Extreme sensitivity of graphene photoconductivity to environmental gases

Abstract

Graphene is a single layer of covalently bonded carbon atoms, which was discovered only 8 years ago and yet has already attracted intense research and commercial interest. Initial research focused on its remarkable electronic properties, such as the observation of massless Dirac fermions and the half-integer quantum Hall effect. Now graphene is finding application in touch-screen displays, as channels in high-frequency transistors and in graphene-based integrated circuits. The potential for using the unique properties of graphene in terahertz-frequency electronics is particularly exciting; however, initial experiments probing the terahertz-frequency response of graphene are only just emerging. Here we show that the photoconductivity of graphene at terahertz frequencies is dramatically altered by the adsorption of atmospheric gases, such as nitrogen and oxygen. Furthermore, we observe the signature of terahertz stimulated emission from gas-adsorbed graphene. Our findings highlight the importance of environmental conditions on the design and fabrication of high-speed, graphene-based devices.

Figure 1. Characterization of graphene samples.

(a) Raman spectroscopy. The relative intensities of the 2D and G peaks suggest monolayer graphene. Inset: photograph of graphene sheet with an area of 1 × 1 cm² on quartz. (b) Surface topography of CVD-grown graphene measured by AFM. The residuals remaining on graphene surface can be attributed to incomplete decomposition or carbonization of PMMA.

Figure 2. Terahertz spectroscopy of graphene in O₂.

(a) Schematic representation of the measurement. Varying t₁ allows the electric field, E_THz, of the terahertz probe pulse to be sampled in the electro-optic (E-O) crystal. The sample can be probed at different times after photoexcitation by altering t₂. (b,c) Terahertz electric field, E_THz, transmitted through the sample (b), and the corresponding photo-induced change in transmission, ΔE (c), as a function of the pump–probe delay time, t₂. (d,e) Real (d) and imaginary (e) parts of the frequency-dependent photoconductivity as a function of t₂. The contours highlight increments of 2 × 10⁻⁵ (Ω/)⁻¹. The thicker contour in e represents Im(Δσ)=0, corresponding to the Lorentzian peak frequency, ω₀. Measurements were performed in pure oxygen at room temperature and pressure.

Figure 3. Environmental dependence of pump-induced change in terahertz photoconductivity.

Here, . From top to bottom: graphene in vacuum, N₂, air and O₂ at room temperature. Photo-induced absorption is observed in vacuum, but photo-induced bleaching is observed in gaseous environments. Inset: schematic representation of the pump–probe measurements of p-doped graphene (ε_F~0.3 eV) in vacuum (top) and in the presence of gases (bottom), with corresponding density of states (g(ε)). The high-energy pump () excites carriers far above the Dirac point, which cool by phonon emission. In gapless graphene, terahertz photons can be absorbed by the photoexcited carriers. In the presence of gases, stimulated emission releases extra terahertz photons, yielding a bleaching pump–probe signal.

Figure 4. Photoconductivity spectra of graphene in different environments.

(a–d) Photoconductivity in vacuum (a), N₂ (b), air (c) and O₂ (d), taken at 2 ps after photoexcitation by the optical pump. Solid orange dots show the real part of the photoconductivity, hollow blue dots show the imaginary part. The solid black line in b–d is a Lorentzian model fit to the conductivity data, demonstrating the Lorentzian form the of photoconductivity in gaseous environments. Shading is used to emphasize the change from positive to negative Im(Δσ). This point corresponds to the peak Lorentzian frequency, ω₀.