Synthesis of Micrometer Length Indium Sulfide Nanosheets and Study

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Synthesis of Micrometer Length Indium Sulfide Nanosheets and Study of Their Dopant Induced Photoresponse Properties Shinjita Acharya,† Mrinal Dutta,‡ Suresh Sarkar,† Durga Basak,‡ Supriyo Chakraborty,§ and Narayan Pradhan*,† †

Centre for Advanced Materials & Department of Materials Science, ‡Solid State Physics, §DST Unit of Nanoscience and Technology, Indian Association for the Cultivation of Science, Jadavpur, Kolkata S Supporting Information *

ABSTRACT: Synthesis of various nanostructured semiconductor materials and processing them for different device fabrications has been at the forefront of research for the last two decades. In comparison to spherical nanoparticles, anisotropic materials e.g. nanorods, nanowires, and nanodisks have been widely explored to obtain a better performance of the devices. In addition, it is also well-known that nanomaterials, on doping with suitable impurities, can enhance the device sensitivity and speed. Combining both, we report here the synthesis of micrometer long In2S3 nanosheets and on doping them with Cu(I), we have studied here their photoresponse properties. These nanosheets are synthesized in a high temperature colloidal method following a catalytic thermal decomposition of a single source precursor of In and S. From various TEM, HRTEM, and HAADF images the growth pattern of these sheets is investigated, and the obtained moiré fringes at the overlapped region are discussed. Finally, the comparative study of the device performance has been carried out with introducing different amounts of copper in these nanosheets. KEYWORDS: nanosheets, indium sulfide, doping, intrinsic vacancy, photodetector

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or two dimensions.14,25,27−31Additionally, due to the defect spinel structure of β-In2S3, it acquires intrinsic vacant sites in the lattice; such vacancies exhibit electron affinity and can act as electron traps.32 This interesting defect structure of β-In2S3 has also paved the way for its application in photovoltaics. It is used in making green or red phosphors for color televisions, dry cells. Recently, it has also been reported that β-In2S3 can replace highly toxic CdS in the buffer layer of solar cells with almost equivalent efficiency.33−36 Hence, the fabrication and designing of different shapes of In2S3 nanomaterials is highly desirable. There are several reports of β-In2S3 in the literature with different morphologies like nanoplates, urchinlike microspheres, dendrites, hollow microspheres consisting of nanoflakes and nanobelts, porous 3D flowerlike structures and nanorods.14,20,25,27−30,37−39 Recently, a report on the synthesis of highly uniform platelet shaped 2D nanostructure has appeared,14 and after doping with Cu this has also been explored for the device fabrication.11 Doping of different suitable foreign ions and creating an electron or hole carrier vacancy in the host has been a known phenomenon.11,40 For In2S3, Cu+ can act as an appropriate dopant with proper affinity for getting incorporated in its lattice to boost the electrical

INTRODUCTION Anisotropic semiconductor nanomaterials have generated a lot of interest in the field of materials science because of their unique patterning ability to influence the physical and electronic properties compared to the spherical particles in nanodimension.1−4 Correlation between the colloidal synthesis of these anisotropic nanostructures and exploiting them for various applications ranging from optical to electronic remains in the frontier research area in recent days.5−10 Among these, 2D semiconductor nanostructures are especially important because of their large surface area and long-range of lattice periodicity that provides a better channel for carrier transportation, enhancing the efficiency of the devices.11−13 Hence, significant research emphasis is on the rise to synthesize such 2D nanostructures14−19 and their implementations in various device based applications.11,12 Over the past few years, group III−VI semiconductor nanomaterials, of which indium sulfide (In2S3) has widely been studied as they are lucrative materials for catalysis, photovoltaics, and solar cells along with other optoelectronic device based applications.20−22 In2S3 exists in three different crystalline forms as a function of temperature: α-In2S3 (defect cubic), β-In2S3 (defect spinel), γ-In2S3 (layered structure).23 Among these, β-In2S3 is an n-type semiconductor with a band gap of 2.0−2.3 eV which is the stable form at room temperature and exists in either cubic or tetragonal crystal structure24−26 and manifests highly anisotropic crystal growth either along one © 2012 American Chemical Society

Received: January 27, 2012 Revised: April 22, 2012 Published: April 26, 2012 1779

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Fabrication of the Device. To make the photodetectors, first the undoped and doped In2S3 nanocrystals dispersed in toluene (2 mg/ mL) were spin coated on precleaned ITO substrates with increasing speed (from 100 to 1000 rpm) for 40 s. The process of spin coating was repeated until the desired thickness ∼1.5 μm measured by Dectak Stylus 6 M profilometer was obtained. These films were then baked at 140 °C for 10 min. Sputter deposited Au electrodes (10 nm) of 1 mm diameter were made on the top surface of the films and annealed at 120 °C for 30 min. Characterizations. TEM images and EDS spectra were taken on a JEOL-JEM 2010 electron microscopy using a 200 kV electron source. TEM images on STEM (HAADF) were taken on a UHR-FEG-TEM, JEOL; JEM 2100 F model using a 200 kV electron source. Specimens were prepared by dropping a drop of nanocrystals solution in chloroform on a carbon coated copper grid and the grid was dried under air. Thermogravimetric analysis (TGA) has been performed with a TA thermal analysis system at heating rate 10 °C/min under N2 environment. XRD of the samples were taken by Bruker D8 Advance powder diffractometer, using Cu Kα (λ = 1.54 Ǻ ) as the incident radiation. For AFM measurement, a diluted purified sample was deposited on a clean mica surface and AFM was measured VEECO dICP -II autoprobe (model AP 0100). The Cu-dopant percentage was determined by ICP-AES using Perkin−Elmer Optima 2100 DV machine. After removing the unreacted precursors by severe washings with chloroform and methanol, the purified nanocrystals were redispersed in chloroform. The chloroform was then evaporated out. Then the dried nanocrystals were digested in concentrated HNO3. The nitric acid solution of the sample was diluted with double distilled water to carry out the measurement I−V Measurements. The I−V characteristics between Au and ITO electrodes were measured using a Keithley 2400 series source meter. The photocurrents were measured under illumination of 400 nm light which was obtained by using a 300 W Xe arc lamp (Oriel) and a monochromator (of model number 74125). The photocurrent transient responses were recorded by illuminating the sample with 400 nm light for 5 s and then switching off the light for another 5 s under the applied bias of 3 V. All measurements were carried out in ambient condition.

properties or more specifically device performance.11,41 Hence, in this respect doped 2D In2S3 nanostructures are one of the ideal materials. However, for doping, materials with high crystallinity are required. Hence, it is important to design wide area grown crystalline 2D In2S3 nanomaterials to address these issues. Exploring the polar organic long chain fatty amines as the capping agent, solvent, and employing catalytic thermal decomposition of a single molecular precursor of indium, we report here the fabrication of two dimensionally grown β-In2S3 nanosheets expanding over micrometer length. These sheets are single crystalline, dispersed in solution, and do not show wrinkling. The crystallinity of these sheets has been supported with XRD, HRTEM, and from the obtained moiré fringes at the overlapped regions. Further, a one-pot postdoping strategy has been employed to introduce Cu+ ions into these highly crystalline sheets, and finally, their photoresponsivity has been studied by fabricating a simple thin film based device.

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EXPERIMENTAL SECTION

Materials. Indium(III) chloride anhydrous (99.999% - In) (InCl3), sodium diethyl dithiocarbamate trihydrate (Na-DDTC), octadecylamine (ODA, 97%), 1-hexadecylamine (HDA, tech., 90%), elemental sulfur powder (S, 99.98%), indium(III) acetate (99.99%-In) (InAc3), 1-octadecene (ODE, tech., 90%), oleylamine (70%), and ITO substrates were purchased from Sigma Aldrich. Copper(II) chloride dihydrate crystals (CuCl2·2H2O) were purchased from Merck chemicals. All the chemicals were used without further purification. Preparation of Indium-Tris Diethyldithiocarbamate (InDDTC) Precursor. One mmol (0.225 g) of InCl3 and 3 mmol (0.676 g) of Na-DDTC were separately dissolved in methanol and stirred for 10 min to get a clear solution of each. Both of the solutions were mixed together with constant stirring which resulted in a white precipitate of the desired complex of In-DDTC at room temperature. The precipitate was filtered, washed, and dried for further use in synthesis. Preparation of Cu Stock Solution. 0.5 mmol (0.089 g) of CuCl2·2H2O was dissolved in 5 mL of oleylamine solution and heated in inert atmosphere in ambient condition to get a bluish green color solution. Reactions were done taking a suitable amount from this stock. Synthesis of In2S3 Nanosheets. 0.1 mmol (0.056 g) of the InDDTC complex was loaded in a three necked flask along with 4 g of hexadecylamine (HDA) or octadecylamine (ODA). The mixture was degassed by purging N2 gas for 15 min to create an inert atmosphere. The temperature was gradually increased to 200 °C, and the solution turned colloidal. The formation of In2S3 was indicated by the formation of the yellow colloidal solution which was annealed first at the same temperature for ∼30 min and then at temperature of 300 °C for another 2 h. Then the solution was cooled down to room temperature, and nanocrystals were purified several times by washing with acetone and chloroform. Further, all the characterizations have been carried out with this purified sample. TEM characterization revealed the formation of long-range uniform nanosheets under this reaction conditions. Synthesis of Cu-Doped In2S3 Nanosheets. For 2% doping of Cu, 0.04 mL from the Cu stock solution was gradually added to the as prepared solution of In2S3 nanosheets, without any purification of the sheets. The injection spanned over a time period of 15 min. The color of the solution slowly changed from yellow to red maintaining the colloidal nature of the reaction medium. In the case of 10% doping, 0.2 mL of the Cu stock solution was added following the same method which resulted in a partly clear solution. The temperature for Cu precursor addition was 220 °C in both cases. After Cu addition in both cases, the reaction was annealed for 30 min more before cooling it down to room temperature. Then the product was obtained following the usual purification method and characterized further by TEM and XRD measurements.

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RESULTS AND DISCUSSION Synthesis and Characterization of 2D In2S3 Nanosheets. Single molecular precursor indium-tris diethyldithio-

Figure 1. Schematic presentation of the synthetic protocol for obtaining organic ligand capped In2S3 nanosheets.

carbamate (In-DDTC) has been utilized to obtain In2S3 sheets (experimental details provided in the Experimental Section). The thermogravimetric (TG) analysis shows that In-DDTC has the decomposition temperature (TD) at 265 °C.42 But it has been observed that the decomposition temperature reduces to as low as 120 °C (Figure S1) in the presence of alkylamine (HDA). This has encouraged us to decompose the precursors and formation of In2S3 at lower temperature to achieve the control of the anisotropic growth of these nanosheets grown over a large area. TG plots of the In-DDTC complex and only HDA has also been provided in the Supporting Information as reference (Figure S2). In a typical synthetic process, when InDDTC is allowed to decompose at 200 °C in hexadecylamine 1780

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STEM image of overlapped sheets. From these images it has been observed that these sheets are ∼50−200 nm in breadth and lengths in a few micrometers. AFM measurements carried out on several samples and the obtained height profiles (Supporting Information, Figure S5) suggest that the thickness of these sheets is within 2 to 3 nm. Further, the detailed crystal structure of the sheets has been investigated by XRD, SAED, and HRTEM. Figure 3a shows the powder XRD of these sheets where most of the peak positions match with cubic β-In2S3. For comparison, indexed XRD peaks from ICSD no. 202353 are provided in the same figure. SAED of the sample shown in Figure 3b also supports the cubic structure where {022} planes having the d-spacing 0.38 nm have been labeled, but more confirmation of the crystal structure is obtained from the HRTEM analysis (Figure 3c and 3d). From the selected area FFT and inverse FFT of the HRTEM the calculated d-spacing (0.38 nm) suggests that these are also the {022} planes of cubic β-In2S3. Moiré Patterns in the Overlapped Region of the Sheets. As these sheets are crystalline, moiré fringes are seen in different overlapped area of the sheets.43 This observation also further strengthens the high crystallinity of these 2D nanosheets. Figure 4(a-d) demonstrates the HRTEM images obtained from different places showing the moiré patterns. This has also been observed in the TEM and HAADF images in Figure 2. Close view of Figure 4a and 4b clearly shows that these patterns are only obtained from the overlapped areas of the sheets. These fringes appear due to the interference of the ordered atomic lattice plane of multiple sheets overlapped in a particular angle as observed in the overlapped area of these sheets in HRTEM images. To know more about these patterns we have carried out the FFT (inset of Figure 4d) from the

Figure 2. (a, b, and c) Represent the TEM images of as synthesized In2S3 nanosheets. (d) Presents the HAADF-STEM image of the overlapped sheets.

(HDA) solvent and further annealed at 300 °C, 2D nanosheets of In2S3 over micrometer length are obtained. The formation of H2S gas even at 150 °C and the change of the solution color to yellow suggest the decomposition of the single source thiocarbamate precursors. The synthetic protocol has been depicted schematically in Figure 1 (and also Figure S3). Figures 2a, 2b, and 2c (Figure S4) show the TEM images of single and overlapped sheets. Figure 2d presents the HAADF-

Figure 3. (a) Presents the powder XRD of the nanosheets. The peaks are mostly overlapped with the bulk cubic β-In2S3 (JCPDF 320456). (b) Represents the SAED of the sheets. (c) and (d) Show the HRTEM images obtained from different sheets. Insets top and bottom in (c) and (d) are the FFT and inverse FFT respectively. The d-spacing of the obtained plane is 0.38 nm, and the planes are intersecting each other at 120 degrees. Hence these planes are {022} planes and the viewing axis is [111]. 1781

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Figure 4. (a) and (b) Show the TEM images of overlapped sheets showing moiré patterns on the overlapping region. (c) and (d) Show the HRTEM images from different overlapped regions. Insert of (d) is the selected area FFT of the HRTEM image in (d). (e) Presents the inverse FFT obtained after masking the spots in the FFT of the inset of (d). (f) Represents the inverse FFT considering the (022) planes of the FFT of the insert in (d). (g) Shows the schematic model of two sheets placed at an angle of 7 degrees and their overlap. The atomic placements are determined considering the cubic structure of In2S3. Insets blue and green images represent the inverse FFT of blue and green masked in the FFT (inset of f).

the free-standing 2D sheets, the reaction needs to be carried out in pure fatty amine solvent. Further, to understand the formation of these crystalline 2D nanosheets, intermediate samples were investigated. Interestingly, all the collected samples within 250 °C using HDA as the solvent are found to be amorphous in nature. TEM images of the samples collected at 200 °C are shown in Figure S8. As these are amorphous, their growth pattern could not be confirmed, but these amorphous structures are confirmed to be In2S3 by EDS analysis (Figure S9) which shows the In:S stoichiometric ratio of ∼1:1.5. This indicates that these structures are indium sulfide and not the unreacted precursors. This means the single source precursors decompose at low temperature (∼150 °C) in the presence of amine and lead to formation of amorphous In2S3 structures which on annealing at higher temperature (∼300 °C) slowly convert into crystalline 2D nanosheets. This transformation has been observed as a slow process and needs sufficient annealing time. When the nucleation for the crystalline sheets begins, the existing amorphous In2S3 in solution supply the monomers and facilitate the sheets to grow in the allowed 2D direction. As a result, highly crystalline β-In2S3 nanosheets are formed. Doping of Cu in the Sheets. These sheets are further explored by doping with copper and the subsequent optoelectrical transport properties measurements. We surprisingly found that introduction of up to 2% of Cu during the synthesis does not affect the shape and phase of the crystal and improves the speed of the photoresponse immensely. To study this, we

overlapped areas which indicate the presence of two planes. For more clarity, inverse FFT has been analyzed which shows the well resolved moiré fringes (Figure 4e) generated due to the (022) planes of two different In2S3 sheets overlapped with an angle of 7 degrees as shown in Figure 4f. A schematic presentation of two such atomic planes and the obtained moiré patterns in the overlapped region are shown in Figure 4g. Mechanism of the Formation of Nanosheets. Systematic analysis of the reaction progress reveals that the reaction medium is very much pivotal. It has been observed that the long-range growth of these nanosheets mostly occurs in fatty amine solvent. When it is diluted with an optimum amount (>20% by weight) of noncoordinating solvent like 1-octadecene (ODE), mostly rolled sheets or flakes are obtained (Figure S6) rather than the free-standing sheets obtained in pure amine solvent. Amines help in the decomposition of the precursors and act as capping ligands dispersing the nanosheets into the solution. An insufficient amount of these amines mostly affect the growth pattern or break the long sheets into fragments due to long time annealing that lead to rolled sheet or flakes like structures. Further, instead of the single source precursor, when different sources for indium and sulfur like In-carboxylates and elemental S respectively are used in alkyl amine, similar irregular nanostructures are observed (Figure S7). This proves that the morphology of these nanostructures remains under the control of solvent polarity, nature of the ligands (coordinating/ noncoordinating), and molecular precursors though the growth is always anisotropic irrespective of all the conditions. But for 1782

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Figure 5. Left Panel: I−V plots for undoped (a), 2% Cu-doped (b), and 10% Cu doped (c) In2S3 nanosheet films under dark and 400 nm monochromatic light illuminations. Right Panel: Temporal photoresponse for undoped (d), 2% Cu doped (e), and 10% Cu-doped (f) In2S3 nanosheets. The upward arrow (↑) and downward arrow (↓) represent the switching on and off step of the illumination respectively.

Table 1. Summary of Device Performance as a Function of Extent of Cu Doping amount of Cu (mole %) 0 2 10

dark current (Amp) 3.94 × 10 3.91 × 10 3.42 × 10

−6 −6 −7

photocurrent (Amp) 6.12 × 10 7.29 × 10 5.42 × 10

−4 −4 −6

have fabricated a very simple device by putting a thin film of the undoped (and doped) materials on ITO and making an Au electrode on the film (details in the Experimental Section). The SEM image of the film has been provided in the Supporting Information (Figure S10). Here, we have adopted a facile onepot postdoping strategy to dope Cu(I) into the nanosheets. First, In2S3 nanosheets are synthesized and without any further purification, CuCl2 dissolved in oleylamine is added into the In2S3 solution dropwise. An optimum concentration of the dopant Cu, which is 2% of the concentration of In precursor is chosen to be incorporated into the sheets that is low enough to facilitate insertion of Cu ions retaining the original sheet structure of the undoped In2S3 nanosheets as observed from the TEM image (Figure S11). We have also doped a higher amount of Cu (∼10%) into the sheet. In this case, overdoping partly destroys the integrity of the nanosheets as shown in Figure S12. In order to ascertain the incorporation of Cu dopant ions into the In2S3 sheet, we carried out ICP-AES measurements, and the copper amounts of 1.7% and 9.2% have been detected in both the samples respectively indicating the incorporation of copper into final products. The oxidation state of Cu in such cases of doping has been under debate for a long time until a few research reports threw light on them and showed that even if the Cu2+ precursor is used as dopant, it reduces to the Cu+ state in the presence of amine and gets incorporated in the +1 state

gain (Iphoton/Idark)

growth time (∼90%)

decay time (∼90%)

155 186 16

2.0 s 0.2 s 0.1 s

0.1 s 0.2 s 0.1 s

into the host lattice.11 We have further performed powder XRD on the optimized Cu (2%) doped sample. The obtained XRD pattern remains similar to the undoped sheets. This suggests that other copper compounds such as CuO or Cu2S are not obtained as the side products. Hence, we can assume here that the Cu+ ions are incorporated into the sheet structure rather than forming any separate phase. Study of Photoresponse Properties of Undoped and Doped Sheets. In Figure 5 we report the I−V characteristics (left panel) and temporal photoresponse (right panel) for the photodetectors made from the doped and undoped In2S3 nanosheets in the dark as well as under light illumination condition. The linear I−V curves confirm the Ohmic nature of the contacts for all the samples. Under illuminated condition, at all bias voltages, the current conduction is more compared to that under dark condition. As shown in Table 1, the photocurrent gain (IPhoton /Idark) in undoped In2S3 is 155, while the value is 186 for 2% Cu doping as calculated from the growth-decay curves in the right panel of Figure 5. By 2% Cu doping there is no significant change in the dark as well as photocurrents, and thus the gain remains almost in a similar order but with a slight increase in the magnitude. However, the value significantly decreases for the 10% doping case. This is possibly due to the creation of excess defects in the crystal lattice by Cu incorporation which act as the traps for the 1783

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Chemistry of Materials carriers.44 Due to formation of trap states, the current decreases in 10% doping sample.45 Here, it is important to note that incorporation of excess Cu in In2S3 indeed enhances the trap or defect states and dismantles the nanosheets to irregularly shaped nanostructures. The photodetector made from this Cudoped In2S3 (10% Cu) with an identical absorbance value as that of the undoped and 2% doped sheets and prepared under similar reaction conditions shows degraded performance: a low gain value. This can be probably attributed to the chances that the excessive concentration of Cu+ can cause competition to In3+ for S ions and thus deforming the sheet structure and degrading the overall device performance. A dramatic improvement in the photoresponse time (for 90% growth of photocurrent) has been noticed with doped samples. An order of magnitude decrease in the growth time has been observed in the case of both 2% and 10% Cu doping (Table 1) samples. The decay time (90% decay in the current) for all three samples however remains in the similar order of magnitude as shown in Table 1. The doping of Cu ions in the vacancies of defect spinel structure of In2S3 leads to a significant increase in the response time and consequently boosts the device speed. We can predict from literature, Cu+ doping narrows the band gap of In2S3 and decreases the distance between valence band-edge of the host and traps.11 Thus, we consider this as the reason for such an enhancement of the photoresponse speed which widens the opportunity of exploiting Cd or Pb free nanosheet structures for device based applications.

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ASSOCIATED CONTENT

ACKNOWLEDGMENTS

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REFERENCES

DST and CSIR of India are acknowledged for funding. S.A. and S.S. acknowledge CSIR, India for a fellowship. N.P. thanks LNJ Bhilwara for a fellowship.

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In summary, an efficient high temperature colloidal technique has been developed to synthesize single crystalline In2S3 nanosheets by decomposition of a single source In-DDTC precursor in fatty amine. Initially amorphous nanostructures with irregular shapes are formed, and on annealing at higher temperature these are transformed to 2D crystalline nanosheets. These amine capped nanosheets are observed freestanding, wrinkle free, and dispersed in solution. Furthermore, Cu has been incorporated into the In2S3 nanosheets varying its amount (0−10%) in the In2S3 nanosheets via a postdoping technique. Finally, comparative optoelectronic studies have been carried out, and it has been found that the In2S3 nanosheet is one of the most desirable materials among the metal sulfides to be implemented in photodetector and further enhancement of the photoresponse properties by 2% of Cu doping and has a better prospect in device applications.

S Supporting Information *

TG plots of HDA, In-DDTC, In-DDTC in HDA, digital scheme, EDS, TEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 1784

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dx.doi.org/10.1021/cm3003063 | Chem. Mater. 2012, 24, 1779−1785