Electrical and optical properties of binary CN x nanocone arrays synthesized by plasma-assisted reaction deposition
© Liu et al.; licensee Springer. 2014
Received: 26 February 2014
Accepted: 9 March 2014
Published: 21 March 2014
Light-absorbing and electrically conductive binary CN x nanocone (CNNC) arrays have been fabricated using a glow discharge plasma-assisted reaction deposition method. The intact CNNCs with amorphous structure and central nickel-filled pipelines could be vertically and neatly grown on nickel-covered substrates according to the catalyst-leading mode. The morphologies and composition of the as-grown CNNC arrays can be well controlled by regulating the methane/nitrogen mixture inlet ratio, and their optical absorption and resistivity strongly depend on their morphologies and composition. Beside large specific surface area, the as-grown CNNC arrays demonstrate high wideband absorption, good conduction, and nice wettability to polymer absorbers.
Since the 1990s, there has been an upsurge in interest in the properties and potential uses of carbon-related nanostructures [1–3]. These unique nanostructures are attractive for nanotechnology applications in photovoltaic devices and photodetectors [4–8]. Many novel thin film solar cells rely on highly light-absorbing and well electrically conductive electrodes for their successful operation and good capability. For example, dye-sensitized solar cells and polymer organic hybrid solar cells exploit titanium oxide as electrodes [7, 8]. But, this material is far from ideal because of poor electrical conduction and limited optical absorption [9, 10]. Carbon-related nanostructures, such as carbon nanotubes and graphene, are attractive electrodes and even absorbers for photovoltaic devices and photodetectors owing to strong optical absorptivity and ultrafast charge transport mobility [6, 11]. Besides, their large specific surface area could greatly increase the donor/acceptor interface, which will effectively increase the separation probability of electrons and holes. Compared with carbon nanotubes and graphene, the binary CN x nanocones (CNNCs) will have good mechanical stability and better electrical and chemical stabilities due to the incorporation of nitrogen. So far, the experimentally synthesized carbon nitride, except our previous reports of the growth of the CNNC arrays , is mainly limited to amorphous or nanosphere CN x thin films and nanobells with low nitrogen content (about 2%) [13–15].
Here, vertically aligned CNNC arrays with high wideband absorption and good electrical conduction were fabricated by an abnormal glow discharge plasma-assisted reaction deposition (GPRD) method which combines highly dense plasma with proper bias enhancement . The methane/nitrogen (CH4/N2) mixture feeding gas ratio, which directly affected the contents and activities of the nitrogen-related and carbon-related precursors in the plasmas, was regulated to control the morphologies and composition of the CNNC arrays. The effects of the morphology, composition, and structure of the CNNC arrays on their optical absorption and electrical conduction were studied. The CNNC arrays with intact shape, high optical absorption, high electrical conduction, and nice wettability to polymer are pursued for potential uses as electrodes or even absorbers in photovoltaic devices and photodetectors.
Optically absorptive and electrically conductive CNNC arrays were grown on nickel-covered silicon (100) substrates by means of the GPRD method, as described previously [12, 16]. The sample preparation involves two steps. In the first step, nickel catalyst layers were deposited on silicon (100) wafers by a pulsed laser deposition method. About 100-nm thick nickel catalyst layers were deposited on the prepared substrates under a base pressure of 1 × 10-3 Pa for 8 min using a Nd:YAG laser to ablate a pure nickel target. The wavelength, pulse energy, and repetition of the Nd:YAG laser were 532 nm, 50 mJ, and 10 Hz, respectively. The distance between the target and substrate was about 4 cm. In the second step, the CNNC arrays were grown by the GPRD method. The plasma source generated reactive plasma just above the substrates through the abnormal glow discharge with a CH4/N2 mixture inlet under a total pressure of 750 Pa. The discharge current, voltage, and time were set to 180 mA, 350 V, and 40 min, respectively. In the CNNC growth, the CH4/N2 inlet ratios were varied from 1/80 to 1/5 in order to obtain the CNNC arrays with different morphologies and compositions. The wettability of the CNNC arrays to poly-3-hexylthiophene mixed with phenyl-C61-butyric acid methyl ester (P3HT:PCBM) layer, which is a commonly used polymer absorber in polymer organic hybrid solar cells, has also been examined by spin coating method using different rotational speeds for different polymer thicknesses.
The morphologies of the samples were characterized by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The crystallinity and composition of the individual CNNCs were characterized by selected-area electron diffraction (SAED) and energy-dispersive X-ray spectroscopy (EDXS). The optical absorption spectra were measured by an ultraviolet spectrophotometer. Longitudinal resistance of the as-grown CNNC arrays was measured by a platinum-cylindrical-tip contacting method using a Power SourceMeter (Keithley Instruments Inc., Beijing, China), and the resistivity of the as-grown CNNCs was obtained by calculating the measured resistance.
Results and discussion
For novel thin film solar cells, such as polymer inorganic hybrid solar cells, the electrodes made from inorganic nanostructures not only require high optical absorption and good electrical conduction but also nice wettability to absorbers, which is almost the main bottleneck of the development of this kind of solar cells. The wettability of the CNNC arrays to P3HT:PCBM (weight ratio of 1:0.8), which is a commonly used polymer absorber in polymer organic hybrid solar cells, was examined by the spin coating method. Figure 1f gives the FESEM image of the surface morphology of the P3HT:PCBM-covered CNNC array. It could be seen in Figure 1f that the P3HT:PCBM layer have fully infiltrated the CNNC arrays, and the several higher CNNC tips protrude from the P3HT: PCBM layer, which indicates that the CNNC arrays have very nice wettability to the P3HT:PCBM absorber layers.
Based on the characterization of morphologies, structures, and composition, the CNNC growth can be outlined as the catalyst-leading growth mode. In this mode, the nickel catalyst layer first melts and fragments into separated hemisphere-like islands under heating of the abnormal glow discharge plasma over the substrate. Then, the incipient CNNCs are formed on the nickel islands due to the deposition of precursors such as CN species, nitrogen atoms, and C2 species from the discharge plasma . As the CN radicals and other reactive species continue to attach, the heights and lateral diameters of the CNNCs increase simultaneously. Meanwhile, the enclosed molten nickel will be sucked to the top and leave the narrow pipelines in the center of the cone bodies by the capillary effect. The catalyst nickel on the tops will lead to the growth of the CNNCs. As the CNNCs increase in height, the ion streams accelerated by a voltage of 350 eV will be focused on the tops by a locally enhanced electric field. The intense ion streams will sputter off the attached species and cut down the diameters of the tops . In this way, the intact CNNC arrays with central pipelines and sharp tips eventually finish the growth. Because the precursors are mainly composed of CN species, nitrogen atoms, and C2 species , the bodies of the as-grown CNNCs are mainly amorphous CN x other than crystalline C3N4 which needs the reaction between atomic C and N without other species involved.
In summary, the vertically aligned CNNC arrays were synthesized on nickel-covered silicon (100) substrates by the GPRD method. The morphologies and composition of the as-grown CNNC arrays are strongly affected by the CH4/N2 feeding gas ratios. The as-grown CNNCs are mainly amorphous CN x , and the atomic content of nitrogen decreases synchronously as the CH4/N2 ratio increases. The CNNC arrays grown at the CH4/N2 ratio of 1/5 have rather perfect cone shapes and good wettability to the polymer P3HT:PCBM. The absorption spectra reveal that the optical absorption of the as-grown CNNC arrays increases with increasing CH4/N2 ratio and show a very good absorption in a wideband of 200 to 900 nm at the CH4/N2 ratio of 1/5. The resistivities of the as-prepared samples decrease as the CH4/N2 ratios increase and reach about 6.45 × 10-5 Ω · m at the CH4/N2 ratio of 1/5, indicating that the as-grown CNNC arrays can have very good conductivity. Due to the large specific surface area, high and wide optical absorption, excellent electrical conduction, and nice wettability (to polymer absorbers) of the as-grown CNNC arrays, such nanocone arrays are supposed to be potential electrodes or even absorbers in the thin film solar cells and photodetectors.
XL, LG, and XF are graduate students major in fabrication of nanometer materials. YZ is an associate professor and MS degree holder specializing in optical devices. JW is a professor and PhD degree holder specializing in optics and nanometer materials. NX is a professor and a PhD degree holder specializing in nanometer materials and devices, especially in nanoscaled super-hard and optoelectronic devices.
binary CN x nanocone
field emission scanning electron microscopy
abnormal glow discharge plasma-assisted reaction deposition
high-resolution transmission electron microscopy
pulsed laser deposition
PCBM: poly-3-hexylthiophene mixed with phenyl-C61-butyric acid methyl ester
selected-area electron diffraction
transmission electron microscopy.
This work is financially supported by the National Basic Research Program of China (973 Program, Grant No. 2012CB934303) and National Natural Science Foundation of China.
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