qCMOS camera has excellent weak signal detection ability because of its excellent characteristics such as very low dark noise. In this experiment, a qCMOS camera and a grating spectrometer were combined, and a new software was developed to control both at the same time for detecting Tip-enhanced Raman scattering (TERS). For this purpose, the detection capability of qCMOS camera on Raman or weak signal Raman is tested, and more application possibilities are explored.
A scanning tunneling microscope (STM) is capable of more than just observing and manipulating the nanoworld with atomic resolution, its tunneling current can also be used as a local source of excitation to produce light from the junction, the so-called STM induced luminescence (STML), which can provide additional information on local electromagnetic properties pertaining to the decay of various excitations. On the other hand, by taking advantage of the strong signal enhancement generated at a metallic tip apex, spatially resolved Raman spectra can be achieved using tip-enhanced Raman scattering (TERS), which can offer chemical recognition ability in real space with high spatial resolution. Furthermore, tip-enhanced photoluminescence (TEPL) allows to explore molecular photo physics and the light-matter interaction as well as nanoscale energy transfer at the sub-molecular scale.
Fig.1 Experiment diagram of Tip-enhanced Raman scattering
For this experiment, the sample (MoS2) was put in STM, and the spectra was excited by 532 nm DPSS laser, then the Raman signal was collected by a home-built cube with optical path, as shown in Fig.1. The qCMOS camera, ORCA-Quest was mounted in a 550 mm focal length spectrometer to collect the Raman spectrum.
As discussed above, the sample (MoS2) was put in STM and excited by a 532 nm DPSS laser. We used a fiber and focusing cube to collect the Raman spectrum of MoS2, shown in Fig.2. The weak Raman spectrum was collected by ORCA-Quest via 180 s exposure time. Due to the low readout noise, the SNR is better than a deep cooling CCD camera. Meanwhile, due to the 4.6*4.6 μm pixel size, the resolution (FWHM) is better than normal CCD cameras whose pixel size is 13 μm, 26 μm or 20 μm.
Fig.2 Optical setup picture
For comparison, another spectrometer acquisition system, liquid nitrogen cooled CCD + 300 mm focal length grating spectrometer, was used to make comparisons with our ORCA-Quest + 550 mm focal length spectrometer. The setup is exactly the same before the collection fiber, with replacement only in the fiber and spectrum acquisition parts.
Fig.3(a) MoS2 Raman signal by qCMOS camera, ORCA-Quest + 550 mm focal length grating spectrometer (f/6.4), 200 μm optical fiber, 300 g/mm grating, 180 s exposure time
Fig.3(b) MoS2 Raman signal by liquid nitrogen cooled CCD + 300 mm focal length grating spectrometer (f/3.9), 400 μm fiber, 600 g/mm grating, 180 s exposure time
From the comparison results, ORCA-Quest + 550 mm focal length grating spectrometer has three highlights, which are
1） Different from traditional sCMOS cameras, which are generally used for short exposure applications, qCMOS cameras have an overall noise level comparable to liquid nitrogen cooled CCD under 180 s exposure time, thus obtaining excellent Raman signal to noise ratio. Even if the influence of fiber and spectrometer on the light collection efficiency is removed, the signal to noise ratio of qCMOS camera is still comparable to that of liquid nitrogen cooled CCD.
2） For CCD detectors, cosmic ray interference at long exposures is difficult to remove. While, under the long integration time, qCMOS has better anti-cosmic ray interference ability than liquid nitrogen cooled CCD.
3） The dark noise suppression level of qCMOS was also validated in the long integration time and is comparable to the level of liquid nitrogen cooled CCD.
Doctor Han Tao graduated from Fudan University in Shanghai in 2005. After many years of technical work in HORIBA Company, he founded Shanghai UPU Optoelectronic Technology Co., Ltd. (http://www.upu-opt.com/) as a co-founder in 2018，who has accumulated nearly 20 years of experience in the field of spectroscopy and spectral measurement.
Shanghai UPU Optoelectronic Technology Co., Ltd. is a high-tech enterprise specializing in the field of optoelectronic technology. The company's business direction is mainly imaging and spectrum. Meanwhile, in recent years, the company have focused on the development and production of confocal Raman spectroscopy and Fluorescence spectroscopy systems. According to the demand of scientific research market, we adopt standardized product ideas, to provide users with a variety of high-performance standardized product modules and customized overall system.
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