Defect-mediated negative differential resistance and photoconductive enhancement in graphene oxide decorated SnSe2 nanosheets
a Department of Physics, Rajiv Gandhi University Doimukh Arunachal Pradesh India pradip.kalita@rgu.ac.in
b Department of Electronics and Communication Technology, Gauhati University Guwahati Assam India
Abstract
Pristine tin diselenide (SnSe2) and graphene oxide-decorated SnSe2 (SnSe2–GO) nanocomposites were synthesized via a facile chemical bath deposition (CBD) method and investigated the photoconductive and nonlinear electrical transport properties. Structural and spectroscopic analyses confirmed the formation of highly crystalline hexagonal SnSe2 and the successful incorporation of GO within the composite structure. Temperature-dependent electrical measurements yielded activation energies of 0.12 and 0.09 eV for pristine SnSe2 and SnSe2–GO, respectively, indicating enhanced carrier transport in the composite. Photoconductivity measurements revealed an approximately 180-fold enhancement in photocurrent under 35 000 lux illumination following GO incorporation. This remarkable improvement, accompanied by an increase in the photoresponse exponent from 0.18 to 0.80, is attributed to more efficient interfacial charge separation, reduced trap-assisted recombination, and enhanced carrier transport. In contrast, pristine SnSe2 exhibited pronounced asymmetric negative differential resistance (NDR) behaviour that was strongly dependent on precursor molarity, Sn stoichiometry, and voltage sweep direction. The systematic shift of the NDR peak with increasing selenium content suggests a defect-mediated transport mechanism governed by selenium-vacancy-related deep-level states. Furthermore, the observed hysteresis and non-zero-bias crossing behaviour indicate a history-dependent electrical response associated with charge accumulation and trapping within the nanosheet network. Interestingly, GO incorporation completely suppressed the NDR response while simultaneously enhancing photoconductivity, which is attributed to partial passivation of electrically active defect states by residual oxygen-containing functional groups in the GO-derived carbonaceous phase and improved interfacial charge transfer. These findings demonstrate that defect engineering and interfacial coupling provide effective strategies for tailoring the photoconductive and nonlinear electrical transport properties of SnSe2-based nanocomposites for optoelectronic and nanoelectronic applications.
Toc
Pristine tin diselenide (SnSe2) and graphene oxide-decorated SnSe2 (SnSe2–GO) nanocomposites were synthesized via a facile chemical bath deposition (CBD) method and investigated the photoconductive and nonlinear electrical transport properties.
Introduction
Layered transition metal chalcogenides (TMCs) have emerged as an important class of two-dimensional (2D) materials due to their remarkable structural, optical, and electronic properties, which make them highly promising for next-generation optoelectronic and nanoelectronic applications. Among these materials, SnSe2, a group IV–VI layered semiconductor, has attracted considerable attention owing to its tunable bandgap (1.0–1.5 eV), high optical absorption coefficient, low resistivity, moderate carrier mobility, and excellent environmental stability.1–4 In addition, its layered crystal structure, strong light–matter interaction, and defect-tolerant electronic characteristics make SnSe2 a versatile platform for investigating charge transport, photoconductive phenomena, and nonlinear electric behaviour. SnSe2 crystallizes in a CdI2-type hexagonal layered structure, where Se–Sn–Se atomic layers are weakly bonded through van der Waals interactions along the c-axis.5 This unique layered configuration facilitates anisotropic carrier transport, efficient exfoliation, and the formation of ultrathin nanosheets with large surface-to-volume ratios, making SnSe2 highly attractive for photodetectors, sensors, energy conversion, and memory devices.
Extensive research has been devoted to the synthesis and optimization of SnSe2 nanostructures through methods such as chemical vapor deposition, Bridgman growth, direct vapor transport, and solution-based approaches.1,6–8 Mukhokosi et al. reported nanostructured SnSe2 thin films exhibiting room-temperature Hall mobilities in the range of 2–8 cm2 V−1 s−1,6 while Su et al. demonstrated SnSe2 field-effect transistors with a mobility of 8.6 cm2 V−1 s−1.1 Similarly, Julien et al.7 and Agarwal et al.8 synthesized high-quality SnSe2 single crystals with carrier concentrations in the order of 1016–1017 cm−3, highlighting the excellent transport characteristics of this layered semiconductor. More recently, the integration of SnSe2 with low-dimensional carbon materials has opened new possibilities for enhancing interfacial charge transport and improving device performance. In particular, GO has emerged as an attractive material owing to its high surface area, abundant oxygen-containing functional groups, tunable conductivity, and strong interfacial coupling capability. The presence of oxygenated functional groups in GO not only provides numerous active sites for interaction with semiconductor nanostructures but also promotes efficient charge separation and carrier migration across the heterointerface. Kishan et al. demonstrated a SnSe2–rGO/MoS2 heterostructure exhibiting enhanced self-powered broadband photodetection due to efficient charge separation at the heterointerface.9 These studies demonstrate that graphene-derived materials are highly effective in improving the optoelectronic performance of SnSe2-based heterostructures through enhanced interfacial charge transfer and suppressed carrier recombination. Similar improvements in carrier transport have also been reported for SnSe2-based van der Waals heterostructures, highlighting the importance of interface engineering in optimizing device performance.10 Beyond optoelectronic applications, SnSe2 has also shown promising potential in sodium-ion batteries, thermoelectric systems, and strain-engineered electronic devices.11–13 These studies collectively highlight the importance of interface engineering as an effective strategy for tailoring the functional properties of SnSe2-based nanostructures.
In addition to conventional electronic transport properties, nonlinear electrical phenomena such as NDR have attracted significant interest because of their potential applications in high-speed switches, multivalued logic circuits, oscillators, and resistive memory devices.14,15 NDR behaviour has been observed across a wide range of material such as conductive polymers,16 conjugated molecules,17 and organic semiconductors.18 Various mechanisms have been proposed to explain the origin of NDR in different systems. Recently, NDR has also been demonstrated in SnSe2-based van der Waals heterostructures, where the phenomenon originates from band-to-band tunnelling across atomically sharp interfaces. For example, Fan et al. reported room-temperature NDR in WSe2/SnSe2 heterostructures by tailoring the heterointerface to enhance tunnelling transport, highlighting the critical role of interface engineering in determining the NDR characteristics.19 In another work, Na et al. demonstrated a gate-tunable tunnelling transistor based on a black phosphorus/SnSe2 heterostructure that exhibits Esaki-diode-like NDR at room temperature, highlighting the important role of interface-controlled tunnelling transport.10 One such mechanism is intervalley scattering, as reported by Tsubaki et al., where they observed NDR in a two-dimensional AlGaAs/GaAs heterostructure due to inter sub band scattering of electrons.20 Esaki et al. reported NDR due to quantum tunnelling across a heavily doped, ultra-narrow p–n junction in silicon or germanium,21 while, Lyo et al. reported tunnelling based NDR in structures at the atomic scale (∼1 nm).22 Salomon et al. reported molecular–electrode interface chemistry as the cause of NDR.23 Electron injection and extraction processes have also been recognized as a source of NDR. Xie et al. observed this effect in perylene tetracarboxylic dianhydride (PTCDA) thin films, attributing the NDR to interface-related mechanisms at the metal/PTCDA contact.24 The behaviour was found to be dependent on voltage scan rate, suggesting the influence of both electronic and ionic components. Although SnSe2-based heterostructures have been extensively explored for high-performance electronic and optoelectronic devices, including tunnel diodes exhibiting NDR through interband tunnelling,25 the influence of GO incorporation on defect-mediated nonlinear electrical transport has remained largely unexplored. In particular, the role of selenium vacancies, defect-mediated transport, and interfacial interactions in governing the emergence or suppression of NDR behaviour has not yet been fully understood.
In the present work, pristine SnSe2 and SnSe2–GO nanocomposites were successfully synthesized via a simple and cost-effective CBD method. The structural, morphological, and vibrational properties of the synthesized nanosheets were systematically investigated using XRD, FESEM, HRTEM, FTIR, and Raman spectroscopy. Furthermore, the photoconductive and nonlinear electrical transport properties of the fabricated planar devices were studied under different illumination conditions. The incorporation of GO significantly enhanced the photocurrent response due to improved charge separation and enhanced carrier transport pathways. Interestingly, pristine SnSe2 exhibited pronounced asymmetric NDR behaviour strongly dependent on precursor molarity and Sn stoichiometry, whereas the SnSe2–GO nanocomposites showed suppression of NDR behaviour, likely due to trap-state passivation by oxygen-containing functional groups in GO. By correlating structural characteristics with electrical transport behaviour, this study provides new insights into the role of defect states and interfacial interactions in governing charge transport in SnSe2-based nanocomposites. The observed hysteresis characteristics further suggest the possibility of memristive-like behaviour in SnSe2 nanosheets, highlighting their potential for future nanoelectronic and resistive memory device applications.
Experimental
Synthetic procedure
Chemicals used
Stannic chloride pentahydrate (SnCl4·5H2O), Selenium powder (Se), ammonia solution (NH4OH), polyvinylpyrrolidone (C6H9NO3), hydrazine hydrate (N2H4) and graphene oxide (GO) were procured from Sigma-Aldrich and used as received without any further purification.
Synthesis of SnSe2 nanosheets
SnSe2 was synthesized via a CBD method using polyvinylpyrrolidone (PVP) as a capping and stabilizing agent. A stock PVP solution was prepared by dissolving 3 g of PVP in 300 mL of deionized water (10 mg mL−1) under continuous magnetic stirring until a clear homogeneous solution was obtained. Of this, 200 mL was used for the synthesis of pristine SnSe2, while the remaining 100 mL was reserved for the preparation of the GO suspension. PVP was employed to regulate nucleation and crystal growth while minimizing particle agglomeration, thereby promoting the formation of uniform nanosheet-like structures. In a typical synthesis, 11 g of SnCl4·5H2O was dissolved in 100 mL of an aqueous PVP solution (∼0.314 M), followed by the addition of 4–5 drops of ammonium hydroxide (NH4OH) under continuous stirring to facilitate complexation of Sn4+ ions. In a separate beaker, 11.9 g of selenium powder was dissolved in 15 mL of hydrazine hydrate (N2H4 H2O) to form a homogeneous selenium precursor solution. Hydrazine hydrate serves both as a solvent and a reducing agent, enabling the in situ generation of reactive selenium species required for SnSe2 formation. The selenium precursor was then added to 250 mL of PVP solution under vigorous stirring. Subsequently, the tin precursor solution was introduced dropwise into the above mixture, resulting in an immediate colour change to reddish-brown, indicating the initiation of nucleation. The reaction mixture was maintained at 80 °C under continuous stirring for 3–4 h, during which the colour gradually deepened to black, confirming the formation of SnSe2 nanosheets. The formation of SnSe2 can be represented by the following simplified reaction:Sb4+ + 2Se2− → SnSe2↓where Se2− ions are generated in situ through the reduction of selenium by hydrazine. The controlled release of Sn4+ and Se2− ions, together with the steric stabilization provided by PVP, promotes the formation of highly crystalline layered SnSe2 nanosheets. After completion of the reaction, the mixture was allowed to stand undisturbed for 20–30 min to facilitate sedimentation. The precipitate was collected by centrifugation, washed, and dried in an oven at 60 °C for 8 h. The obtained product was designated as E1 (pure SnSe2). The typical isolated yield after centrifugation, washing, and drying was approximately 2 g. A schematic illustration of the synthesis process is presented in Fig. 1.
Synthesis of SnSe2–GO nanocomposites
For the preparation of the SnSe2–GO nanocomposites, the remaining 100 mL of the stock PVP solution was used to prepare the GO suspension by dispersing 3 g of graphene oxide powder (30 mg mL−1) under continuous magnetic stirring at 600 rpm and 80 °C for 4 h until a homogeneous and stable suspension was obtained. The abundant oxygen-containing functional groups present on the GO sheets facilitate strong interfacial interactions with the growing SnSe2 nanostructures and provide additional nucleation sites for heterogeneous growth.
The as-prepared GO suspension was subsequently mixed with the SnSe2 reaction mixture in different volume ratios of 1 : 0.5, 1 : 1, and 1 : 2 (SnSe2–GO). The immediate colour change to deep black indicated effective incorporation of GO within the growing SnSe2 nanosheet network. The presence of GO is expected to influence the nucleation kinetics and growth behaviour of SnSe2 while simultaneously providing conductive pathways that can facilitate charge transport in the resulting composite structure. The composite mixtures were subjected to identical CBD conditions, i.e., stirring at 80 °C for 3–4 h followed by a settling period of 20–30 min. The resulting precipitates were collected via centrifugation and dried at 60 °C for 8 h. The final products were labelled as E2 (1 : 0.5), E3 (1 : 1), and E4 (1 : 2), respectively. The isolated yield of the SnSe2–GO composites typically ranged between 2.5 and 3.5 g, depending on the GO content.
The overall synthesis strategy enables the formation of SnSe2–GO hybrid nanostructures through a facile, low-cost, and scalable solution-based route, making it attractive for large-area optoelectronic and nanoelectronic device fabrication.
Characterization techniques
The structural and optical properties of the synthesized SnSe2 and SnSe2–GO nanocomposite samples were comprehensively investigated using a range of analytical techniques. The crystal structure was examined by X-ray diffraction (XRD, X'Pert PRO). Surface morphology and microstructural features were analysed using field emission scanning electron microscopy (FESEM, MAIA3 XMH), while detailed lattice and nano-structural information were obtained from high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F). The optical properties were evaluated using UV-visible absorption spectroscopy (Agilent Cary 60 UV-vis spectrophotometer) and photoluminescence (PL) spectroscopy (Cary Eclipse fluorescence spectrophotometer). Functional groups and bonding characteristics were analysed using Fourier transform infrared spectroscopy (FTIR, Shimadzu IR Affinity), whereas vibrational and structural disorder analysis was carried out using Raman spectroscopy (Alpha300RA AFM & Raman system). Electrical properties were assessed through current–voltage (I–V) measurements using a Keithley source meter (Model 2450), which provides high precision with a basic accuracy of 0.012% and a resolution of 6½ digits.
Results and discussion
XRD
The XRD patterns of pristine SnSe2 (E1) and SnSe2–GO nanocomposites (E2–E4) are shown in Fig. 2. The diffraction peaks of the as-synthesized SnSe2 are in good agreement with the standard hexagonal phase of SnSe2 (JCPDS card no. 23-0602), confirming the successful formation of a well-crystallized layered structure.26 The prominent diffraction peaks located at 2θ ≈ 23.5°, 30.0°, 41.9°, 43.8°, and 51.2° are indexed to the (100), (101)/(011), (102), (003), and (110) crystallographic planes, respectively, which are characteristic of hexagonal SnSe2.
The sharp and well-defined nature of these peaks indicates good crystallinity of the synthesized material. Such a high degree of crystallinity is desirable for electronic and optoelectronic applications, as it minimizes grain-boundary scattering and facilitates efficient carrier transport through the layered SnSe2 network. Notably, the intense peak around 2θ ≈ 30° corresponding to the (101)/(011) plane suggests preferential growth along this direction, which is consistent with the anisotropic layered structure of SnSe2. The preferential growth behaviour can be attributed to the intrinsic crystal symmetry and the controlled nucleation environment provided by the CBD process. Furthermore, the absence of any additional impurity peaks confirms the phase purity of the synthesized sample. No diffraction features corresponding to elemental selenium, tin oxide, or other secondary tin selenide phases are observed, indicating the successful formation of phase-pure SnSe2 nanosheets.
Upon incorporation of GO, the SnSe2–GO nanocomposites (E2–E4) retain the characteristic diffraction peaks of SnSe2 without any significant shift in peak positions, indicating that the crystal structure of SnSe2 remains intact after composite formation. However, a slight reduction in peak intensity and marginal peak broadening can be observed for the composites. This behaviour may be attributed to a reduction in crystallite size, increased lattice strain, and interfacial interactions between SnSe2 nanosheets and GO. The presence of GO can influence the nucleation and growth kinetics of SnSe2 during the CBD process by providing heterogeneous nucleation sites, thereby restricting excessive crystal growth while preserving the overall crystal structure. A distinct diffraction peak appears at around 2θ ≈ 11.9° in the composite samples, corresponding to the (001) plane of graphene oxide. This peak originates from the enlarged interlayer spacing of oxygen-functionalized GO sheets and serves as a characteristic signature of graphene oxide incorporation.27 The intensity of this peak becomes more evident with increasing GO content, further supporting the successful integration of GO within the composite matrix.
The coexistence of SnSe2 and GO diffraction features, without the formation of any additional crystalline phases, suggests that the composite formation occurs through strong interfacial coupling and physical integration between the two components rather than through structural transformation of the SnSe2 lattice. Such intimate interfacial contact between SnSe2 and GO is expected to promote efficient charge transfer across the heterointerface, which can play a crucial role in enhancing the photoconductive response and modifying the electrical transport behaviour of the composite, as discussed in subsequent sections.
FESEM and HRTEM
Field emission scanning electron microscopy was employed to investigate the surface morphology of pristine SnSe2 and GO-decorated SnSe2 nanocomposites (SnSe2–GO), as presented in Fig. 3a–d. Fig. 3a shows the FESEM image of pristine SnSe2 (E1), revealing a clustered morphology composed of flake-like nanosheets. These structures appear as thick, stacked assemblies formed by the aggregation of thinner individual layers, which can be attributed to the intrinsic layered nature of SnSe2 and the weak van der Waals interactions between adjacent sheets.28 Such morphology is typical for layered chalcogenide materials and supports the formation of nanosheet-like architectures. The formation of interconnected nanosheet assemblies is advantageous for charge transport, as the large exposed surface area and layered arrangement can facilitate carrier migration and enhance interaction with incident light during photoconductive measurements.
Fig. 3b depicts the FESEM image of the SnSe2–GO nanocomposite (E4, 1 : 2 ratio). The image clearly exhibits sheet-like features along with interconnected nanoflakes that tend to self-assemble into hierarchical clusters. Importantly, the incorporation of GO does not significantly alter the overall morphology of SnSe2, indicating that the composite retains its characteristic layered structure while enabling effective integration of GO sheets within the matrix. The preservation of the nanosheet morphology after GO incorporation suggests that the composite formation process does not disrupt the crystal growth of SnSe2. Instead, GO acts as a supporting scaffold that promotes intimate interfacial contact between the two components. Such interfacial coupling is expected to facilitate charge separation and transport by providing additional conductive pathways throughout the composite network.
Elemental composition and chemical purity were further analysed using energy-dispersive X-ray (EDX) spectroscopy for both pristine SnSe2 (E1) and the SnSe2–GO composite (E4). The EDX spectrum of E1 confirms the presence of Sn and Se as the primary constituents, consistent with the formation of stoichiometric SnSe2. The absence of any detectable impurity elements further supports the phase purity inferred from the XRD analysis. In contrast, the EDX spectrum of the composite sample (E4) exhibits additional peaks corresponding to carbon (C) and oxygen (O), along with Sn and Se, thereby confirming the successful incorporation of graphene oxide into the SnSe2 framework. The simultaneous presence of Sn, Se, C, and O indicates the coexistence of both SnSe2 and GO within the composite structure without the formation of undesirable secondary phases. The oxygen signal originates from the oxygen-containing functional groups of GO, which are known to contribute to strong interfacial interactions with semiconductor nanostructures. Such interactions can influence carrier trapping, charge transfer, and recombination processes, thereby playing an important role in the electrical and photoconductive behaviour of the composite. The quantitative elemental compositions, including the atomic weight percentages of the constituent elements, are summarized in Fig. 3c and d for SnSe2 and SnSe2–GO, respectively.
The high-resolution transmission electron microscopy (HRTEM) images of pristine SnSe2 (E1) and SnSe2–GO nanocomposites (E4, 1 : 2 ratio) are presented in Fig. 4a–f. The pristine SnSe2 sample (Fig. 4a) exhibits well-dispersed nanosheet-like structures, confirming the formation of layered morphology. The corresponding HRTEM image (Fig. 4b) reveals clear lattice fringes with an interplanar spacing of ∼0.29 nm, which is indexed to the (101) plane of hexagonal SnSe2, consistent with the XRD results. The formation of such nanosheets can be attributed to anisotropic growth along the crystallographic c-axis, governed by weak van der Waals interactions between adjacent layers. The nanosheets appear nearly square-shaped with an average lateral size of approximately 3 nm. The presence of well-defined lattice fringes demonstrates the high crystalline quality of the synthesized nanosheets and indicates the successful formation of ordered SnSe2 domains. Such crystallinity is expected to facilitate efficient charge transport by reducing structural disorder and carrier scattering within the nanosheets. The selected area electron diffraction (SAED) pattern (Fig. 4c) displays distinct diffraction rings/spots corresponding to the crystalline planes of SnSe2, further confirming its high crystallinity and agreement with XRD analysis.
In the case of the SnSe2–GO nanocomposite (E4), the HRTEM image (Fig. 4d) shows nanosheets with a relatively reduced lateral size of ∼2 nm, uniformly distributed over the GO sheets. The composite exhibits well-resolved lattice fringes corresponding to both SnSe2 and GO. As shown in Fig. 4e, the measured interplanar spacings of ∼0.30 nm and ∼0.78 nm is attributed to the (101) plane of SnSe2 and the (001) plane of GO, respectively. The simultaneous observation of lattice fringes associated with both constituents provides direct evidence of intimate interfacial contact between SnSe2 nanosheets and GO sheets. Such nanoscale interfacial coupling is crucial for facilitating charge transfer across the heterointerface and can contribute significantly to the enhanced photoconductive response observed in the composite. The reduction in nanosheet size in the composite is likely associated with GO-mediated nucleation control and growth restriction, wherein the GO sheets act as heterogeneous nucleation sites that suppress excessive crystal growth and promote the formation of smaller, uniformly distributed SnSe2 nanocomposites.
Furthermore, the SAED pattern of the composite (Fig. 4f) exhibits characteristic diffraction features corresponding to both SnSe2 and GO, confirming the coexistence of both phases and their crystalline nature. The absence of any additional diffraction features indicates that no secondary crystalline phases are formed during the composite synthesis process. These observations are in good agreement with the XRD results, validating the successful formation of SnSe2–GO nanocomposites. The combined HRTEM and SAED analyses therefore provide strong evidence for the formation of highly crystalline SnSe2 nanosheets intimately integrated with GO, a structural configuration that is favourable for efficient carrier transport and interfacial charge separation.
FTIR & Raman spectroscopy
FTIR spectroscopy was employed to investigate the functional groups present in pristine SnSe2 (E1) and SnSe2–GO nanocomposites (E4, 1 : 2 ratio), as shown in Fig. 5a and b, respectively, over the wavenumber range of 4000–500 cm−1. For pristine SnSe2 (E1), broad absorption bands observed at ∼3868 and 3796 cm−1 are attributed to O–H stretching vibrations, which arise from adsorbed moisture or residual hydroxyl groups associated with the aqueous synthesis process.29 The band at ∼1631 cm−1 corresponds to N–H bending vibrations or H–O–H bending of adsorbed water molecules. Weak features observed around ∼2029 cm−1 are likely associated with atmospheric CO2 or combination bands rather than intrinsic vibrational modes. Additional peaks at ∼1094 and 948 cm−1 may be attributed to C–O stretching vibrations or residual organic species originating from PVP used during synthesis. A distinct absorption band at ∼654 cm−1 is assigned to Sn–O–Sn vibrations, indicating slight surface oxidation or interaction with oxygen species.30 The presence of surface hydroxyl and oxygen-containing species suggests the existence of surface-active sites that can influence carrier trapping and interfacial charge-transfer processes in SnSe2 nanosheets.
For the SnSe2–GO nanosheet (E4) similar O–H stretching vibrations are observed at 3944.48 and 3714.16 cm−1, confirming the presence of hydroxyl groups.29 A weak band around ∼2307 cm−1 is attributed to atmospheric CO2 absorption. The prominent peak at ∼1651 cm−1 corresponds to C <svg xmlns="http://www.w3.org/2000/svg" version="1.0" width="13.200000pt" height="16.000000pt" viewBox="0 0 13.200000 16.000000" preserveAspectRatio="xMidYMid meet"><metadata> Created by potrace 1.16, written by Peter Selinger 2001-2019 </metadata><g transform="translate(1.000000,15.000000) scale(0.017500,-0.017500)" fill="currentColor" stroke="none"><path d="M0 440 l0 -40 320 0 320 0 0 40 0 40 -320 0 -320 0 0 -40z M0 280 l0 -40 320 0 320 0 0 40 0 40 -320 0 -320 0 0 -40z"/></g></svg> C stretching vibrations of graphitic domains, along with possible contributions from bending vibrations of adsorbed water molecules. Compared with pristine SnSe2, the appearance and enhanced intensity of carbon-related vibrational bands provide evidence for the successful incorporation of GO within the composite structure.
Importantly, the strong peak at ∼1090 cm−1 and the band at ∼1415 cm−1 are assigned to C–O–C and C–OH stretching vibrations, respectively, which are characteristic of oxygen-containing functional groups in GO.31,32 Furthermore, the presence of Sn–O–Sn vibrational modes is confirmed by peaks at ∼640 and 796 cm−1, consistent with the SnSe2 framework. The simultaneous observation of GO-related oxygen functionalities and SnSe2-associated vibrational modes confirms the formation of a hybrid nanostructure while preserving the chemical integrity of both components. Moreover, the oxygen-containing groups of GO can act as defect-modulating centres and provide additional charge-transfer pathways, which may contribute to the enhanced photoconductive response and altered electrical transport behaviour observed in the composite samples. The coexistence of SnSe2-related vibrational modes and GO-specific functional group signatures clearly demonstrates the successful formation of SnSe2–GO nanocomposites.
The Raman spectra of pristine SnSe2 (E1) and the SnSe2–GO nanocomposite (E4) are presented in Fig. 6a and b, respectively. For pristine SnSe2, characteristic peaks corresponding to the in-plane Eg mode and out-of-plane A1g mode are observed in the low-frequency region (∼118 cm−1–241 cm−1), confirming the formation of layered hexagonal SnSe2.33 These vibrational modes originate from the characteristic lattice dynamics of the Se–Sn–Se layers and are consistent with the crystalline phase identified from the XRD analysis. The presence of well-defined Raman modes further indicates good crystallinity and structural ordering within the synthesized SnSe2 nanosheets.
For the SnSe2–GO composite (E4), two prominent peaks located at ∼1350 cm−1 and ∼1580 cm−1 correspond to the D band and G band of graphene oxide, respectively.34 The D band arises from defects and disorder in the sp2 carbon network, while the G band is associated with the in-plane vibration of sp2-hybridized carbon atoms.35 The intensity ratio of the D to G band ID/IG provides insight into the degree of structural disorder in GO. From the spectrum, the ID/IG ratio is close to unity, indicating a significant level of defects and oxygen-containing functional groups in the GO sheets. Such defect sites can act as active centres for interfacial interaction with SnSe2 nanosheets and are expected to influence carrier trapping, charge transfer, and recombination dynamics within the composite. Furthermore, the oxygen-containing functional groups associated with these defect sites can facilitate strong electronic coupling between GO and SnSe2, thereby enhancing interfacial charge transport.
The coexistence of SnSe2 vibrational modes and GO-related D and G bands confirms the successful formation of the SnSe2–GO nanocomposite without altering the intrinsic crystal structure of SnSe2. Notably the composite still exhibits peaks near 118 cm−1 and 185 cm−1 indicating that SnSe2 remains part of the structure. However, a reduction in the intensity of the SnSe2-related modes is observed after GO incorporation. This behaviour may arise from the coverage of SnSe2 nanosheets by GO sheets, interfacial interactions between the two components, and attenuation of the Raman signal due to the presence of the carbonaceous layer.36 The simultaneous observation of SnSe2 and GO Raman signatures, together with the defect-rich nature of GO, provides strong evidence for the formation of an interfacially coupled hybrid structure. Such interfacial coupling is expected to promote efficient charge separation and carrier transport, which correlates well with the enhanced photoconductive response and modified electrical transport characteristics observed for the composite samples.
It should be noted that the present Raman and FTIR measurements confirm the presence of a disordered, oxygen-functionalized carbonaceous component derived from the starting GO; however, they do not unambiguously distinguish fully oxidized GO from partially reduced GO-derived carbon. Quantitative determination of the carbon phase and C/O ratio would require XPS analysis, while independent electrical conductivity measurements would further clarify the transport properties of the carbonaceous component. Therefore, in the following discussion, the carbon phase is conservatively referred to as a GO-derived oxygen-functionalized carbonaceous phase where its exact chemical state is relevant.
Temperature dependence of dark current
The activation energy represents the minimum energy required to initiate activity in atoms or molecules within the material in initiate the electronic process. The activation energy of the prepared samples can be determined using the Arrhenius equation37 given by eqn (1).Here, I0 is the pre-exponential factor, Ea is the activation energy, K is the Boltzmann constant, and T is the temperature in Kelvin. Taking logarithm on both sides in eqn (1) we get
The temperature dependence of dark current is measured at a constant bias voltage of 10 V. With the increase in temperature from 298.15 K to 338.15 K, the dark current (ID) for both pristine SnSe2 (E1) and SnSe2–GO nanocomposite (E4) was recorded. The activation energy was estimated from the slope of a linear fit of ln (ID) vs. 1000/T as shown in Fig. 7. Based on the linear fitting of ln (Id) vs. 1000/T for both samples, the calculated activation energies were found to be 0.12 eV for SnSe2 and 0.09 eV for SnSe2–GO nanocomposite. These findings are consistent with the earlier reports by Jaegermann et al.,38 Patil et al.,39 and Hare et al.40 The relatively low activation energies indicate that charge transport is dominated by shallow defect states and localized energy levels within the bandgap rather than by intrinsic band-to-band excitation. Such behaviour is characteristic of defect-assisted conduction in layered chalcogenide semiconductors.
The lower activation energy observed for the SnSe2–GO nanocomposite suggests that GO incorporation facilitates charge transport by reducing the energy barrier for carrier hopping and promoting carrier delocalization through interfacial electronic coupling. The presence of conductive GO sheets can provide additional charge transport pathways, thereby lowering the thermal energy required for carrier activation. Furthermore, the observed activation energies indicate that the investigated temperature range primarily probes the extrinsic conduction regime, where thermally activated defect states significantly influence electrical transport.
Photoconductivity measurement technique
The photoconductivity measurements were carried out using a planar device geometry fabricated on a clean glass substrate, as schematically illustrated in Fig. 8. Initially, glass substrates (3 cm × 3 cm) were thoroughly cleaned with ethanol to remove surface contaminants and impurities. Copper electrodes were then deposited on the substrate surface in a parallel configuration, maintaining an inter-electrode gap of approximately 2 mm and an effective channel length of about 5 mm.
The planar devices were fabricated by drop-casting 25 µL of the as-prepared nanomaterial dispersion onto the electrode gap (≈2 mm) using a calibrated micropipette, followed by drying at 50 °C for 2 h. The deposition volume and dispersion concentration were kept identical for all devices to ensure reproducible film coverage and minimize variations in the active layer. Quantitative film-thickness measurements were not performed in the present study. The current–voltage (I–V) characteristics were recorded by sweeping the applied voltage across the device using a source meter. The planar electrode configuration provides a simple and effective platform for evaluating lateral charge transport through the nanomaterial film. In addition, the exposed device geometry ensures efficient interaction between incident light and the active layer, thereby enabling reliable assessment of photogenerated charge-carrier dynamics. All electrical measurements were performed under identical experimental conditions to ensure consistent comparison between pristine SnSe2 and SnSe2–GO devices.
Photocurrent light-intensity measurements
To investigate the photoconductive behaviour of the E1 and E4 devices, I–V measurements were performed under dark conditions and under white light illumination intensities of 5 K, 15 K, 25 K, and 35 K lux. A halogen lamp was employed as the illumination source, and all measurements were carried out in a dark-room environment to minimize background interference. The corresponding I–V characteristics are presented in Fig. 9a and b.
As the illumination intensity increases, the measured current increases significantly for both devices, indicating efficient photogeneration of charge carriers. The enhanced current under illumination originates from the generation of electron–hole pairs upon photon absorption, which increases the carrier concentration and consequently the electrical conductivity of the devices. Notably, the SnSe2–GO nanocomposite exhibits a more pronounced photoresponse compared to pristine SnSe2, suggesting improved charge separation and carrier transport in the composite system.
The I–V curves shows that the photocurrent increases over dark as the light intensity rises from 5000 to 35 000 lux. In the present case the photocurrent (Iph) is defined as Iph = IL − Id where IL and Id are total and dark current respectively. The variation of photocurrent with light intensity is shown in Fig. 10a. In both samples, the photocurrent increases continuously with increasing illumination intensity, confirming efficient photoconductive behaviour. The nature of photo-response can be determined from the plot Fig. 10b transforming into a log scale where the power law can be expressed by Iphα φr. Here r is an exponent that determines the nature of photoelectronic processes in these materials. The extracted values of (r) were found to be approximately 0.18 for pristine SnSe2 and 0.80 for the SnSe2–GO composite.
For pristine SnSe2, the significantly sublinear photoresponse (r < 1) indicates that photoconduction is dominated by defect-assisted trapping and recombination processes. Upon illumination, photons with energy greater than the band gap of SnSe2 generate electron–hole pairs within the nanosheets. However, a considerable fraction of the photogenerated electrons becomes trapped at selenium-vacancy-related deep-level states, while the remaining carriers undergo rapid recombination, thereby limiting the increase in photocurrent with increasing illumination intensity. Such behaviour is characteristic of defect-rich layered semiconductors exhibiting trap-controlled photoconduction. Following GO incorporation, the photoresponse exponent increases substantially to ∼0.80, approaching nearly linear behaviour. This improvement is consistent with more efficient interfacial carrier transport across the SnSe2–GO interface and reduced trap-assisted recombination. The intimate interfacial coupling between SnSe2 and the GO-derived carbonaceous phase may facilitate charge separation and carrier extraction, thereby suppressing electron–hole recombination and increasing the fraction of photogenerated carriers contributing to the photocurrent. However, the present measurements do not directly establish the direction of charge transfer or the specific band alignment at the SnSe2/carbonaceous interface. Simultaneously, the oxygen-containing functional groups of GO partially passivate selenium-vacancy-related trap states, thereby reducing trap-assisted recombination and increasing the fraction of photogenerated carriers contributing to the photocurrent. The combined effects of efficient interfacial charge separation, prolonged carrier lifetime, reduced trap-assisted recombination, and improved carrier transport account for the approximately 180-fold enhancement in photocurrent observed for the SnSe2–GO nanocomposite under 35 000 lux illumination. A similar enhancement mechanism has been reported for SnSe2-based heterostructures, where efficient interfacial charge separation and suppressed carrier recombination improve the photoresponse. However, unlike these reports, the present SnSe2–GO nanocomposite demonstrates that GO-induced trap passivation not only enhances photocurrent but also suppresses defect-mediated NDR, establishing a direct correlation between defect passivation and nonlinear electrical transport.41
It should be noted that the present measurements were performed under illumination specified in lux and were primarily intended to compare the relative photoconductive response of pristine SnSe2 and SnSe2–GO under identical experimental conditions. Because the incident optical power at the sample surface and the effective illuminated device area were not determined, quantitative photodetector parameters such as responsivity, specific detectivity, and external quantum efficiency cannot be reliably extracted from the present data. Accordingly, the observed approximately 180-fold increase in photocurrent is interpreted as a relative photoconductive enhancement rather than a fully quantified device-performance metric. Future measurements using calibrated optical power and a defined active area will be required for a quantitative assessment of photodetector performance.
The observed improvement is consistent with previous reports on SnSe2-RGO heterostructures, where graphene derivatives facilitate efficient carrier extraction and suppress electron–hole recombination. However, unlike these earlier studies, the present work demonstrates that GO incorporation not only enhances the photoconductive response but also suppresses defect-mediated NDR by passivating selenium-vacancy-related trap states, thereby establishing a direct correlation between defect passivation, enhanced photocurrent, and nonlinear electrical transport of SnSe2.42
The enhanced photocurrent can also be correlated with the structural characteristics of the synthesized nanomaterials. XRD, FESEM, and HRTEM analyses confirmed the formation of highly crystalline nanosheet structures with intimate interfacial contact between SnSe2 and GO. The combination of high crystallinity and efficient interfacial charge transfer minimizes carrier scattering and recombination losses, enabling more effective utilization of photogenerated carriers. These observations are in excellent agreement with the structural and spectroscopic analyses, which indicate that GO promotes efficient interfacial coupling while reducing the influence of defect-assisted recombination. The transition of the photoresponse exponent from 0.18 to 0.80 further corroborates the suppression of trap-limited photoconduction and the establishment of more efficient photocarrier generation and collection following GO incorporation. Consequently, under identical measurement conditions, the SnSe2–GO nanocomposite exhibits a substantially enhanced relative photoconductive response compared to pristine SnSe2.
The I–V characteristics of the Cu/SnSe2/Cu and Cu/SnSe2–GO/Cu devices exhibit linear behaviour at low bias and nonlinear behaviour at high bias under both dark and illuminated conditions. The linear region at low bias indicates relatively efficient carrier transport across the interfaces, but the onset of nonlinearity at higher voltages suggests the presence of a small potential barrier at the Cu–SnSe2 or Cu–SnSe2–GO interface. The work function of SnSe2 (∼5.2 eV)43 is higher than that of copper (∼4.6 eV),44 which theoretically favours ohmic contact formation. However, the observed deviation from linearity at higher bias suggests the involvement of defect-assisted transport processes, interface states, and charge trapping effects, which become increasingly significant under strong electric fields. These processes are closely related to the NDR behaviour discussed in the following section. All photoelectronic measurements were performed at 2 V within the linear operating region to ensure reliable comparison of the photoconductive response.
Nonlinear electrical transport and negative differential resistance behaviour
The nonlinear electrical transport characteristics of SnSe2 nanosheets were investigated through I–V measurements under different synthesis conditions. Particular attention was given to the emergence of NDR, a phenomenon of considerable interest for nonlinear electronic devices, logic circuits, oscillators, and neuromorphic systems.
To examine the influence of precursor concentration on the electrical response, SnSe2 nanosheets were synthesized using different precursor molarities (0.01 M, 0.1 M, 0.5 M, and 1 M), and their corresponding I–V characteristics were recorded by sweeping the voltage from −15 V to +15 V with an average scan rate 3.0 V s−1. As shown in Fig. 11a, samples prepared at lower molarities (0.01 M and 0.1 M) exhibit nearly linear I–V characteristics, indicating predominantly ohmic conduction. In contrast, samples synthesized at higher precursor concentrations (0.5 M and 1 M) display a pronounced NDR region during the positive voltage sweep. The current initially increases with increasing bias, reaches a peak value, and subsequently decreases despite further increases in the applied voltage, which is the characteristic signature of NDR. Fig. 11b presents the cyclic I–V measurements. An interesting asymmetry is observed between the forward and reverse voltage sweeps. It shows the two (I–V) does not meet at zero bias that may be attributed to the capacitive effect and as a result appreciable current exist. During the negative-to-zero voltage region, the current remains relatively stable, whereas a distinct NDR peak develops during the subsequent positive sweep. Similarly, during the reverse scan from +15 V to −15 V, the current remains comparatively stable until the bias approaches zero, after which another NDR region appears. Such polarity-dependent behaviour suggests that the NDR process is governed by bias-dependent carrier injection and extraction mechanisms involving defect-mediated charge trapping and detrapping.
The emergence of NDR only at higher precursor molarities demonstrates that the synthesis conditions strongly influence the nonlinear electrical response of SnSe2. Although quantitative film-thickness measurements were not performed in the present study, the observed molarity dependence cannot therefore be directly attributed to thickness variations. Instead, the results indicate that changes induced by precursor concentration, such as variations in nanosheet morphology, layer stacking, interparticle connectivity, and defect-state distribution, may influence carrier trapping and transport. Among these factors, the subsequent stoichiometry-dependent measurements provide further evidence that defect-related trap states play an important role in governing the observed NDR behaviour.
To further investigate the origin of NDR, the Sn : Se precursor ratio was varied from 1 : 2 to 1 : 5. The resulting I–V characteristics are shown in Fig. 12a and b with an average scan rate 2.85–3.0 V/S. A systematic shift of the NDR peak towards lower voltages is observed with increasing selenium content. Specifically, the 1 : 2 sample exhibits the NDR peak at the highest voltage, whereas the 1 : 5 sample shows the NDR onset at significantly lower voltages. This behaviour strongly suggests that the NDR response is closely linked to stoichiometry-dependent defect states. Variations in the Sn ratio are expected to modify the concentration and distribution of selenium vacancies, which act as electrically active trap centres within the SnSe2 lattice. As the defect landscape evolves, the voltage required to fill, activate, or release trapped carriers changes, resulting in the observed shift of the NDR peak position. The polarity-dependent behaviour observed in Fig. 12 closely resembles that found in the molarity-dependent measurements, indicating a common conduction mechanism. The results collectively suggest that carrier trapping at defect states dominates the low-bias regime, while subsequent field-assisted detrapping and redistribution of charge carriers at higher bias give rise to the NDR phenomenon.
The origin of the observed NDR can be understood in terms of defect-assisted transport involving deep-level trap states associated with selenium vacancies. These defects can strongly influence carrier injection, trapping, and extraction processes under an applied electric field. Previous studies have shown that NDR in layered chalcogenide heterostructures is commonly associated with interband or band-to-band tunnelling across engineered van der Waals interfaces. For example, BP/SnSe2 and WSe2/SnSe2 heterostructures exhibit tunnelling-induced NDR arising from broken-gap band alignment,10,19 while WS2/SnS2 vertical heterostructures demonstrate NDR originating from a finite tunnelling window across the heterojunction.45 These reports establish interface-controlled quantum tunnelling as the dominant mechanism in heterostructure devices. In contrast, the present SnSe2 nanosheets exhibit NDR in the absence of any engineered heterointerface, indicating that the observed nonlinear transport is governed by intrinsic defect-assisted carrier dynamics rather than tunnelling across a junction. A similar mechanism was proposed by Roy et al. for ZnO nanorod/p-Si heterostructures,46 where oxygen-vacancy-related defect states were found to induce NDR through trap-assisted carrier transport. Their findings establish a direct correlation between defect states and nonlinear electrical response, supporting the interpretation that selenium-vacancy-related traps play a central role in the present SnSe2 system. The proposed mechanism therefore differs fundamentally from tunnelling-mediated NDR reported in SnSe2-based van der Waals heterostructures, where current reduction originates from the collapse of the tunnelling window as the applied bias increases. Instead, the NDR observed here results from carrier trapping, field-assisted detrapping, and redistribution of space charge at selenium-vacancy-related defect states, highlighting an alternative defect-controlled pathway for achieving nonlinear electrical transport in layered SnSe2.
Based on the experimental observations, a defect-assisted transport mechanism governed by selenium-vacancy-related deep-level states is proposed. Under low applied bias, electrons injected from the Cu electrodes are transported through the interconnected SnSe2 nanosheets, while a small fraction becomes captured by isolated selenium-vacancy-related trap states. Since the trap occupancy remains relatively low, the free-carrier concentration continues to increase with applied voltage, leading to the initial increase in current. As the applied electric field increases, progressively more deep-level trap states become occupied, resulting in the accumulation of localized space charge within the nanosheet network. The trapped charge screens the local electric field and reduces the efficiency of further carrier injection, thereby decreasing the effective free-carrier density available for conduction. Consequently, the current decreases despite increasing applied voltage, giving rise to the negative differential resistance region. At higher bias, the stronger electric field promotes field-assisted detrapping together with activation of additional conduction pathways. Consequently, previously localized carriers are released into the conduction band, increasing the free-carrier density and restoring the current beyond the NDR minimum.
The pronounced asymmetry of the NDR behaviour observed during cyclic voltage sweeps can be attributed to polarity-dependent carrier injection and extraction processes together with the non-uniform spatial distribution of selenium-vacancy-related trap states within the SnSe2 nanosheet network. During opposite bias polarities, the occupation of deep-level traps and the resulting space-charge distribution evolve differently, producing different carrier injection barriers and detrapping conditions. Consequently, the voltage corresponding to the NDR peak depends on the direction of the applied electric field, giving rise to the experimentally observed asymmetric NDR characteristics.
Another notable feature of the cyclic I–V characteristics is the presence of finite current near zero bias together with non-zero crossing behaviour, indicating a history-dependent electrical response associated with charge accumulation and/or trapping within the defect-rich SnSe2 nanosheet network. The observed hysteresis cannot, on the basis of the present I–V measurements alone, be unambiguously assigned to a memristive mechanism, since capacitive effects, charge trapping, ionic motion, and other electrochemical processes may also contribute to hysteretic behaviour. Moreover, the non-zero crossing of the cyclic I–V loop does not satisfy the canonical pinched-hysteresis criterion generally associated with an ideal memristive system. Therefore, the present results are more appropriately described as hysteretic and charge-storage-related electrical behaviour. Further frequency-dependent measurements and dedicated switching characterization would be required to establish a memristive response.
An important outcome of the present study is that photoconductivity enhancement and NDR suppression originate from the same defect-engineering process rather than from two independent phenomena. In pristine SnSe2, selenium-vacancy-related deep-level states simultaneously act as carrier trapping centres under illumination and as active trap sites governing the carrier trapping–detrapping dynamics responsible for NDR. Upon incorporation of the GO-derived oxygen-functionalized carbonaceous phase, residual oxygen-containing functional groups are proposed to partially passivate these electrically active defect states, thereby reducing trap-assisted recombination and increasing the lifetime of photogenerated carriers, which enhances the photocurrent. At the same time, the reduced density of deep-level trap states suppresses the space-charge accumulation required for defect-mediated NDR. Thus, GO incorporation effectively shifts the transport mechanism from a defect-dominated regime towards an interfacial-carrier-transport-dominated regime, providing a unified explanation for the simultaneous enhancement of photoconductivity and suppression of NDR.
Overall, the experimental results consistently indicate that defect states and interfacial interactions play a decisive role in governing the electrical transport behaviour of the synthesized nanostructures. The relatively low activation energies, defect signatures observed from Raman spectroscopy, and the pronounced NDR behaviour in pristine SnSe2 collectively provide strong indirect evidence that carrier transport is strongly influenced by defect-assisted trapping and detrapping processes, most likely associated with selenium vacancies and deep-level localized states. Upon incorporation of the GO-derived carbonaceous phase, residual oxygen-containing functional groups are proposed to partially passivate selenium-vacancy-related trap states, while the carbonaceous network promotes efficient interfacial carrier transport, thereby simultaneously enhancing photoconductivity and suppressing defect-mediated NDR. These findings demonstrate that interfacial engineering through GO incorporation provides an effective strategy for tailoring the balance between photoconductive and nonlinear electrical responses in SnSe2-based nanostructures. Although the proposed mechanism is strongly supported by the combined structural, spectroscopic, and electrical characterization together with the systematic suppression of NDR following GO incorporation, direct verification of selenium-vacancy-related defect states by techniques such as XPS, PL spectroscopy, and electrode-dependent transport measurements would provide additional confirmation of the proposed transport mechanism.
Conclusion
Pristine SnSe2 and GO-decorated SnSe2 (SnSe2–GO) nanocomposites were successfully synthesized via a simple chemical bath deposition method. Structural and spectroscopic analyses confirmed the formation of highly crystalline hexagonal SnSe2 and the successful incorporation of GO within the composite structure without altering the crystal phase of SnSe2. The SnSe2–GO nanocomposite exhibited a lower activation energy (0.09 eV) than pristine SnSe2 (0.12 eV), indicating improved carrier transport arising from interfacial interactions between SnSe2 and GO. Photoconductivity studies revealed a significant increase in photocurrent with increasing illumination intensity for both samples, with the SnSe2–GO nanocomposite exhibiting a substantially enhanced photoresponse. The improved photoconductive performance is attributed to more efficient charge separation and charge transport facilitated by the GO network. In contrast, pristine SnSe2 exhibited pronounced asymmetric NDR behaviour that was strongly dependent on precursor molarity, Sn stoichiometry, and voltage sweep direction. The observed NDR is most likely associated with selenium-vacancy-mediated trap states and defect-assisted carrier trapping/detrapping processes. Increasing selenium content systematically shifted the NDR onset towards lower voltages, highlighting the important role of defect states in governing nonlinear electrical transport. Interestingly, GO incorporation suppressed the NDR response while simultaneously enhancing photoconductivity, suggesting partial passivation of electrically active defect states by residual oxygen-containing functional groups associated with the GO-derived carbonaceous phase. Furthermore, the observed hysteresis and non-zero-bias crossing behaviour in pristine SnSe2 indicate a history-dependent electrical response associated with charge-storage and trapping effects within the nanosheet network. Overall, the present study demonstrates that defect engineering and interfacial coupling provide effective routes for tailoring the photoconductive and nonlinear electrical properties of SnSe2-based nanocomposites, highlighting their potential for future optoelectronic and nanoelectronic applications. Although the proposed defect-mediated transport mechanism is strongly supported by the combined structural, spectroscopic, and electrical characterization, further investigations using XPS, PL spectroscopy, and electrode-dependent transport measurements would provide direct verification of the proposed role of selenium-vacancy-related defect states.
Conflicts of interest
There are no conflicts to declare.
Supplementary Material
Acknowledgements
The authors sincerely thank Gauhati University for providing the PL, UV-vis spectroscopy and XRD facilities; Mahatma Gandhi University, Kottayam for FESEM and Raman spectroscopy facilities; IIT Mumbai for providing the HRTEM facility; and NERIST, Nirjuli for FTIR characterization.
Data availability
The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6ra05598b.