Third-order nonlinear optical studies of carbon nanotubes developed by floating catalyst technique

Highlights

• Growth rate of carbon nanotubes (CNT's) rises at higher temperatures and graphite sheets build up with a lower defect.

• Crystalline perfection of CNT's was improved with increase in temperature.

• CNT's display strong nonlinear absorption coefficient of 29.9 cm/GW.

• CNT's exhibits excellent optical limiting with a limiting threshold of 3 J/cm2.

Abstract

Carbon nanotubes were developed via floating catalyst technique using acetylene and ferrocene as carbon and catalyst precursors respectively at temperatures varying from 700 to 1150 °C. Structure and morphology were characterized via SEM, TEM and Raman spectroscopy. The dimensions and yield of grown CNT's were observed to be same in all temperatures. The crystalline perfection of CNT's was improved with an increase in temperature. It is observed that the temperature of the synthesis could affect the degree of nanotubes graphitization. Catalytic growth of CNT's was elucidated on the basis of the model of vapour-liquid-solid and the proposed model was supported by TEM. Growth rate of nanotubes rises at higher temperatures and graphite sheets builds with lower defect. Nonlinear optical and limiting characteristics were determined using ns pulses at 532 nm. CNT's display strong nonlinear absorption coefficient of 29.9 cm/GW. CNT's exhibiting excellent optical limiting with a limiting threshold of 3 J/cm 2.

Introduction

One-dimensional carbon nanotubes gained significant interest in modern technology and seem to be the material for next-generation technology [1,2]. Due to their low density, unique microstructure, excellent electrical and mechanical properties and attractive optical properties that allowing this material to apply in different potential technological applications, such as bio-imaging, nonlinear optics, photonics, photovoltaics, optoelectronic devices, nanoelectronics devices, hydrogen storage, sensors [[3], [4], [5], [6], [7], [8], [9]]. CNTs exhibit many significant characteristic properties such as nonlinear Kerr-effect, reverse saturable absorption, saturable absorption etc. The large nonlinear optical phenomena possessing in this material enable applications in all-optical signal processing, optical limiting, high-speed optical switches etc [6]. The large optical nonlinearity in CNT's is due to the presence of delocalized π-electrons along the tube axes [10].

The one dimensional CNT's have been developed with various synthetic approaches such as chemical vapour deposition (CVD), laser ablation, arc discharge, etc [11]. Compared to other techniques, the CVD technique is an easy and inexpensive method to develop CNTs with controlled manner [12]. CVD technique needs a carbon source and metal catalyst to produce one-dimensional CNTs. Adding of catalyst splits CVD systems into floating catalyst technique in which a catalyst is in gaseous phase. The advantage of floating catalyst technique over the fixed catalyst technique is that the catalyst preparation stage is not necessary. The catalyst is continually produced in the chamber and the problem of deactivation of catalyst is eluded. Although, a lot of work has been reported on the synthesis of CNT's using ferrocene as a catalyst precursor, here we have carried out a detailed investigation into the synthesis of CNT's by optimizing the synthesis temperature using the ferrocene double-stage CVD method.

In this paper, we presented a simple way for the synthesis of CNT using a floating catalyst process. The temperature effect on CNT synthesis has been elaborated. CNT's' catalytic growth was clarified on the basis of the VLS model and the proposed model was validated by studies of transmission electron microscopy (TEM). Further, the NLO and optical limiting properties of the CNT's were investigated using Z-scan technique at nanosecond laser pulses.