains unclear, recent results from WS 2 flakes suggest a prominent role for ML regions in the formation of free carriers possibly due to reduced exciton binding energy and traps at lateral interfaces. 41…”

Evaluation-oriented exploration of photo energy conversion systems: from fundamental optoelectronics and material screening to the combination with data science

“…Pulse radiolysis using a pulsed electron beam from a high-energy accelerator is an effective method for quantitative, homogeneous injection of charges in a solution and block sample [28,37], whereas light injection has an uncertainty of φ that depends on the sample, wavelength, intensity, and time range. Other notable TRMC options include field-induced TRMC [65], TRMC under pressure in a hydrostatic medium (<0.15 GPa) [66], steady-state microwave conductivity using chopped light and a lock-in amplifier [67,68], and spatiotemporal imaging of microwave conductivity [69], highlighting the diversity and versatility of GHz spectroscopies.…”