Original Paper
phys. stat. sol. (b) 243, No. 13, 3505 – 3509 (2006) / DOI 10.1002/pssb.200669163
Laser-assisted electron emission from CVD nano-graphite films D. A. Lyashenko*, 1, 2, E. D. Obraztsova2, A. N. Obraztsov3, and Yu. P. Svirko1 1 2 3
Department of Physics, University of Joensuu, 7 Yliopistokatu, Joensuu, 80101, Finland Natural Sciences Center, General Physics Institute, 38 Vavilov street, 119991, Moscow, Russia Department of Physics, M. V. Lomonosov Moscow State University, 119992, Moscow, Russia
Received 20 April 2006, revised 21 August 2006, accepted 21 August 2006 Published online 10 October 2006 PACS 68.37.Vj, 79.20.Ds We demonstrate that irradiation with nano-second light pulses results in significant enhancement of the electron emission from nano-graphite films. The observed emission current density is as high as 10 A/cm2 at applied field of 2 V/µm. The duration of the emission pulse depends on the applied DC voltage and laser intensity. However, in our experimental conditions, the temporal profile of the electron pulse nearly reproduces that of the incident laser pulse. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1
Introduction
Nano-graphite (NG) has recently emerged as a promising material for opto- and vacuum electronics. In particular, the enhanced field electron emission from NG makes it possible to create cold cathodes that can produce electron beam with current densities of up to 1 A/cm2 at applied electric field of about 6.5 V/µm [1]. This opens new opportunities in development of cathodoluminescent light sources with record high efficiency and reliability. Very recently we have demonstrated that irradiation of the NG films by nanosecond laser pulse results in generation of a strong DC current in the film due to the optical rectification and photovoltaic effects [2]. The temporal profile of this light-induced DC current reproduces that of the light pulse in a wide spectral range. This interesting nonlinear optical phenomenon can be employed in advanced detectors of the laser radiation that will be sensitive to both power and polarization of the nanosecond laser pulse. In this paper we report that when the NG cathode is irradiated by a nanosecond light pulse with energy in the mJ range, the DC current in the film is accompanied by pronounced emission of electrons from the film surface into surrounding vacuum. The temporal profile of the electron emission pulse that emerges from the film may vary depending of the applied voltage and the light intensity, however in our experimental conditions, the pulse has approximately same duration as the incident nanosecond laser pulse of moderate intensity. The obtained magnitude of the emission current density is as high as 10 A/cm2 at the incident laser pulse intensity of 50 MW/cm2 and DC field of 2 V/µm.
2 Experiment NG film with thickness of 3–4 µm is grown on the 25 × 25 mm Si substrates using chemical vapor deposition (CVD) technique, which is described elsewhere [2]. One can observe from Fig. 1. that the film consists of graphite crystallites with thickness of 2–20 nm (it corresponds to 5–50 graphite atomic layers) and length of 1–3 µm. The crystallites are randomly distributed in the film and are separated from *
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© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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D. A. Lyashenko et al.: Laser-assisted electron emission from CVD nano-graphite films
(a)
(b)
Fig. 1 SEM images of CVD nano-graphite film: (a) Top view of the sample. The film consists of nanosized graphite crystallites, which are preferably orientated along the surface normal. Each crystallite contains from 5 to 50 graphene sheets and is of few microns length. (b) Cross-section of the sample; Si substrate is seen as a grey area.
one another by 0.5–1 µm. The graphite atomic layers are preferably oriented along the substrate normal. The width of the angular distribution of the graphite flakes with respect to the substrate normal is about 20 degrees. In the experiment, we investigated electron emission from NG cathode under illumination by an OPO that operated at wavelength λ = 1.6 µm with full width at half maximum (FWHM) pulse width of 10 ns. Cathode is placed into vacuum lamp with transparent ITO anode. The transparency of the anode at λ = 1.6 µm is 80% for normal incidence. The distance between the NG cathode and ITO anode is 500 µm, while the applied voltage is varied from 0 to 1300 V. The irradiated area on the cathode surface does not exceed 0.1 cm2. In our experimental conditions, we did not observe degradation of the NG cathode caused by the irradiation with intense laser pulse. This indicates that energy density in the incident laser pulse is below the damage threshold [3]. The temporal profile of the emission pulse that emerges from the film may vary depending of the applied voltage and the laser intensity, however in our experimental conditions, its duration is comparable with that of the incident laser pulse when applied electric field is above than 0.06 V/µm. The shapes of the electron emission and laser pulses are shown in Fig. 2. 100
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Fig. 2 Temporal profile of the incident laser pulse and emission current at E = 0.5 V/µm and I = 24 MW/cm2.
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Original Paper
phys. stat. sol. (b) 243, No. 13 (2006)
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Fig. 3 (a) Visualisation of field emission (FE) and laser-assisted emission (LAE) in the cylindrical vacuum lamp with the phosphor-coated anode. The LAE is observed in the vicinity of the cathode area, which is irradiated by the laser pulse. (b) Statistical distribution of laser-assisted emission current magnitude for 2439 laser pulses at laser intensity I = 33.6 MW/cm2 and applied electric filed E = 0.5/µm. Inset shows the emission current as a function of the pulse number in the time-ordered set.
When the applied electric filed is higher than 0.06 V/µm, the laser-assisted emission shows significant fluctuation of the current from pulse to pulse. The statistical properties of the laser-assisted emission from the nano-graphite resemble those of thermionic emission. A typical statistical distribution of the emission current is shown in Fig. 3b. Both expectation value and dispersion of the emission current increases with laser pulse intensity. When the electric fields lower than 0.06 V/µm, fluctuations of emission current decreases down to 5–10%, which is typical values for the field emission. In order to visualize the thermionic nature of the laser-assisted electron emission from NG cathode we perform independent measurements using a vacuum diode of cylindrical geometry. In this device (see Fig. 3a), the cathode is a Ni wire of 1 mm in diameter coated by NG film, while the anode is located on the inner part of the cylindrical vacuum tube and is coated by a phosphor. The cathode-to-anode distance is 10 mm. In terms of the applied voltage, the field emission threshold for this cathodoluminescence
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© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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D. A. Lyashenko et al.: Laser-assisted electron emission from CVD nano-graphite films
lamp is about 5 kV. We clearly observed typical for field emission inhomogeneous phosphorescence, at applied voltage of 12–18 kV. However, the characteristics of the electron emission change dramatically when the cathode is irradiated by 10 ns laser pulse. Specifically, the phosphorescence is observed when the applied voltage is considerably lower than the field emission threshold of 5 kV. However, the phosphorescence pattern in the vicinity of the irradiated cathode area is rather homogeneous similarly to that in for the thermionic emission. The phosphorescence at applied voltage of U = 2 kV and laser pulse energy of 18 mJ is shown on Fig. 3a. At low electric field, the magnitude of the emission current is proportional to the applied electric field. However, this dependence shows signs of saturation at electric fields above 0.5 V/µm (see Fig. 4a). When the irradiated area of the cathode is 0.07 cm2, the magnitude of the emission pulse measured at I = 36 MW/cm2 and E = 1 V/µm is as high as 0.35 A. By focusing the laser beam further we achieve the current density of 10 A/cm2 at the intensity of I = 50 MW/cm2. The electron emission is essentially nonlinear function of the light intensity. It grows exponentially as soon as the intensity of the incident pulse exceeds the threshold value, which depends on the applied electric field (see Fig 4b). In our experimental conditions, the threshold intensity of the nonlinear electron emission from the NG film decreases from Ith = 18 MW/cm2 at the cathode-anode electric field of E = 0.005 V/µm down to Ith = 16 MW/cm2 at E = 2 V/µm. Since the photon energy in our experiment is much lower than the work function of graphite, the observed results can be described in terms of the thermionic emission [4] from the NG cathode. In this process, laser heats the electron gas in nano-sized graphite flakes. The heated electrons subsequently equilibrate with the lattice, i.e. we observe emission from the Fermi–Dirac thermal distribution shifted by the photon energy. The emission current density can be presented in terms of the Richardson–Dushman equation,
{(
}
)
J µ T 2 exp - ϕ - e3 β E / kBT ,
where T is the lattice temperature, e is the electron charge, β is the local field enhancement factor. Since in the nanosecond time scale the temperature is proportional to the incident pulse intensity, T ∝ I, one may expect that ln (J/I 2) is a linear function of 1/I. One can observed from Fig. 5 that our experimental results are satisfactory described by this model, however ln (J/I 2) is nearly independent on the electric field at moderate light intensities, i.e. the field-induced correction to the NG work function is negligible. In contrast, we observe a considerable departure from the Richardson-Dushman equation and strong influence of the static electric field on the emission current at higher light intensities. Such a departure can be caused by the nonlinearities in T with incident laser intensity and its time dependence, which complicate the theoretical description of our experimental findings.
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Fig. 5 Electron emission as a function of the light intensity in Richardson variables. The electric current density J is shown in [A/cm2].
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© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Original Paper
phys. stat. sol. (b) 243, No. 13 (2006)
3509
3 Conclusion In conclusion, we demonstrate a high performance of the CVD nano-graphite film in electron emission under irradiation by nanosecond light pulses. The obtained results indicate that this novel nano-carbon material has a strong potential in manufacturing long-lived and reliable cathodes that are capable of producing nano-coulomb electron bunches (e.g. for RF photo-injectors and X-ray tubes) under illumination of nano- and pico-second pulses of the mid-IR spectral range. Acknowledgements This work was supported by the INTAS (grant #04-84-297), RFBR (#04-02-17618), and RAS Presidium Programs.
References [1] A. N. Obraztsov, A. P. Volkov, Al. A. Zakhidov, D. A. Lyashenko, Yu. V. Petrushenko, and O. P. Satanovskaya, Appl. Surf. Sci. 215, 214 – 221 (2003). [2] A. N. Obraztsov, A. A. Zolotukhin, A. O. Ustinov, A. P. Volkov, and Yu. P. Svirko, Carbon 41, 836 – 840 (2003). [3] A. N. Obraztsov, O. Gröning, A. A. Zolotukhin, Al. A. Zakhidov, and A. P. Volkov, Diam. Relat. Mater. 15, 838 (2006). [4] F. A. M. Koeck, J. M. Garguilo, and R. J. Nemanich, Diam. Relat. Mater. 13, 2052 (2004).
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