Low temperature synthesis, optical and photoconductance properties of nearly monodisperse thin In₂S₃ nanoplatelets

Abstract

We report on the synthesis of nearly monodisperse In₂S₃ nanoplatelets ∼7 nm thick by a low temperature polyol process using 1-thioglycerol as the capping chemical. The synthesized nanoplatelets crystallized in bulk tetragonal structure and show preferential crystallographic growth within a range of diameters from ∼75–85 nm. Quantum confinement was observed by spectroscopic analyses with a determined band gap of 3.4 eV. Strong UV luminescence corroborated valence-conduction band transition in these confined nanoplatelets. Room temperature photoconductance measurements performed on the prototype device fabricated from the nanoplatelets revealed noticeable enhancement of conductivity under illumination as compared to the dark condition.

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Introduction

The preparation of monodisperse nanocrystals, passivated by organic molecules permits studies to distinguish truly novel properties obtained for nano-scale structures compared to their polydisperse counterparts.^1–6 Shape-controlled monodisperse nanocrystals have recently been in focus since building blocks for electronic and optical nano-scale devices often require assembly of nanocrystals with non-spherical morphologies.^7–12 In semiconductor materials, quantum confinement allows size-tunability in optical and electrical properties once a critical dimension threshold is attained.¹³ These effects have been studied in a wide range of materials systems,^2,3,7 among which chalcogenides are of particular interest. This interest has been fueled by the high degree of reproducibility and control that is currently available in the fabrication and manipulation of chalcogenide quantum-confined structures.^14,15 Among many important chalcogenide semiconductors, In₂S₃ is a promising optoelectronic material with superior photoconductive properties.^16,17 Due to its stability and nontoxicity, the material is also potentially being explored for use as a buffer layer in CuIn(Ga)S₂(Se₂) heterojunction solar cells,^18,19 and phosphors for color displays.^20–23 Within the different phases that exist for In₂S₃, the defect spinel type with a cation-ordered structure obtained in tetragonal form is the most important.^24–26 Tetragonal In₂S₃ is a typical n-type semiconductor with bulk band gap of 2.0–2.3 eV and a large Bohr exciton diameter (33.8 nm) which makes it a feasible candidate for the exploration of quantum confinement effects by varying shapes and sizes.^24,27 Although a large number of diverse morphologies in nanocrystalline forms of In₂S₃ have been investigated for shape-dependent versatile properties,^28–43 there are only a few reports on the synthesis and properties of monodisperse In₂S₃ nanocrystals.^44,45 Moreover, these In₂S₃ monodisperse nanocrystals are synthesized by arrested-precipitation method using organic reaction media at high temperatures and may need a ligand–exchange process to remove the organic surfactants from the surface of these nanocrystals.^44,45 However, in order to fabricate device using these In₂S₃ nanocrystals, ligand removal is likely to cause degradation of the quality of the nanocrystals.

In this paper we report the synthesis of nearly monodisperse, thin In₂S₃ nanoplatelets by 1-thioglycerol assisted, low temperature polyol process. Our simple synthesis approach allows production of the nanocrystals in high yield and quality in a single-step. We studied the room temperature optical properties of the dispersed colloidal nanocrystals and measured the electrical conductance behavior of the In₂S₃ nanoplatelets both in light and dark conditions. Though the photoconductivity properties of undoped and doped In₂S₃ single crystals and thin films have been studied well,^{16,17,46–50} to the best of our knowledge the photoconductance behavior of monodisperse nano sized In₂S₃ has not been reported before. We observed a strong quantum confinement from the optical measurements of the synthesized platelets due to their low thickness and the photocurrent showed noticeable enhancement as compared to that measured in the dark. We expect that the observed optical and photoconductivity behavior in these high quality nanocrystals will be of electronic grade for proficient optoelectronic applications.

Experimental section

Preparation of monodisperse In₂S₃ nanoplatelets

All chemicals were used as received without further purifications. The grades of the chemicals were all above ACS 99+%. In a typical synthesis process of In₂S₃ nanoplatelets, anhydrous InCl₃ (Alfa Aesar, USA) was first dissolved in 50 ml anhydrous ethylene glycol (Acros Organics, USA) with 2 ml 3-Mercaptopropane-1,2-diol (1-thioglycerol; Sigma Aldrich, USA) and the temperature was raised to 70 °C under vigorous stirring and reflux process. After 0.5 h, thiourea (Fisher Scientific, USA) was added to the solution of 1.5 times molar quantity to that of the dissolved InCl₃ and the reflux was continued under stirring. The temperature was increased to 140 °C and then held for 1 h. In the end, the solution was cooled down to room temperature and an equal volume of hexane/ethanol (Alfa Aesar, USA) was added to allow precipitation. The precipitate was collected by centrifugation and repeated washing with anhydrous ethanol, and finally vacuum dried. A yield of 0.8 gm was obtained per reaction batch.

Characterization

Microstructural characterization of In₂S₃ nanoplatelets was carried out by scanning electron microscopy (SEM, JEOL JSM 6390 LV) and high resolution transmission electron microscopy (TEM-HRTEM, FEI Tecnai F20 S-Twin TEM). Composition analysis was obtained by energy-dispersive X-ray spectroscopy equipped with the SEM (EDS, Oxford Instruments INCAX sight). X-ray diffraction (XRD) study was performed by Bruker AXS D8 diffractometer equipped with Lynx Eye position-sensitive detector. The optical characteristics of the specimen were examined by absorption and photoluminescence spectroscopies. Room temperature optical absorption spectra were recorded with a UV-vis spectrophotometer (Shimadzu) and steady-state photoluminescence (PL) spectra were recorded in a Fluoro Max-P (Horiba Jobin Yvon) luminescence spectrometer by dispersing the In₂S₃ nanoplatelets in ethanol.

For the measurement of photoconductance, the prototype device was fabricated on 300 nm thick Au sputtered 1 cm × 1 cm quartz (SiO₂) substrates. Before Au sputtering the quartz substrates were cleaned by ultrasonic bath using acetone and water and N₂ dried in vacuum. A CRC-100 sputtering system (Plasma Sciences Inc.) was used for the desired thickness of Au sputtering. A deep groove of ∼1 mm in diameter was scribed along the central portion of the Au-coated quartz substrate using a diamond tip glass cutter, in order to form a discontinuation in the Au coating on the substrate. Care was taken not to scratch the Au coating on both sides of the groove. The Au films on both sides of the groove then served as the electrode pads. Electrical conductivity was checked both before and after the groove was made to ensure electrical continuity on the substrate. In₂S₃ nanoplatelets were mixed with ethanol to obtain a smooth thick paste. Then, the paste was coated on the non-contact area (i.e. groove) and uniformly spread on both sides of the Au electrodes. The paste was allowed to dry naturally in air to form a thin film of In₂S₃ nanoplatelets with a typical thickness of 0.2 mm. The contacts on the film were made laterally with a separation of ∼2 mm on the Au electrodes for photocurrent measurement. A two probe contact configuration was created and the device was placed in a vacuum chamber. A dc source was connected between the two Au electrodes and the I–V measurements were performed both in dark and light conditions with an Agilent 34405A 51/2 digit digital multimeter and Keithley 2400 voltage sourcemeter at room temperature.

Results and discussion

Fig. 1(a) shows a typical XRD spectrum of In₂S₃ nanoplatelets with well defined crystalline peaks characteristic of tetragonal In₂S₃ (JCPDS Card No. 25-0390). As evident from the pattern, two sets of crystalline peaks were observed in the nanocrystals, one of which is perpendicular to the b-axis (planes (109), (206), (00) and (10)) and the other parallel to the plane (116) (planes (116) and (22)) of tetragonal In₂S₃. By calculating the full width at half maxima of the peaks and employing Debye–Scherrer formula, the crystallite sizes (diameters) were calculated for peaks (10) and (22) to be 70 nm and 80 nm, respectively. Microstructural study of the synthesized product by SEM was performed by depositing few monolayers of nanocrystals on copper coated glass substrates from a dilute dispersion of the nanocrystals in acetone with ∼0.08 absorbance units of optical density at 225 nm (photograph of the dispersed platelets shown in the inset of Fig. 1(a)). The washed and dried In₂S₃ nanoplatelets powder was lime-yellow in color and was easily dispersible in acetone, ethanol and water. Fig. 1(b) shows the SEM image of the deposited thin film of In₂S₃ nanocrystals indicating that the nanocrystals are round and plate-like in shape with a range of lateral diameters from ∼75–85 nm with a std. deviation of ≤5%. EDS spectra collected from over 40 to 50 different nanocrystals confirmed the chemical composition of In₂S₃. Average In: S atomic ratio of 2 : 2.9 (std. deviation ± 0.1) recorded from the nanocrystals indicate near stoichiometric composition of our In₂S₃ specimen. A representative EDS spectrum is shown in the inset of Fig. 1(b).

Fig. 1 (a) Powder XRD spectrum of In₂S₃ nanoplatelets. The inset shows the photograph of the stable dispersion of the nanoplatelets in water. (b) SEM image of few layers thick In₂S₃ nanoplatelets deposited on Cu coated glass substrate. The inset shows a representative EDS spectrum of In₂S₃ nanoplatelets. The average In:S atomic ratio obtained from the several EDS spectra is 2 : 2.9, which is near stoichiometric.

Low magnification TEM image of the In₂S₃ specimen (Fig. 2(a)) revealed that the nanocrystals are almost round, nanoplatelet-like in shape and have nearly monodisperse distribution. The size distribution of the nanoplatelets is represented in a histogram shown in the inset of Fig. 2(a). The average diameter of these nanoplatelets was around 85 nm and conforms to our calculations from the XRD and SEM analyses. A higher resolution TEM image from the zone marked with an orange square is shown in Fig. 2(b). The monodispersity of In₂S₃ nanoplatelets is clearly visible from the image as well as the size and morphology of the platelets. The platelets have slightly corrugated surfaces with not so sharply defined boundaries. This could possibly be due to the thinness of the nanoplatelets.

Fig. 2 (a) Low magnification TEM image of nearly monodisperse In₂S₃ nanoplatelets synthesized using 1-thioglycerol as the capping chemical. Inset shows the nanoplatelet diameter histogram. The regions in the figure marked with orange and red squares are shown in (b) and (d), respectively. (b) Higher magnification TEM image showing monodispersity, round shapes and thin nature of In₂S₃ nanoplatelets. (c) HRTEM image of a single nanoplatelet indicating highly crystalline nature of the structure. A calculated lattice fringe spacing of 0.33 nm indicates (109) plane of tetragonal In₂S₃. Single crystal SAED pattern shows the reflections from major crystallographic planes of tetragonal In₂S₃. (d) High resolution TEM image studied from a zone clustered with overlapping In₂S₃ nanoplatelets (zone marked with red square in (a)) showing some regions where the thin sidewalls are exposed. Inset shows two parallel In₂S₃ nanoplatelets in upright or tilted positions exposing the thickness (∼7 nm) of the nanoplatelets. Lattice fringe spacing of 0.33 nm is calculated from their thinnest sides.

In order to understand the crystallinity of an individual In₂S₃ nanoplatelet, lattice fringe pattern and selected area electron diffraction (SAED) pattern were analyzed from a single platelet, which is shown in Fig. 2(c) and in the inset of Fig. 2(c), respectively. Continuous and sharp lattice fringes indicate highly crystalline nature of the platelet and rule out the presence of structural defects in the crystals. A lattice fringe spacing of 0.33 nm matches with the (109) plane of tetragonal In₂S₃. SAED pattern reveals single crystal structure of the nanoplatelet with clear linear dotted reflections from the planes defined in tetragonal In₂S₃. Slightly distorted and overlapping diffraction spots in the SAED pattern are contributions from adjacent overlying In₂S₃ nanoplatelets. Thicknesses of the In₂S₃ nanoplatelets were then selectively measured from an area where the patterned distributions of the nanoplatelets were destroyed due to clustering (Fig. 2(d), the area marked with red square). The inset of Fig. 2(d) shows the HRTEM image of two upright and/or tilted In₂S₃ nanoplatelets arranged parallel to each other. As observed from the image, the average thickness of these In₂S₃ nanoplatelets measured is around 7 nm with a std. deviation of 7–10%. A lattice fringe spacing of 0.33 nm again represented the (109) plane in tetragonal In₂S₃. The observed monodispersity in size and thickness of In₂S₃ nanoplatelets may have occurred due to the use of 1-thioglycerol as the capping chemical. In 1-thioglycerol, the presence of strong S–H group at one end and OH group at the other end linked to the C atoms allows shape confinement in In₂S₃ nanoplatelets by bonding the S end to the platelet surface.⁵¹ When 1-thioglycerol was not used to cap the In₂S₃ nanoplatelets, platelet like shapes still form in In₂S₃ but larger sizes and broader size distribution makes them polydispersed, as was noticed from Fig. 3(a). In this case, the size range of the platelets without being capped by 1-thioglycerol ranges from 30 nm to 250 nm. SAED pattern collected from clustered In₂S₃ platelets also shows polycrystalline pattern (inset of Fig. 3(a)) with crystalline diffraction rings visible from major planes in tetragonal In₂S₃. Higher resolution TEM image (Fig. 3(b)) shows that the thicknesses of the larger platelet structures are ∼10 nm. Fast Fourier transform (FFT) pattern collected from the platelets (inset of Fig. 3(b)) additionally confirms the crystalline nature of the nanoplatelets.

Fig. 3 TEM images of the nanostructures formed in a similar chemical environment without using any capping chemical. (a) Clustered polydispersed In₂S₃ nanoplatelets were observed with the inset showing highly crystalline polycrystalline pattern collected from the nanocrystals. (b) A higher magnification image of the clustered region shows the thinness of the platelets and FFT in the inset confirms polycrystallinity in these structures.

Fig. 4(a) displays the absorption spectrum of In₂S₃ nanoplatelets in the UV-vis range. A dilute dispersion of the nanoplatelets in water with ∼0.05 absorbance units of optical density at 225 nm was used for collecting the spectrum. A strong absorption peak is observed at 276 nm with a weak shoulder at 365 nm (Fig. 4(a)). This suggests the presence of two energy band positions corresponding to the respective absorption edges in the In₂S₃ nanoplatelets. The energy gap of bulk In₂S₃ is ∼2.2 eV with the corresponding absorption edge ∼560 nm.^20,27,52,53 As observed from Fig. 4(a), the absorption edge is blue-shifted by almost ∼195 nm compared to the absorption edge in bulk In₂S₃, which may be related to the strong quantum size confinement in these thin nanoplatelets. With the thickness (7 nm) of the In₂S₃ nanoplatelets being almost 5 times less than the Bohr diameter of bulk In₂S₃ (33.8 nm),^27,54 quantum confinement in these nanocrystals is expected. The step-like characteristic of the absorption spectrum correlates well with that from the In₂S₃ nanocrystals prepared by many other researchers using organic capping media and have been explained to be due to valence-conduction band transition in In₂S₃.^27,45,53,55 Room temperature PL spectra for aqueous dispersion of In₂S₃ nanoplatelets are shown in Fig. 4(b). Unlike bulk undoped In₂S₃, nano-sized undoped In₂S₃ are reported to show enhanced luminescence properties.^27,53 Using an excitation wavelength of 280 nm, In₂S₃ nanoplatelets show a strong UV luminescence peak centered around 383 nm, which is only 18 nm Stokes-shifted compared to the absorption peak at 365 nm. The closeness of the absorption and emission peak in In₂S₃ nanoplatelets is suggestive of the small size dispersion in the specimen. The origin of electronic transitions in emissive In₂S₃ has remained debatable. While the UV luminescence was related to band-edge emission,⁵³ there are reports that associate it with indirect transitions.²⁰ In many different nano-sized crystals, the emission wavelength maximum was reported to vary with the excitation wavelength because of the size distribution of nanocrystals in a specimen.⁵⁶ Generally, the shift in the emission peak maximum increases more with the larger crystal size distribution as well as with smaller size of the nanocrystals. We recorded the emission spectra of In₂S₃ nanoplatelets by varying excitation wavelengths from 220 nm to 320 nm as shown in the inset of Fig. 4(b). As observed from the spectra there are little or no shifts in the emission peak maxima from In₂S₃ nanoplatelets. This effect may be related to the mono-size dispersion of the diameter (≤5%) of the In₂S₃ nanoplatelets and contribution from the narrow distribution of thickness (7–10%) of the nanoplatelets in the specimen. A quantum yield of 2% was measured using Horiba JobinYvon QuantumYield calculator from the dispersion of In₂S₃ nanoplatelets. Currently we are in the process of performing lifetime decay measurements of the luminescence from these In₂S₃ nanoplatelets that will help us understand more about the transition processes in these thin nanostructures and will be reported elsewhere.

Fig. 4 (a) Optical absorbance spectrum collected from the dispersion of In₂S₃ nanoplatelets shows the presence of step-like pattern. A band gap of 3.4 eV calculated from the lower absorption edge is ∼1.2 eV higher than that reported in the bulk tetragonal In₂S₃. (b) Room temperature PL spectrum of the In₂S₃ nanoplatelets shows strong UV emission. As shown in the inset the emission peaks remain unchanged on varying excitation wavelength in the specimen.

Quantum confinement in In₂S₃ nanoplatelets was also expected to affect the electrical properties of the material. Since In₂S₃ shows important photoconductance properties in bulk with potential applications in various optical devices including solar cells, we measured the photo I–V properties of our synthesized In₂S₃ nanoplatelets. Photoconductance (Photo I–V) measurement of the In₂S₃ nanoplatelets was carried out using a tungsten halogen lamp as the white light solar simulator. Fig. 5(a) shows the schematic configuration of the device fabricated from the In₂S₃ nanoplatelets. The device was made as discussed in the experimental section and was stable for over a month without any degradation of the material or contacts. Fig. 5(b) shows the I–V characteristic of the hybrid system in the dark and under illumination with the bias ranging from −5 V to 5 V. The I–V curve in the dark exhibited a linear increase from −40 pA to 40 pA resulting in a dark conductivity (σ_dark) of 3.33 × 10⁻¹⁰ Ω⁻¹ cm⁻¹. As the device was illuminated, the conductance was greatly enhanced and the I–V curve showed a superlinear behavior with a breakdown voltage of ∼4.6 V. Under the highest bias voltage, the current reached above 0.022 μA, resulting in a photoconductivity (σ_photo) to a dark conductivity ratio (σ_photo/σ_dark) of more than 10² times.

Fig. 5 (a) A schematic diagram of the prototype device constructed for measuring I–V of the In₂S₃ nanoplatelets in light and dark conditions. (b) An I–V graph of the In₂S₃ nanoplatelets measured in dark and under illumination by a halogen light as the solar simulator.

We know that generally in semiconductor systems, σ_photo show linear behavior assuming that neither the mobility nor the lifetime of free carriers in the system is bias dependent.^57,58 Previously, we observed non-linear Schottky behavior in In₂S₃ zigzag nanowires due to contact work function offset.³⁹ The difference between the work functions of In₂S₃ zigzag nanowires and Au electrodes was responsible for lowering the barrier across the junction and injected electrons from Au-to-In₂S₃ nanowires.³⁹ However, it is understood that mobility of carriers could also be enhanced under illumination due to barrier height lowering and barrier width narrowing.^59–61 In thin nanostructures,^62–64 surface adsorption of free ionic species may introduce barriers at the adsorbent sites. The adsorbents upon partial release under illumination lead to an enhancement in the mobility of the carriers. More release of the free ionic species at larger bias may happen due to increase in surface temperature of the nanostructures under longer illumination.^63,64 Owing to low thickness of In₂S₃ nanoplatelets this possibility may not be ruled out. Furthermore, the crystal structure of In₂S₃ has a considerable number of cation vacancies.^24,27 In a vacancy profuse system like In₂S₃, In atoms may leave their ordered positions and occupy crystallographically ordered vacancies upon excitation (illumination in this case) resulting in an increase in the difference in concentration of ionized donors and acceptors. Consequently, the carrier concentration may increase leading to an enhanced conductivity upon excitation (illumination). The detail mechanism behind the conductivity enhancement in In₂S₃ nanoplatelets under illumination has yet to be understood, which may furthermore increase the potentiality of our material for direct device applications.

Conclusion

To conclude, we have demonstrated the high yield synthesis of nearly monodisperse In₂S₃ nanoplatelets of an average of 85 nm diameter and 7 nm uniform thickness by a simple, low temperature, single-step polyol process using ethylene glycol as the solvent medium and 1-thioglycerol effectively as the capping chemical. Without using the capping chemical, nanoplatelet like morphology still formed, but had more polydispersed size and shape. XRD analysis revealed that the nanoplatelets had some selective growth along b-axis and along (116) plane of tetragonal In₂S₃. Electron microscopy unambiguously confirmed the high crystallinity and uniformity in sizes and shapes of the synthesized nanoplatelets. Optical absorbance revealed large shift in the band gap as compared to the bulk suggesting a strong quantum confinement effect in these thin nanoplatelets. Nearly monodisperse and surface defect free In₂S₃ nanoplatelets showed steady band edge UV luminescence and almost unchanged PL peak position on varying excitation wavelength. Room temperature photoconductivity measurement performed on a prototype device prepared from the In₂S₃ nanoplatelets showed an enhancement of more than two orders of magnitude under illumination as compared to that observed in the dark condition. This may be related to the enhancement of mobility of carriers by removal of surface ionic species in these thin nanoplatelets with increasing bias voltage. The present study is intended to open up a new approach in the synthesis of monodisperse In₂S₃ nanocrystals for efficient optoelectronic applications, such as in solar cells^18,19 and as phosphors.^20–23

Acknowledgements

This work was partially supported by the Florida Cluster for Advanced Smart Sensor Technologies (FCASST) and the United States Army (Grant No. W81XWH1020101/3349).

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