Background

Graphene-like two-dimensional nanomaterials are of great scientific and technological interests [1, 2]. Currently, there has been growing research interests in developing low-dimensional materials that exhibit unique photoelectric properties [3] and quantum dots (QDs) have gained much attraction [4]. Indium sulfide (In2S3) QDs, which belong to the group III–VI semiconductor materials [5], have many unique optoelectrical, thermal, and mechanical properties, which are suitable for numerous potential applications. For example, sulfide nanomaterials have experienced rapid development for use in solar cells [6], photodetectors [7, 8], biological imaging [9], and photocatalytic degradation [10]. There are various ways of preparing sulfide QDs, and they can be divided into two main categories, namely, ‘top-down’ and ‘bottom-up’ [11].

However, commonly used bottom-up methods, such as hydrothermal [12], template[13, 14], and microwave methods [15], have many limitations that restrict the widespread application of sulfide QDs [16]. To ensure the successful application of sulfide QDs, it is of paramount importance to develop low-cost, facile preparation method that can produce stable, reliable, and high-quality QDs material [17]. In this article, a novel preparation method that allows synthesis of In2S3 QDs at atmospheric temperature conditions has been developed by using indium chloride and sodium sulfide as indium and sulfur source respectively. The physical and photoelectric properties of the as-prepared In2S3 QDs were investigated using multiple characterization techniques.

Photoelectric device based on the In2S3 QDs were fabricated, and results show the detectivity of the device stabilizes at 1013 Jones under 365 nm UV irradiation at room temperature, which demonstrates In2S3 QDs have great potential applications in photodetectors. Compared with other growth methods, the reported approach is mild, facile, environmentally friendly, rapid, and cheap. Therefore, it is suitable for low-cost large-scale production of the device that also yields excellent performances. This work demonstrates a low-cost, effective fabrication technique for future application of sulfide QDs in the field of photoelectric detection.

Methods

Materials

Sodium sulfide (Na2S·9H2O) was purchased from Tianjin Wind Ship Chemical Testing Technology Co. Ltd., Tianjin China. Indium chloride (InCl3·4H2O) was obtained from Shanghai Aladdin Biochemical Technology Co. Ltd Shanghai, China. Sodium dodecyl sulfate was purchased from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China. Dialysis bag (USA spectrum lab’s regenerated cellulose membrane, Mw = 300) was purchased from Shanghai Yibai Economic and Trade Co. Ltd. All of the materials above were purchased commercially and used without further purification.

In2S3 QDs Fabrication

In2S3 QDs were prepared using the fabrication process as shown in Fig. 1a. Na2S (0.1 mol/L) and InCl3 (0.1 mol/L) were first dissolved in deionized water. The same volume of Na2S and SDS(CMC 0.008 mol/L)solutions were mixed using magnetic stirrer for 20 min at 1500 rpm. A mixture of InCl3 and SDS was prepared in the same way. The addition of SDS is to obtain a monodispersed, passivated QDs under a controlled synthesis process. The Na2S mixture was then added to the InCl3 mixture solution in a beaker to initiate the chemical reaction, which resulted in yellowish products after 10 min. Deionized water was added to the reacted solution and then followed by centrifugation at 3000 rpm for 5 min. The products were washed three times and purified using dialysis bag. The prepared In2S3 QDs were collected in the dialysis bag.

Fig. 1
figure 1

a The schematic illustration of the preparation of In2S3 QDs. b TEM image and size distribution (inset) the white line is the Gaussian fitting curve. c–e HRTEM images, inset of FFT image of a selected red area. f The SEM image. g XRD spectrum. h Raman spectrum. i The line profile of the diffraction fringes in (d). j The AFM image. k The height analysis of randomly selected In2S3 QDs labeled as A, B, C, and D in j

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Characterization

Transmission electron microscope (TEM) images were obtained with a JEM-2100 high-resolution trans-mission microscope operating at 200 kV. The surface morphology and phase image of photovoltaic devices were determined by scanning electron microscope (SEM, FEI Quanta 200) and AFM (atomic force microscope, SPA-400), respectively. XRD analysis was investigated using a Rigaku D/Max-RA X-ray diffractometer with Cu Ka radiation. Raman spectrum was recorded at ambient temperature on a Renishaw in via Raman microscope with an argon-ion laser at an excitation wavelength of 514.5 nm. Optical properties were characterized by UV-vis, UV-vis-NIR (UV-3600), and fluorescence (Hitachi F-7000) spectrometers. Functional groups on the surface of the In2S3 QDs were verified by XPS (X-ray photoelectron spectroscopy) (PHI Versa Probe II) using 72 W, mono Al Ka radiation. J-V and C-V were measured using Keithley 2400 source meter and semiconductor device analyzer (Keysight B1500A), respectively.

Results and Discussion

Structure and Morphology Studies

TEM images of the In2S3 QDs are shown in Fig. 1b–e. It can be seen that In2S3 QDs are evenly distributed and exhibit spheroid morphology. Its particle size distribution follows the Gaussian distribution with size ranging from 1 to 3 nm and FWHM of 1.12 nm. The particle has an average size of 2.02 nm. Figure 1c–e are HRTEM images of the In2S3 QDs showing its lattice fringes for d = 0.271 nm, 0.311 nm, and 0.373 nm, corresponding to the cubic crystal system of 400, 222, and 220 lattice planes respectively [18]. Figure 1i shows a longitudinal profile of the lattice fringes shown in Fig. 1d. The fast Fourier transform (FFT) pattern of the selected region (red dotted square) is shown in Fig. 1d insert, which reveals six bright spots from the 400 plane diffraction, indicating the crystalline structure of the hexagonal system. The scanning electron microscopy (SEM) image of the as-prepared In2S3 QDs is shown in Fig. 1f. As shown, the In2S3 QDs agglomerated to form a relatively compact structure in order to reduce its surface energy. X-ray diffraction (XRD) planes at 400, 222, and 220 of the In2S3 QDs are shown in Fig. 1g and the calculated particle size using the Sheer formula is in good agreement with the measured size from the 400 plane of HRTEM image. Figure 1h shows Raman spectrum of the In2S3 QDs with typical peaks at 304 cm−1 and 930 cm−1 [19]. Atomic force microscopy (AFM) was performed on four randomly selected In2S3 QDs, marked as A, B, C, and D as shown in Fig. 1j, with measured heights of 1.53 nm, 2.35 nm, 1.35 nm, and 2.32 nm (shown in Fig. 1k), respectively. The average height of 1.94 nm from the AFM measurement is very close to that obtained from the TEM.

The estimated band gap of In2S3 QDs is 3.50 eV, which is larger than its bulk value of 2.3 eV, due to the quantum effect. The band gap was calculated using the Brus equation:

$$ {E}_{np}\approx {E}_{g(0)}+\frac{{\overline{h}}^2{\pi}^2}{2{R}^2}\left(\frac{1}{{m_e}^{\ast }}+\frac{1}{{m_h}^{\ast }}\right)-\frac{1.8{e}^2}{4\pi \varepsilon R} $$
(1)

where Enp is the bandgap of the QDs, Eg is the band gap of bulk In2S3 (2.3 eV), \( \overline{h} \) =h/2π is the reduced Planck constant, e is the electron charge, me* is the effective mass of electron, mh* is the effective mass of hole, me*= mh*(0.25 × 10−28g), R is the radius of the particle and ε is the dielectric constant (ε = 11).

Figure 2a shows ultraviolet-visible (UV-vis) absorption spectra of the In2S3 QDs. There are two characteristic peaks of absorption located at 225 nm and 283 nm [20]. Since In2S3 is a direct bandgap material, its optical band gap can be expressed by the following equation:

$$ \alpha hv=A{\left( hv- Eg\right)}^{1/ 2} $$
(2)

where α is the absorption coefficient, A is a constant, hv is the photo energy, and Eg is the band gap energy.

Fig. 2
figure 2

a UV-vis absorption spectra of In2S3 QDs aqueous solution. Inset: an estimation of band gap energy (Eg). b PL emission spectra. c PL excitation (PLE) spectra, inset: luminescence image under visible and 365 nm light source. d The XPS full-scan spectrum. e XPS S2p spectrum. f XPS In3d3/2 and In3d5/2 spectrum

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The band gap energy of the QDs can be estimated from the curve of (αhv)2 vs. photo energy (hv). The estimated Eg of 3.54 eV, as shown in the inset of Fig. 2a, is very close to the calculated value using the Brus equation (Enp=3.50 eV). Photoluminescence (PL) and photoluminescence excitation (PLE) [21] studies were performed to investigate the optical properties of the In2S3 QDs. It can be seen from Fig. 2b that there is an emission peak at a wavelength between 300 and 450 nm, and the strongest peak intensity is centered at ~ 390 nm under the excitation of Ex = 250 nm. PLE spectra in Fig. 2c show that wavelengths of the characteristic excitation peaks are shorter than the receiving wavelengths (500–540 nm). The broadening of energy gap of In2S3 QDs compared to its bulk material may also be demonstrated by PL and PLE results. The fluorescence of the In2S3 QDs under visible light and 365 nm UV light are shown in Fig. 2c insert. This demonstrates that the In2S3 QDs possess good UV fluorescence properties. X-ray photoelectron spectroscopy (XPS) was also performed to study the chemical bonds of the In2S3 QDs. Figure 2d shows the XPS full scan spectrum, which consists of S2p at 162.5 eV, In3d5/2 at 444.5 eV, and In3d3/2 at 452.5 eV. Besides, there are residual Cl, Na, O, and C from the surfactant and reactant. Core level peaks of S2p and In3d are shown in Fig. 2e, f respectively. The deconvoluted peaks reveal the bonding states of S2p (In-S, C-S), In3d5/2. (In-S, In-O), and In3d3/2 (In-S, In-O).

As the In2S3 QDs demonstrated excellent ultraviolet absorption properties, UV photodetector based on the In2S3 QDs was fabricated and investigated. The preparation process is illustrated in Fig. 3a.

Fig. 3
figure 3

a Schematic diagram illustrating the fabrication process of the In2S3 QDs UV photovoltaic detector. b Electrode without QD. c–d Optical microscopic images of the In2S3 QDs photodetector at different magnifications. e–h Performance of the In2S3 QDs detector. e J-V curves. f Log (J)-V curves. g R (responsivity)-V curves. h D*

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The specification of the Au interdigitated electrodes is similar to that reported by Tang. et al. [22], consisting of electrodes with a thickness of 400 nm, a length of 120 μm, and width and spacing of 10 μm. Figure 3b shows an optical image of empty electrodes. Fig. 3c, d shows the optical microscopic images showing the spacing of the electrodes filled with the In2S3 QDs, which acted as a photosensitive layer. The measured current density against voltage (J-V) and log (J-V) curves of the device in dark condition, irradiated by 0.16 mW cm−2 and 0.47 mW cm−2 power density of 365 nm UV light are shown in Fig. 3e, f respectively. An increase in the current density is observed when the irradiated power density increases, hence demonstrating the characteristics of a rectifier. The responsivity (R) and detectivity (D*) of the photodetector are calculated using the following equations:

$$ R={J}_{\mathrm{ph}}/{P}_{\mathrm{opt}} $$
(3)
$$ D\ast =\frac{R}{\sqrt{2q/ jd}} $$
(4)

where Jph is the photocurrent density, Popt is the photo power density, q is the absolute electron charge (1.6 × 10−19 coulombs), and Jd is the dark current density [23]. From Fig. 3g, the maximum value of R is 4.13 A W−1, which is significantly larger than that of graphene and many other two-dimensional nanomaterial devices [24, 25] and is seen to increase with an increase in the reverse bias voltage. As shown in Fig. 3h, the D* is stabilized at around 1013 Jones.

Fig. 4
figure 4

a Photodetector with In2S3 QDs as an active layer. b Plot of R-T at 1 V and 2 V. c Plot of ln (ρ)-1/T-based devices at 1 V. d C-F curves measured at room temperature. e The C-V curves (40 MHz)-based photodetector in the dark condition. f Variation of the capacitance with applied voltages and plots of 1/C2 vs. V of the device

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The optical images of empty electrodes and those filled with In2S3 QDs are shown in Fig. 4a. The plot of R-T measured from the In2S3 QDs-based photodetector at a voltage of 1 V and 2 V is shown in Fig. 4b. It shows that an increase in temperature has led to a decrease in the resistance; however, it does not exhibit a simple linear relationship. In order to understand the electrical properties of the In2S3 QDs, the ln(ρ)-1/T of the device was attained and the results are shown in Fig. 4c. By using the two model equations [26]:

$$ \rho =R\frac{\left(N-1\right)\kern0em wd}{l} $$
(5)
$$ \mathrm{In}\ \left(\rho \right)\kern0.5em =\kern0.5em \mathrm{In}\kern0.5em (A)\kern0.5em +\kern0.5em {E}_a/\kern0.5em \left({k}_b\cdot T\right) $$
(6)

where N is the number of interdigitated electrodes, w is the overlapping length, l is the spacing, and d is the thickness of the film [27]. Using a simple linear regression, the calculated thermal activation energy (Ea) is 0.011 eV and the finger-leading factors (A) is 4.16 × 108 Ω°cm. The thermal activation energy of In2S3 QDs could be reduced as long as the obtained energy is sufficient for the carriers to participate in conduction, which can result in lower resistivity and higher conductivity.

Generally, C-V measurements can provide many important information on the nature of the semiconductor interface and charge transport. Fig. 4d shows that capacitance decreases with increasing frequency and the decrease in capacitance is significant at low frequencies. This is due to the interface states, which respond to the alternating current signal, and the presence of the interface states would suppress the AC signal at high frequency, hence resulting in a weakened trend or a constant capacitance. Figure 4e shows the C-V curves of the In2S3 QDs-based photodetector at room temperature with a frequency of 40 MHz. The C-V relationship under a bias can be expressed as [28]

$$ {C}^{-2}=\frac{2\left({V}_{bi}-V\right)}{q{\varepsilon}_0{\varepsilon}_r{NS}^2} $$
(7)

where Vbi is the built-in potential at zero bias, ε0 is the permittivity of vacuum, εr is the relative permittivity of a material, N is the carrier concentration in the depletion layer and S is the photosensitive area (3.3 mm2). The x-intercept is Vbi = 0.6 V, and the carrier concentration N can be calculated from the slope of the linear section of 1/C2 vs. V plot [29]: \( N=\frac{-2}{q{\varepsilon}_0{\varepsilon}_r{A}^2}{\left[\frac{\partial \left({C}^{-2}\right)}{\partial V}\right]}^{-1} \), and the calculated N=4.3 × 1019 cm−3. The depletion width (Wd) is between the electrode and the In2S3 QDs layer, expressed as \( {W}_d={\left[\frac{2{\varepsilon}_0{\varepsilon}_r\left({V}_{bi}-V\right)}{qN}\right]}^{1/2} \), the calculated Wd = 12.34 nm. These physical parameters are shown in Fig. 4f. It is evident that the Vbi and Wd are the same as similar QDs devices (such as the graphene quantum dots) [30], but the N is larger by an order of magnitude at zero bias. This explains the excellent performances of the device as compared to other QDs device [31].

Conclusions

A novel and facile preparation method to produce high crystal quality In2S3 QDs was developed. The structural, optical, electrical, and photovoltaic properties of the In2S3 QDs have been studied. In the dark field condition, the activation energy (Ea), finger-leading factor (A), built-in potential (Vbi), and depletion layer width (Wd) of the UV photodetector based on In2S3 QDs were obtained. In2S3 QDs were used as the sole photoactive material in the fabricated photodetector that exhibits the highest detectivity (D*) of 2 × 1013 Jones at room temperature under 365 nm UV light illumination without preamplifier. This method is ideal in developing high performance, large array of In2S3 QDs-based UV photoelectric detector at very low cost.