Spontaneous formation of gold nanoparticles on MoS2 nanosheets and its impact on solution-processed optoelectronic devices

Summary Understanding size-dependent properties of 2D materials is crucial for their optimized performance when incorporated through solution routes. In this work, the chemical nature of MoS2 as a function of nanosheet size is investigated through the spontaneous reduction of chloroauric acid. Microscopy studies suggest higher gold nanoparticle decoration density in smaller nanosheet sizes, resulting from higher extent of reduction. Further corroboration through surface-enhanced Raman scattering using the gold-decorated MoS2 nanosheets as substrates exhibited an enhancement factor of 1.55 × 106 for smaller nanosheets which is 7-fold higher as compared to larger nanosheets. These plasmonic-semiconductor hybrids are utilized for photodetection, where decoration is found to impact the photoresponse of smaller nanosheets the most, and is optimized to achieve responsivity of 367.5 mAW-1 and response times of ∼17 ms. The simplistic modification via solution routes and its impact on optoelectronic properties provides an enabling platform for 2D materials-based applications.


INTRODUCTION
Two-dimensional (2D) materials have been emerging to the forefront for numerous enticing applications. This large family of materials is found to be versatile and diverse in properties, encompassing interesting materials such as topological insulators, semiconductors, and superconductors (Choudhary et al., 2017;Lu et al., 2019;Sun et al., 2011;Teweldebrhan et al., 2010). Semiconducting 2D materials like transition metal dichalcogenides have shown promise for several applications in memory and computing circuits, and for optoelectronic devices such as photodetectors, light-emitting devices, and solar cells (Dathbun et al., 2017;Huh et al., 2020;Jariwala et al., 2014;Tsai et al., 2014;Wang et al., 2012aWang et al., , 2012b. The vast scope for their applicability and possible commercialization mandates the need to produce high-quality 2D materials in large quantities. Among synthetic protocols, solution routes offer significant advantages through scalability, ambient processability, speedy, and additive manufacturability, and are less demanding compared to traditional techniques (Bonaccorso et al., 2016;Hassan et al., 2020;Nicolosi et al., 2013;Varrla et al., 2015). In this regard, the protocol of liquid-phase exfoliation has enabled the processing of numerous 2D materials (Bellani et al., 2021;Bonaccorso et al., 2016;Cao et al., 2016;Coleman et al., 2011;Ma and Sasaki, 2015). Detailed investigations have probed the underlying aspects of solvent suitability, and suggest a predominant role of Hansen solubility parameters and surface tension components to enable stable and concentrated nanosheet dispersions (Coleman et al., 2011;O'Neill et al., 2012;Wang et al., 2017;Zhou et al., 2011). These insights have enabled the production of dispersions in suitable organic solvents and mixed-solvent systems. Aqueous dispersions have also been produced, most often through the use of additives such as surfactants in order to obtain stability (Guardia et al., 2014;Lotya et al., 2009;Maleski et al., 2017;McManus et al., 2017).
An inherent attribute of liquid-exfoliated nanosheets is the large polydispersity in size, both in lateral dimensions and in number of layers. The physiochemical properties of 2D materials have been found to be highly size dependent, thus making it vital to address the polydispersity in liquid-exfoliated nanosheets. For example, addition of larger nanosheets as fillers has been shown to exhibit enhanced mechanical properties in polymer nanocomposites when compared to smaller nanosheets (O'Neill et al., 2012). Similarly, the electrocatalytic activity in hydrogen evolution reaction is found to increase with decreasing flake size (Harvey et al., 2015; Seo et al., 2015;Varrla et al., 2015;Wang et al., 2013). In photodetectors, the Aqueous dispersions offer several obvious advantages in processing, although the use of stabilizing additives could have detrimental effects on properties such as conductivity (Secor et al., 2013(Secor et al., , 2015. In this regard, aqueous additive-free dispersions hold vast potential for a diverse variety of applications, especially those involving biological systems (Kaur et al., 2018;Lobo et al., 2021). In addition to this, aqueous dispersions enable suitable modification of nanosheets for application-tailored properties . Similarly, modification of 2D materials using other nanomaterials has also been evaluated and is found to synergistically augment their capabilities. Of special interest are 2D material systems with noble metal nanoparticles to form hybrids. Such modification could have interesting implications through the introduction of phenomena, such as enhanced light absorption through plasmonics (Tu and Wu, 2021;Yin et al., 2014), and better electron transport through efficient charge transfer and increased conductivity (Goswami et al., 2019). To synthesize such systems, several approaches have been made. Physical deposition approaches such as sputtering have been used, although this method is limited to coating prefabricated films of 2D materials on substrates (DiStefano et al., 2020;Li et al., 2020b;Rahmati et al., 2019). Direct addition of nanoparticles is another adopted route, although the process for first growing such nanoparticles forms a separate synthesis step and involves the use of reducing agents (Pramanik et al., 2019). In this regard, transition metal dichalcogenides possess a rich chemistry, which enables them to reduce metal precursors such as HAuCl 4, resulting in Au nanoparticle decoration through solution-processable protocols (Grieger et al., 2020;Kim et al., 2013;Polyakov et al., 2014). The spontaneity of this reduction reaction arises from the intrinsic chemical nature of the nanosheets and effectively bypasses the need for additional synthetic processes and associated use of chemicals. For example, the extent of HAuCl 4 reduction has been shown to be influenced by the phase of MoTe 2 nanosheets obtained from chemical vapor deposition with the 1T 0 phase being more energetically favorable, while the dechlorination on 2H-MoTe 2 occurs favorably only on defect sites (Tao et al., 2021). Other 2D materials such as GaTe obtained from micromechanical cleavage have also demonstrated spontaneous reducing ability leading to Au nanoparticle decoration and has been attributed to monoclinic structure and defects . Given the tremendous influence Au nanoparticle decoration could have on properties of semiconducting transition metal dichalcogenide nanosheets, this simplistic synthetic protocol could have a lot to offer. The extent of such implications on the properties of 2D materials leverages the need to understand the factors of control, in order to develop suitable methods to obtain such hybrids. Although a variation in chemical properties based on nanosheet size is known to occur, its influence on such spontaneous reduction reactions remains unexplored and draws interest, given the scope for tunability in properties through optimal modification with noble metal nanoparticles.
In this work, we explore the ability to suitably modify MoS 2 nanosheets by carrying out spontaneous reactions in an aqueous surfactant-free media. The role of nanosheet size on the chemical activity is probed through the reduction of HAuCl 4 to Au nanoparticles by MoS 2 . Size-selected nanosheets were systematically reacted with different amounts of Au precursor without the need for additional reducing agents, and the extent of Au nanoparticle decoration was investigated using transmission electron microscopy. A higher coverage of the nanosheets with the Au nanoparticles was observed in smaller MoS 2 nanosheets, and could be resulting from their relatively higher defect density. To support these observations, the decorated MoS 2 nanosheets have been used as substrates in surface-enhanced Raman scattering and the enhancement factors were found to increase with decreased nanosheet size reaching 1.55 3 10 6 . In addition to this, the role of nanoparticle decoration on light-matter interactions in this plasmonic-semiconductor Au-MoS 2 system is evaluated in the domain of optoelectronics using solution-processed photodetectors. Significant enhancements in photoresponse were observed with increasing Au decoration, especially in the case of smaller nanosheets. Optimal Au loading was achieved simply by dipping MoS 2 -coated devices in HAuCl 4 , and the fabricated devices exhibited responsivity up to 367.5 mAW -1 , with a rise and decay time of $17 ms. This simplistic protocol powered by the intrinsic size-dependent chemical nature of liquid-exfoliated nanosheets provides a design factor for precise tailoring of light-matter interactions in 2D materials.

RESULTS AND DISCUSSIONS
To investigate the implications of nanosheet dimensions on their chemical reactivity in the spontaneous reduction of HAuCl 4 , obtaining nanosheets of well-defined size is crucial. Here, size selection was carried out by a cascaded centrifugation protocol. The resulting sediments fractionated at successively higher centrifugation speeds from such protocols are shown to systematically decrease in lateral dimensions and thickness (Backes et al., 2016a(Backes et al., , 2016b(Backes et al., , 2016c(Backes et al., , 2017. To determine the dimensions of the size-selected nanosheets, various methods were employed. Figure 1A shows the x-ray diffractogram of nanosheets obtained at successively progressing sedimentation speeds from 1.2 krpm to 5 krpm. The intensity of the (002) reflection of MoS 2 appearing at $14.4 is found to reduce relative to other characteristic material peaks and increase in full-width at half maximum ( Figure S1), which further indicates a reduction in nanosheet thickness separated at higher centrifugation speeds. The extinction spectra of MoS 2 as shown in Figure 1B contain characteristic peaks from exciton transitions arising from splitting of the valence band at the high symmetry point. This, along with the Raman spectra ( Figure S2) indicates retention in the chemical nature of MoS 2 after exfoliation and re-dispersion in water. Through extensive experimental corroborations between UV-visible spectroscopy and atomic force microscopy, the iScience Article position of the A-exciton transition (observed at $ 670 nm) has been correlated to the number of layers through an empirical formula (Backes et al., 2014). To determine the number of layers, the double-derivative of the A-exciton peak with respect to incident wavelength was fitted with Lorentzian profiles, as shown in Figure 1C. The number of layers obtained by this metric suggested the separation of thick nanosheets between 1 and 1.2 krpm to comprise $21 layers, subsequently reducing to $17 layers between 1.2 and 2.5 krpm, $14 layers between 2.5 and 3 krpm, and $8 layers between 3 krpm and 5 krpm, as represented in Figure 1D. A general trend observed in liquid exfoliation is the production of thicker nanosheets with larger lateral dimensions, which reduce in size as they get thinner. To determine the lateral dimensions of the nanosheets, field emission scanning electron microscopy (FESEM) image analysis, as well as dynamic light scattering (DLS) were carried out. FESEM images of the size-selected nanosheets were analyzed using ImageJ software to obtain distribution histograms. For the sample obtained between 1.2 and 2 krpm, the obtained distribution is as shown in Figure 1E, along with a lognormal fitting. The hydrodynamic diameter inferred from DLS for this sample is as shown in Figure 1F. Respective plots for the other sizes are represented in the supporting information ( Figure S3). The lateral sizes as determined by FESEM statistics were found to be 290 G 95 nm (1-1.2 krpm), 217 G 70 nm (1.2-2.5 krpm), 178 G 56 nm (2.5-3 krpm), and 137 G 31 nm (3-5 krpm). Similarly, by DLS, the hydrodynamic diameters were determined to be 352 nm (1-1.2 krpm), 219 nm (1.2-2.5 krpm), 198 nm (2.5-3 krpm), and 161 nm (3-5 krpm). Shown in Figure 1G are the lateral dimensions of the four samples, showing good agreement between both techniques. Thus by using centrifugation based separation protocols in a cascaded sequence, well size-separated dispersions can be obtained with reduced polydispersity. For ease, the samples are hereby referred to as extra-large (XL-MoS 2 , 1-1.2 krpm), large (L-MoS 2 , 1.2-2.5 krpm), medium (MÀMoS 2 , 2.5-3 krpm), and small (S-MoS 2 , 3-5 krpm) accordingly. In addition to probing size, the zeta potentials of the nanosheets were obtained in aqueous media ( Figure S4). The nanosheets in all samples were found to bear a negative charge which could be arising from the dangling bonds along the edges, resulting in dispersion stability (Coleman, 2009).
Upon obtaining dispersions with controlled and well-characterized nanosheet size, we carried out Au nanoparticle decoration through a spontaneous reduction of HAuCl 4 using MoS 2 . This reaction is enabled through a suitable alignment of the energy levels between MoS 2 and AuCl 4 À , which results in a spontaneous electron transfer (Grieger et al., 2020;Kim et al., 2013;Polyakov et al., 2014;Song et al., 2018;Yuan et al., 2019). As also observed for other 2D materials, the reduction occurs preferably at defect sites owing to favorable energetics Tao et al., 2021), and results in the growth of Au nanoparticles. Here, we show extensive characterization for the size-selected MoS 2 nanosheets. The UV-visible extinction spectra of MoS 2 dispersions when mixed with increasing amounts of HAuCl 4 are as shown in Figure S5, showing a heightening intensity of the Au plasmonic peak around 550 nm, which is red-shifted with increasing Au concentration. By tuning the reaction concentrations, the plasmonic activity of Au nanoparticles prepared using this method can be suitably tuned (Lin et al., 2020). In order to probe the effect of nanosheet dimensions on HAuCl 4 reduction, the size-selected MoS 2 nanosheets were reacted with 2 and 4 equivalents of HAuCl 4 , taken in equal volumes of 1 mL. The x-ray diffractogram of the decorated MoS 2 nanosheets (obtained by reacting with 4 equivalents of HAuCl 4 ) are as shown in Figure S6, suggesting crystallinity of the nanoparticles. In the case of XL-MoS 2 and L-MoS 2 , the (002) peak from MoS 2 presents itself along with reflections from the Au lattice. These observations of reducing agent-free crystalline nanoparticle growth on MoS 2 nanosheets occur at room temperature and without the use of demanding synthetic conditions, thus strengthening the capabilities of aqueous dispersions in performing modifications.
Transmission electron microscopy (TEM) was used to evaluate the decoration density and crystallinity of the Au nanoparticles grown on the nanosheets. The nucleation and growth of the Au nanoparticles is dependent on the chemical activity of the nanosheets and by the amount of Au precursor available for reaction. In order to probe the effect of nanosheet size on these aspects, we have analyzed micrographs obtained using TEM. Shown in Figures 2A and 2B are representative TEM images of the XL-MoS 2 nanosheets reacted with 2 and 4 equivalents of HAuCl 4 , respectively. Similarly, shown in Figures 2C and 2D are representative images of S-MoS 2 nanosheets decorated under similar concentrations. An increased decoration density was observed at 4 equivalents as compared to 2 equivalents, which could be due to the higher availability of Au precursor for the reaction. Similar observations can be made for both XL-MoS 2 and S-MoS 2 with respect to the reaction concentrations. On comparing the decoration based on nanosheet size, a much higher decoration density is found to have occurred in S-MoS 2 as compared to XL-MoS 2 for both 2 and 4 equivalents of HAuCl 4 . As HAuCl 4 reduction is observed to occur preferentially along edges and defects Polyakov et al., 2014), its higher density in smaller nanosheets could be the driving factor ll OPEN ACCESS iScience 25, 104120, April 15, 2022 iScience Article for the reaction and subsequent Au nanoparticle decoration. In addition to the decoration density of Au nanoparticles populating the nanosheets, a further evaluation of the effect of nanosheet dimensions on nanoparticle size was performed by comparing the statistical histograms of $350 nanoparticles ( Figure S7). The Au nanoparticles grown on XL-MoS 2 were found to have a slight increase as reaction concentrations increased from 2 to 4 equivalents, with mean diameters of 2.35 and 2.67 nm, respectively. As for the case of S-MoS 2 , slightly larger Au nanoparticles with a mean diameter of 3.35 nm for 2 equivalents and 3.69 nm for 4 equivalents were found, respectively. In the case of S-MoS 2 , some large as well as free Au particles were also observed ( Figure S8). Thus, Au nanoparticle decoration on MoS 2 nanosheets through the spontaneous reduction of HAuCl 4 is found to be dependent not only on the precursor concentration but also on the MoS 2 nanosheet dimensions, suggesting their size-dependent reducing ability. Figure 2E shows a representative high-resolution TEM image of Au-decorated MoS 2 . The crystallinity of Au nanoparticles (circled in yellow) can be inferred from the periodic spacing which was found to be 0.24 nm, and can be indexed to the (111) plane. The periodicity in the supporting MoS 2 nanosheet can also be seen (as shown in red) with a spacing of 0.27 nm, which can be indexed to the (100) plane. This periodicity for both MoS 2 and Au is also observed in the Fourier transformed image (inset) as well. A processed magnified inset of the region marked by a square shows the hexagonal lattice formed by the Mo and S atoms. The lattice spacing observed through high-resolution TEM images is in agreement with the selected area electron diffraction (SAED) and x-ray diffraction patterns, thereby confirming the crystalline nature of both MoS 2 and the Au nanoparticles. Similar observations on nanosheet and nanoparticle crystallinity were observed for both investigated nanosheet sizes.
In order to further investigate the extent of Au decoration on the size-selected nanosheets and their impact on light-matter interactions, we explored the use of the decorated nanosheets as substrates for surfaceenhanced Raman scattering. Plasmonic nanoparticles have been shown to enhance the intensity of Raman signals, even down to single molecule level by enhancing the local electromagnetic field, while non-plasmonic semiconducting platforms can also result in enhancements through transitions occurring via charge transfer (Camden et al., 2008;Lee et al., 2016;Birke, 2012, 2014;Sun et al., 2014). A dependence of phase of the transition metal dichalcogenide has also been shown to dictate the extent of decoration, and also affect the contributions of chemical enhancement (Tao et al., 2021). To investigate the

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iScience 25, 104120, April 15, 2022 5 iScience Article contrasting decoration densities observed by TEM, the decorated size-selected nanosheets were used as substrates for SERS with methylene blue dye as a reporter molecule.
The SERS activity of the substrates was evaluated by determining the enhancement factors (EF), and a comparison was made between the size-selected nanosheets decorated to different densities. The EF for an SERS substrate is calculated using the formula,

EF = (I SERS /I Raman ) 3 (N Raman /N SERS )
where, I SERS and I Raman represent the intensity of the SERS signal from the reporter molecule on the Au-MoS 2 substrate (taken at a concentration of 10 mM), and the intensity of the reporter molecule powder, respectively. N Raman and N SERS represent the number of molecules in the powder, and on the SERS substrate under illumination, respectively. Here, the Raman peak at 1624 cm À1 was considered for calculations, which arise from the stretching of the C-C bonds in the aromatic ring (Nicolai and Rubim, 2003;Xiao and Man, 2007). Further details regarding the calculation of the EF are provided in the supporting information.
The spectra of methylene blue on various sample substrates are presented in Figure 3A. It was observed iScience Article that with denser Au nanoparticle decoration, the intensity of the Raman signals was found to be increased. In addition to this, smaller nanosheets reacted under similar conditions to that of larger nanosheets were found to have higher intensities. The enhancement factors were calculated, and are as represented in Figure 3B. For XL-MoS 2 (2 eq.), the enhancement factor was calculated to be 2.15 3 10 5 (G1 3 10 4 ), while for S-MoS 2 (4eq.) the value was found to be approximately 7 times higher at 1.55 3 10 6 (G26.9 3 10 4 ). S-MoS 2 reacted with 2 equivalents had an enhancement factor which is $2.2 times higher than that of XL-MoS 2 reacted with 4 equivalents. Such enhancement in EF is a result of optimized Au precursor concentration and the size of MoS 2 nanosheets. The EF calculated here through size-selection of MoS 2 nanosheets is in par with previously reported protocols, although to note that it is achieved via an all solution route without the need for demanding synthetic conditions or chemicals Lv et al., 2020;Su et al., 2014;Tegegne et al., 2020).
The peak intensity for lower concentrations of methylene blue was investigated on the S-MoS 2 (4 equivalents) substrates as shown in Figure 3C, and were found to detect concentrations as low as 1 nM. The dependence of peak intensity on analyte concentration is as shown in the inset of Figure 3C. In order to find the variation in intensity across various positions on the substrate, multiple areas were probed (40 points). The normalized peak intensity at 1624 cm À1 for all points is as represented in Figure 3D, demonstrating a relative standard deviation (RSD) of only about 17.2%. In addition, the stability of the Au-MoS 2 substrates was also investigated. By comparing freshly prepared substrates to 5-month-old substrates stored in ambient conditions, the signal intensities were found to be highly reproducible and showcase the stability of the decorated nanosheets ( Figure S9). These observations suggest the plausible usage of these substrates in analyte detection.
To evaluate the influence of plasmonic noble metal nanoparticles on the semiconducting MoS 2 platform, their applicability in photodetection was studied. The Au decoration was carried out as depicted in the schematic in Figure 4A. MoS 2 dispersions were spray coated onto Au gap electrodes on glass substrates, which were then dipped into HAuCl 4 solution for different intervals of time in order to carry out nanoparticle decoration. The interest is to obtain high-performance photodetectors with as low Au metal loading. Optimization of the dipping time is essential to prevent a direct pathway for conduction through interconnected Au nanoparticles that could result in electrical shorting, while also ensuring as minimal Au metal content as possible. Presented in Figure S11 are the photoresponses of devices comparing bare MoS 2 to those decorated for 1 and 2 min, for both XL-MoS 2 and S-MoS 2 . Higher dark currents were observed in XL-MoS 2 nanosheet films as compared to S-MoS 2 nanosheet films and could possibly be due to the lesser number of inter-nanosheet junctions in larger nanosheets films (Kelly et al., 2021). Upon illumination, higher photocurrents were observed in bare XL-MoS 2 (18.5 pA) as compared to bare S-MoS 2 (3.8 pA). After 1 min of dipping the XL-MoS 2 and S-MoS 2 devices, the photocurrent was found to be significantly enhanced to 600 and 352 pA, respectively. However, after 2 min of dipping, XL-MoS 2 shows a marginal rise to 619 pA, whereas S-MoS 2 further increased to 676 pA. The improvement in photocurrents seen in S-MoS 2 as compared to XL-MoS 2 could be a result of the higher reducing ability which leads to denser Au nanoparticle decoration. Therefore, further optimizations with respect to dipping time were carried out with S-MoS 2 alone. It was found that a dipping time of 5 min produced a significant improvement in photocurrents, beyond which the performance reduced and could be coming from electron-hole recombination in the films. The I-V characteristics for S-MoS 2 decorated by dipping for 5 min are as shown in Figure 4B. Significantly higher currents were observed under illuminated conditions. Compared to the bare MoS 2 device (inset), the dark currents were also considerably higher, suggesting decreased resistance from the Au nanoparticle decoration. Photocurrent responses obtained under increasing bias potentials are shown in Figure 4C. The photocurrent response as a function of incident optical power was evaluated at a bias potential of 15 V and the figures of merit for photodetectors calculated. The photocurrent exhibited a dependence on optical power with a factor of 0.31. The responsivity and photocurrents at different incident optical power are as represented in Figure 4D. The responsivity was found to vary exponentially with the incident power by a factor of À0.68. At an optical input of 1.46 mW mm À2 , the device exhibited a responsivity of 367.5 mA W À1 , and a specific detectivity of 2.95*10 8 Jones. The response time of the photodetectors was also probed ( Figure S12). The rise time was found to be 17.8 ms and the decay time was found to be 16.8 ms. A comparison of the figures of merit for our photodetectors and literature reported photodetectors are provided in the supporting information (Table S1). The devices demonstrated here perform superior to reported MoS 2 based photodetectors with Au nanoparticles in terms of responsivity and response time ( iScience Article Comparisons with reports on solution-processed transition metal dichalcogenide-based photodetectors have also been made (Abid et al., 2019;Alzakia et al., 2020aAlzakia et al., , 2020bKim et al., 2021;Li et al., 2020a;Liu et al., 2019;McManus et al., 2017;Pulikodan et al., 2020;Seo et al., 2019).
A few of the plausible mechanisms through which enhancement in device performance is observed through Au nanoparticle decoration are as outlined in Figure 4E. The plasmonic resonance from the Au nanoparticles could be increasing the optical absorbance as compared to bare MoS 2 , as suggested by the UV-visible absorbance spectra ( Figure S5). Additionally, the fields generated between plasmonic nanoparticles that produce an electromagnetic enhancement in SERS could be operating in an analogous fashion by greatly amplifying the localized electric fields in the semiconducting MoS 2 nanosheets, thereby enhancing the photogeneration of electrons upon irradiation (Lin et al., 2013;Miao et al., 2015). Besides these plasmonic-induced effects, the Au nanoparticles that are rooted to the underlying nanosheet substrates form a good interface for charge transfer from the optically active MoS 2 nanosheets (Singha et al., 2018;Sreeprasad et al., 2013). This is supported by observations of shifted (and enhanced) Raman peaks after decoration as shown in Figures S5 and S10. This was found to be more prominent in the case of S-MoS 2 as compared to XL-MoS 2 . The presence of the metal nanoparticles also substantially increases the electrical conductivity of the film, when compared to bare MoS 2 nanosheets as observed from the I-V characteristics. The photoresponse of MoS 2 photodetectors was thus improved by a great margin through Au nanoparticle decoration through a facile process of dipping the device in HAuCl 4 solution. The synthetic protocol reported here for Au nanoparticle decoration was also found to be applicable to other aqueously dispersed transition metal dichalcogenides such as MoSe 2 , WS 2 , and WSe 2 , suggesting the broad scope for modifying 2D materials (Figures S13 and S14). Crystalline nanoparticles were found to grow spontaneously on the nanosheet surfaces as inferred from x-ray diffractograms as shown in Figure S13A. The absorption characteristics are similar to that of MoS 2 and the presence of the plasmonic nature is attributed to the formation of the Au nanoparticles ( Figures S13B and S13D). The Raman spectra of the decorated nanosheets also suggested a retention in their chemical nature (Figures S13E-S13G). Transmission electron microscopy further confirms the decoration of the nanosheets with crystalline Au nanoparticles ( Figure S14). The investigations presented here focus on the scope for modification of MoS 2 nanosheets for optoelectronic applications. By utilizing size-selected nanosheets for the spontaneous chemical reduction of chloroauric acid, Au nanoparticle decoration on the nanosheets was obtained in aqueous medium under ambient conditions without the use of reducing agents. Crystalline nanoparticle growth with varying decoration densities was observed through investigations by electron microscopy and SERS, suggesting a size dependent chemical nature of MoS 2 nanosheets. Smaller decorated nanosheets were found to exhibit 7-fold higher enhancement factors as compared to the larger nanosheets, reaching as high as 1.55 3 10 6 . The implications of Au nanoparticle decoration on MoS 2 nanosheets in photodetection were evaluated through solution routes, by dipping spray coated MoS 2 devices into a solution of HAuCl 4 . Similar to the observations from SERS, smaller nanosheets were found to be affected profoundly. Optimized Au metal loading yielded devices with a fast response time of $17 ms, while achieving a responsivity of 367.5 mAW -1 . These findings suggest the scope for suitable modifications of various 2D materials through spontaneous metal nanoparticle formation together with size-control could enhance their performance in various applications using solution-processed techniques.

Limitations of the study
The protocol for Au decoration outlined in our work relies on the higher defect density in smaller nanosheets, although in order to gain a further understanding into the underlying phenomenon, the kinetics of nucleation and growth need detailed investigation. Theoretical studies may in this case provide a deeper understanding in to the observed higher activity of smaller nanosheets as compared to larger nanosheets. In order to investigate the implications of decoration density, SERS has been used as a tool here, although the underlying factors warrant a detailed investigation in order to discern electromagnetic and chargetransfer contributions. Enhanced photoresponse of nanosheets with Au nanoparticle decoration has been evaluated in this work; however, there is a scope for further optimization for better figures of merit in solution-processed photodetectors, to be in par with photodetectors obtained using other synthetic strategies such as chemical vapor deposition and mechanical cleavage.

STAR+METHODS
Detailed methods are provided in the online version of this paper and include the following:  To obtain the number of molecules excited in this volume, N Raman = V * r MB * N A /M MB Where r MB is the density of methylene blue (1757 kg m À3 ), N A is the Avogadro number and M MB is the molecular weight of methylene blue (319.9 g mol À1 ).
The area of one methylene blue molecule = 0.8723 nm 2 The area under the laser spot is = pr 2 = 1.9 * 10 À12 m 2 Therefore, the number of molecules under illumination (assuming monolayer coverage on the substrate) is found to be = 1.9 * 10 À12 m 2 /0.8723 nm 2 = 2.18 * 10 6 By substituting these computed values, a factor of 43.4 was obtained for multiplication with the observed I SERS to calculate the value of EF.

Solution-processed photodetectors
Photodetectors were fabricated by spray coating 5 mL of dispersions (0.2 mg mL À1 ) of the undecorated MoS 2 nanosheets onto glass slides with Au gap electrodes. The substrates were held at 80 C on a hotplate at a distance of 10 cm, and spraying was carried out at a pressure of 10psi. The spray coated devices were then decorated with Au nanoparticles by dipping the substrates in to a solution of 1 mM HAuCl 4 for a defined period of time. After dipping, the substrates were immediately washed with running deionized water and dried on a hotplate held at 80 C and later stored under dessication. The electrical characterization of the devices were carried out on a Keithley 4200SCS paired with a Cascade Microtech EPS-150 probe station. A 532 nm laser was used as the light source.