Effects of environmental conditions on the ultrafast carrier dynamics in graphene revealed by terahertz spectroscopy

A thorough understanding of the stability of graphene under ambient environmental conditions is essential for future graphene-based applications. In this paper, we study the effects of ambient temperature on the properties of monolayer graphene using terahertz time-domain spectroscopy as well as time-resolved terahertz spectroscopy enabled by an optical-pump/terahertz-probe technique. The observations show that graphene is extremely sensitive to the ambient environmental conditions and behaves differently depending on the sample preparation technique and the initial Fermi level. The analysis of the spectroscopic data is supported by van der Pauw and Hall effect measurements of the carrier mobility and carrier density at temperatures comparable to those tested in our THz spectroscopic experiments.

Figure 1

THz-TDS measurements at different temperatures: the transmitted THz pulses (a) and (c) and the transmitted peak field normalized to the transmission at room temperature of $22{}^{\circ }\mathrm{C}$ (b) and (d) for the highly $p$-doped CVD graphene sample CVD-1 (a) and (b), and the lightly $n$-doped epitaxial graphene EPTX-1 (c) and (d), respectively.

Figure 2

The OPTP response of the graphene samples at different temperatures: the THz differential transmission $\mathrm{\Delta }T/T_0$ as a function of the pump-probe delay time at various temperatures for (a) CVD, and (b) epitaxial graphene samples, and the temperature dependence of the peak values of $\mathrm{\Delta }T/T_0$ for various (c) CVD and (d) epitaxial graphene samples tested in our experiments.

Figure 3

Recovery of the OPTP response after cooling down back to the room temperature for two epitaxial graphene samples (a) EPTX-1 and (b) EPTX-2.

Figure 4

The VDP-HE measurements of the carrier concentration and mobility as functions of temperature for (a) $p$-doped CVD and (b) $n$-doped epitaxial graphene, and the corresponding dc scattering times in (c) and (d), respectively. Please note that the $x$-axis range for parts (b) and (d) is different compared to parts (a) and (b).

Figure 5

Heating effect on the Fermi level $E_F$ in (a) $p$-doped CVD graphene and (b) $n$-doped epitaxial graphene, exemplified by desorption of water and oxygen molecules surrounding graphene, leaving extra electron doping.

Figure 6

Two-cycle measurements of the heating effects on the OPTP response of the epitaxial graphene sample EPTX-3 in dry air environment, (a) the first cycle, (b) the second cycle, and (c) the peak of the differential transmission $\mathrm{\Delta }T/T_0$ as a function of temperature for the two cycles.

Figure 7

The temperature dependence of the THz field-induced carrier dynamics; absolute THz conductivity and scattering time for the CVD-1 graphene sample in (a) and (c), respectively, and for the EPTX-1 graphene sample in (b) and (d), respectively.