Near-field optical characterization of low-dimensional nanomaterials

Abstract

Due to the wave nature of light, the spatial resolution of traditional far-field optical microscopy is fundamentally prohibited to be much smaller than the wavelength, thus cannot fulfill the requirement of nano-characterization of low-dimensional materials. This diffraction limit can be circumvented by the optical version of scanning probe microscopy via incorporating the near-field optical information into the imaging process. In the last one and a half decades, scanning near-field optical microscopic techniques, especially scattering-type scanning near-field optical microscopy (s-SNOM), have undergone tremendous development. The wavelength independent spatial resolution of s-SNOM goes far beyond the diffraction limit, leading to an explosive amount of applications that spread throughout the field of materials science. The instrument of s-SNOM is a delicate combination of optical and scanning probe techniques. It utilizes the sharp tip of an atomic force microscope (AFM) probe to achieve nanoscale spatial resolution. Specifically, the light beam from a source is focused onto the apex of the AFM tip by a parabolic mirror. The tip serves as a nanoscale light confiner and enhancer, inducing a strong electromagnetic field (hot spot) underneath the tip apex. When the tip is brought into contact with the investigated sample, this field is modified by its interaction with the sample and carries near-field information of the sample. Then the tip acting as an optical antenna scatters the near-field information into far-field. The back-scattered near-field signals are feed into a photodetector to register a near-field image of the sample. By modulating the near-field signal via tapping the AFM tip and demodulating at the high order harmonics of the tip-tapping frequency, the noise from the background and stray light can be greatly suppressed. Since the near-field hot spot is on the same scale with the tip apex, the spatial resolution of s-SNOM is predominantly defined by the dimensions of the tip; a value of 20 nm is routinely achievable with commercially available and economical AFM tips. In principle, s-SNOM is a permittivity-sensitive technique, thus its most typical application is to imaging the surface permittivity distributions of low-dimensional nanomaterials. In the literatures, s-SNOM has been used to realize material-specific identification of zero-dimensional nanoparticles; nanoscale-resolved mapping of the free-carrier distribution along a one-dimensional nanowire and two-dimensional nanoimaging of the phase boundaries of organic thin films have been reported. Surprisingly, sub-surface imaging of objects with a depth of tens of nanometers has also been proved possible. By using the pump-probe method, the ultrafast dynamical properties of low-dimensional materials can be characterized with simultaneously high spatial and temporal resolutions. The s-SNOM tip can also serve as a momentum matcher between the free-space light and the intrinsic electromagnetic modes supported by low-dimensional materials, therefore, s-SNOM can be used to excite and image these modes. Both field mapping of localized modes in zero-dimensional materials and fringe pattern imaging of propagating modes in one- and two-dimensional materials have been reported. What is more, direct imaging of the wave packets has also been achieved by coupling s-SNOM to ultrafast optics. s-SNOM can provide spectral information of the sample if a broadband light source is used. A Fourier transform infrared (FTIR) spectrometer based on s-SNOM (nano-FTIR) has been developed. The spectra obtained by nano-FTIR have been demonstrated to be in good agreement with those acquired by conventional FTIR. Applications of the nano-FTIR including in-situ monitoring of chemical reactions and quick mapping of the dispersion of polaritons have been reported. In summary, s-SNOM has contributed significantly to the field of low-dimensional material characterizing. Nonetheless, it still has a large potential to expand its scope of applications. This is especially true at extreme conditions like ultrahigh vacuum, ultralow temperature, and strong magnetic field. If s-SNOM operating in these extreme conditions can be developed, it would be of great help in solving the enigma of strongly correlated materials.