Studying Two-Dimensional (2D) Materials Using Time-Resolved Differential Transmission Spectroscopy
Dr. Kenan Gundogdu’s research group in the Physics Department at North Carolina State University studies ultrafast dynamics and develops new spectroscopy methods for studying semiconductors.
The group investigates organic electronics, hybrid semiconducting systems, and low-dimensional materials such as graphene and transition metal dichalcogenides (TMDCs), which — because of their unique electronic and mechanical properties at atomic-scale thicknesses — are candidates for next-generation materials for energy harvesting, spintronics, and photonics applications.
“FERGIE delivers very fast data acquisition.”
— Dr. Kenan Gundogdu
Dr. Gundogdu, who earned his Ph.D. at the University of Iowa, joined the North Carolina State faculty following postdoctoral work in the Chemistry Department at the Massachusetts Institute of Technology. His research group in Raleigh investigates structural and electronic dynamics in condensed matter systems using ultrafast and nonlinear optical spectroscopy techniques.
Specifically, the group focuses on dynamics that are relevant to solar energy conversion. Research questions include the role of coherent and incoherent electron motion in energy conversion, how energy transport happens in interfaces that involve inorganic and organic materials, and the identification of the physical properties of optical excitations in such hybrid materials.
Dr. Gundogdu and his colleagues have authored a paper titled “Dense Electron-Hole Plasma Formation and Ultra-Long Charge Lifetime in Monolayer MoS2 via Material Tuning” that details their recent study of condensed matter systems and electron dynamics in optically excited 2D semiconductors (Bataller et al. Nano Lett., Just Accepted Manuscript, DOI: 10.1021/acs.nanolett.8b04408, Web Publication Date: 04 Jan 2019).
In this work, they used several different spectroscopic methods to study monolayer MoS2, a TMDC material. Raman spectroscopy was utilized to measure the band structure change in MoS2 as a function of temperature; time-resolved photoluminescence and differential transmission spectroscopies were employed to measure the formation and evolution of electron-hole plasma in the 2D material. The time-resolved differential transmission measurements were facilitated by FERGIE.
“FERGIE is easy to use, install, configure, and set up.”
FERGIE in Action
Figure 1 is a diagram of the experimental setup the group utilized to perform time-resolved differential transmission measurements of monolayer MoS2.
Figure 1. A TTL-controlled pulsed beam excites the sample into a dense electron-hole plasma (EHP) state. A weak broadband probe pulse (i.e., a white light continuum generated using a Ti:sapphire amplifier) probes the differential transmission.
White light from an optical parametric amplifier pumped by an amplified femtosecond laser was used as the probe signal. A microscope objective focused the probe light and, after passing through the sample and exiting the vacuum chamber, the light was collected by a second microscope objective and imaged onto the FERGIE’s entrance slit.
Operating in its kinetics spectroscopy mode, FERGIE was electronically triggered to acquire every probe pulse. For each time step, alternating probe and pump+probe spectra from FERGIE were recorded and then processed to yield ΔT/T.
Figure 2 presents resultant data.
Figure 2. (a) Time-resolved differential transmission spectrum with 500 ns square-wave photoexcitation (green dashed lines). Spectral line-outs at various times during EHP formation (b) and decay (c). (d) Temporal line-outs are shown at A (black) and B (green) exciton energies, and at energies corresponding to dense plasma (red) and conduction band (yellow) states affected by bandgap renormalization. The temperature (blue dashed) dynamics for the material center is shown alongside the differential transmission. All energy channels are normalized to the steady-state EHP intensity averaged between 300 and 500 ns. Data courtesy of Dr. Kenan Gundogdu (North Carolina State University). First published in DOI: 10.1021/acs.nanolett.8b04408.
“FERGIE offers a lot of convenience, all in one system.”
Three Quick Questions
Princeton Instruments: Are there any other features of FERGIE that impressed you?
Kenan Gundogdu: The system also comes with nice CUBE attachments that enable a lot of different spectroscopies… they come in handy when you need them.
PI: For what additional research application(s) would FERGIE be particularly well suited?
Kenan Gundogdu: Fast Raman spectroscopy.
PI: Will you recommend FERGIE to your colleagues?
Kenan Gundogdu: Yes, specifically to people in materials research and chemistry departments.
Associate Professor Kenan Gundogdu
North Carolina State University