Density Detection of Aligned Nanowire Arrays Using Terahertz Time-Domain Spectroscopy
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A rapid technique is necessary to quantitatively detect the density of nanowire (NW) and nanotube arrays in one-dimensional devices which have been identified as useful building blocks for nanoelectronics, optoelectronics, biomedical devices, etc. Terahertz (THz) time-domain spectroscopy was employed in this research to detect the density of aligned Ni NW arrays. The transmitted amplitude of THz peaks and optical thickness of NW arrays was found to be the effective parameters to analyze the density change of NW arrays. Owing to the low multiple scattering and high order of Ni NW arrays, a linear relationship was observed for the transmitted amplitude and optical thickness regarding NW density, respectively. Therefore, THz technique may be used as a promising tool to characterize the density of one-dimensional structures in the large-scale integrated nanodevice fabrication.
KeywordsNanowire density Terahertz time-domain spectroscopy Transmitted amplitude Optical thickness
PACS42.25.Bs 68.55.JK 75.50.Cc 78.47.+p 78.67.-n
Arrays of one-dimensional (1D) nanostructures such as nanowires (NWs) and nanotubes (NTs) are a very attractive option to be used as building blocks for nanoscale electronic, biosensor, chemical detector, etc. [1, 2, 3]. To realize their full potential in applications, however, NWs and NTs must be integrated efficiently into various device architectures. Many factors, such as NW/NT numbers, diameter, and alignment, should be considered in the process of developing large-scale device fabrication. To control the lateral density of NW and NT arrays, since it can influence device performance, is one of the most important challenges. A number of experimental studies have been reported to explore the impact of NW and NT density on device performance in the context of photoelectric response, nanowire electrical transport studies, gas-phase chemical sensing, and field emission [4, 5, 6, 7]. The fundamental requirements for these studies are to obtain quantitatively meaningful relationships between the NW and NT density and device properties. Meanwhile, many approaches have been suggested to control the density of NWs [8, 9]. However, these methods only focus on the vertically grown NWs, and the NW density was controlled during the preparation of NWs.
Though there have lots of existing methods such as directed self-assembly, flow-assisted alignment, contact printing, Langmuir–Blodgett technique, and blown bubble methods [10, 11, 12, 13, 14] to fabricate the large-scale lateral-aligned NW/NT devices, it is still difficult to precisely control the density of NWs/NTs incorporated into each device. Especially, when the NW arrays have a high density, it is impossible to quantitatively analyze the density distribution of large-scale assembly of NWs and NTs.
Terahertz time-domain spectroscopy (THz-TDS) is a normal and significant THz method based on the THz electric field with time resolution. As a newly developed spectral technique, some spectral features can be used as the standard to qualitatively and quantitatively analyze the material structures and physical properties of tested samples. Ramanandan’s group investigated the oxidation kinetics of nanometer-thick copper films using the in situ THz transmission spectroscopy . Balci’s group investigated the complex refractive index, dielectric function, and conductivity of ZnO NWs using temperature-dependent THz-TDS combined with calculations . In this work, we investigated the relationship between the optical properties and the aligned Ni NW density using the THz-TDS technique. These results indicated that THz-TDS technique was effective to realize the detection of NW density.
Results and Discussion
Usually, scanning electron microscope (SEM) is an appropriate way to describe the density, diameter, and alignment of NWs and NTs in nanodevices. Li et al. have fabricated the Si NW-based field-effect transistor biosensors using nanomanipulation inside a SEM system and the number of NWs in each device is only 1, 4, and 7, respectively . The multiple In2O3 nanowire devices for gas detection have been reported by Zhang et al., and the SEM image showed that the number of NWs in a device is estimated to be 100~200 . It can be found that SEM system is difficult to detect the exact density of lateral-aligned NWs if the number of NWs in each device is higher than the order of 100 of NW density. Recently, conventional optical technology in the UV-IR region has been used to study the properties of nanostructures. However, the quantitative analysis of the NW density in NW arrays remains difficult. Because the light wavelength is approximately equal to the size of NWs, the nonlinear interaction between light and NWs is strongly dependent on the numbers, shape, and alignment of NWs induced by the multiple scattering effect. In In2S3 nanostructure arrays, it is found that the springs, screws, and vertical rods have an enhanced absorption compared to zigzags and tilted rods investigated by UV-NIR spectroscopy . Tena-Zaera et al. investigated the optical scattering effect of ZnO NW arrays in visible wavelength range and found that the increase in diameter of NWs induces a considerable redshift in the reflectance maximum . THz-TDS technique was considered to quantitatively estimate the NW density because of its advantages compared to the SEM technique and UV-NIR spectroscopy. The wavelength range of THz signal is from 30 to 1000 μm, which is much larger than the diameter of NWs and the distance between the neighboring NWs. Therefore, the multiple scattering in NW arrays has not been distinctively enhanced with NW density increasing. However, the lower NW density and alignment of NWs will influence the accuracy and measuring resolution of THz-TDS technique.
In summary, the practicability was demonstrated about the THz-TDS being applied to quantitatively measure the density of aligned NW arrays. The density variation of Ni NW arrays as the function of the transmitted amplitude and optical thicknesses of all samples was analyzed, and the linear relationship was found. It is indicated that THz-TDS represents a powerful tool for the fabrication of 1D nanoscale devices in future.
This work was supported by the National Nature Science Foundation of Beijing (4142047), Beijing Higher Education Young Elite Teacher Project (YETP0684), and Excellent Young Teachers Program of China University of Petroleum (ZX20150108).
WFX and XW designed and planned all the experiments. XW performed the NW synthesis and THz measurements. XW and WFX performed the characterization of samples, analyzed the results, and prepared the manuscript. YL, JQZ, and KZ helped in the preparation of the manuscript. WFX and KZ developed the conceptual framework, supervised the whole work, and finalized the manuscript. All authors discussed the results and reviewed the manuscript. All authors read and approved the final manuscript
The authors declare that they have no competing interests.
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