Nanographite films produce a strong DC response under laser irradiation

Carbon nanotubes (CNTs) are generating significant interest due to unique electronic and optical properties that have made them very attractive for various nonlinear optical and opto-electronic applications.¹ However, the mass production of CNTs has overshadowed other interesting graphene-based nanocarbon materials, such as the planar nanographite crystals (NGCs).²

NGCs consist of a surface inlaid with a graphene sheet of regular hexagons of carbon atoms that remains planar on the micron or sub-micron scale. In CNTs, this surface is scrolled into a cylinder that can be as long as several microns with nanometer or even sub-nanometer diameter. CNTs also exhibit the electronic properties of 1D objects, while graphene is essentially a 2D carbon nanostructure.

Since the optical properties of both NGCs and CNTs arise from their conjugated π-electrons systems, NGCs are expected to be as promising as CNTs for electronic, optoelectronic and photonic applications. In our work, we demonstrated that a strong nanosecond electric pulse can be generated in a NGC film under laser irradiation. The conversion efficiencies achieved in the IR to UV spectral region show that NGCs are promising materials for the fabrication of fast ultra-broadband light sensors.

We grew a NGC film with a thickness of 3–4μm using chemical vapor deposition (CVD)² on 25×25mm silicon substrates. The film, shown in Figure 1, is 1-3μm-long and consists of graphite crystallites with a thickness of 2-20nm, equivalent to (∼)5-50 graphene sheets. The crystallites are randomly distributed in the film and are separated from one another by a distance of 0.5-1μm. However, the graphite atomic layers are preferentially oriented along the normal to the substrate.

Figure 1. Scanning electron microscope images of a nanographite crystal film at different resolutions (a, b) and transmission electron microscope image of the nano-graphite crystallites (c). The film has a length of a few microns.

We measured the voltage generated by a laser pulse irradiating the film between two parallel metal electrodes in electrical contact with the film (see Figure 2). The resistance between the electrodes varied between 20 and 200Ω and the interelectrode capacitance was less than 1pF. We were able to demonstrate that the light-induced DC signal was proportional to F(α)sin2α cosβ where (α) is the angle of incidence, β is the angle between the incidence plane and the normal to the electrodes, and where F(α) is a slowly varying function (see Figure 3).³

Figure 2. Experimental setup used to measure the DC response of the nanographite crystal film.

Figure 3. Dependence of the amplitude of the DC signal on the azimuthal angle β (a) and on the incident angle α (b) at λ = 1064nm.³

The observed electrical signal is nearly independent of the beam diameter. This suggests that it could originate from photon drag and optical rectification effects, which are determined by the power rather than the intensity of the laser pulse. The measured efficiency of the light power/voltage conversion was as high as 500 mV/MW at λ = 1064nm, while the shape of the electrical signal reproduced the shape of the incident light pulse on the nanosecond time scale.

To investigate the spectral dependence of the electrical signal, we also performed measurements in the UV to mid-IR spectral range with a p-polarized excitation beam at 50^o incidence using 10ns pulses. In the 266-1907nm spectral range, the conversion efficiency increased linearly with photon energy. In the near- and mid-IR spectral range, however, it was nearly wavelength-independent in the 1300-4000nm interval, displaying a sharp increase at longer wavelength (see Figure 4).

Figure 4. Wavelength dependence of the conversion efficiency.⁴

The strong optical nonlinearity of the NGC film implies that its electronic properties differ dramatically from those of bulk graphite. This difference is probably the result of the medium-range order arising from π-bonding that distinguishes NGCs from σ-bonded amorphous semiconductors. In particular, the observed effect originates from the drastically enhanced mobility of the conduction electrons along graphene sheets.⁵ Similar effects can also be observed in aligned CNTs that can be bound into yarns.⁶

In conclusion, we demonstrated that a nanographite film can display strong second-order nonlinearity in a wide spectral range spanning from the UV to the mid-IR region. More interestingly, the generated DC response reproduces the shape of the nanosecond laser pulse and shows a light-to-signal conversion efficiency up to 1μV/W in the mid-IR and visible spectral regions.