In-Situ Chemical Thinning and Surface Doping of Layered Bi2Se3

As a promising topological insulator, two-dimensional (2D) bismuth selenide (Bi2Se3) attracts extensive research interest. Controllable surface doping of layered Bi2Se3 becomes a crucial issue for the relevant applications. Here, we propose an efficient method for the chemical thinning and surface doping of layered Bi2Se3, forming Se/Bi2Se3 heterostructures with tunable thickness ranging from a few nanometers to hundreds of nanometers. The thickness can be regulated by varying the reaction time and large-size few-layer Bi2Se3 sheets can be obtained. Different from previous liquid-exfoliation methods that require complex reaction process, in-situ and thickness-controllable exfoliation of large-size layered Bi2Se3 can be realized via the developed method. Additionally, the formation of Se nanomeshes coated on the Bi2Se3 sheets remarkably enhance the intensity of Raman vibration peaks, indicating that this method can be used for surface-enhanced Raman scattering. The proposed chemical thinning and surface-doping method is expected to be extended to other bulk-layered materials for high-efficient preparation of 2D heterostructures.


Introduction
After successful exfoliation of bulk graphite into atomic-thick graphene [1], increasing fascinated properties derived from 2D materials have been demonstrated in recent decades [2][3][4], such as the superconductivity of twisted bilayer graphene [5], tunable bandgap of the transition-metal dichalcogenides (TMDs) [6], and anisotropic photoelectric properties of black phosphorus [7]. In most situations, these extraordinary electronic and optical properties can be observed only when the thickness of the van der Waals solids decreases to one or a few layers. Taking the few-layer bismuth chalcogenides (A 2 B 3 , "A" represents Bi and "B" represents Se, Te) as typical examples, they exhibit anisotropic electrical properties, such as having the insulated state along the c-axis but the metallic state along the surface [8][9][10]. Due to superior thermoelectric performance, Bi 2 Se 3 has become one of the most studied topological materials [11]. The thickness of Bi 2 Se 3 is closely related to electrical properties, such as the thickness-dependent topological phase transition in Bi 2 Se 3 crystal [12].
Scalable fabrication of high-quality, large-size 2D materials is becoming more and more important for industrial applications [13]. Many preparation methods have been developed to obtain single or few-layer Bi 2 Se 3 sheets for exploring their properties [14][15][16]. Bottom-up approaches including chemical vapor deposition [17], vapor-solid techniques [18] and solvothermal synthesis [19] are proposed to fabricate few-layer Bi 2 Se 3 sheets. Multiple upbottom methods are also developed to realize the exfoliation of bulk Bi 2 Se 3 into few-layer sheets [15,[20][21][22][23]. Although high-quality few-layer Bi 2 Se 3 can be obtained by mechanical exfoliation with assistance of adhesive force of the tapes or the tip force of the atomic force microscopy (AFM), they also suffer from low efficiency [15]. Liquid exfoliation methods Nanomaterials 2022, 12, 3725 2 of 10 including lithium intercalation [14], electrochemical exfoliation [20,21], and ionic liquidinduced forces receive enormous interest due to the advantages of high efficiency [22]. However, the complex operation procedure, as well as the difficulty for fabricating largesize Bi 2 Se 3 sheets on target substrates, limits their applications in integrated optoelectronic devices [24]. Therefore, it is crucial to develop a facile method for in-situ exfoliation of layered Bi 2 Se 3 sheets on various target substrates.
Novel properties can be introduced into 2D materials by surface doping. The doping of Bi 2 Se 3 has been extensively studied for the regulation of the electric and optical properties [25][26][27]. For example, Sb-Doped Bi 2 Se 3 nanoplates grown in solution were reported to be ambipolar topological insulators with high carrier mobility [28]. Ag was doped into Bi 2 Se 3 by melt-growth method to tune the Fermi level of Bi 2 Se 3 upward [29]. Controllable surface-doping of Bi 2 Se 3 has been demonstrated to be a powerful method for modulating their physical properties and extending their applications.
Herein an ion-exchange-driven exfoliation method is proposed for in-situ chemical thinning and surface doping of layered Bi 2 Se 3 sheets on the target substrates, step by step. The thickness of the layered Bi 2 Se 3 sheets can be regulated from a few nanometers to hundreds of nanometers by controlling the cycling number of chemical thinning. Notably, large-size few-layer Bi 2 Se 3 sheet with a lateral size more than 200 µm can be achieved via the developed exfoliation method. Importantly, self-shedding of the Bi 2 Se 3 sheets from the bulk crystal can be induced by the expansion stress derived from the intercalation and cation exchange of heavy Ru 3+ . By contrast, complex operation process and highpower ultrasonication are needed for the exfoliation of layered Bi 2 Se 3 via previous liquidexfoliation methods. The proposed chemical thinning and surface doping method may find applications in the exfoliation and modulation of other bulk-layered materials.

Chemical Thinning Methods
Bulk-layered Bi 2 Se 3 were micromechanically exfoliated from a synthetic bulk crystal on the SiO 2 /Si substrate. To improve the contact between the Bi 2 Se 3 sheets and the substrate, the obtained layered Bi 2 Se 3 was annealed at 100 • C for 2 h under vacuum. Ruthenium (III) chloride (RuCl 3 ) solution (5 mM) can be obtained by mixing 6.5 mg ruthenium (III) chloride hydrate and 5 mL acetone at about 50 • C for 1 h. Then, the Bi 2 Se 3 sheets on the SiO 2 /Si substrate were placed in the Ruthenium (III) chloride solution for chemical thinning until the thickness reaches the designed value. After that, the sample was removed from the solution and rinsed with acetone and ethanol. The thickness of the Bi 2 Se 3 sheets can be regulated by controlling the cycling number of chemical thinning.

Characterization
Optical images of the Bi 2 Se 3 sheets with various thickness were taken by a Nikon (ECLIPSE LV 150N, Tokyo, Japan) camera that was focused by a 50× objective lens (Nikon Tu Plan Fluor, Tokyo, Japan) and imaged by FLY-CU3E630SP. The thickness and surface morphology of the samples were characterized by AFM system (Bruker Innova, USA) under an ambient atmosphere operating in the tapping mode. The thickness quoted below is averaged from an interior area of the sample. Surface morphologies of the samples were characterized by scanning electron microscopy (SEM) (MIRA3 TESCAN, Brno, The Czech Republic). X-ray photoelectron spectroscopy (XPS) analysis was carried out using X-ray photoelectron spectrometer (PHI 5000Versaprobe-III, Japan). The Raman spectroscopy were conducted with an assembled system using an exciting laser wavelength of 532 nm. The laser was focused by a 100× objective lens (LEICA DM 2700M, Wetzlar, Germany) before irradiating the samples. The reflected light of the sample was collected into the spectrometer with 1800 lines (ANDOR SR-500i, Britain) on the order of 2 s with 200 averaged spectra.

Results and Discussion
3.1. In-Situ Chemical Thinning and Surface Doping of Bulk-Layered Bi 2 Se 3 Figure 1a depicts the chemical thinning process of bulk Bi 2 Se 3 into few-layer sheets. First, bulk Bi 2 Se 3 were transferred onto the target substrates (e.g., SiO 2 /Si) for chemical thinning. Then, the bulk Bi 2 Se 3 was immersed into the RuCl 3 solution (5 mM). After that, the solution was heated to 50 • C and stayed for 3 h. At beginning, the Ru 3+ diffuses onto the surface of Bi 2 Se 3 and the cation exchange between Ru 3+ from the solution and Bi 3+ from the bulk Bi 2 Se 3 can be induced, forming unstable Bi 2 Ru 2 -xSe 3 compound. With further increase of the reaction time, Se 2− can be oxidized into Se due to strong oxidants, resulting the formation of Se nanomeshes on the surface of bulk Bi 2 Se 3 . During the reaction process, surface wrinkling can be induced by the expansion stress derived from cation-substitutioninduced lattice mismatch. The wrinkling layer spontaneously sheds from the bulk Bi 2 Se 3 , leading to a decrease in the sheet thickness.
Nanomaterials 2022, 12, x FOR PEER REVIEW photoelectron spectrometer (PHI 5000Versaprobe-III, Japan). The Raman spectro were conducted with an assembled system using an exciting laser wavelength of 53 The laser was focused by a 100× objective lens (LEICA DM 2700M, Wetzlar, Germ before irradiating the samples. The reflected light of the sample was collected in spectrometer with 1800 lines (ANDOR SR-500i, Britain) on the order of 2 s with 200 aged spectra.

In-Situ Chemical Thinning and Surface Doping of Bulk-Layered Bi2Se3
Figure 1a depicts the chemical thinning process of bulk Bi2Se3 into few-layer s First, bulk Bi2Se3 were transferred onto the target substrates (e.g., SiO2/Si) for che thinning. Then, the bulk Bi2Se3 was immersed into the RuCl3 solution (5 mM). Afte the solution was heated to 50 °C and stayed for 3 h. At beginning, the Ru 3+ diffuse the surface of Bi2Se3 and the cation exchange between Ru 3+ from the solution and Bi 3 the bulk Bi2Se3 can be induced, forming unstable Bi2Ru2-xSe3 compound. With furth crease of the reaction time, Se 2− can be oxidized into Se due to strong oxidants, res the formation of Se nanomeshes on the surface of bulk Bi2Se3. During the reaction pr surface wrinkling can be induced by the expansion stress derived from cation-sub tion-induced lattice mismatch. The wrinkling layer spontaneously sheds from the Bi2Se3, leading to a decrease in the sheet thickness.  The thickness of the Se-doped Bi 2 Se 3 sheets can be regulated by controlling the cycling number of the chemical thinning. Each cycle of the reaction time is fixed at 3 h. As shown in Figure 1b, in-situ chemical thinning of a heart-shaped Bi 2 Se 3 sheet on a SiO 2 /Si substrate Nanomaterials 2022, 12, 3725 4 of 10 was taken as an example to demonstrate the superiority of the proposed exfoliation method. The thickness of the Bi 2 Se 3 sheet gradually decreases from 351 to 6 nm by increasing the reaction time from 0 to 15 h (5 cycles). Interestingly, the Se-doped Bi 2 Se 3 sheets exhibit various colors with the chemical thinning process. The color of pristine Bi 2 Se 3 sheet changes from off-white to brown after 3 h reaction (1st cycle), and then the color turns to purple-red when the reaction time reaches 6 h (2nd cycle). With further increase of the reaction time to 9 (3rd cycle) and 12 h (4th cycle), the Se-doped Bi 2 Se 3 sheets exhibit orange and blue color, respectively. Impressively, after 15 h reaction (5th cycle), the color of the Se-doped Bi 2 Se 3 sheet changes into a purple that is similar to that of the substrate, indicating that the bulk Bi 2 Se 3 sheet is chemically-thinned into a few layers.

Controllable Exfoliation of Large-Size Layered Bi 2 Se 3
AFM characterization was conducted to see the thickness and morphology evolution of the Bi 2 Se 3 sheet with increasing the chemical thinning cycles. As shown in Figure 2, the sheet thickness decreases with increases in the reaction time from 3 to 15 h. For example, a decrease of 60 nm in the thickness can be induced by the first cycle treatment (3 h), and the second cycle treatment (6 h) can cause 70 nm chemical thinning. After the fifth cycle treatment (15 h), the thickness can be reduced to~6 nm. For the heart-shaped Bi 2 Se 3 sheet with a lateral size of~150 µm, the average rate of the chemical thinning is about 23 nm/h. ation method. The thickness of the Bi2Se3 sheet gradually decreases from 351 to 6 nm by increasing the reaction time from 0 to 15 h (5 cycles). Interestingly, the Se-doped Bi2Se3 sheets exhibit various colors with the chemical thinning process. The color of pristine Bi2Se3 sheet changes from off-white to brown after 3 h reaction (1st cycle), and then the color turns to purple-red when the reaction time reaches 6 h (2nd cycle). With further increase of the reaction time to 9 (3rd cycle) and 12 h (4th cycle), the Se-doped Bi2Se3 sheets exhibit orange and blue color, respectively. Impressively, after 15 h reaction (5th cycle), the color of the Se-doped Bi2Se3 sheet changes into a purple that is similar to that of the substrate, indicating that the bulk Bi2Se3 sheet is chemically-thinned into a few layers.

Controllable Exfoliation of Large-Size Layered Bi2Se3
AFM characterization was conducted to see the thickness and morphology evolution of the Bi2Se3 sheet with increasing the chemical thinning cycles. As shown in Figure 2, the sheet thickness decreases with increases in the reaction time from 3 to 15 h. For example, a decrease of 60 nm in the thickness can be induced by the first cycle treatment (3 h), and the second cycle treatment (6 h) can cause 70 nm chemical thinning. After the fifth cycle treatment (15 h), the thickness can be reduced to ~6 nm. For the heart-shaped Bi2Se3 sheet with a lateral size of ~150 μm, the average rate of the chemical thinning is about 23 nm/h. These results demonstrate that the developed chemical thinning method is suitable for in-situ layer-by-layer exfoliation of large-size layered Bi2Se3, and the thickness of the Bi2Se3 can be regulated from a few nanometers to hundreds of nanometers via changes in the reaction time. The color evolution of the sheet with increases in the reaction time can also be attributed to the decreasing sheet thickness resulting from the chemical thinning. Notably, the surface of the pristine Bi2Se3 sheet is smooth and the edge is sharp (Figure  2a), while numerous nanoparticles and nanomeshes can be observed on the surface of the sheet after chemical thinning (Figure 2b-f).  These results demonstrate that the developed chemical thinning method is suitable for in-situ layer-by-layer exfoliation of large-size layered Bi 2 Se 3 , and the thickness of the Bi 2 Se 3 can be regulated from a few nanometers to hundreds of nanometers via changes in the reaction time. The color evolution of the sheet with increases in the reaction time can also be attributed to the decreasing sheet thickness resulting from the chemical thinning. Notably, the surface of the pristine Bi 2 Se 3 sheet is smooth and the edge is sharp (Figure 2a), while numerous nanoparticles and nanomeshes can be observed on the surface of the sheet after chemical thinning (Figure 2b-f).

Raman Analysis of the Exfoliated Samples
Raman characterizations of the exfoliated sheets were carried out to investigate the effect of chemical thinning on the structure of the layered Bi 2 Se 3 . Figure 3 shows the Raman spectroscopy of the pristine Bi 2 Se 3 crystal and the exfoliated sheets after chemical thinning of various time durations. The pristine bulk Bi 2 Se 3 exhibits three characteristic Raman peaks including 72, 131 and 173 cm −1 , which can be assigned to the A 1g 1 mode (out of plane stretch), E g 2 mode (in-plane stretch) and A 1g 2 mode (out of plane stretch) [30,31]. After the first cycle chemical thinning (3 h), the sample also shows typical Raman peaks of Bi 2 Se 3 , but the intensity of characteristic peaks increases remarkably, which can be attributed to the formation of nanoparticles and nanomeshes on the surface. The formation of nanostructures generates numerous nanoscale gaps that are beneficial for surface-enhanced Raman scattering. It is worth noting that a new peak at 253 cm −1 appeared after chemical thinning, which can be assigned to the characteristic peak of amorphous Se [32][33][34][35]. This result suggests the formation of amorphous Se on the surface during the chemical thinning process, which will be further demonstrated by the latter XPS analysis. Impressively, the characterized peaks of Bi 2 Se 3 almost disappeared after the fifth cycle of chemical thinning (15 h), and only the Raman signal of amorphous Se can be observed. This result suggests that ultrathin Se films can be obtained when the reaction time is enough.
of Bi2Se3, but the intensity of characteristic peaks increases remarkably, which can be at-tributed to the formation of nanoparticles and nanomeshes on the surface. The formation of nanostructures generates numerous nanoscale gaps that are beneficial for surface-enhanced Raman scattering. It is worth noting that a new peak at 253 cm −1 appeared after chemical thinning, which can be assigned to the characteristic peak of amorphous Se [32][33][34][35]. This result suggests the formation of amorphous Se on the surface during the chemical thinning process, which will be further demonstrated by the latter XPS analysis. Impressively, the characterized peaks of Bi2Se3 almost disappeared after the fifth cycle of chemical thinning (15 h), and only the Raman signal of amorphous Se can be observed. This result suggests that ultrathin Se films can be obtained when the reaction time is enough.
With increase of the reaction time to 12 h, the intensities of A1g 1 , Eg 2 and A1g 2 modes reduce due to the decrease of the sheet thickness, while, within the former 12 h, the intrinsic Bi2Se3 characteristic peak in the reacted Bi2Se3 was enhanced relative to the intrinsic Bi2Se3, presenting surface-enhanced Raman. It is worth mentioning that the peak of A1g 1 shows a ∼3 cm −1 shift to lower wavenumber compared to that of pristine Bi2Se3 (Figure 3 insert). This shift can be attributed to the lower degree of the vibrations in the exfoliated Bi2Se3, since the A1g modes that correspond to the out-of-plane vibrations of the Se and Bi atoms parallel to the c-axis are very sensitive to the thickness [36][37][38]. Besides, the broadening of the Eg 2 peak can be observed for the exfoliated samples, which may be caused by the enhancement of electron-phonon coupling in the few QL regime [30].

Surface Morphology and Element Content Analysis of the Exfoliated Samples
To figure out the element composition of the exfoliated samples, SEM and EDS mapping were carried out to characterize the surface morphology and element contents. The With increase of the reaction time to 12 h, the intensities of A 1g 1 , E g 2 and A 1g 2 modes reduce due to the decrease of the sheet thickness, while, within the former 12 h, the intrinsic Bi 2 Se 3 characteristic peak in the reacted Bi 2 Se 3 was enhanced relative to the intrinsic Bi 2 Se 3 , presenting surface-enhanced Raman. It is worth mentioning that the peak of A 1g 1 shows a ∼3 cm −1 shift to lower wavenumber compared to that of pristine Bi 2 Se 3 (Figure 3 insert). This shift can be attributed to the lower degree of the vibrations in the exfoliated Bi 2 Se 3 , since the A 1g modes that correspond to the out-of-plane vibrations of the Se and Bi atoms parallel to the c-axis are very sensitive to the thickness [36][37][38]. Besides, the broadening of the E g 2 peak can be observed for the exfoliated samples, which may be caused by the enhancement of electron-phonon coupling in the few QL regime [30].

Surface Morphology and Element Content Analysis of the Exfoliated Samples
To figure out the element composition of the exfoliated samples, SEM and EDS mapping were carried out to characterize the surface morphology and element contents. The optical and SEM images in Figure 4a,b show that the pristine Bi 2 Se 3 sheet without chemical thinning has a smooth surface. The EDS mapping result demonstrates that the atomic ratio of Bi to Se is~2/3, which corresponds to the stoichiometric ratio of Bi 2 Se 3 . After chemical thinning, the formation of nanoparticles and nanomeshes can be induced on the surface of the exfoliated samples, forming rough coatings (Figure 4c-h). Additionally, Ru element can be observed on the surface of the exfoliated samples after chemical thinning. Compared with the exfoliated sample with a thickness of 585 nm, the mapping signals for Bi and Se decreases with reducing the sheet thickness to 34 and 24 nm (Figure 4f-h). optical and SEM images in Figure 4a,b show that the pristine Bi2Se3 sheet without chemical thinning has a smooth surface. The EDS mapping result demonstrates that the atomic ratio of Bi to Se is ~2/3, which corresponds to the stoichiometric ratio of Bi2Se3. After chemical thinning, the formation of nanoparticles and nanomeshes can be induced on the surface of the exfoliated samples, forming rough coatings (Figure 4c-h). Additionally, Ru element can be observed on the surface of the exfoliated samples after chemical thinning. Compared with the exfoliated sample with a thickness of 585 nm, the mapping signals for Bi and Se decreases with reducing the sheet thickness to 34 and 24 nm (Figure 4f-h). As shown in Figure 5, the atomic ratio of Se to Bi is no longer 1.5 for the exfoliated samples. For the exfoliated sample with a thickness of 585 nm, the atomic ratio of Se to Bi is about 7.6. When the sheet thickness decreases to 34 nm, the ratio further increases to 26.9, which is much higher than that of the pristine Bi2Se3. Impressively, the atomic content of Bi declined, even to 0%, when the sheet thickness decreased to 24 nm. By contrast, the atomic content of Se is always higher than 77%, regardless of the decrease of sheet thickness. These results demonstrate that cation exchange between Bi 3+ and Ru 3+ can be induced during the chemical thinning process. The chemical states of various elements will be further verified by XPS characterization in the next section. As shown in Figure 5, the atomic ratio of Se to Bi is no longer 1.5 for the exfoliated samples. For the exfoliated sample with a thickness of 585 nm, the atomic ratio of Se to Bi is about 7.6. When the sheet thickness decreases to 34 nm, the ratio further increases to 26.9, which is much higher than that of the pristine Bi 2 Se 3 . Impressively, the atomic content of Bi declined, even to 0%, when the sheet thickness decreased to 24 nm. By contrast, the atomic content of Se is always higher than 77%, regardless of the decrease of sheet thickness. These results demonstrate that cation exchange between Bi 3+ and Ru 3+ can be induced during the chemical thinning process. The chemical states of various elements will be further verified by XPS characterization in the next section.

XPS Characterization of the Exfoliated Samples
The chemical compositions of the exfoliated samples during the chemical thinning were traced by XPS to investigate the surface chemical states. To prepare the exfoliated samples, the pristine bulk Bi2Se3 sheets were immersed into 5 mM RuCl3 solution with durations of 30 and 50 min, respectively. Figure 6 shows the characterized XPS peaks of the pristine Bi2Se3 and the exfoliated samples. All the peaks are calibrated by the reference carbon peak at 284.8 eV. For the pristine Bi2Se3 without chemical thinning (0 min), two characterized peaks at 157.8 and 163.1 eV that represent Bi4f7/2 and Bi4f5/2 are observed ( Figure 6b). Besides, two peaks at 53.3 and 54.1 eV appear, which can be assigned to Se3d5/2 and 3d3/2, demonstrating the chemical states of layered Bi2Se3 [33,34].

XPS Characterization of the Exfoliated Samples
The chemical compositions of the exfoliated samples during the chemical thinning were traced by XPS to investigate the surface chemical states. To prepare the exfoliated samples, the pristine bulk Bi 2 Se 3 sheets were immersed into 5 mM RuCl 3 solution with durations of 30 and 50 min, respectively. Figure 6 shows the characterized XPS peaks of the pristine Bi 2 Se 3 and the exfoliated samples. All the peaks are calibrated by the reference carbon peak at 284.8 eV. For the pristine Bi 2 Se 3 without chemical thinning (0 min), two characterized peaks at 157.8 and 163.1 eV that represent Bi4f7/2 and Bi4f5/2 are observed ( Figure 6b). Besides, two peaks at 53.3 and 54.1 eV appear, which can be assigned to Se3d5/2 and 3d3/2, demonstrating the chemical states of layered Bi 2 Se 3 [33,34].
After immersing the bulk Bi 2 Se 3 into the RuCl 3 solution, the intensity of Se3d5/2 and 3d3/2 peaks decreased rapidly with the reaction time while the peaks of Se 0 clearly increased (Figure 6a). The increase of Se 0 with the soaking time could be further demonstrated from the increase of Se3p3/2 (161.2 eV) and Se3p1/2 (166.8 eV) in Figure 6b [39]. These results indicate that Se 2tends to be oxidized to zero-valent Se 0 during the chemical thinning process. As shown in Figure 6b, the peaks of Bi4f7/2 and 4f5/2 decrease with extensions of the soaking time, indicating the dissociation of Bi 3+ from Bi 2 Se 3 , which is in agreement with the result of the EDS mapping. The dissociation of Bi 3+ is resulted by the cation exchange between Bi 3+ and Ru 3+ [40,41]. Similarly, the substitution of Bi 3+ of Bi 2 Se 3 with Cu + cation has been demonstrated in previous study [38]. When the reaction time reaches more than 30 min, two peaks at 280.5 eV and 284.6 eV that represent the Ru3d5/2 and Ru3d3/2 can be observed, suggesting the formation of zero-valent Ru 0 (Figure 6c). This result can be further confirmed by the characterized peaks of Ru3p3/2 (462.0 eV) and Ru3p1/2 (484.05 eV), as shown in Figure 6d   After immersing the bulk Bi2Se3 into the RuCl3 solution, the intensity of Se3d5 3d3/2 peaks decreased rapidly with the reaction time while the peaks of Se 0 clea creased (Figure 6a). The increase of Se 0 with the soaking time could be further d strated from the increase of Se3p3/2 (161.2 eV) and Se3p1/2 (166.8 eV) in Figure 6 These results indicate that Se 2-tends to be oxidized to zero-valent Se 0 during the ch thinning process. As shown in Figure 6b, the peaks of Bi4f7/2 and 4f5/2 decreas extensions of the soaking time, indicating the dissociation of Bi 3+ from Bi2Se3, whic agreement with the result of the EDS mapping. The dissociation of Bi 3+ is resulted cation exchange between Bi 3+ and Ru 3+ [40,41]. Similarly, the substitution of Bi 3+ of with Cu + cation has been demonstrated in previous study [38]. When the reactio reaches more than 30 min, two peaks at 280.5 eV and 284.6 eV that represent the R and Ru3d3/2 can be observed, suggesting the formation of zero-valent Ru 0 (Figu This result can be further confirmed by the characterized peaks of Ru3p3/2 (462.0 e Ru3p1/2 (484.05 eV), as shown in Figure 6d [42].

Conclusions
In summary, a facile and efficient chemical thinning method is proposed for by-layer in-situ exfoliation and surface doping of large-size bulk Bi2Se3. Layered Se-Bi2Se3 sheets with tunable thickness ranging from a few nanometers to hundreds nometers can be achieved by controlling the reaction time. As opposed to previous exfoliation methods that require complex reaction processes, thickness-controllabl liation of large-size layered Bi2Se3 can be realized via the developed method. In ad the formation of Se nanomeshes on the Bi2Se3 sheets remarkably enhance the inten

Conclusions
In summary, a facile and efficient chemical thinning method is proposed for layerby-layer in-situ exfoliation and surface doping of large-size bulk Bi 2 Se 3 . Layered Sedoped Bi 2 Se 3 sheets with tunable thickness ranging from a few nanometers to hundreds of nanometers can be achieved by controlling the reaction time. As opposed to previous liquid-exfoliation methods that require complex reaction processes, thickness-controllable exfoliation of large-size layered Bi 2 Se 3 can be realized via the developed method. In addition, the formation of Se nanomeshes on the Bi 2 Se 3 sheets remarkably enhance the intensity of Raman peaks, demonstrating that the proposed chemical thinning method may find applications in surface-enhanced Raman scattering. The developed method is expected to be extended in a controllable manner to other bulk-layered materials for highly efficient preparations of 2D heterostructures for diverse applications.