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Li Wang, Shen-long Jiang, Qun Zhang, Yi Luo. Multi-domain High-Resolution Platform for Integrated Spectroscopy and Microscopy Characterizations[J]. Chinese Journal of Chemical Physics , 2020, 33(6): 680-685. doi: 10.1063/1674-0068/cjcp2006093
Citation: Li Wang, Shen-long Jiang, Qun Zhang, Yi Luo. Multi-domain High-Resolution Platform for Integrated Spectroscopy and Microscopy Characterizations[J]. Chinese Journal of Chemical Physics , 2020, 33(6): 680-685. doi: 10.1063/1674-0068/cjcp2006093

Multi-domain High-Resolution Platform for Integrated Spectroscopy and Microscopy Characterizations

doi: 10.1063/1674-0068/cjcp2006093
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  • Corresponding author: Shen-long Jiang, E-mail: poetjsl@ustc.edu.cn
  • Received Date: 2020-06-12
  • Accepted Date: 2020-07-02
  • Publish Date: 2020-12-27
  • In recent decades, materials science has experienced rapid development and posed increasingly high requirements for the characterizations of structures, properties, and performances. Herein, we report on our recent establishment of a multi-domain (energy, space, time) high-resolution platform for integrated spectroscopy and microscopy characterizations, offering an unprecedented way to analyze materials in terms of spectral (energy) and spatial mapping as well as temporal evolution. We present several proof-of-principle results collected on this platform, including in-situ Raman imaging (high-resolution Raman, polarization Raman, low-wavenumber Raman), time-resolved photoluminescence imaging, and photoelectrical performance imaging. It can be envisioned that our newly established platform would be very powerful and effective in the multi-domain high-resolution characterizations of various materials of photoelectrochemical importance in the near future.
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Multi-domain High-Resolution Platform for Integrated Spectroscopy and Microscopy Characterizations

doi: 10.1063/1674-0068/cjcp2006093

Abstract: In recent decades, materials science has experienced rapid development and posed increasingly high requirements for the characterizations of structures, properties, and performances. Herein, we report on our recent establishment of a multi-domain (energy, space, time) high-resolution platform for integrated spectroscopy and microscopy characterizations, offering an unprecedented way to analyze materials in terms of spectral (energy) and spatial mapping as well as temporal evolution. We present several proof-of-principle results collected on this platform, including in-situ Raman imaging (high-resolution Raman, polarization Raman, low-wavenumber Raman), time-resolved photoluminescence imaging, and photoelectrical performance imaging. It can be envisioned that our newly established platform would be very powerful and effective in the multi-domain high-resolution characterizations of various materials of photoelectrochemical importance in the near future.

Li Wang, Shen-long Jiang, Qun Zhang, Yi Luo. Multi-domain High-Resolution Platform for Integrated Spectroscopy and Microscopy Characterizations[J]. Chinese Journal of Chemical Physics , 2020, 33(6): 680-685. doi: 10.1063/1674-0068/cjcp2006093
Citation: Li Wang, Shen-long Jiang, Qun Zhang, Yi Luo. Multi-domain High-Resolution Platform for Integrated Spectroscopy and Microscopy Characterizations[J]. Chinese Journal of Chemical Physics , 2020, 33(6): 680-685. doi: 10.1063/1674-0068/cjcp2006093
  • In recent decades, a huge advancement in the field of materials science has been witnessed. For instance, the size of semiconductor devices is pushed to a few nanometers [1, 2], the length scale in the measurements of electron transport properties reaches the order of optical wavelength [3, 4], and the time scale in the measurements of charge carrier dynamics enters a few picoseconds or even femtoseconds [5, 6]. The in-situ microscopic and real-time dynamic processes, which usually play key roles in the functionalization of materials, have aroused great attention from both experimentalists and theorists in a wide spectrum of fundamental and application research fields. The development of multi-domain (energy, space, time) high-resolution characterization techniques has become a topical trend in the field of high-precision measurement of material systems [7]. These techniques can lay a solid foundation for the accurate determination of important basic parameters and processes, such as physical and chemical structures, excited-state properties, charge-carrier interactions, and enigmatic quantum effects under external fields. The confocal Raman microscopy is a robust tool for investigating the form and structure of materials [8]. Benefited from some innovations, its spatial resolution has been routinely improved to hundreds of nanometers. Besides, if a pulsed ultrashort laser (picosecond or femtosecond) is used as the excitation source and a delayed synchronized acquisition is arranged, it can also achieve a high temporal resolution. Herein, we describe a multi-domain (energy, space, time) high-resolution platform for integrated spectroscopic and microscopic characterizations of materials. The proof-of-principle results collected on this platform demonstrate its powerful functionalities, promising extensive applications toward multi-domain characterizations of various materials with high spectral, spatial, and temporal resolutions.

  • FIG. 1 shows the schematic diagram of our newly established multi-domain platform, which is partly based on a WITec alpha300 RAS+ confocal Raman microscopy (WITec GmbH) but with some important modifications.

    Figure 1.  The Schematic diagram of the multi-domain (energy, space, time) high-resolution platform for integrated spectroscopy and microscopy characterizations.

    As for the function of scanning near-field optical microscopy (SNOM), we succeed in improving its spatial resolution from $ \sim $100 nm to $ \sim $60 nm by using an aperture whose pinhole diameter is reduced as much as possible without significantly reducing the amount of light transmission; meanwhile, the sample substrate is modified by an aperture array made of Au nano-gaps [9], as can be clearly visualized from an atomic-force microscopy (AFM) image shown in FIG. 2(a). Such an arrangement turns out to greatly improve the signal intensity and signal-to-noise ratio due to the gap-plasmon effect. As such, the near-field resolution is further improved from $ \sim $60 nm to $ \sim $20 nm, as revealed in FIG. 2 (b) and (c).

    Figure 2.  (a) AFM image of an Au nano-gaps aperture array. (b) SNOM image corresponding to (a). (c) The variation of AFM height (red line) and SNOM intensity (black line) with distances along the white dashed lines in (a) and (b), respectively.

    As for the laser systems used, we achieve a convenient switching among the following three: (ⅰ) a diode-pumped continuous-wave (CW) laser (355 or 532 nm, Cobalt Laser); (ⅱ) a picosecond pulsed diode laser (PDL 800-D, 373 nm, PicoQuant GmbH); (ⅲ) a tunable femtosecond laser, which is delivered by an optical parametric amplifier (TOPAS-Prime, 240$ - $2600 nm, Light Conversion) excited by a Ti:sapphire regenerative amplifier (Astrella, center wavelength 800 nm, pulse duration 35 fs, pulse energy 7 mJ, repetition rate 1 kHz, Coherent).

    As for the Raman or photoluminescence (PL) measurements, the polarization of the excitation laser is adjusted by a half-wave plate and a Glan-Taylor prism, and the polarization of the signal is adjusted by another Glan-Taylor prism. The optical delay line, which is used to variably adjust the time delays between the excitation and probe lasers, comprises several mirrors with low group-delay dispersion and a linear DC motor stage (Parker GmbH). The excitation laser is reflected by a dichroic mirror into an objective or a tip with a tiny hole of 60/90/100 nm diameter and then is focused onto the sample. The movement of the sample is controlled by a piezo stage. The diffraction-limited spot size of the CW 532-nm excitation laser is determined to be $ \sim $300 nm (with a 100$ \times $ objective, NA = 0.90 where NA stands for numerical aperture). The probe laser dispersed by an $ f $ = 600 mm spectrometer equipped with 150, 600, or 1800 grooves/mm grating (UHTS 300). The collected signals are detected using a back-illuminated charge-coupled device (CCD) camera (DU970N-BV, Andor) that is thermoelectrically cooled down to $ - $60 $ ^{\circ} $C. It is worth noting here that the dichroic mirror can be replaced by BragGrate notch filters (OptiGrate Corp.) with an optical density of 3 and a spectral bandwidth of $ \sim $5$ - $10 cm$ ^{-1} $ such that the Rayleigh line can be effectively suppressed to aid in the acquisition of low-wavenumber Raman spectra.

    As for the micro-zone photoelectrical performance measurements, a source meter (Keithley 2400, Tektronix Inc.) is used as the power supplier to provide the background potential difference. When a photovoltaic device is irradiated, the separation of photogenerated electrons and holes gives rise to a weak photocurrent, which is amplified by a current amplifier (FEMTO Messtechnik GmbH) and then sent to a computer for processing and analysis. Note that all of the following proof-of-principle tests are performed under ambient conditions.

  • To examine the spatial and spectral resolutions accomplished on our platform, we perform measurements on an ultrathin graphene sample, which is prepared using a routine mechanical exfoliation method [10]. As shown in FIG. 3(a), the area of Raman imaging (approximately 20 µm$ \times $20 µm) is surrounded by the blue dotted lines. Through a point-by-point scanning, we can readily analyze the spectral profile of each point to obtain the structural information of interest. Displayed in FIG. 3 (b)-(d) are the Raman mapping results for the G band of graphene ($ \sim $1582 cm$ ^{-1} $, in-plane C$ - $C stretching) [11], including the spatial distributions of its intensity, peak frequency ($ \omega_ \rm{G} $), and full width at half maximum (FWHM, $ \Gamma_ \rm{G} $), respectively. From these spatial mappings one can clearly see that the degree of wrinkling-induced stress varies across the large-scale graphene [12, 13]. Moreover, the spectral structures of the 2D band ($ \sim $2677 cm$ ^{-1} $, the first overtone of D mode) [11] in the labelled regions (FIG. 3(e)) exhibit an obvious position dependence with rich evolution information. These results would enable a further understanding of the correlation between energy space and real space.

    Figure 3.  (a) Optical imaging of the crystal structure showing the tested graphene sample. Spatially resolved Raman mappings of the G-band (b) intensity, (c) peak frequency, and (d) FWHM, all corresponding to the blue rectangle region in (a). (e) Left panel: spatially resolved Raman imaging of the 2D-band intensity distributions corresponding to the green rectangle region in (a), right panel: position-dependent spectral evolution of 2D band. (f) Polar plot of the G-band intensity $ I_ \rm{G} $ as a function of laser polarization angle $ \theta $ for configurations of back-scattering geometry. (g) Low-wavenumber Raman spectra showing Stokes and anti-Stokes branches for the C and ZO$ ' $ modes.

    Angle-resolved polarization Raman spectroscopy can be employed to identify the Raman modes (based on crystal symmetry and Raman selection rules) and to determine the crystallographic orientation of anisotropic materials [14, 15]. In our back-scattering geometry ($ Z $-in and $ Z $-out for the light), the electric polarization vectors of the incident $ e_ {\rm{i}} $ and scattered $ e_ {\rm{s}} $ light are in the $ X $-$ Y $ plane. By setting the angle between the $ e_ {\rm{i}} $ polarization and the $ X $ axis as $ \theta $ and fixing the $ e_ {\rm{s}} $ polarization to be parallel with the $ X $ axis, we have

    According to the relationship of Raman intensity

    where $ \; {R} $ is the Raman tensor [16], we have

    As an example, given that the Raman tensors of the G band are $ \tilde{R_1} $ and $ \tilde{R_2} $ [17]:

    the calculated $ I_ \rm{G} $ equals to a constant of $ c^2 $, independent of laser polarizations and sample azimuth angles. As such, the G band manifests itself as an isotropic distribution on the basal plane, as shown in FIG. 3(f).

    Low vibrational-frequency (typically below 140 cm$ ^{-1} $) micro-Raman spectroscopy has been used to unveil such details as stacking order and interlayer interactions in two-dimensional materials [18-20]. The reflective-volume Bragg-grating-based notch filters can be adopted for facilitating the acquisition of low-wavenumber Raman spectra. Such a notch filter can reflect light with a bandwidth as narrow as 10 cm$ ^{-1} $, not affected by other wavelengths passing through and with an overall transmittance up to 95%. By integrating it into the platform, we achieve simultaneous recording of the Stokes and anti-Stokes Raman spectra featuring low wavenumbers. As a demonstration, FIG. 3(g) shows a representative result on the two important rigid-plane Raman modes in bilayer graphene, i.e., shear mode (or C mode, $ \sim $28 cm$ ^{-1} $) [21] and out-of-plane interlayer optical photon breathing mode (or ZO$ ' $ mode, $ \sim $85 cm$ ^{-1} $) [22], linking directly to the interlayer interactions of interest therein.

    The above proof-of-principle characterizations demonstrate that our platform has been of high caliber in the spectral (energy) and spatial domains. To exploit its functionality in the temporal domain, we replace the CW light source with a pulsed laser (pulse duration 40 ps) such that the time-resolved imaging measurements can be executed with the aid of an external synchronization system. FIG. 4(a) shows the optical imaging for the crystal structure of a typical perovskite material of CsPbBr$ _3 $ with square and stick shapes. FIG. 4(b) exhibits a representative PL emission spectrum excited with the 373-nm pulsed laser, showing a strong excitonic emission band peaking at $ \sim $526 nm [23]. Considering that such an excitonic band may consist of contributions from bright and dark exciton states [24], we use 528-nm short/long-wave-pass filters to record the asymmetrically distributed PL emissions, as divided into spectral regions A and B in FIG. 4(b), mainly reflecting the bright and dark excitonic nature, respectively [24]. The time-correlated single photon counting technique [25] is adopted to map out the PL lifetimes ($ \tau_{ \rm{PL}} $) of the two spectral regions, with an instrument response function of $ \sim $100 ps. FIG. 4(c) shows a typical exponential fitting to yield the specific $ \tau_{ \rm{PL}} $ of region A for the square-shaped sample. The resulting $ \tau_{ \rm{PL}} $ mappings for the square- and stick-shaped samples are displayed in FIG. 4 (d)-(f) and FIG. 4 (g)-(i), respectively. Obviously, the two samples exhibit distinctly different patterns of spatially non-uniform $ \tau_{ \rm{PL}} $ distributions. As for the square-shaped sample, one can see from FIG. 4 (d) and (e) that the upper and left edges emit longer-lived PL than the lower and right edges for both regions A and B. Nevertheless, the lifetime differences ($ \Delta $$ \tau $$ _{ \rm{PL}} $) between regions A and B (point-by-point), as plotted in FIG. 4(f), manifest as an interesting nested pattern where the narrow exterior with negatively-valued $ \Delta $$ \tau $$ _{ \rm{PL}} $ (i.e., region-A emissions are shorter-lived than region-B emissions) surrounds the large-area interior with positively-valued $ \Delta $$ \tau_{ \rm{PL}} $ (i.e., region-A emissions are longer-lived than region-B emissions). As for the stick-shaped sample, one can see from FIG. 4 (g) and (h) that longer-lived PL emissions appear at its two ends for both regions A and B. Remarkably, the corresponding $ \Delta $$ \tau $$ _{ \rm{PL}} $ mapping, as plotted in FIG. 4(i), exhibits also a nested pattern but with exactly opposite $ \Delta $$ \tau $$ _{ \rm{PL}} $ signs. These proof-of-principle tests (taking time-resolved PL as an example) highlight the utility of time-domain mapping function integrated in our multi-domain platform. Along this line, we are currently making efforts to further extend to higher temporal resolution by introducing femtosecond light sources, such as the broadband tunable TOPAS-Prime system (see Instrumentation section). By doing so, in conjunction with the already achieved high spectral (energy) and spatial resolutions, we expect that our multi-domain (energy, space, time) platform would be capable of producing a wealth of high-quality data full of new physics to be explored, such as the elusive interaction, correlation, and interplay involved in a variety of exotic material systems.

    Figure 4.  (a) Optical imaging of the crystal structure of CsPbBr$ _3 $ perovskite material with different shapes. (b) PL emission spectrum (excitation at 373 nm). (c) Time-resolved PL kinetics (excitation at 373 nm). (d) Lifetime mapping of the square-shaped CsPbBr$ _3 $ for region A in (b). (e) Lifetime mapping of the square-shaped CsPbBr$ _3 $ for region B in (b). (f) Lifetime difference pattern derived from (d) and (e). (g) Lifetime mapping of the stick-shaped CsPbBr$ _3 $ for region A in (b). (h) Lifetime mapping of the stick-shaped CsPbBr$ _3 $ for region B in (b). (i) Lifetime difference pattern derived from (g) and (h).

    Last but not least, the function of photocurrent imaging is also integrated in our multi-domain high-resolution platform, as schematically illustrated in FIG. 5(a). The spatially resolved photocurrent imaging is operated in a micro-zone scanning mode. As a proof of principle, we conduct an evaluation of photoelectrical performance on a test sample of silicon electrode material (FIG. 5(b)). FIG. 5(c) shows a typical photocurrent response (excitation at 532 nm) under laser on/off conditions. FIG. 5(d) maps out the spatial distribution of photocurrent for a specific zone of the test sample (as marked by the red rectangle in FIG. 5(b)). Such a mapping, in combination with the above-described multi-domain functions, would be very useful for gleaning in-situ information from certain material devices. Hence, the relationship between the photoelectrical performances and the physical structures, properties, and effects can be scrutinized and elucidated, thereby providing instructive guidance for rational design of materials and material devices.

    Figure 5.  (a) Schematic configuration of the photocurrent mapping setup that is also integrated in our multi-domain platform. (b) Optical imaging of a test sample of silicon electrode material. (c) A representative test recording photocurrent response (excitation at 532 nm). (d) Photocurrent mapping for the red rectangle zone in (b).

  • To summarize, we have successfully established a multi-domain high-resolution platform for integrated spectroscopic and microscopic characterizations, in which the best resolution values are about 10 cm$ ^{-1} $ (energy), 20 nm (space), and 100 ps (time). The multiple functions of our platform are carefully tested with a set of proof-of-principle measurements including Raman imaging (high-resolution Raman, polarization Raman, low-wavenumber Raman), time-resolved photoluminescence imaging, and photoelectrical performance imaging, under in-situ and/or real-time operating conditions. Such a multi-domain high-resolution platform, with further modifications (e.g., by introducing femtosecond time-resolved light sources, external fields, and temperature-controlling systems) that are currently underway in our laboratory, would enable more comprehensive, robust, and effective characterizations in a highly integrated fashion. The obtained multi-domain high-resolution data would allow us to explore new physics such as the elusive interaction, correlation, and interplay involved in photoelectrochemical, photonic, and plasmonic material systems as well as the related prototypical devices.

  • This work is supported by the National Key Research and Development Program of China (No.2016YFA0200602, No.2017YFA0303500, and No.2018YFA0208702), the National Natural Science Foundation of China (No.21573211, No.21633007, No.21803067, and No.91950207), the Anhui Initiative in Quantum Information Technologies (AHY090200), and the USTC-NSRL Joint Funds (UN2018LHJJ).

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