2D semiconducting films display a significant change in conducting and optical behavior based on the composition of the ambient, suggesting great potential in their use for the next generation of gas sensing devices. While physical adsorption on pristine surfaces has typically shown minimal effect on conductivity, this can be increased by doping with noble metals at vacancy sites, e.g., S- or Mo-vacancies in MoS₂ [1]. By selecting the type and concentration of a dopant atom through substitutional doping, the selectivity of the 2D film can be tuned, e.g., Au and Pt doping of MoS₂ improves sensitivity towards ammonia and humidity, respectively [1]. This allows for the design of a new generation of chemiresistive, optical, and FET-based sensors whose selectivity and specificity is engineered through targeted atomic-level doping.

© Lado Filipovic

A prototype device structure of mono-layered MoS₂ for gas sensing [2].

Goals

We will combine ab-initio and charge transport simulations of pristine and doped 2D monolayers in order to understand how doping with targeted metal atoms improves selectivity towards target target gases, mainly aiming at CO, CO₂, and humidity, for air quality and breath analysis [M4]. In this PhD project we will (a) investigate the interaction between ambient gas molecules and 2D heterostructures with relevant defects (e.g., S- and Mo-vacancies in MoS₂) and dopants using ab-initio calculations. We will then (b) model the change in the electrical, transport, and optical behavior of thin films due to defect and dopant interactions with gas molecules. We will further (c) study the temporal baseline drift in these films by imaging the surface before and after exposure using STEM. The observed changes will be studied theoretically using ab-initio calculations. Finally, we aim to (d) study back-gated FET devices whose channels are made of pristine, defected, and doped MoS₂ layers using both experiments and modeling techniques. The primary goal is to understand the qualitative and quantitative impact of different dopants on the microscopic and macroscopic behavior of metal-doped 2D films (e.g., Au, Cu, Ni, Pt, Ti) towards selective gas sensing applications and to understand the baseline drift in these novel devices.

Methods

MoS₂ films will be provided by TU-D partners Holzer and Müller, which will be scanned for presence of native defects and vacancies. Additional defects will be introduced in a controlled manner and the films will be doped by the Wilhelm and Eder groups, respectively. First principles with density functional theory (DFT) will be applied to study favorable adsorption configurations, adsorption energies, and the impact on the energy bands, when molecules of interest interact with near-pristine, controlled-defected, and selectively doped monolayers. Non-equilibrium Green’s function (NEGF) calculations will subsequently be applied to study the impact of defects and dopants on conduction in the 2D layer before and after exposure to ambient molecules. The DFT studies of the molecule-surface interactions will be supported by experimental characterizations from the Diebold and Kotakoski groups.

Collaborations

Holzer and Müller will provide exfoliated and CVD grown MoS₂ films, respectively. Wilhelm will use ion irradiation to generate vacancies and Eder will provide controlled attachment of metal and metal-oxide clusters at these defect sites. Lendl will provide spatially resolved AFM-Raman/IR and Parkinson atomically resolved STM/TPD/IR studies. Madsen will provide DFT calculation for the material system prior to molecular adsorption. Marchetti-Deschmann will measure electron transport in MoS₂. Foelske (XPS) and Diebold (STM, LEEDS) will provide material composition and interface information during molecular binding. Kotakoski will provide STEM measurements of MoS₂ films and Weber will build the back-gated FET devices. 

Supervisor

Lado Filipovic’s group studies CMOS-integrated semiconductor-based environmental sensors aimed for low-power and wearable technologies. The principal goal of the research is the application of 2D films in room-temperature low-power gas sensors and their integration with CMOS technology. His group also simulates the effect of molecular adsorption on the conductive and optical behavior of pristine, defected, and doped 2D films such as TMDs, phosphorene, arsenene, MXenes and works on designing associated nanoscale devices .

Website

Group of Prof. Filipovic, opens an external URL in a new window

Literature

1. D. Burman, H. Raha, B. Manna, P. Pramanik, and P. K. Guha. Substitutional doping of MoS₂ for superior gas-sensing applications: A proof of concept. ACS Sensors 6, 3398–3408 (2021). DOI: 10.1021/acssensors.1c01258.
2. W. Jiang, K. Chen, J. Wang, D. Geng, N. Lu, and L. Li. Understanding the adsorption behavior of small molecule in MoS₂ device based on first-principles calculations. Materials Research Express 8, 055010 may (2021). DOI: 10.1088/2053-1591/ac021d.
3. S. Fatemeh, M. Moradinasab, U. Schwalke, and L. Filipovic. Superior sensitivity and optical response of blue phosphorene and its doped systems for gas sensing applications. ACS Omega 6, 18770–18781 (2021). DOI: 10.1021/acsomega.1c01898.
4. L. Filipovic and S. Selberherr. Gas sensing with two-dimensional materials beyond graphene. In 2021 IEEE 32nd Int. Conf. Microelectronics, pages 29–36, (2021).DOI: 10.1109/MIEL52794.2021.9569088.