Exfoliated layered nanosheets of orthorhombic SnS, subjected to 90 MeV carbon ion irradiation

1. Introduction

Interdisciplinary research involving two-dimensional (2D) materials has grown in prominence in the recent decades following the groundbreaking discovery of graphene in 2004.¹ This discovery opened new avenues for exploring the unique properties of 2D materials and sparked a surge of interest in investigating other layered systems such as transition metal dichalcogenides (TMDCs), MXenes, phosphorene, etc. Among the vast and ever-expanding array of 2D materials, recently, group IV-VI metal monochalcogenides (MMCs) have joined as a stellar candidate due to their earth abundancy, affordability and eco-friendly features.² This group has garnered significant attention from the research community owing to their distinct and exciting properties, as well as their broad application perspectives.³ The MMCs are a class of layered, p-type semiconductors which generally possess an orthorhombic crystal structure, a 1:1 stoichiometry and the chemical formula MX, where M represents group-IV metals such as Ge, Sn and Si and X stands for chalcogens such as S, Se, Te.³ Among these, tin sulfide (SnS) is a notable member of the MMC family and was first reported by R. Herzenberg, a German mineralogist.⁴ The SnS system adopts a characteristic puckered layered structure akin to black phosphorus (BP), which gives rise to pronounced in-plane anisotropy.⁵ In its bulk state, the material exhibits an indirect band gap that ranges from 1.08eV to 1.27eV, whereas the monolayer form would emerge with a characteristic band gap of $\sim$1.9eV.^6,7 Thus, a layer-dependent band gap tunability similar to other 2D materials is possible, rendering it a potential candidate for various electronic and optoelectronic applications.⁸ It has been observed that the SnS has an orthorhombic crystal structure (Pnma) under ambient conditions but it makes transition to a more symmetric configuration (Cmcm) at higher temperatures.⁹ Such intriguing physical properties complemented with a large absorption coefficient ($>$10⁴${\text{cm}}^{-1}$) and high carrier mobility make SnS a versatile candidate to be deployed in photovoltaic devices,¹⁰ field-effect transistors (FETs),¹¹ gas sensors,¹² energy storage,¹³ thermoelectric devices,¹⁴ etc.

Advancements in nanomaterial modifications using high-energy particles, particularly swift heavy ion (SHI) irradiation, have been explored for their significance in fundamental research as well as practical applications.¹⁵ The use of high-energy irradiation offers a unique approach to tailor material properties at the nanoscale level.¹⁶ When the energetic ions pass through solids, they transfer a significant amount of energy to the target atoms via two distinct mechanisms: elastic collisions, which result in nuclear energy loss ($S_n$), and inelastic collisions, leading to electronic energy loss ($S_e$). The extent of change brought in by SHI irradiation is profoundly influenced by the parameters such as ion energy, ion fluence as well as target material composition.¹⁷ Some of the prominent effects include the formation of latent tracks, or ion tracks and the creation of atomic displacements in the target material, point defects and defect clusters.¹⁸ The structural modification is expected to alter the material properties, such as its mechanical strength, electrical conductivity, thermal conductivity as well as optical responses. The advantage of SHI irradiation has already been realized for technological benefits, such as nano-filtration membranes for water purification,¹⁹ nanoelectronic devices,²⁰ and nanopores for detecting single DNA molecules.²¹

Despite the growing interest in ion irradiation-induced modification observed in diverse materials including oxides and chalcogenides,^22,23,24,25 its effect on layered and nano-dimensional SnS has not been highlighted yet. Herein, we discuss 90MeV carbon ion irradiation-led changes in structural, morphological, vibrational and electrical properties of nanoscale, exfoliated SnS systems. The insights gained from our study will help pave the way for further developments in the field of ion-beam engineering of layered, 2D materials like MMCs in pure and functional forms.

2. Experimental: Materials and Methods

2.1. Processing of SnS nanosheets

All chemicals and reagents were procured from standard companies through a local supplier. Granular tin (II) sulfide (SnS, $>$99.5%) was purchased from Alfa ${\text{Aesar}}^{\circledR }$ and used without further purification steps. 2-propanol (IPA, (CH₃)₂CHOH, $>$99%) was received from ${\text{Merck}}^{\circledR }$. Initially, tin (II) sulfide granules were mechanically ground using an agate mortar for 15min. In the following, the ground residue was then mixed in IPA (1mg/ml). Afterward, the whole solution was subjected to ultrasonication using a bath sonicator (${\text{RoHS}}^{\circledR }$) for 8h under the power of 100W and a frequency of 40kHz. Throughout the ultrasonication process, the temperature of the bath sonicator was kept under 30^∘C. The final product was collected from the sonicated solution by centrifugation at approximately, 6000rpm for 15min, and then washed with deionized (DI) water and ethanol several times. Finally, the obtained material was dried in a hot air oven at a temperature of 50^∘C, for 12h. The schematic diagram depicting the liquid phase exfoliation process is presented in Fig. 1(a) for reference.

2.2. Irradiation experiment

A schematic representation of the ion irradiation process is shown in Fig. 1(b). To facilitate irradiation, the initially prepared, exfoliated SnS dried powder was carefully dispersed onto a thin circular teflon pellet, possessing a typical diameter of $\sim$13mm. To ensure uniformity, the powder was firmly compressed onto the pellet’s surface with the help of a hydraulic press exerting a pressure of 40kg/cm². Subsequently, the resulting samples were rendered suitable for irradiation. The irradiation was performed within the Material Science chamber using a 90MeV ${\text{C}}^{6+}$ ion beam as the irradiation source. This ion beam with current 1pnA (particle nano-Ampere) was made available at the 15 UD pelletron accelerator situated at the Inter University Accelerator Centre (IUAC) in New Delhi. The irradiation process was conducted at four distinct fluences, namely $1\times 10^{10}$, $1\times 10^{11}$, $1\times 10^{12}$ and $1\times 10^{13}$ions/cm². The typical thickness of the SnS specimen on top of the teflon dough was $\sim$100$\mu$m, good enough for the carbon ion beam passage having a projectile range of $\sim$104$\mu$m.

2.3. Characterization techniques

The irradiation effects on SnS nanosystems were investigated through a comprehensive analysis of their structural, morphological and electrical properties. X-ray diffraction (XRD) was carried out using a BRUKER AXS, D8 FOCUS, X-ray diffractometer from Germany, employing ${\text{CuK}}_{\alpha }$ radiation with a wavelength ($\lambda$) of 1.543Å. The XRD measurements covered a 2$\theta$ range from 20^∘ to 80^∘, allowing the observation of any changes in the crystalline structure of the nanosystems. To analyze the vibrational modes of the samples, a Raman spectrometer from Renishaw, UK was used. This spectrometer utilized a 2.5mW ${\text{Ar}}^{+}$ ion laser with an excitation wavelength of 514nm. The surface roughness of the SnS samples before and after irradiation was studied using an atomic force microscope (AFM) manufactured by NTEGRA Vita, NDMDT. The AFM technique could help assess changes in the topographic features and surface properties caused by radiation exposure. Furthermore, morphological changes of the samples were analyzed utilizing a field emission scanning electron microscope (FESEM, JSM-7200F). Lastly, the electrical conductivity measurements were carried out using a two-probe measuring setup on a Keithley ${2400}^{\circledR }$ source meter. This setup could help provide useful insights into the electrical behavior of the post-irradiated SnS nanosystems on a comparative basis.

2.4. SRIM/TRIM calculations

The Stopping and Range of Ions in Matter (SRIM) software was employed to calculate the electronic ($S_e$) energy loss and the nuclear energy loss ($S_n$) for a carbon ion traversing through the SnS material.²⁶ Primarily, the energy loss of the ions was chalked up to the inelastic collisions ($S_e$) with the target material rather than the elastic collisions ($S_n$). The results from SRIM indicated that the electronic energy loss ($S_e$) was $\sim$56eV/Å, while the nuclear energy loss ($S_n$) was much less amounting to $3\times 10^{-2}$eV/Å, as presented in Table S1 in Supplementary section. Additionally, a Transport of Ions in Matter (TRIM) calculation was performed, simulating the behavior of 30,000 carbon ions penetrating the SnS material to a depth of 200$\mu$m in order to investigate the ion distribution. The depth distribution profiling of ions provides valuable insights into the extent to which the projectile ion loses its energy within the lattice while passing through the target material.

In our case, 90MeV ${\text{C}}^{6+}$ ions were used to incident at a normal angle onto SnS material. The graph displaying the variation of $S_e$ and $S_n$ with ion energy is shown in Fig. 2(a). The electronic energy loss ${[dE∕dx]}_e$ is found to be largely predominant over the nuclear energy loss process ${[dE∕dx]}_n$. Ion trajectories i.e., the depth profiling as regards the energy loss of the projectile ions within the SnS lattice can be found in Fig. 2(b), while a transverse view can be seen in Fig. S1(a). Moreover, a 3D plot illustrating the ion distribution is shown in the Supplementary section (Fig. S1(b)). This plot clearly indicates that the projectile range within the SnS material is approximately, 104$\mu$m, with the lateral straggling of $\sim$1.4$\mu$m. The plot showing the target displacements and vacancies generated by each ion penetrating the material is shown in Fig. S1(c). Comprising of exfoliated SnS, the samples for this study had a thickness of $\sim$100$\mu$m. Accordingly, 90MeV carbon ion beam was specifically opted for to ensure complete irradiation of the entire sample and full-depth thickness.

3. Results and Discussion

3.1. Surface morphological features

The surface morphology of bulk, pristine and irradiated SnS was carefully examined using FESEM and can be found in Figs. 3(a)–3(f). The pristine SnS, in its native form, exhibits a layered morphology spread over a wider, irregular area with stacks of multiple sheets interspersed with smaller fragments. Noticeably, the pristine sheet displays a lateral size of approximately, 9–10$\mu$m. On the other hand, the lateral size of the irradiated sheets is observed to be shortened up to the range, of 2–4$\mu$m and the morphological features of these sheets under different fluences are depicted in Figs. 3(b)–3(f). The irradiation effect has caused fragmentation and de-stacking of the wider SnS layers into smaller units. An increase in the ion fluence seems to have resulted in progressive fragmentation of the nanosheets. This morphological change is primarily due to the impact of the ions with the target specimen. In fact, as the carbon ions were bombarded on the SnS layers, they transferred a significant amount of energy to the lattice atoms, resulting in atomic displacements and leading to the fragmentation of larger SnS layers into smaller elements.

The AFM study was employed to compare the surface topography of the SnS samples under investigation followed by analysis using the WSxM software^©.²⁷ 2D and three-dimensional (3D) views, covering a scan area of 2.5$\mu \mathrm{\text{m}}\times 2.5$$\mu$m, have clearly revealed the presence of nanosheet-like structures in both the pristine and irradiated nanoscale SnS systems. The AFM image of the pristine sample, as illustrated in Fig. 4(a), appeared devoid of any distinct surface features. However, upon irradiation with ${\text{C}}^{6+}$ ions, noticeable alterations to the SnS surface texture and augmented height profiling were observed, as can be noticed in Figs. 4(b)–4(e). The RMS roughness values of pristine and irradiated SnS are given in Table S2 (see Supplementary section). The RMS roughness as a function of ion fluence, as displayed in Fig. 4(f), exhibits a logarithmic growing trend with increasing ion fluence.

Noticeably, this ion irradiation-induced change was accompanied by a substantial increase in the uneven microscopic surface construct in the SnS system. The enhanced roughness can be attributed to the introduction of damage and higher defect densities generated by the high-energy carbon ions over the exfoliated nanosheets.²⁸

3.2. Crystal structure and lattice parameters

XRD measurements were carried out to explore the crystal structure of both pristine and irradiated SnS systems. The XRD patterns in Fig. 5(a) presented prominent peaks located at the Bragg’s angle, $2\theta \sim 26.1^{\circ }$, 32.1^∘, 44.9^∘, 45.7^∘ and 66.9^∘, which are indexed as the (120), (040), (141), (002) and (080) planes of the orthorhombic crystal structure of SnS, respectively (reference JCPDS Card No. 39-0354).²⁹ Describing the preferred orientation of crystallites, the magnified view of the sharp (040) peak is shown in Fig. 5(b). Notably, an observable peak shift towards lower 2$\theta$ values is evident with increasing fluence, thereby indicating possible lattice expansion. The average crystallite size (D) was determined through the popular Scherrer’s equation relevant for single line fitting³⁰

where $K,\lambda ,\beta ,\theta$ denote the Scherrer’s constant, X-ray wavelength, full width at half maxima (FWHM) and Bragg’s angle; respectively. The variation of average crystallite size as a function of ion fluence is shown in Fig. 5(c). A sharp increase in D was noted at the initial fluence of $1\times 10^{10}$ions/cm², indicating that high energy imparted to the crystallites resulted in spontaneous coalescence, also known as particle growth.³¹ However, at higher fluences, a subsequent reduction in the average crystallite size can also be seen, possibly due to fragmentation resulting from inelastic collisions between the incident ions and the target material.^32,33 It was invoked that the observed anomalous trend on the size of the average crystallites with increasing ion fluence can arise as a consequence of competitive coalescence and fragmentation effects upon radiation exposure.

The lattice parameters ($a\ne b\ne c$) of the orthorhombic phase of the samples were determined by using Eq. (2)³⁴

where $d_{hkl}$ represents the interplanar spacing, ($hkl$) are the Miller indices and a, b, c are the lattice parameters. Additionally, for microstrain ($\eta$), the Eq. (3) was used,³⁵

The structural parameters for both pristine and irradiated samples are summarized in Table 1. It also showcases the variation of microstrain with ion fluence, exhibiting a decrease in $\eta$ when compared to the pristine system. These lattice parameters were then used to calculate the orthorhombic unit cell volume, $v_c$ which displayed an increase in volume alongside ion fluence, further corroborating the lattice expansion aspect (Fig. 5(c)).

3.3. Manifested Raman active optical modes

Raman spectroscopy was employed to conduct vibrational analysis of the pristine and irradiated SnS samples. Note that, the MMC possesses an orthorhombic crystal structure with $D_{2h}^{16}$ symmetry (Pnma).³⁶ In a structure like that, there exist a total of 24 phonon modes at the center of the Brillouin zone, which can be represented as $\mathrm{\Gamma }=4A_g+2B_{1g}+4B_{2g}+2B_{3g}+2A_u+4B_{1u}+2B_{2u}+4B_{3u}$.³⁷ Among these, 12 are Raman active modes (4$A_g$, 2$B_{1g}$, 4$B_{2g}$ and 2$B_{3g}$). Recorded within the range of 100–600${\text{cm}}^{-1}$, the Raman spectra of nanoscale pristine and irradiated SnS are depicted in Fig. 6(a). Observably, there exist significant spectral changes following the irradiation effect. Examining the pristine SnS spectrum, four distinct peaks emerged at $\sim$154, 183.5, 213.2 and 302.7${\text{cm}}^{-1}$. These peaks were associated with distinct Raman modes, with the phonon peaks at $\sim$183.5${\text{cm}}^{-1}$ and 213.2${\text{cm}}^{-1}$ that corresponded to $A_g$(2) and $A_g$(3) modes; respectively.³⁸ Whereas, the peak at $\sim$154${\text{cm}}^{-1}$ can be attributed to the $B_{3g}$ mode, a minor peak at 302.7${\text{cm}}^{-1}$ was also detected in the pristine case, which might be due to the presence of a Sn₂S₃ phase as an impurity (Fig. 6(b)).^39,40 The peak positions of the $B_{3g}$, $A_g$(2) and $A_g$(3) Raman modes in the irradiated samples can be found in Fig. 6(c).

At the highest fluence, the $A_g$(3) peak disappeared, indicating a modification in the crystal structure resulting from an excessive high energy transfer due to the ion irradiation effect. This suggests that high-energy ions would introduce sufficient energy to cause structural rearrangements, or damage within the SnS lattice, leading to the loss of the characteristic Raman signature associated with the $A_g$(3) mode. To be mentioned, as the irradiation fluence increases the peak positions would shift towards higher wavenumbers, indicating a blue-shift.

Upon exposure to the ion fluence of $1\times 10^{10}$ions/cm², there is an augment in the intensity of the Sn₂S₃ impurity peak. The increase might be due to a potential transformation or redistribution of the Sn₂S₃ phase caused by ion irradiation, resulting in a stronger Raman signal. However, with further escalation in fluence, the peak gradually diminishes and eventually disappears at the highest fluence. The disappearance of the Raman peaks at higher fluences signifies the occurrence of ion irradiation-induced amorphization.⁴¹ The interaction between high-energy ions and the SnS material disrupts the crystalline order, potentially resulting in the formation of defects within the crystal lattice, which leads to the creation of highly disordered zones. Consequently, the material gradually loses its crystallinity, giving way to the development of adequate amorphized regions, locally.

3.4. Electrical characteristics of irradiated SnS system

The current–voltage (I–V) characteristics acquired for the pristine and 90MeV carbon ion irradiated samples are shown in Fig. 7(a). It is evident from the figure that the I–V plots seem to follow a linear trend, indicating non-rectifying Ohmic nature of the specimens, with a maximal slope observed for the mid-fluence case ($1\times 10^{11}$ions/cm²).

When the ion fluence increases, a noticeable rise in the slope ($dI∕dV$) of the I–V curves can be observed. This nearly three-fold increase in the slope from a value of $0.88\times 10^{-6}$S to $2.45\times 10^{-6}$S can be compared in Table 2. Then at the highest fluence ($1\times 10^{13}$ions/cm²), however, the slope is reduced to $1.38\times 10^{-6}$S. In other words, at higher fluences ($>$${10}^{11}$ions/cm²) irradiated nanoscale SnS offers more resistance to the passage of current due to the local structural disorder and defect-mediated carrier scattering events, affecting the carrier mobility.

The electrical conductivity of each sample was determined by measuring the slope of the linear regions of the I–V curves, or conductance ($G=\mathrm{\Delta }I∕\mathrm{\Delta }V$). We have,^42,43

where $\sigma$ represents conductivity in units of S${\text{cm}}^{-1}$, l denotes the inter-probe distance ($\sim$0.5cm), w is the width of the sample and h is the thickness of the sample.

We know that conductivity in terms of mobility can be written as⁴⁴

Here, n is the carrier concentration,⁴⁵ and $\mu$ is the carrier mobility. The values for $\mathrm{\Delta }\mu$ were calculated (see Supplementary section) and provided in Table 2. The plot of $\mathrm{\Delta }\mu$ versus $\text{log}(1+f)$ is shown in Fig. S2. On the other hand, Fig. 7(b) displays the change in conductivity of all the SnS samples before and after irradiation. It was observed that for $1\times 10^{11}$ fluence, the conductivity would shoot up substantially. This rise in conductivity at lower fluences can be attributed to the introduction of a small number of defects, such as vacancies or interstitials, into the SnS material through ion irradiation. These defects lead to an increase in the carrier concentration, thereby enhancing the conductivity of the material.⁴⁶ However, as the ion fluence increases, carrier mobility degrades owing to multiple scattering events, resulting in reduced conductivity. Thus, the anomalous feature of conductivity is a consequence of irradiation-led microstructural disorder affecting carrier scattering events and consequently, mobility.

4. Conclusion

In conclusion, this study demonstrated the structural, vibrational and carrier transport properties of exfoliated SnS upon energetic carbon ion irradiation. The combined use of XRD and Raman analysis revealed structural modifications within the SnS nanosystem upon ion irradiation, particularly higher fluences caused radiation-induced structural disorder or amorphization. The difference in the average crystallite size of the irradiated SnS would tend to vary up to $\sim$8.4nm compared to the pristine case. The AFM and FESEM analyses signify morphological changes in the irradiated SnS samples. Notably, the AFM images showed an increase in the RMS roughness. Concurrently, the study of electrical properties exhibited an anomalous trend in conductivity for the pristine and irradiated samples. These findings suggest opportunities for controlled modifications of SnS using ion irradiation. Such adjustments may lead to tailored material properties and new applications for SnS and related monochalcogenides, potentially contributing to a wide array of multifaceted applications.

Acknowledgments

The authors gratefully acknowledge the financial support provided by IUAC, New Delhi (UFR-73330/2023). We extend sincere appreciation to the UFR-66303/2019 project for facilitating the irradiation experiment. Special thanks are due to SAIC, Tezpur University, for extending their XRD and Raman facilities. We also gratefully acknowledge the DST-FIST program for providing the FESEM facility. Additionally, the authors express gratitude to the SAIC IASST, Guwahati, for extending the AFM facility. We acknowledge Dr. Ankush Medhi and Ms. Bhupali Deka for their assistance during the irradiation experiment, and we appreciate the support and valuable suggestions received from peers and colleagues during the entire process.

ORCID

Stuti Tamuli  https://orcid.org/0009-0004-5551-4305

D. Mohanta  https://orcid.org/0000-0002-1750-7620