High-Performance Detectors Based on the Novel Electronic and Optoelectronic Properties of Crystalline 2D van der Waals Solids
Authors:   , Saenz Saenz Gustavo Alberto 
Published: February 2025

In this work, we study the properties and device applications of MoS2, black phosphorus, MoOx, and NbSe2. We first start with the design, fabrication, and characterization of ultra-high responsivity photodetectors based on mesoscopic multilayer MoS2. The device architecture is comprised of a metal-semiconductor-metal (MSM) photodetector, where Mo was used as the contact metal to suspended MoS2 membranes. The dominant photocurrent mechanism was determined to be the photoconductive effect, while a contribution from the photogating effect was also noted from trap-states that yielded a wide spectral photoresponse from UV-to-IR with an external quantum efficiency (EQE) ~ 104. From time-resolved photocurrent measurements, a fast decay time and response time were obtained with a stream of incoming ON/OFF white light pulses. Another interesting semiconductor 2D material that has attracted special attention due to its small bandgap and ultra-high hole mobility is the black phosphorus. An analysis of the optoelectronic properties and photocurrent generation mechanisms in two-dimensional (2D) multilayer crystallites of black phosphorus (BP) was conducted from 350 K down to cryogenic temperatures using a broad-band white light source. The Mo-BP interface yielded a low Schottky barrier "φ" _"SB" ~ -28.3 meV and a high photoresponsivity R of ~ 2.43 x 105 A/W at a source-drain bias voltage of ~ 0.5 V (300 K, and incident optical power ~ 3.16 μW/cm2). Our report is the first to highlight the empirical use of Mo as a contact metal with BP. From the analysis conducted on the BP devices, the thermally driven photocurrent generation mechanism arising from the photobolometric effect (PBE) dominated the carrier dynamics for T > 181 K since the photocurrent Iph and the bolometric coefficient β undergo a transition in polarity from positive to negative. Our results show the promise of BP to potentially advance thermoelectric and optoelectronic devices stemming from this mono-elemental, direct bandgap 2D van der Waals solid. Another intriguing metallic 2D material is superconducting 2H-NbSe2. Here we present the temperature-dependent Raman spectroscopy and electronic transport on bulk NbSe2, carried out to investigate the scattering mechanisms. We report on the photoresponse of direct probed mesoscopic 2H-NbSe2 as a function of laser energy for lasers at 405 nm, 660 nm, and 1060 nm wavelengths used to irradiate the device, where the modulation from the superconducting-to-normal-state is detected through photomodulation. Additionally, the various oxidation levels of molybdenum oxide have interesting optical and electrical properties as a function of the oxygen vacancy and stoichiometry. The substoichiometric MoOx (2 < x < 3) behaves as a high work function conductor due to its metallic defect band. As a result, one of the potential applications of MoOx is for electrical contacts providing high hole injection or extraction. In this work, we have synthesized MoOx nanosheets via chemical vapor deposition and a four-terminal device was fabricated via e-beam lithography and electronic transport was measured as a function of temperature. Outstanding properties were obtained from our MoOx nanosheets, including a high conductivity of ~ 6,680.3 S cm-1, a superior temperature coefficient of resistance ~ -0.10%, and a high sensitivity based on the bolometric coefficient β of ~ 0.152 mS K-1. In summary, this work pushes the state-of-the-art in enabling 2D van der Waals materials for next-generation high-performance detectors.