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Effect of Tm dopant on luminescence, photoelectric properties and electronic structure of In2S3 quantum dots
Journal of Luminescence 217 (2020) 116775
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Journal of Luminescence journal homepage: http://www.elsevier.com/locate/jlumin
Effect of Tm dopant on luminescence, photoelectric properties and electronic structure of In2S3 quantum dots Yi Liu, Liyong Du, Kuikun Gu, Mingzhe Zhang * State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun, 130012, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Tm doped In2S3 Photoluminescence spectra Photoelectric properties Electronic structure calculations
Doping is an effective method to tune the properties of semiconductor nanomaterials. Particularly rare earth ion doping has attracted increasing attention due to its excellent optical properties. However, the design and syn thesis of the indium sulfide (In2S3) quantum dots (QDs) with controllable optical electronic structure remain a challenge. Herein, Tm-doped In2S3 QDs are synthesized using a gas-liquid chemical deposition method. Fluo rescence intensity of In2S3: Tm QDs has great enhancement, which can be controlled by regulating doping concentrations of Tm3þ ions. The photoelectric properties of In2S3: Tm QDs are also researched and the results demonstrate enhanced sensitivity to light, short response/recovery times and long-term stability. Furthermore, based on first-principles calculations, the change of electronic structure and enhancement of optical properties in In2S3: Tm QDs are mainly attributed to Tm3þ ions doping. The as-synthesized In2S3 QDs with luminescent and photoelectric properties are expected to have extensive applications in photovoltaic, photoelectric and sensor fields.
1. Introduction Quantum dots (QDs) have attracted wide interest in recent years due to their noticeable optical properties and potential applications in photodetectors, electrochemistry, solar cells, photocatalysis, etc [1–4]. Great effects have been made to design and synthesize a variety of semiconductor QDs with tunable optical properties. Sulphide materials are considered for a broad range of applications including optoelec tronic device, lasers, bio imaging and photovoltaics due to their unique luminescent and photoelectric properties [5–8]. Doping semiconductor QDs are a crucial process and commonly employed method for acquiring tunable electronic states and improving their performance in diverse fields [9]. Nowadays, an enormous amount of attention is focused on sulphide doped with rare-earth ions. The rare-earth ions doped materials are well developed as optical devices because of high luminescence quantum yield and chemical stability, corresponding to 4f-4f and 4f-5d transitions of rare-earth elements [10–12]. Furthermore, it is necessary to select suitable host materials doped with rare-earth elements to improve their optical properties. Indium sulfide with excellent optical, photoconductive and acoustic properties is an important n-type III-VI sulfide because of its special defect structure [13–15]. In2S3 has been found in three different
crystalline structures: α-In2S3 (defective cubic structure), β-In2S3 (defective spinel structure), γ-In2S3 (layered hexagonal structure) [16]. β-In2S3 with a band gap of 2.0–2.3 eV is a very stable phase at room temperature and shows great potential applications in the visible light range [17]. These properties relate to its defect structure, as shown in Fig. 1a. Its unit cell consist of tetrahedral and octahedral sites where In3þ occupies all octahedral sites and two-thirds of tetrahedral sites. The remaining one-third of tetrahedral sites are unoccupied which result in cation vacancy [18]. Therefore, β-In2S3 is an appropriate host material for various dopants. Moreover, due to the low toxicity of In2S3, it can serve as a substitute for CdS in the buffer layer of solar cells. John et al. reported that CuInS2/In2S3 solar cell obtained a relatively high effi ciency (9.5%) without any anti-reflection coating [19]. Kilani et al. re ported that Sn doping In2S3 films resulted in a widening of the optical gap and enhanced the conductivity [20,21]. Li et al. reported that the rare earth Dy3þ and Tb3þ ions co-doped β-In2S3 showed enhanced photoluminescence emission peaks and room temperature ferromagne tism [22]. However, no relevant reports about the synthesis, optical and photoelectrical properties of Tm3þ doped In2S3 have been published so far. Hence, it is necessary to deeply explore the relationship between its characteristics and changes in internal structures. In this work, the Tm doped In2S3 QDs are fabricated via a gas-liquid
* Corresponding author. E-mail address: [email protected] (M. Zhang). https://doi.org/10.1016/j.jlumin.2019.116775 Received 22 March 2019; Received in revised form 20 September 2019; Accepted 23 September 2019 Available online 24 September 2019 0022-2313/© 2019 Elsevier B.V. All rights reserved.
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Journal of Luminescence 217 (2020) 116775
Fig. 1. (a) Crystal structure of the β-In2S3 (b) XRD patterns of the pristine In2S3 and the In2S3: Tm QDs. (c) HRTEM image of the 1.22 at% In2S3: Tm QDs. The inset corresponding to SAED pattern.
chemical deposition method and their optical and photoelectric prop erties are investigated and discussed in details. Tm doped In2S3 QDs demonstrate tunable bandgap and enhanced luminescence and light sensitivity by UV–visible spectroscopy, photoluminescence and elec trical measurements. Furthermore, first-principles calculations are per formed to account for doping Tm3þ ions induced structural and electronic changes of In2S3. The experimental and theoretical results indicate that the as-synthesized In2S3 QDs may be a promising material for use in solar cells and optoelectronic fields.
the crystal growth process, a proposed growth mechanism is shown in Fig. S1 and the reaction formula analysed as follows, 3HOCH2 CH2 SH þ InðCH3 COOÞ3 →ðHOCH2 CH2 SÞ3 In þ 3CH3 COOH 3HOCH2 CH2 SH þ TmðCH3 COOÞ3 →ðHOCH2 CH2 SÞ3 Tm þ 3CH3 COOH
ðHOCH2 CH2 SÞ3 In þ ðHOCH2 CH2 SÞ3 Tm þ H2 S↑→In2 S3 : Tm↓
(1) (2) (3)
The growth of the In2S3: Tm QDs mainly involves three steps. Step 1: the preparation of the reaction solution. There are In3þ cations, Tm3þ cations and HOCH2CH2SH in the reaction solution. The addition of acetic acid is beneficial to the dissolution of indium acetate. Moreover, the pH value of the reaction solution cannot be too small, otherwise it will inhibit the reaction between the H2S gas and the solution. The complexes of (HOCH2CH2S)3In and (HOCH2CH2S)3Tm are formed by HOCH2CH2SH combining with In3þ cations and Tm3þ cations in the process of mixing the reaction solutions (Reaction (1), (2)). Step 2: the reaction solution reacted with H2S gas. The dissolved S2 anions in the reaction solution cause the gradual decomplexation of (HOCH2CH2S)3In and (HOCH2CH2S)3Tm complexes to form the ionic bonds between the In3þ/Tm3þ cations and S2 anions (Reaction (3)). This process eventu ally leads to Tm3þ cations doping into the host In2S3 lattices and the formation of multiple nucleation. The In2S3: Tm crystal nuclei attract the surrounding ions to further growth. In the crystal growth stage, exces sive HOCH2CH2SH surrounds the In2S3: Tm crystal nuclei can prevent the agglomeration of crystal nuclei and facilitate the homogeneous growth of crystal nuclei in every direction. HOCH2CH2SH plays an important role in reducing reaction rate, supporting Tm3þ cations doping and controlling homogeneous nucleation. Step 3: the growth of crystal grains. Under the action of ultrasonic bath, the immature In2S3: Tm crystal grains can avoid agglomeration and remove the surrounding HOCH2CH2SH. After the Ostwald ripening, the In2S3: Tm crystal grains gradually grow to stable and homogeneous nanoparticle, which
2. Experiment section 2.1. Sample preparation and growth mechanism The pristine In2S3 QDs and the In2S3: Tm QDs were prepared by using a gas-liquid phase chemical deposition method. Indium acetate (In (COOCH3)3⋅3H2O), Thulium acetate hydrate (Tm(COOCH3)3⋅4H2O) and mercaptoethanol (HOCH2CH2SH) were mixed with deionized water as a reactive solution. All reagents in this work were commercially available and used without further purification. The molar ratios of Tm3þ and In3þ were 0, 3, 6, 9 and 12 at.% in the mixed solution, respectively. HOCH2CH2SH (60 mmol L 1) were dissolved in the above reaction so lutions with magnetic stirring. The pH of the well-mixed reaction solu tion was adjusted to 2.5–3 using acetic acid (CH3COOH) and then was placed in a chamber with circulating water (30 � C). The H2S gas was synthesized by reaction of HCl and Na2S solution according to the molar ratio of 2:1. In the reaction process, the chamber was situated in an ultrasonic bath to avoid the nanoparticles agglomeration. The H2S gas was directly introduced into the reaction solution by using gas pump until the colour of the reaction solution was unchanged. Finally, the precipitates were collected using centrifugation, washed thrice with deionized water and anhydrous alcohol respectively and then dried at 60 � C in a nitrogen atmosphere. To further understand the growth mechanism of the In2S3: Tm QDs in 2
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Journal of Luminescence 217 (2020) 116775
Fig. 2. XPS results of the 1.22 at% In2S3: Tm QDs: (a) Survey spectrum, (b) In 3d binding energy spectrum, (c) S 2p binding energy spectrum, (d) Tm 4d binding energy spectrum.
eventually precipitate at the bottom of the reaction chamber.
2.4. Calculation details In order to deeply investigate the electronic structures of the pristine In2S3 and the In2S3: Tm QDs, the first-principles calculations based on density functional theory (DFT) were carried out using the projected augmented wave (PAW) method implemented in the Vienna Ab initio Simulation Package (VASP) package. Generalised Gradient Approxi mation (GGA) of Perdew-Burke-Ernzerhof (PBE) was used to treat the exchange-correlation functional. The electronic wave functions were expanded using plane waves with a cut off energy of 400 eV. All supercells were optimized and relaxed with energy and force tolerances of 10 4 eV and 0.05 eV Å 1. A 3 � 3 � 1 Monkhorst-Park mesh was used for the Brillouin zone integration. The Hubbard parameter U (GGA þ U) was used to take care of the Coulomb repulsion and exchange interac tion, the U ¼ 5 eV and U ¼ 7.3 eV were applied to the In d orbital and the Tm f orbital separately.
2.2. Preparation of the pristine In2S3 and In2S3: Tm nanoparticles films 2 mg of the pristine In2S3 and In2S3: Tm (1.22 at.%) nanoparticles were each dissolved in 20 ml of ethanol solution and sonicated to form a uniform suspension. Then the suspension was separately sprayed on the precleaned p-type Si by means of ultrasonic spray. The films were calcined at 500 � C for 2 h in nitrogen-filled environment. 2.3. Characterization X-ray diffraction spectra (XRD) were determined by a Rigaku D/Max2550 X-ray diffractometer with a Cu-Kα line of 1.54056 Å. The dopant concentrations of the as-synthesized samples were obtained by energydispersive X-ray spectroscopy (EDS) provided by scanning electron mi croscopy (Magellan-400). High resolution transmission electron micro scopy (HRTEM) and selected area electron diffraction (SAED) images were recorded by a transmission electron microscopy (JEOL JEM2200FS). The chemical composition and valence states of the samples were characterized using X-ray photoelectron spectroscopy (XPS) (ESCALAB MK II). Room temperature UV–visible spectra were recorded using a UV-2802S spectrophotometer. Photoluminescence (PL) spectra were carried out using a spectrofluorometer (Shimadzu RF-5301PC) with a Xe lamp as an excitation source. The luminescence decay curves were recorded using an Horiba QuantaMaster 8000 fluorescence spectrophotometer. The photoelectrical properties measurement was conducted by Lake-share probe station and Keysight B1500A.
3. Results and discussion The chemical composition and the crystal size of the pristine In2S3 and In2S3: Tm QDs are analysed by XRD pattern, as shown in Fig. 1b. The dopant concentrations of Tm3þ ions are determined by EDS. The dopant concentrations obtained by statistical calculations are around 0.72, 1.22, 1.54 and 2.10 at.%, respectively. The details are shown in the supplementary information. The EDS results (Fig. S2) indicate that the atomic ratio of In and S is close to 1: 1.5, which conforms to stoichio metric proportion of In2S3. All the identified diffraction peaks XRD patterns of the as-synthesized In2S3 QDs are consistent with the standard tetragonal In2S3 phase (JCPDS, No. 73–1366) with the I41/amd space 3
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Journal of Luminescence 217 (2020) 116775
Fig. 3. (a) UV–vis diffuse reflectance spectra of the pristine In2S3 and In2S3: Tm QDs. The inset shows the related energy level diagram of the Tm3þ ions. (b) (αhν)1/2 as a function of photon energy for the pristine In2S3 and In2S3: Tm QDs.
group. These diffraction peaks corresponding to the (109), (00 12) and (22 12) crystal planes of a tetragonal phase In2S3. No impurity phases are detected such as thulium sulfide, thulium oxide, indium oxide, indicating the samples are high pure and the Tm3þ ions substitude for the In3þ ions without altering the crystal structure of the host material In2S3. The Scherrer’s equation is used to calculate the average size of the samples. The average crystallite sizes of as-obtained samples are roughly 2.8 nm, indicating no significant influence on crystallite size with Tm doping. Fig. 1c displays the HRTEM of the 1.22 at.% In2S3: Tm QDs. The lattice fringes with a spacing of 0.267 nm correspond to the (00 12) plane of the tetragonal In2S3. The average size of the particles is 3–5 nm which is corresponding to the XRD analysis. The SEAD pattern in the inset of Fig. 1c displays polycrystalline diffraction rings consistent with (109), (00 12) and (22 12) planes of In2S3. To determine the chemical composition and valence state of the sample, the 1.22 at.% In2S3: Tm QDs are characterized by XPS. The In2S3: Tm QDs consist of In, Tm, S, C and O which are confirmed by survey spectrum shown by Fig. 2a. The observed C and O in the sample may be attributed to the reference and absorbed gaseous molecules. Fig. 2b shows the high resolution XPS spectrum of In 3d, the peaks at 445.15 eV and 452.72 eV are assigned to In 3d5/2 and In 3d3/2 mainly corresponding to the In3þ in In2S3 [23]. Fig. 2c represents the high resolution S 2p fitted XPS spectrum, the S 2p3/2 and S 2p1/2 peaks positioned at 161.61 eV and 162.2 eV can be assigned to S ion in S2 state in In2S3 [24]. The Tm 4d peak centered at the binding energy of 177.86 eV which confirms the presence of Tm3þ in the In2S3 QDs, as
shown in Fig. 2d [25]. Therefore, the results of XPS analysis reveal that Tm element was successfully incorporated into the In2S3 host crystal lattice. Fig. 3a shows UV–vis diffuse reflectance spectra of the pristine In2S3 and In2S3: Tm QDs. The related energy level diagram of the Tm3þ ions is shown in the inset of Fig. 3a. As seen from Fig. 3a, the strong absorption edge is exhibited in the visible region. Absorption bands at 686 nm and 789 nm associated to the transition of Tm3þ ions from the ground state (3H6) to the excited states (3F2,3, 3H4) [26]. The bandgap values of the synthesized samples are determined from the Tauc relation: �n ðαhνÞ ¼ A hν Eg where hν is the energy of the incident photons, α is the absorption co efficient, A is the absorption constant, Eg is the optical bandgap value and n is the power, which characterizes the band gap transition. n ¼ 2 for an indirect allowed transition and n ¼ 1/2 for a direct allowed transition. The bandgap values can be obtained by the intercept of the linear portion in plotting (αhν)1/2 versus hν and are shown in Fig. 3b. The bandgap value of In2S3 QDs with Tm3þ dopant concentrations of 0 at.%, 0.72 at.%, 1.22 at.%, 1.54 at.% and 2.10 at.% are 2.03eV, 1.96 eV, 1.94 eV, 1.97 eV and 1.99 eV. The bandgap narrowing for In2S3: Tm QDs can be explained using the many-body effects [27]. The inter action between the conduction electrons of In2S3 and the localized f electrons of Tm3þ ions which causes the shrinkage of the bandgap. The variation of bandgap value with increasing Tm3þ concentration further
Fig. 4. (a) PL emission spectra of the pristine In2S3 and In2S3: Tm QDs excited at 279 nm. (b) Dependence and linear fitting of log (I/x) versus log(x) beyond the quenching concentration. 4
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multipole-multipole interaction based on the Dexter’s theory. The emission intensity (I) per activator concentration (x) follows the energy transfer formula (5) [36]: i 1 (5) I=x ¼ k½1 þ βðxÞθ=3 where k and β are constants for a given host crystal in the same exci tation condition; θ ¼ 6, 8, and 10, corresponding to dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interactions, respec tively. Fig. 4b shows that the dependence of log (I/x) (emission intensity (I) at 687 nm) versus log(x) is quiet linear and the line slope (-θ/3) is 1.9842, The value of θ is approximately 6, which reveals that dipoledipole interaction is the concentration quenching mechanism of Tm3þ ions in the In2S3: Tm QDs. To further investigate the energy transfer process, fluorescence decay curves in the In2S3 QDs with different Tm3þ concentrations excited at 283 nm and monitored at 470 nm are shown in Fig. 5. The decay curves can be fitted well using the following three-exponential function (6) [29]: I ¼ A1 exp ð
t = τ1 Þ þ A2 exp ð
t = τ2 Þ þ A3 exp ð
t = τ3 Þ
(6)
where A1, A2 and A3 are the fitting decay constants, τ1 τ2 and τ3 are the decay time for exponential components. The average lifetime (τav) is calculated according to the following equation (7): P 2 Aτ τav ¼ P i i (7) Ai τ i The calculated τav are 54.13 ns, 50.96 ns, 50.81 ns, 43.36 ns, and 40.10 ns, corresponding to 0 at.%, 0.72 at.%, 1.22 at.%, 1.54 at.% and 2.10 at.%, respectively. The fluorescence lifetimes of the as-synthesized In2S3 and In2S3: Tm QDs are longer than previously reported [18,37,38], which may be due to defects in the prepared samples cause trap states near the edge of the conduction band. The conduction electrons in trap states can relax without recombination, which results in extended car rier lifetimes [39,40]. It can be found that the decay lifetimes decrease gradually with increasing Tm3þ dopant concentrations. The decrease of fluorescence lifetimes in the In2S3: Tm QDs indicate that the energy transfer process takes place from the host In2S3 to the dopant Tm3þ ions [29]. In order to futher investigate the energy transfer process between the host In2S3 and Tm3þ ions, the energy transfer efficiency (ηT) can be estimated by the following equation (8):
Fig. 5. The fluorescence decay curves of the 470 nm emissions in the pristine In2S3 and In2S3: Tm QDs under 283 nm excitation.
indicates the successful doping of Tm3þ ions. In order to investigate In2S3: Tm QDs with different doping con centrations for its optical properties, the room temperature photo luminescence spectrum upon 279 nm excitation is depicted in Fig. 4. The emission spectra features broadly consist of five peaks at 417 nm, 470 nm, 493 nm, 530 nm, 586 nm. The emission peak at 417 nm results from the surface oxidation states with higher energy levels [28]. The emission peak at 470 nm is ascribed to exciton recombination while the emission peak at 493 nm, 530 nm and 586 nm are assigned to near-impurity/near-defect excitonic luminescence [29–31]. For Tm doped In2S3 QDs, the emission peak at 682 nm and 764 nm are assigned to the 3F2,3 → 3H6 and 1G4→3H5 electronic transition of Tm3þ, respec tively [32,33]. Tm3þ ions doping causes defects that form many defec tive energy levels. The defective energy levels can participate in and affect the emission process, thus the near-defect excitonic luminescence at 493 nm for the In2S3: Tm QDs is more obvious than that of the pristine In2S3 QDs. The intensities of PL spectra increased with increasing the Tm3þ ions concentration to 1.22 at.%. When the doping concentration is further increased, the intensities are gradually decreased due to the concentration quenching. The concentration quenching is ascribed to the non-radiative energy transfer between the activator (donor) and quenching site (acceptor) [34]. The critical energy transfer distance (Rc) can be expressed by eqn (4) [35]: � �1 3V 3 Rc � 2 4πxc N
ηT ¼ 1
τ τ0
(8)
where τ and τ0 correspond to the decay lifetime of the host In2S3 with and without energy acceptor (Tm3þ) ion present, respectively. The values of the energy transfer efficiency are calculated to be 5.9%, 6.1%, 19.9% and 25.9%, corresponding to Tm3þ dopant as 0.72 at.%, 1.22 at. %, 1.54 at.% and 2.10 at.%, respectively. The lower energy transfer ef ficiencies suggest that more nonradiative relaxations occur in In2S3: Tm QDs. The energy transfer efficiencies increase with increasing Tm3þ dopant concentration, which indicates an efficient energy transfer process. Current-voltage (I–V) characteristics curves can further prove that the effective doping of Tm3þ ions in In2S3 and enhanced conductivity in In2S3: Tm 3þ QDs. Fig. S4 shows I–V curves of the pristine In2S3 and In2S3: Tm (1.22 at.%) QDs, which reflects that the current increases linearly under the bias voltage. Tm3þ doped In2S3 QDs display higher current value than the pristine In2S3 under the same voltage, indicating increased conductivity. This means that the doped Tm3þ ions are beneficial to improve charge conduction and electrical properties of In2S3. Their photoelectric properties are studied through fabricating ptype Si/In2S3 heterostructures under illumination with 405 nm wave length and different power densities at room temperature to explore the potentials for photoelectronic applications. Fig. 6a depicts the schematic
(4)
in which V is the volume of the unit cell, N is the number of cation sites in the unit cell, and xc is the critical concentration. According to the above experimental values of V and N (1880.44 Å3, 32) into eqn (4), the Rc of Tm3þ in In2S3: Tm QDs is calculated to be about 20.9 Å from the critical concentration of Tm3þ (1.22 at.%). This indicates that exchange interaction plays little role in energy transfer because the critical transfer distance of exchange interaction is limited to less than 5 Å. Therefore, the process of energy transfer is controlled by electric 5
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Fig. 6. (a) The schematic diagram of the p-Si/In2S3 heterostructure. (b) The energy band illustration of the fabricated heterostructure under light illumination. I–V curves of (c) p-Si/In2S3 heterostructure and (d) p-Si/In2S3: Tm (1.22 at.%) heterostructure under dark and illumination with different light intensities.The inset shows enlarged version of the I–V curves for reverse -biased region under dark and illumination with different light intensities.
diagram of the p-Si/In2S3 heterostructures. The energy band illustration of the fabricated heterostructures under light illumination is presented in Fig. 6b. The electron affinity of p-Si and In2S3 are 4.05 eV and 4.25 eV, respectively [41]. The bandgap values (Eg) of p-Si and In2S3 correspond to 1.12 eV and 2.2 eV respectively, therefore, the valence band offset is 0.28 eV (ΔEv) and the conduction band offset is 0.2 eV (ΔEc). When the light illuminates the heterojunction, electron-hole pairs are generated, and then the constraint of electrons (e) and holes (h) are broken by the built-in electric field. Free electrons and holes are collected under the action of electric field. Fig. 6c and d presents I–V curves of p-Si/In2S3 and p-type Si/In2S3:
Tm (1.22 at.%) heterostructures under dark, and illumination with 405 nm wavelength. The I–V curves of the samples show obvious rectifying characteristics under dark conditions. Under the reverse bias, the dark current is roughly 10 9 nA, which is derived from the generated recombination current by the hot carrier in p-Si under thermal excitation state [42]. Under the forward bias, the current increases rapidly, which is caused by the increase of the concentration of the minority carrier near the boundary under forward injection. Under illumination condi tion, the photon energy of light is larger than the bandgap of In2S3, and excitions are generated from In2S3 and p-Si. In the forward bias, pho togenerated electron-hole pairs are dissociated to form photoexcited
Fig. 7. Time-dependent photocurrent response of (a) p-Si/In2S3 and (b) p-Si/In2S3: Tm (1.22 at.%) heterostructures under light illumination. 6
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field, resulting in a larger photocurrent [43]. In the reverse bias, the increase of the depletion region and the larger potential barrier lead to the fact that the photocarrier transfer is limited by the high barrier at the interface. The photogenerated electron-hole pairs are not effectively separated and collected, therefore the increase of the reverse photo current is not obvious under illumination [44]. The p-Si/In2S3 hetero structure has a low photocurrent at the reverse bias, and enlarged version of the I–V curves for reverse-biased region under dark and illumination with different light intensities are shown in the inset of Fig. 6c and d. Furthermore, the photocurrent increase with the enhancement of laser intensity suggesting that the photocurrent de pends on the number of photoexcited carriers. At the same forward bias, the photocurrent value of p-type Si/In2S3: Tm (1.22 at.%) hetero structure is larger than p-Si/In2S3 heterostructure, indicating that Tm3þ ions play an important role in improving the sensitivity of light. The photo-sensing application of the two heterostructures are tested by measuring time dependence of the photocurrent by switching the light on/off at room temperature, as shown Fig. 7. The dark current and photocurrent of the p-Si/In2S3 heterostructure are 0.31 μA and 0.78 μA at 2 V bias. However, the dark current and photocurrent of the ptype Si/In2S3: Tm (1.22 at.%) heterostructure are 0.44 μA and 1.48 μA respectively at 2 V bias. Obviously, the photocurrent of doped Tm3þ ions is larger than the pristine In2S3, which is attributed to the fact that effective dopant of Tm3þ ions improves the carrier concentration of In2S3. In addition, the photoresponsivity of the heterostructures is also keep constant after multiple cycles. The photocurrent returned to the original value when the illumination is stopped, demonstrating the short response/recovery times and excellent long-term stability of the heter ostructures. Based on the above photoelectrical testing of the hetero structures reveals that Tm doping in In2S3 can effectively improve the photoelectric performances, such as enhancing the conductivity and the sensitivity to visible light. To gain insight into the electronic structures of the pristine In2S3 and In2S3: Tm QDs, two types of models are built. Fig. 8a and b are corre sponding to In32S48 (the ideal system of pristine In2S3) and In31TmS48 (one Tm dopant atom), respectively. The total density of states (TDOS) and partial density of states (PDOS) are studied and illustrated in Fig. 9. The Fermi level is aligned at 0 eV. For pristine In2S3 (Fig. 9a), the top of the valence band (VB) is composed of S-3p and In-5p, 4d states, the bottom of conduction band (CB) is mainly contributed by S-3p and In-5s
Fig. 8. Two calculation models: (a) In32S48 (the ideal system of pristine In2S3); (b) In31TmS48 (one Tm dopant atom).
carriers under the effect of built-in electric field, and the valence band offset of In2S3 also promotes the transfer of photoexcited holes to the interface. Eventually, these photoexcited carriers drift under the electric
Fig. 9. The total density of states (TDOS) and partial density of states (PDOS) of two models: (a) In32S48; (b) In31TmS48. 7
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states, in agreement with previous results [15]. Compared with the pristine In2S3, it can be clearly seen from Fig. 9b that incorporation of the Tm3þ ions substantially changes the TDOS of the pristine In2S3. Substituting In3þ for Tm3þ lead to bandgap narrowing and the move ment of the Fermi level towards the conduction band, which is attrib uted to the increased carrier concentration induced by Tm doping. Therefore, an extra carrier induced by the doped Tm impurity is the main reason of fluorescence and photoelectric propertied enhancement. From the PDOS, the top of the VB is composed of S-3p, In-5p, 4d and Tm-4f states, and the bottom of CB is mostly composed of S-3p, In-5p and Tm-4f states. The main contribution of the doped Tm comes from the f orbital, while the contribution of d orbital is relatively weaker. The overlap of the Tm-4f and In-5s, 5p, 4d orbitals with S-3p levels in the same range of energies of the PDOS indicates that some covalent bonds exist between In, Tm and S atoms. Moreover, the Tm-4f states are more dispersed towards the Fermi level, which means more covalent nature of the Tm–S bond. It is also pointed out from the two-dimensional charge density (Fig. S5) that there are electron clouds overlap and charge transfer between Tm and S atoms stimulates the formation of Tm–S ionic bond [45].
[5]
[6] [7] [8] [9] [10] [11] [12] [13]
4. Conclusions
[14]
In conclusion, the pristine In2S3 and In2S3: Tm QDs are synthesized using a gas-liquid phase chemical deposition method. The effects of Tm doping on the optical and structural characteristics of In2S3 QDs are investigated. The UV–visible spectra indicate that the Tm3þ ions doping lead to the shrinkage of the bandgap. The luminescence intensity can be tuned by regulating Tm3þ ions doping concentrations. There are con centration quenching in In2S3: Tm QDs for highly Tm3þ ions doping concentrations, which is attributed to the non-radiative energy transfer. The photoelectric performances are measured by fabricating p-Si/In2S3 heterostructures and the results show that Tm3þ ions as effective doping can significantly enhance sensitivity of In2S3 to light. Theoretical anal ysis reveal that the doped Tm3þ ions increase carrier concentration in the In2S3 host materials and lead to more covalent nature of the Tm–S bond. The above results suggest that Tm doped In2S3 QDs have a great potential in photovoltaic cells and photoelectric device applications.
[15] [16] [17]
[18] [19] [20] [21]
Conflicts of interest
[22]
The authors declare no competing financial interest.
[23]
Acknowledgments
[24]
This work was financially supported by the National Natural Science Foundation of China, no. 11474124. Our calculation works were sup ported by the High Performance Computing Center of Jilin University, China.
[25] [26]
Appendix A. Supplementary data
[27]
Supplementary data to this article can be found online at https://doi. org/10.1016/j.jlumin.2019.116775.
[28]
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Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: http://www.elsevier.com/locate/jlumin
Effect of Tm dopant on luminescence, photoelectric properties and electronic structure of In2S3 quantum dots Yi Liu, Liyong Du, Kuikun Gu, Mingzhe Zhang * State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun, 130012, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Tm doped In2S3 Photoluminescence spectra Photoelectric properties Electronic structure calculations
Doping is an effective method to tune the properties of semiconductor nanomaterials. Particularly rare earth ion doping has attracted increasing attention due to its excellent optical properties. However, the design and syn thesis of the indium sulfide (In2S3) quantum dots (QDs) with controllable optical electronic structure remain a challenge. Herein, Tm-doped In2S3 QDs are synthesized using a gas-liquid chemical deposition method. Fluo rescence intensity of In2S3: Tm QDs has great enhancement, which can be controlled by regulating doping concentrations of Tm3þ ions. The photoelectric properties of In2S3: Tm QDs are also researched and the results demonstrate enhanced sensitivity to light, short response/recovery times and long-term stability. Furthermore, based on first-principles calculations, the change of electronic structure and enhancement of optical properties in In2S3: Tm QDs are mainly attributed to Tm3þ ions doping. The as-synthesized In2S3 QDs with luminescent and photoelectric properties are expected to have extensive applications in photovoltaic, photoelectric and sensor fields.
1. Introduction Quantum dots (QDs) have attracted wide interest in recent years due to their noticeable optical properties and potential applications in photodetectors, electrochemistry, solar cells, photocatalysis, etc [1–4]. Great effects have been made to design and synthesize a variety of semiconductor QDs with tunable optical properties. Sulphide materials are considered for a broad range of applications including optoelec tronic device, lasers, bio imaging and photovoltaics due to their unique luminescent and photoelectric properties [5–8]. Doping semiconductor QDs are a crucial process and commonly employed method for acquiring tunable electronic states and improving their performance in diverse fields [9]. Nowadays, an enormous amount of attention is focused on sulphide doped with rare-earth ions. The rare-earth ions doped materials are well developed as optical devices because of high luminescence quantum yield and chemical stability, corresponding to 4f-4f and 4f-5d transitions of rare-earth elements [10–12]. Furthermore, it is necessary to select suitable host materials doped with rare-earth elements to improve their optical properties. Indium sulfide with excellent optical, photoconductive and acoustic properties is an important n-type III-VI sulfide because of its special defect structure [13–15]. In2S3 has been found in three different
crystalline structures: α-In2S3 (defective cubic structure), β-In2S3 (defective spinel structure), γ-In2S3 (layered hexagonal structure) [16]. β-In2S3 with a band gap of 2.0–2.3 eV is a very stable phase at room temperature and shows great potential applications in the visible light range [17]. These properties relate to its defect structure, as shown in Fig. 1a. Its unit cell consist of tetrahedral and octahedral sites where In3þ occupies all octahedral sites and two-thirds of tetrahedral sites. The remaining one-third of tetrahedral sites are unoccupied which result in cation vacancy [18]. Therefore, β-In2S3 is an appropriate host material for various dopants. Moreover, due to the low toxicity of In2S3, it can serve as a substitute for CdS in the buffer layer of solar cells. John et al. reported that CuInS2/In2S3 solar cell obtained a relatively high effi ciency (9.5%) without any anti-reflection coating [19]. Kilani et al. re ported that Sn doping In2S3 films resulted in a widening of the optical gap and enhanced the conductivity [20,21]. Li et al. reported that the rare earth Dy3þ and Tb3þ ions co-doped β-In2S3 showed enhanced photoluminescence emission peaks and room temperature ferromagne tism [22]. However, no relevant reports about the synthesis, optical and photoelectrical properties of Tm3þ doped In2S3 have been published so far. Hence, it is necessary to deeply explore the relationship between its characteristics and changes in internal structures. In this work, the Tm doped In2S3 QDs are fabricated via a gas-liquid
* Corresponding author. E-mail address: [email protected] (M. Zhang). https://doi.org/10.1016/j.jlumin.2019.116775 Received 22 March 2019; Received in revised form 20 September 2019; Accepted 23 September 2019 Available online 24 September 2019 0022-2313/© 2019 Elsevier B.V. All rights reserved.
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Fig. 1. (a) Crystal structure of the β-In2S3 (b) XRD patterns of the pristine In2S3 and the In2S3: Tm QDs. (c) HRTEM image of the 1.22 at% In2S3: Tm QDs. The inset corresponding to SAED pattern.
chemical deposition method and their optical and photoelectric prop erties are investigated and discussed in details. Tm doped In2S3 QDs demonstrate tunable bandgap and enhanced luminescence and light sensitivity by UV–visible spectroscopy, photoluminescence and elec trical measurements. Furthermore, first-principles calculations are per formed to account for doping Tm3þ ions induced structural and electronic changes of In2S3. The experimental and theoretical results indicate that the as-synthesized In2S3 QDs may be a promising material for use in solar cells and optoelectronic fields.
the crystal growth process, a proposed growth mechanism is shown in Fig. S1 and the reaction formula analysed as follows, 3HOCH2 CH2 SH þ InðCH3 COOÞ3 →ðHOCH2 CH2 SÞ3 In þ 3CH3 COOH 3HOCH2 CH2 SH þ TmðCH3 COOÞ3 →ðHOCH2 CH2 SÞ3 Tm þ 3CH3 COOH
ðHOCH2 CH2 SÞ3 In þ ðHOCH2 CH2 SÞ3 Tm þ H2 S↑→In2 S3 : Tm↓
(1) (2) (3)
The growth of the In2S3: Tm QDs mainly involves three steps. Step 1: the preparation of the reaction solution. There are In3þ cations, Tm3þ cations and HOCH2CH2SH in the reaction solution. The addition of acetic acid is beneficial to the dissolution of indium acetate. Moreover, the pH value of the reaction solution cannot be too small, otherwise it will inhibit the reaction between the H2S gas and the solution. The complexes of (HOCH2CH2S)3In and (HOCH2CH2S)3Tm are formed by HOCH2CH2SH combining with In3þ cations and Tm3þ cations in the process of mixing the reaction solutions (Reaction (1), (2)). Step 2: the reaction solution reacted with H2S gas. The dissolved S2 anions in the reaction solution cause the gradual decomplexation of (HOCH2CH2S)3In and (HOCH2CH2S)3Tm complexes to form the ionic bonds between the In3þ/Tm3þ cations and S2 anions (Reaction (3)). This process eventu ally leads to Tm3þ cations doping into the host In2S3 lattices and the formation of multiple nucleation. The In2S3: Tm crystal nuclei attract the surrounding ions to further growth. In the crystal growth stage, exces sive HOCH2CH2SH surrounds the In2S3: Tm crystal nuclei can prevent the agglomeration of crystal nuclei and facilitate the homogeneous growth of crystal nuclei in every direction. HOCH2CH2SH plays an important role in reducing reaction rate, supporting Tm3þ cations doping and controlling homogeneous nucleation. Step 3: the growth of crystal grains. Under the action of ultrasonic bath, the immature In2S3: Tm crystal grains can avoid agglomeration and remove the surrounding HOCH2CH2SH. After the Ostwald ripening, the In2S3: Tm crystal grains gradually grow to stable and homogeneous nanoparticle, which
2. Experiment section 2.1. Sample preparation and growth mechanism The pristine In2S3 QDs and the In2S3: Tm QDs were prepared by using a gas-liquid phase chemical deposition method. Indium acetate (In (COOCH3)3⋅3H2O), Thulium acetate hydrate (Tm(COOCH3)3⋅4H2O) and mercaptoethanol (HOCH2CH2SH) were mixed with deionized water as a reactive solution. All reagents in this work were commercially available and used without further purification. The molar ratios of Tm3þ and In3þ were 0, 3, 6, 9 and 12 at.% in the mixed solution, respectively. HOCH2CH2SH (60 mmol L 1) were dissolved in the above reaction so lutions with magnetic stirring. The pH of the well-mixed reaction solu tion was adjusted to 2.5–3 using acetic acid (CH3COOH) and then was placed in a chamber with circulating water (30 � C). The H2S gas was synthesized by reaction of HCl and Na2S solution according to the molar ratio of 2:1. In the reaction process, the chamber was situated in an ultrasonic bath to avoid the nanoparticles agglomeration. The H2S gas was directly introduced into the reaction solution by using gas pump until the colour of the reaction solution was unchanged. Finally, the precipitates were collected using centrifugation, washed thrice with deionized water and anhydrous alcohol respectively and then dried at 60 � C in a nitrogen atmosphere. To further understand the growth mechanism of the In2S3: Tm QDs in 2
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Fig. 2. XPS results of the 1.22 at% In2S3: Tm QDs: (a) Survey spectrum, (b) In 3d binding energy spectrum, (c) S 2p binding energy spectrum, (d) Tm 4d binding energy spectrum.
eventually precipitate at the bottom of the reaction chamber.
2.4. Calculation details In order to deeply investigate the electronic structures of the pristine In2S3 and the In2S3: Tm QDs, the first-principles calculations based on density functional theory (DFT) were carried out using the projected augmented wave (PAW) method implemented in the Vienna Ab initio Simulation Package (VASP) package. Generalised Gradient Approxi mation (GGA) of Perdew-Burke-Ernzerhof (PBE) was used to treat the exchange-correlation functional. The electronic wave functions were expanded using plane waves with a cut off energy of 400 eV. All supercells were optimized and relaxed with energy and force tolerances of 10 4 eV and 0.05 eV Å 1. A 3 � 3 � 1 Monkhorst-Park mesh was used for the Brillouin zone integration. The Hubbard parameter U (GGA þ U) was used to take care of the Coulomb repulsion and exchange interac tion, the U ¼ 5 eV and U ¼ 7.3 eV were applied to the In d orbital and the Tm f orbital separately.
2.2. Preparation of the pristine In2S3 and In2S3: Tm nanoparticles films 2 mg of the pristine In2S3 and In2S3: Tm (1.22 at.%) nanoparticles were each dissolved in 20 ml of ethanol solution and sonicated to form a uniform suspension. Then the suspension was separately sprayed on the precleaned p-type Si by means of ultrasonic spray. The films were calcined at 500 � C for 2 h in nitrogen-filled environment. 2.3. Characterization X-ray diffraction spectra (XRD) were determined by a Rigaku D/Max2550 X-ray diffractometer with a Cu-Kα line of 1.54056 Å. The dopant concentrations of the as-synthesized samples were obtained by energydispersive X-ray spectroscopy (EDS) provided by scanning electron mi croscopy (Magellan-400). High resolution transmission electron micro scopy (HRTEM) and selected area electron diffraction (SAED) images were recorded by a transmission electron microscopy (JEOL JEM2200FS). The chemical composition and valence states of the samples were characterized using X-ray photoelectron spectroscopy (XPS) (ESCALAB MK II). Room temperature UV–visible spectra were recorded using a UV-2802S spectrophotometer. Photoluminescence (PL) spectra were carried out using a spectrofluorometer (Shimadzu RF-5301PC) with a Xe lamp as an excitation source. The luminescence decay curves were recorded using an Horiba QuantaMaster 8000 fluorescence spectrophotometer. The photoelectrical properties measurement was conducted by Lake-share probe station and Keysight B1500A.
3. Results and discussion The chemical composition and the crystal size of the pristine In2S3 and In2S3: Tm QDs are analysed by XRD pattern, as shown in Fig. 1b. The dopant concentrations of Tm3þ ions are determined by EDS. The dopant concentrations obtained by statistical calculations are around 0.72, 1.22, 1.54 and 2.10 at.%, respectively. The details are shown in the supplementary information. The EDS results (Fig. S2) indicate that the atomic ratio of In and S is close to 1: 1.5, which conforms to stoichio metric proportion of In2S3. All the identified diffraction peaks XRD patterns of the as-synthesized In2S3 QDs are consistent with the standard tetragonal In2S3 phase (JCPDS, No. 73–1366) with the I41/amd space 3
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Fig. 3. (a) UV–vis diffuse reflectance spectra of the pristine In2S3 and In2S3: Tm QDs. The inset shows the related energy level diagram of the Tm3þ ions. (b) (αhν)1/2 as a function of photon energy for the pristine In2S3 and In2S3: Tm QDs.
group. These diffraction peaks corresponding to the (109), (00 12) and (22 12) crystal planes of a tetragonal phase In2S3. No impurity phases are detected such as thulium sulfide, thulium oxide, indium oxide, indicating the samples are high pure and the Tm3þ ions substitude for the In3þ ions without altering the crystal structure of the host material In2S3. The Scherrer’s equation is used to calculate the average size of the samples. The average crystallite sizes of as-obtained samples are roughly 2.8 nm, indicating no significant influence on crystallite size with Tm doping. Fig. 1c displays the HRTEM of the 1.22 at.% In2S3: Tm QDs. The lattice fringes with a spacing of 0.267 nm correspond to the (00 12) plane of the tetragonal In2S3. The average size of the particles is 3–5 nm which is corresponding to the XRD analysis. The SEAD pattern in the inset of Fig. 1c displays polycrystalline diffraction rings consistent with (109), (00 12) and (22 12) planes of In2S3. To determine the chemical composition and valence state of the sample, the 1.22 at.% In2S3: Tm QDs are characterized by XPS. The In2S3: Tm QDs consist of In, Tm, S, C and O which are confirmed by survey spectrum shown by Fig. 2a. The observed C and O in the sample may be attributed to the reference and absorbed gaseous molecules. Fig. 2b shows the high resolution XPS spectrum of In 3d, the peaks at 445.15 eV and 452.72 eV are assigned to In 3d5/2 and In 3d3/2 mainly corresponding to the In3þ in In2S3 [23]. Fig. 2c represents the high resolution S 2p fitted XPS spectrum, the S 2p3/2 and S 2p1/2 peaks positioned at 161.61 eV and 162.2 eV can be assigned to S ion in S2 state in In2S3 [24]. The Tm 4d peak centered at the binding energy of 177.86 eV which confirms the presence of Tm3þ in the In2S3 QDs, as
shown in Fig. 2d [25]. Therefore, the results of XPS analysis reveal that Tm element was successfully incorporated into the In2S3 host crystal lattice. Fig. 3a shows UV–vis diffuse reflectance spectra of the pristine In2S3 and In2S3: Tm QDs. The related energy level diagram of the Tm3þ ions is shown in the inset of Fig. 3a. As seen from Fig. 3a, the strong absorption edge is exhibited in the visible region. Absorption bands at 686 nm and 789 nm associated to the transition of Tm3þ ions from the ground state (3H6) to the excited states (3F2,3, 3H4) [26]. The bandgap values of the synthesized samples are determined from the Tauc relation: �n ðαhνÞ ¼ A hν Eg where hν is the energy of the incident photons, α is the absorption co efficient, A is the absorption constant, Eg is the optical bandgap value and n is the power, which characterizes the band gap transition. n ¼ 2 for an indirect allowed transition and n ¼ 1/2 for a direct allowed transition. The bandgap values can be obtained by the intercept of the linear portion in plotting (αhν)1/2 versus hν and are shown in Fig. 3b. The bandgap value of In2S3 QDs with Tm3þ dopant concentrations of 0 at.%, 0.72 at.%, 1.22 at.%, 1.54 at.% and 2.10 at.% are 2.03eV, 1.96 eV, 1.94 eV, 1.97 eV and 1.99 eV. The bandgap narrowing for In2S3: Tm QDs can be explained using the many-body effects [27]. The inter action between the conduction electrons of In2S3 and the localized f electrons of Tm3þ ions which causes the shrinkage of the bandgap. The variation of bandgap value with increasing Tm3þ concentration further
Fig. 4. (a) PL emission spectra of the pristine In2S3 and In2S3: Tm QDs excited at 279 nm. (b) Dependence and linear fitting of log (I/x) versus log(x) beyond the quenching concentration. 4
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multipole-multipole interaction based on the Dexter’s theory. The emission intensity (I) per activator concentration (x) follows the energy transfer formula (5) [36]: i 1 (5) I=x ¼ k½1 þ βðxÞθ=3 where k and β are constants for a given host crystal in the same exci tation condition; θ ¼ 6, 8, and 10, corresponding to dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interactions, respec tively. Fig. 4b shows that the dependence of log (I/x) (emission intensity (I) at 687 nm) versus log(x) is quiet linear and the line slope (-θ/3) is 1.9842, The value of θ is approximately 6, which reveals that dipoledipole interaction is the concentration quenching mechanism of Tm3þ ions in the In2S3: Tm QDs. To further investigate the energy transfer process, fluorescence decay curves in the In2S3 QDs with different Tm3þ concentrations excited at 283 nm and monitored at 470 nm are shown in Fig. 5. The decay curves can be fitted well using the following three-exponential function (6) [29]: I ¼ A1 exp ð
t = τ1 Þ þ A2 exp ð
t = τ2 Þ þ A3 exp ð
t = τ3 Þ
(6)
where A1, A2 and A3 are the fitting decay constants, τ1 τ2 and τ3 are the decay time for exponential components. The average lifetime (τav) is calculated according to the following equation (7): P 2 Aτ τav ¼ P i i (7) Ai τ i The calculated τav are 54.13 ns, 50.96 ns, 50.81 ns, 43.36 ns, and 40.10 ns, corresponding to 0 at.%, 0.72 at.%, 1.22 at.%, 1.54 at.% and 2.10 at.%, respectively. The fluorescence lifetimes of the as-synthesized In2S3 and In2S3: Tm QDs are longer than previously reported [18,37,38], which may be due to defects in the prepared samples cause trap states near the edge of the conduction band. The conduction electrons in trap states can relax without recombination, which results in extended car rier lifetimes [39,40]. It can be found that the decay lifetimes decrease gradually with increasing Tm3þ dopant concentrations. The decrease of fluorescence lifetimes in the In2S3: Tm QDs indicate that the energy transfer process takes place from the host In2S3 to the dopant Tm3þ ions [29]. In order to futher investigate the energy transfer process between the host In2S3 and Tm3þ ions, the energy transfer efficiency (ηT) can be estimated by the following equation (8):
Fig. 5. The fluorescence decay curves of the 470 nm emissions in the pristine In2S3 and In2S3: Tm QDs under 283 nm excitation.
indicates the successful doping of Tm3þ ions. In order to investigate In2S3: Tm QDs with different doping con centrations for its optical properties, the room temperature photo luminescence spectrum upon 279 nm excitation is depicted in Fig. 4. The emission spectra features broadly consist of five peaks at 417 nm, 470 nm, 493 nm, 530 nm, 586 nm. The emission peak at 417 nm results from the surface oxidation states with higher energy levels [28]. The emission peak at 470 nm is ascribed to exciton recombination while the emission peak at 493 nm, 530 nm and 586 nm are assigned to near-impurity/near-defect excitonic luminescence [29–31]. For Tm doped In2S3 QDs, the emission peak at 682 nm and 764 nm are assigned to the 3F2,3 → 3H6 and 1G4→3H5 electronic transition of Tm3þ, respec tively [32,33]. Tm3þ ions doping causes defects that form many defec tive energy levels. The defective energy levels can participate in and affect the emission process, thus the near-defect excitonic luminescence at 493 nm for the In2S3: Tm QDs is more obvious than that of the pristine In2S3 QDs. The intensities of PL spectra increased with increasing the Tm3þ ions concentration to 1.22 at.%. When the doping concentration is further increased, the intensities are gradually decreased due to the concentration quenching. The concentration quenching is ascribed to the non-radiative energy transfer between the activator (donor) and quenching site (acceptor) [34]. The critical energy transfer distance (Rc) can be expressed by eqn (4) [35]: � �1 3V 3 Rc � 2 4πxc N
ηT ¼ 1
τ τ0
(8)
where τ and τ0 correspond to the decay lifetime of the host In2S3 with and without energy acceptor (Tm3þ) ion present, respectively. The values of the energy transfer efficiency are calculated to be 5.9%, 6.1%, 19.9% and 25.9%, corresponding to Tm3þ dopant as 0.72 at.%, 1.22 at. %, 1.54 at.% and 2.10 at.%, respectively. The lower energy transfer ef ficiencies suggest that more nonradiative relaxations occur in In2S3: Tm QDs. The energy transfer efficiencies increase with increasing Tm3þ dopant concentration, which indicates an efficient energy transfer process. Current-voltage (I–V) characteristics curves can further prove that the effective doping of Tm3þ ions in In2S3 and enhanced conductivity in In2S3: Tm 3þ QDs. Fig. S4 shows I–V curves of the pristine In2S3 and In2S3: Tm (1.22 at.%) QDs, which reflects that the current increases linearly under the bias voltage. Tm3þ doped In2S3 QDs display higher current value than the pristine In2S3 under the same voltage, indicating increased conductivity. This means that the doped Tm3þ ions are beneficial to improve charge conduction and electrical properties of In2S3. Their photoelectric properties are studied through fabricating ptype Si/In2S3 heterostructures under illumination with 405 nm wave length and different power densities at room temperature to explore the potentials for photoelectronic applications. Fig. 6a depicts the schematic
(4)
in which V is the volume of the unit cell, N is the number of cation sites in the unit cell, and xc is the critical concentration. According to the above experimental values of V and N (1880.44 Å3, 32) into eqn (4), the Rc of Tm3þ in In2S3: Tm QDs is calculated to be about 20.9 Å from the critical concentration of Tm3þ (1.22 at.%). This indicates that exchange interaction plays little role in energy transfer because the critical transfer distance of exchange interaction is limited to less than 5 Å. Therefore, the process of energy transfer is controlled by electric 5
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Journal of Luminescence 217 (2020) 116775
Fig. 6. (a) The schematic diagram of the p-Si/In2S3 heterostructure. (b) The energy band illustration of the fabricated heterostructure under light illumination. I–V curves of (c) p-Si/In2S3 heterostructure and (d) p-Si/In2S3: Tm (1.22 at.%) heterostructure under dark and illumination with different light intensities.The inset shows enlarged version of the I–V curves for reverse -biased region under dark and illumination with different light intensities.
diagram of the p-Si/In2S3 heterostructures. The energy band illustration of the fabricated heterostructures under light illumination is presented in Fig. 6b. The electron affinity of p-Si and In2S3 are 4.05 eV and 4.25 eV, respectively [41]. The bandgap values (Eg) of p-Si and In2S3 correspond to 1.12 eV and 2.2 eV respectively, therefore, the valence band offset is 0.28 eV (ΔEv) and the conduction band offset is 0.2 eV (ΔEc). When the light illuminates the heterojunction, electron-hole pairs are generated, and then the constraint of electrons (e) and holes (h) are broken by the built-in electric field. Free electrons and holes are collected under the action of electric field. Fig. 6c and d presents I–V curves of p-Si/In2S3 and p-type Si/In2S3:
Tm (1.22 at.%) heterostructures under dark, and illumination with 405 nm wavelength. The I–V curves of the samples show obvious rectifying characteristics under dark conditions. Under the reverse bias, the dark current is roughly 10 9 nA, which is derived from the generated recombination current by the hot carrier in p-Si under thermal excitation state [42]. Under the forward bias, the current increases rapidly, which is caused by the increase of the concentration of the minority carrier near the boundary under forward injection. Under illumination condi tion, the photon energy of light is larger than the bandgap of In2S3, and excitions are generated from In2S3 and p-Si. In the forward bias, pho togenerated electron-hole pairs are dissociated to form photoexcited
Fig. 7. Time-dependent photocurrent response of (a) p-Si/In2S3 and (b) p-Si/In2S3: Tm (1.22 at.%) heterostructures under light illumination. 6
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Journal of Luminescence 217 (2020) 116775
field, resulting in a larger photocurrent [43]. In the reverse bias, the increase of the depletion region and the larger potential barrier lead to the fact that the photocarrier transfer is limited by the high barrier at the interface. The photogenerated electron-hole pairs are not effectively separated and collected, therefore the increase of the reverse photo current is not obvious under illumination [44]. The p-Si/In2S3 hetero structure has a low photocurrent at the reverse bias, and enlarged version of the I–V curves for reverse-biased region under dark and illumination with different light intensities are shown in the inset of Fig. 6c and d. Furthermore, the photocurrent increase with the enhancement of laser intensity suggesting that the photocurrent de pends on the number of photoexcited carriers. At the same forward bias, the photocurrent value of p-type Si/In2S3: Tm (1.22 at.%) hetero structure is larger than p-Si/In2S3 heterostructure, indicating that Tm3þ ions play an important role in improving the sensitivity of light. The photo-sensing application of the two heterostructures are tested by measuring time dependence of the photocurrent by switching the light on/off at room temperature, as shown Fig. 7. The dark current and photocurrent of the p-Si/In2S3 heterostructure are 0.31 μA and 0.78 μA at 2 V bias. However, the dark current and photocurrent of the ptype Si/In2S3: Tm (1.22 at.%) heterostructure are 0.44 μA and 1.48 μA respectively at 2 V bias. Obviously, the photocurrent of doped Tm3þ ions is larger than the pristine In2S3, which is attributed to the fact that effective dopant of Tm3þ ions improves the carrier concentration of In2S3. In addition, the photoresponsivity of the heterostructures is also keep constant after multiple cycles. The photocurrent returned to the original value when the illumination is stopped, demonstrating the short response/recovery times and excellent long-term stability of the heter ostructures. Based on the above photoelectrical testing of the hetero structures reveals that Tm doping in In2S3 can effectively improve the photoelectric performances, such as enhancing the conductivity and the sensitivity to visible light. To gain insight into the electronic structures of the pristine In2S3 and In2S3: Tm QDs, two types of models are built. Fig. 8a and b are corre sponding to In32S48 (the ideal system of pristine In2S3) and In31TmS48 (one Tm dopant atom), respectively. The total density of states (TDOS) and partial density of states (PDOS) are studied and illustrated in Fig. 9. The Fermi level is aligned at 0 eV. For pristine In2S3 (Fig. 9a), the top of the valence band (VB) is composed of S-3p and In-5p, 4d states, the bottom of conduction band (CB) is mainly contributed by S-3p and In-5s
Fig. 8. Two calculation models: (a) In32S48 (the ideal system of pristine In2S3); (b) In31TmS48 (one Tm dopant atom).
carriers under the effect of built-in electric field, and the valence band offset of In2S3 also promotes the transfer of photoexcited holes to the interface. Eventually, these photoexcited carriers drift under the electric
Fig. 9. The total density of states (TDOS) and partial density of states (PDOS) of two models: (a) In32S48; (b) In31TmS48. 7
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Journal of Luminescence 217 (2020) 116775
states, in agreement with previous results [15]. Compared with the pristine In2S3, it can be clearly seen from Fig. 9b that incorporation of the Tm3þ ions substantially changes the TDOS of the pristine In2S3. Substituting In3þ for Tm3þ lead to bandgap narrowing and the move ment of the Fermi level towards the conduction band, which is attrib uted to the increased carrier concentration induced by Tm doping. Therefore, an extra carrier induced by the doped Tm impurity is the main reason of fluorescence and photoelectric propertied enhancement. From the PDOS, the top of the VB is composed of S-3p, In-5p, 4d and Tm-4f states, and the bottom of CB is mostly composed of S-3p, In-5p and Tm-4f states. The main contribution of the doped Tm comes from the f orbital, while the contribution of d orbital is relatively weaker. The overlap of the Tm-4f and In-5s, 5p, 4d orbitals with S-3p levels in the same range of energies of the PDOS indicates that some covalent bonds exist between In, Tm and S atoms. Moreover, the Tm-4f states are more dispersed towards the Fermi level, which means more covalent nature of the Tm–S bond. It is also pointed out from the two-dimensional charge density (Fig. S5) that there are electron clouds overlap and charge transfer between Tm and S atoms stimulates the formation of Tm–S ionic bond [45].
[5]
[6] [7] [8] [9] [10] [11] [12] [13]
4. Conclusions
[14]
In conclusion, the pristine In2S3 and In2S3: Tm QDs are synthesized using a gas-liquid phase chemical deposition method. The effects of Tm doping on the optical and structural characteristics of In2S3 QDs are investigated. The UV–visible spectra indicate that the Tm3þ ions doping lead to the shrinkage of the bandgap. The luminescence intensity can be tuned by regulating Tm3þ ions doping concentrations. There are con centration quenching in In2S3: Tm QDs for highly Tm3þ ions doping concentrations, which is attributed to the non-radiative energy transfer. The photoelectric performances are measured by fabricating p-Si/In2S3 heterostructures and the results show that Tm3þ ions as effective doping can significantly enhance sensitivity of In2S3 to light. Theoretical anal ysis reveal that the doped Tm3þ ions increase carrier concentration in the In2S3 host materials and lead to more covalent nature of the Tm–S bond. The above results suggest that Tm doped In2S3 QDs have a great potential in photovoltaic cells and photoelectric device applications.
[15] [16] [17]
[18] [19] [20] [21]
Conflicts of interest
[22]
The authors declare no competing financial interest.
[23]
Acknowledgments
[24]
This work was financially supported by the National Natural Science Foundation of China, no. 11474124. Our calculation works were sup ported by the High Performance Computing Center of Jilin University, China.
[25] [26]
Appendix A. Supplementary data
[27]
Supplementary data to this article can be found online at https://doi. org/10.1016/j.jlumin.2019.116775.
[28]
References
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