Imaging molecular adsorption and desorption dynamics on graphene using terahertz emission spectroscopy

Abstract

Being an atomically thin material, graphene is known to be extremely susceptible to its environment, including defects and phonons in the substrate on which it is placed as well as gas molecules that surround it. Thus, any device design using graphene has to take into consideration all surrounding components, and device performance needs to be evaluated in terms of environmental influence. However, no methods have been established to date to readily measure the density and distribution of external perturbations in a quantitative and non-destructive manner. Here, we present a rapid and non-contact method for visualizing the distribution of molecular adsorbates on graphene semi-quantitatively using terahertz time-domain spectroscopy and imaging. We found that the waveform of terahertz bursts emitted from graphene-coated InP sensitively changes with the type of atmospheric gas, laser irradiation time, and ultraviolet light illumination. The terahertz waveform change is explained through band structure modifications in the InP surface depletion layer due to the presence of localized electric dipoles induced by adsorbed oxygen. These results demonstrate that terahertz emission serves as a local probe for monitoring adsorption and desorption processes on graphene films and devices, suggesting a novel two-dimensional sensor for detecting local chemical reactions.

Figure 1. Time-domain waveforms of terahertz radiation emitted from graphene-coated InP under different conditions.

(a), A schematic diagram of the experimental geometry. (b), Time evolution of THz waveforms from graphene-coated InP under continuous excitation by femtosecond near-infrared pulses from a Ti:sapphire laser. The curves are intentionally shifted vertically. (c), THz waveforms from InP (no graphene) under continuous excitation by femtosecond near-infrared pulses from a Ti:sapphire laser. The waveform remains the same even after ten minutes of continuous excitation. (d), THz waveforms from graphene-coated InP in vacuum under continuous excitation by femtosecond near-infrared pulses from a Ti:sapphire laser The waveform remains the same even after ten minutes of continuous excitation. (e), Time evolution of THz waveforms from graphene-coated InP under continuous excitation by femtosecond near-infrared pulses from a Ti:sapphire laser. The vacuum pump is turned on at 10 minutes and the waveform changes drastically.

Figure 2. Time evolution of THz emission from graphene-coated InP in different environmental gases.

(a), Air, (b), vacuum, (c), oxygen, and (d), oxygen. In (d), the sample was pre-annealed to remove water before exposure to oxygen. In all cases, after 10 minutes of continuous THz emission measurements (Region I), the sample was illuminated by ultraviolet UV light (365 nm) for 5 minutes (Region II). After the UV light is switched off (Region III), the THz waveform slowly tends towards the original form.

Figure 3. Band diagrams explaining the sign change of THz amplitude due to adsorbed oxygen molecules.

(a), Band diagram for pristine graphene on InP. The photo excited current flows towards the substrate. (b), Band diagram for graphene on InP with adsorbed oxygen molecules between them. The charge transfer between graphene and oxygen creates dipoles, which modify the band bending. The photoexicted current now flows towards the surface.

Figure 4. Laser-induced desorption dynamics of oxygen molecules from graphene observed through THz waveforms.

(a), Two kinds of waveforms emitted from graphene-coated InP, one taken under high vacuum (red line) and the other taken after exposure to air for several days (black line). (b), The time dependent relative weights of the waveforms taken after exposure to air used for the simulation of experimental data. (c), THz waveforms for graphene-coated InP in air at different times. (d), Simulated waveforms consisting of a superposition of two observed waveforms of different relative weights. The simulated waveforms agree with the observed waveforms very well, with appropriately changing relative weights in the superposition.

Figure 5. THz imaging of adsorbed oxygen on graphene.

(a), Amplitude mapping of THz radiation emitted from graphene-coated InP after exposure to air for a few weeks. (b), THz waveforms corresponding to Point 1 and Point 2 indicated in the map as well as that for a bare InP substrate. The waveform at Point 1 indicates a highly polarized surface due to adsorption of oxygen molecules, and the waveform at Point 2 is qualitatively the same as that from the bare InP.