Since the term ‘nanotechnology’ was used for the first time in 1974, the era of nanotechnology has lasted for several decades [1,2]. Nanotechnology has provided a new paradigm in many fields of science and engineering. An important breakthrough in the growth of nanotechnology has been the development of various high-resolution microscopes that can reveal the nanoscale world by overcoming the limitations of optical microscopes. In particular, the invention of the scanning probe microscope (SPM), which uses a sharp tip to image nanoscale structures on a sample surface, has accelerated the development of nanotechnology. The first SPM was the scanning tunneling microscope (STM) invented by Binnig and Rohrer of IBM in 1981 [3]. An STM based on the feedback of the tunneling current between a metallic tip and a metallic sample provides atomic resolution imaging performance [3]. Various techniques based on STM have been developed, including scanning tunneling spectroscopy, inelastic electron tunneling spectroscopy, and atomic manipulation [4–6]. With the development of these STM-based techniques, researchers are able to image the morphology of nanostructures, manipulate them, and measure their electronic or chemical properties at the nanoscale [4–6]. However, STM has critical weaknesses from the point of view of wide usability because the sample for STM study must be conductive, and it is mostly operated in tricky conditions such as ultrahigh vacuum and low temperature. Several years after the invention of STM, a new type of SPM, atomic force microscopy (AFM), was developed in 1986 by three scientists, including an STM inventor, Binnig, in 1986 [7]. Two years later, Meyer and Amer developed an AFM consisting of a laser beam, cantilever, and position-sensitive detector, which is a common AFM type of the present time [8]. Because AFM is operated based on the feedback of the atomic force between a sharp tip and a sample, the sample and tip do not need to be conductive. Although the resolution of common AFM is lower than that of STM, the AFM has also the advantage that it can be operated in various conditions of ambient air, vacuum, and specific gas. Furthermore, the most significant strength of AFM is that there are many AFM-based application modes, such as Kelvin probe force microscopy (KPFM), conductive AFM, and scanning spreading resistance microscopy [9–17]. These application modes make it possible to measure the nanoscale electrical properties of a sample, such as the work function, electrical current, electrical resistance, as well as the surface morphology [9–17].

Light absorption and emission phenomena are attractive topics in a wide range of research fields because they can be applied in the development of various types of optoelectronic devices, including solar cells, photodetectors, and light-emitting diodes. In particular, recently emerging 2D materials have shown attractive optoelectronic properties that can be applied in next-generation optoelectronic devices [18,19]. Nanoscale measurement tools with light illumination are important for the investigation of optoelectronic properties. There have been many efforts to illuminate the AFM tip-sample junction, with side and bottom illumination being most common [20–25]. The side-illumination type has the advantage that a non-transparent sample can be studied using this method; however, it is difficult to align the light spot exactly on the junction between the AFM tip apex and the sample surface because the light paths are restricted by the bulky AFM cantilever. In the case of the bottom-illumination type, the light spot can be aligned easily on the AFM junction, although the sample substrates are restricted to transparent materials such as bare or indium tin oxide (ITO)-coated glass. Figure 1 shows a simple schematic of the bottom-illumination setup used for the AFM. In this study, we designed and fabricated a compact bottom-illumination-type light illumination module with a laser diode that can be successfully installed in a commercial AFM instrument. Using this module, we can shine green laser light on a ribbon-like MoS₂ flake locally from its bottom. AFM measurements clearly revealed the thickness and size of the MoS₂ flakes. In addition, photoconductive AFM (pc-AFM) can be conducted by measuring the current between the tip and the sample in the dark and under illumination. The most important advantage of this light illumination module is that it can illuminate a specific sample area locally without illuminating the rest of the sample area. This module can be used for pc-AFM or KPFM studies under illumination for various optoelectronic materials and devices.

Figure 1. The schematic of the AFM setup with illumination from the bottom.

We designed a compact light illumination module using the AutoCAD inventor 3D modeling software. The parts for the light illumination module were produced in a professional machine shop and assembled by us. We prepared an ITO/glass sample covered with MoS₂ flakes to test the light-illumination module. The MoS₂ flakes was exfoliated from the bulk MoS₂ source and transferred onto the ITO/glass substrate by the typical ‘Scotch-tape’ method which is exactly same as the method used to obtain graphene flakes from highly oriented pyrolytic graphite [26]. The prepared MoS₂/ITO/glass test sample was observed under an optical microscope to determine the appropriate MoS₂ flakes. After the target MoS₂ flake was selected, we performed Raman spectroscopy and mapping measurements to confirm that it is MoS₂, not dust, using a commercial Raman spectroscopy instrument (XperRAM C Series, Nanobase). The test sample was then transferred to a commercial AFM instrument (NX10, Park Systems) with our new light illumination module and investigated in PinPoint AFM mode [27]. In particular, our NX10 AFM instrument was installed in an acrylic glove box with a gas purification system (MB 10 COMPACT, MBRAUN) to control artifacts induced by ambient gases such as oxygen and water. For the AFM test, we used a conductive diamond-coated AFM tip (CDT-NCHR, NanoWorld) for reliable measurements, minimizing the data distortion caused by tip degradation.

Figures 2(a) and 2(b) show the top and side views of the schematic diagram of our compact light illumination module design, respectively. Figures 2(c) and 2(d) show a 3D model of the light illumination module design and drawings of its components, respectively. Our light illumination module consists of four components: i) a main frame, ii) a long mirror support, iii) a short mirror support, and iv) a laser diode holder, as shown in Figs. 2c) and 2(d)(. The light illumination module was designed for installation on the sample stage of a commercial NX10 AFM instrument. According to the specifications of our AFM instrument, the maximum weight of the sample (including the sample holder) should be less than 500 g. Therefore, the light illumination module must not be too heavy. Among the four components shown in Figs. 2(c) and 2(d), the main frame and the laser diode holder were fabricated from aluminum because it is a metal which is relatively light and cheap, as well as easily machinable. In addition, aluminum has high thermal conductivity (237 Wm⁻¹K⁻¹) which aids in dissipation of the heat generated by the laser diode. The remaining two components in Figs. 2(c) and 2(d), the long and short mirror supports, respectively, were fabricated from polyether ether ketone, because these two components should slide with ease on the main frame. As shown in Fig. 2(a), the laser beam enters the light illumination module horizontally. We used a commercial green laser diode (DJ532 series, Thorlabs) with a wavelength of 532 nm attached to the laser diode holder. The laser diode can be turned on, and its intensity can be adjusted using a typical power supply. As shown in Figs. 2(c) and 2(d), both the short and long mirror supports have an inclined plane of 45°, and the mirrors are attached to these inclined planes. The incident laser beam inside the light illumination module was successively reflected against two mirrors, as shown in Fig. 2(a). The incident laser beam travels horizontally with a turning angle of 90° after reflection at the short mirror support, and it vertically escapes from the light illumination module after reflection at the long mirror support. As indicated by the red arrows in Fig. 2(a), the long and short mirror supports can be moved in the x- and y-directions, respectively, by turning two threaded rods (2 mm dia.) connected to them. Therefore, we can change the position of the vertical exit laser beam within an x-y range of 18 × 40 mm².

Figure 2. (a) The top and (b) side view of the schematic of the compact light illumination module. (c) 3D model design of the light illumination module and (d) drawings of four components for it.

After assembling the four components, a magnetic stainless-steel disk of 1 mm thickness was glued to the bottom of the main frame to install the light illumination module on the sample stage of our commercial AFM instrument with a magnetic force. In addition, the top of the main frame was covered with a 2 mm thick glass plate, and a sample was attached to the light-illuminated region of the glass plate using double-sided adhesive tape. The lateral size of our light illumination module is 70 × 80 mm² including a laser diode and its height is 14 mm. Our NX10 AFM instrument has a restriction in the gap between the AFM tip and sample stage. The height of the sample in the AFM system should be less than 20 mm. Therefore, our 14 mm thick light illumination module could be installed on the sample stage of our AFM system, and the allowable sample height was reduced to ~6 mm, which is acceptable for most optoelectronic device samples. Figure 3(a) shows a photograph of our assembled light illumination module; the green laser light is also shown. A photograph of the light illumination module mounted on the AFM sample stage is shown in Fig. 3(b).

Figure 3. Photographs of (a) the light illumination module and (b) the AFM setup equipped with the light illumination module.

We prepared MoS₂ flakes dispersed on an ITO/glass substrate as the test sample for our light-illumination module. An optical microscopy image of the MoS₂ flakes on the ITO/glass is shown in Fig. 4(a). By the ‘Scotch tape’ method typically used for preparing 2D nanomaterials such as graphene, 2D nanomaterials are exfoliated randomly in terms of shape, size, and thickness. Therefore, we need to find an appropriate flake by repeated trials of the ‘Scotch tape’ method. Figure 4(b) shows the target MoS₂ flake selected from among the various MoS₂ flakes shown in Fig. 4(a). For the MoS₂/ITO/glass test sample, we performed Raman spectroscopy and microscopy measurements. The Raman spectrum acquired for the target flake is shown in Fig. 4(c), and two characteristic Raman peaks of MoS₂ for $E_{2g}^1$ and A₁₉ modes were observed near ~381 and ~406 cm⁻¹ [28]. Figure 4(d) shows the Raman mapping image for the target MoS₂ flake acquired by mapping on the basis of the Raman shift at 406 cm⁻¹. Raman spectroscopy and microscopy results confirmed that the target flake shown in Fig. 4(b) was MoS₂.

Figure 4. (a) Optical microscopy image of MoS₂ flakes dispersed on an ITO/glass substrate. (b) Enlarged optical image of the MoS₂ flake selected in (a). (c) Raman spectrum and (d) mapping acquired on the MoS₂ flake in (b).

After the Raman experiments, the MoS₂/ITO/glass test sample was transferred to the new light-illumination module installed on the sample stage of our commercial AFM system. First, we aligned the target MoS₂ flake shown in Fig. 4(b) right under the AFM tip by moving the sample stage of the AFM instrument. The vertical exit laser beam was aligned underneath the AFM tip by moving the two mirror supports in the light-illumination module. After completing the alignment of the three parts (AFM tip, target MoS₂ flake, and exit laser beam), we acquired optical images using the microscope in the AFM instrument. Figures 5(a) and 5(b) show optical images with the laser diode on and off, respectively, confirming that the exit laser beam was successfully aligned underneath the AFM tip. The AFM topographic image acquired of the target MoS₂ flake is shown in Fig. 5(c); its root mean square surface roughness value was 2.8 nm, which is dominated by the surface roughness of the ITO/glass substrate. The shape of the target MoS₂ flake is clearly shown in Fig. 5(c) and is the same as the shape shown in Fig. 4(b) or Fig. 4(d), which indicates that we exactly aligned the target MoS₂ flake, as well as the exit laser beam, underneath the AFM tip. The length of the target MoS₂ flake which looks like a ribbon was ~23 µm. Figure 5(d) shows the line profile extracted from the red line in Fig. 5(c) and, as indicated by two blue dotted, the thickness of the target MoS₂ flake is ~3.4 nm which is close to the thickness of fivelayer MoS₂ by considering that thickness of monolayer MoS₂ is 0.65 nm. The rough structure of the line profile in Fig. 5(d) is originated from the surface roughness of ITO/glass substrate. Furthermore, it was possible to perform pc-AFM operation using this light illumination module by measuring the dark current and photocurrent (under illumination) flowing through the AFM tip-sample junction by turning the laser light off and on, respectively. As shown in Figs. 5(e) and 5(f). the current inside the MoS₂ flakes increases slightly under illumination, which is consistent with previously reported results [20].

Figure 5. (a), (b) Optical images acquired in the AFM instrument equipped with the light illumination module when the laser light is turned (a) on and (b) off. (c) AFM topographic image of the MoS₂ flake selected in Fig. 4. (d) Line profile extracted from the red line in (c). (e) Dark current (acquired in the dark) and (f) photocurrent (acquired under illumination) images of the sample area indicated as the green dotted square in (c).

In this study, we designed and fabricated a compact light illumination module using a laser diode as the light source for application in a commercial AFM instrument. Using this module, a ribbon-like MoS₂ flake can be illuminated locally from the bottom. AFM measurements revealed the thickness and size of the MoS₂ flakes. In addition, pc- AFM can be performed by measuring the current between the tip and sample in the dark and under illumination. The most important advantage of this light illumination module is that a specific sample area can be illuminated locally without illuminating the rest of the sample area. This module can be used for pc-AFM or KPFM studies under illumination of various optoelectronic materials and devices.

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (Grant No. 2020R1A2C1005299).

The authors declare no conflicts of interest.