Ultra-Small Bismuth Quantum Dots: Facile Liquid- Phase Exfoliation

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Ultra-Small Bismuth Quantum Dots: Facile Liquid-Phase Exfoliation, Characterization, and Application in High-Performance UV-Vis Photo-detector chenyang xing, Weichun Huang, zhongjian xie, Jinlai Zhao, Dingtao Ma, Taojian Fan, Weiyuan Liang, Yanqi Ge, Biqin Dong, Jianqing Li, and Han Zhang ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01211 • Publication Date (Web): 20 Nov 2017 Downloaded from http://pubs.acs.org on November 21, 2017

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Ultra-Small Bismuth Quantum Dots: Facile LiquidPhase Exfoliation, Characterization, and Application in High-Performance UV-Vis Photo-detector

Chenyang Xing1,‡ Weichun Huang1,‡ Zhongjian Xie,1 Jinlai Zhao2, Dingtao Ma2, Taojian Fan1, Weiyuan Liang1, Yanqi Ge1, Biqin Dong3, Jianqing Li2 and Han Zhang1 * 1

Shenzhen Engineering Laboratory of Phosphorene and Optoelectronics, International

Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of Ministry of Education, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China. 2

Faculty of Information Technology, Macau University of Science and Technology, Macao

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School of Civil Engineering, Guangdong Province Key Laboratory of Durability for Marine

Civil Engineering, Shenzhen University, Shenzhen 518060, China KEYWORDS: Bismuth, quantum dot, liquid phase exfoliation, self-driven, photo-detection

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ABSTRACT

Two-dimensional (2D) mono-elemental bismuth (Bi) crystals, one of the pnictogens (group VA), has recently attracted increasing interest because of its intriguing characteristics. Here, uniformly sized 2D Bi quantum dots (BiQDs) with an average diameter (thickness) of 4.9±1.0 nm (2.6±0.7 nm) were fabricated through a facile liquid-phase exfoliation (LPE) method, and the corresponding photo-response was evaluated using photoelectrochemical (PEC) measurements. The as-fabricated BiQDs-based photodetector not only exhibits an appropriate capacity for selfdriven broadband photoresponse but also shows high-performance photoresponse under low bias potentials ranging from UV to visible light in association with long-term stability of the ON/OFF switching behavior. In terms of these findings, it is further anticipated that the resultant BiQDs possess promising potential in UV-visible photodetection as well as in liquid optoelectronics. Our work may open a new avenue for delivering high-quality monoelemental pnictogen QDs from their bulk counterparts, thereby expanding interest in 2D monoelemental materials.


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The previous two decades have witnessed rapid exploration and development of van der Waals materials (vdWMs) since the path-breaking discovery of graphene was reported in 2004.1-13 Twodimensional (2D) materials are defined as layered structures with a unique anisotropy stemming from strong differences in the bonding type between inter-layers (weak vdW interactions) and intra-layers (strong covalent bonding). As a consequence, 2D crystals with ultrathin or even atomic thickness have emerged as an appealing alternative to traditional 3D bulk crystal because of their excellent transparency, flexibility, and malleability. As a star member of the 2D family, graphene has found significant applications in electronic devices as a result of its high carrier mobility and linear dispersion relation of its Dirac cone and is now considered an alternative to silicon as a new generation of electronic components. However, this unique Dirac cone results in a permanent conductive state of graphene, that is, zero bandgap (Ep), leading to a poor capacity to absorb light and weak electronic on/off ratio. The zero-Ep of graphene may limit its applicability in optoelectronic applications. Consequently, other 2D crystals with non-zero Eg are believed to be vital supplements to graphene. Recently, black phosphorus (BP) in group VA (group 15, the pnictogens) has gained tremendous attention because of its thickness-dependent gaps (0.3 eV to 2.0 eV) and a high carrier mobility, approximately 103-104 cm2 V-1 s-1.14-18 Xu et al. fabricated BP quantum dots (BPQDs) and evaluated their nonlinear optical response.19 Ren et al. reported a BP nanosheet (BPNF)-based self-powered photodetector and observed a robust photoresponse stability for BPNFs in alkaline electrolytes.20 Li et al. successfully fabricated fewlayer BP-based field effect transistors.14 Importantly, because of the intriguing properties of BP, other elemental 2D pnictogen materials, such as arsenene (As), antimonene (Sb), and bismuthene (Bi), have also been considered. Theoretical investigations demonstrate that the pnictogen monolayers have a broad range of Ep, rendering them suitable for broadband photo-response.21


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For instance, their Eg were calculated to be 1.68 eV (α-arsenene), 1.43 eV (α-antimonene), 0.36 eV (α-bismuthene), and 0.99 eV (β-bismuthene). In addition, it has also been demonstrated that puckered arsenene and buckled bismuthene hold carrier mobilities as high as several thousand cm2 V-1 s-1.21 With regard to their broad range of spectral band-gaps, as well as high carrier mobilities, the pnictogens are reasonably considered to have unique potential for broadband photo-response-related applications. Bismuth (Bi), in this context, has recently gained additional attention among other monoelemental 2D pnictogen materials. First, bismuthene is theoretically predicted to be an intrinsic quantum spin Hall (QSH) insulator in view of its strong spin-orbit coupling (SOC) strength,22-24 while arsenene and antimonene are normal semiconductors in the ground state.25-27 Second, bismuth offers high scalability and compatibility with existing silicon-based technology.28 For example, single-crystalline and polycrystalline bismuth films can be readily grown on Si (111) and Si (001) substrates through molecular beam epitaxy (MBE)29-33 and annealing processes,34 respectively. To date, many chemical routes have been developed for synthesizing metallic Bi nanomaterials, such as photochemical,35 metal-organic precursor,36-39 reduction,40-45 solvothermal,46,47 microwave,48 and laser ablation methods.49,50 The photochemical and metal-organic precursor methods have been widely employed to synthesize high-quality nanoparticles. W. Heiss37 and W. E. Buhro et al.38 made improvements via the photochemical method and metal-organic precursor method, respectively, but the strict experimental procedure, high temperature, toxic organometallic precursors, and long reaction time limit their continued development. Different from chemical strategies, liquid-phase exfoliation (LPE) can realize large-scale fabrication of Bi nanosheets. Gusmão et al. reported exfoliated Bi nanosheets via shear exfoliation with the aid of an aqueous surfactant.51 However, these Bi nanosheets suffered


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from the uncontrollability of the lateral size and thickness, as well as unwanted oxidation. Compared with Bi nanosheets, Bi quantum dots (BiQDs) with uniform size can be considered an excellent candidate for fabrication using a LPE method, which has not been documented in the literature. Herein, BiQDs with uniform size distribution (approximately 5 nm) were fabricated via a LPE route. The as-prepared ultrasmall BiQDs were developed as a building block for photoelectrochemical (PEC)-type photodetectors, and we found that the BiQDs-based photodetectors exhibited an appropriate self-driven photoresponse performance in KOH electrolytes. Moreover, the photoresponse behavior can be significantly enhanced by applying an external bias potential. Additionally, long-term stability measurements verified that BiQDsbased photodetectors showed appealing durability in alkaline electrolytes. Because of their facile synthesis, excellent self-driven photoresponse, and durability in alkaline electrolytes, this BiQDs-based photodetector may shed light on new designs for QDs-based PEC-type photodetectors.

Experimental section Preparation of BiQDs. A typical liquid-phase exfoliation (LPE) method was used to fabricate BiQDs, as shown in Scheme 1. Briefly, bulk Bi powder was mixed with N-methyl pyrrolidone (NMP) with a concentration of 5 mg/mL. The mixture was then subjected to bath sonication with a power of 400 W for 48 hours. The temperature was fixed at 5 °C by built-in cooling water equipment. To obtain a BiQDs/NMP solution, the above mixture was first filtrated through a porous 100-nmpore-diameter anodized aluminum oxide (AAO) membrane to remove large-size Bi nanomaterials and then filtered again through a 20-nm-pore-diameter AAO membrane, yielding


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Bi nanomaterials with smaller size (Scheme 1a). The BiQDs/NMP solution was then centrifuged with a speed of 18,000 rpm for 30 min. The obtained precipitate was dried in a vacuum drying oven. To prepare photoelectrochemical (PEC)-based samples, BiQDs (1 mg) were dispersed into 1 ml of PVDF/NMP (10 mg/100 mL) by bath sonication (400 W, 30 min). The mixture was then directly dropped onto the conductive side of ITO glass, and the BiQDs-coated ITO was dried in a vacuum drying oven at 80 °C for 3 hours. Characterization Transmission electron microscopy (TEM, Tecnai G2 Spirit 120 kV) was used to characterize the morphology and height distribution of BiQDs. High-resolution transmission electron microscopy (HRTEM) was also used to determine the atomic arrangement. Atomic force microscopy (AFM, Bruker) was used to evaluate the height of BiQDs with 512 pixels per line. High-resolution confocal Raman microscope (HORIBA LabRAM HR800) was operated to record the Raman spectra for BiQDs at room temperature with an excitation wavelength of 633 nm. UV-vis-NIR absorption spectra for BiQDs were measured in the range of 200-1100 nm using a UV-vis absorbance spectrometer (Cary 60, Agilent). Photoresponse activity The typical photoresponse behavior of the BiQDs was evaluated using a PEC measurement with a standard three-electrode system, as shown in Scheme 1b. The BiQDs-coated ITO glass, platinum wire, and saturated calomel were functioned as the working electrode (i.e. photoanode), counter electrode (photocathode), and reference electrode, respectively. The KOH (0.1 M, 0.5 M and 1.0 M) and Na2SO4 (0.1 M and 0.5 M) aqueous solutions were used as electrolytes. Lights with wavelengths of 350 nm, 365 nm, 380 nm, 400 nm, 475 nm, 520 nm, 550 nm, 650 nm and 700 nm (the certain wavelengths of lights were realized by using their corresponding optical


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filters) were used to illuminate the BiQDs sample. And a gradually increasing light intensities of these lights with labels of level I, II, III, IV, and VI were applied (please see Table S1 in the Supporting Information). As a control, pure ITO glass was also irradiated by a simulated light (a mixed light from 300 nm to 800 nm) under the same conditions. Linear sweep voltammetry (LSV) was carried out at bias potential from 0 V to 1.0 V at a scan speed of 0.01 V/s. Amperometric current-time (I-t) curves were recorded at bias potential of 0 V, 0.3 V, and 0.6 V under gradually increasing light irradiation with a sampling interval of 5 s. Electrochemical impedance spectrum (EIS) were determined in the frequency range from 105 to 100 Hz with an amplitude of 0.005 V.


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Scheme 1 The experimental schematic of the BiQDs-based photodetectors: (a) a liquid phase exfoliation (LPE) strategy to obtain the ultra-small BiQDs; (b) a typical PEC system built for evaluating the photoresponse behaviors of BiQDs.


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Results and Discussion Figure 1 illustrates typical characterizations of the as-prepared BiQDs. From the TEM image in Figure 1a, BiQDs were uniformly exfoliated with an average size of 4.9 ± 1.0 nm (Figure 1f). High-resolution TEM images show clear lattice fringes of 0.21 nm (Figure 1b) and 0.33 nm (Figure 1c), the latter of which can be ascribed to the (012) plane of Bi crystal.52 In addition, an obvious and strong fast Fourier transform (FFT) photograph (Figure 1d) as well as a selected area electron diffraction (SAED) pattern (Figure 1e) demonstrate that the crystal features of BiQDs are well-preserved during the liquid exfoliation process. The topographic morphology of BiQDs was obtained using AFM measurements, as seen in Figure 1g. The measured heights of 2.3 nm, 3.7 nm, and 4.7 nm in Figure 1h correspond to 7, 11, and 14 layers, respectively, considering that one layer is regarded as an average inter-atomic spacing in bulk Bi of 0.34 nm.53 The average thickness of BiQDs was measured to be 2.6 ± 0.7 nm (Figure 1i). The Raman spectra of bulk Bi and BiQDs are shown in Figure 1j. Bulk Bi has two main resonances, at 68.9 cm-1 and 95.3 cm-1, corresponding to the Eg and A1g first-order Raman modes of Bi crystals.54 Additionally, other weak peaks, such as those at 117.9 cm-1, 138.0 cm-1, 185.6 cm-1, 208.4 cm-1, and 308.9 cm-1, were observed for bulk Bi, which could be traced back to the formation of αBi2O3,

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Compared with bulk Bi, BiQDs have differences in the Eg as well as A1g modes,

showing an obviously decreased peak intensities ratio. For instance, the value of peak intensities ratio of I(Eg)/I(A1g) was calculated to be around 0.61 for BiQDs, much lower than that of 1.09 for Bulk Bi. This dependence of the Raman intensities may be ascribed to the flake thickness of the exfoliated nanosheets or quantum dots,

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and this observation for BiQDs is in agreement

with those for previously reported Bi nanosheets.51 The optical characterization of the BiQDs is shown in Figure 1k. A single but sharp peak can be observed at 264 nm, suggesting a good


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uniform dimension distribution of BiQDs. Additionally, BiQDs show a broad absorption ranging from 200 nm to 600 nm, implying their potential for application in UV-visible photoelectric devices. This result is in good accordance with previously reported results on mono-dispersed spherical Bi nanoparticles.50


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Figure 1. Characterization of BiQDs: (a) TEM image illustrating the quantum size; (b) and (c) high-resolution TEM (HRTEM) images showing crystalline lattices; (d) fast Fourier transform (FFT) photograph; and (e) selected area electron diffraction (SED) pattern; (f) statistical analysis of the dimensions of BiQDs measured from the TEM image; (g) AFM image of BiQDs; (h) height profiles along the white lines in Figure 1g; (i) statistical analysis of the heights of BiQDs measured from the AFM image; (j) Raman spectra recorded by an excitation laser with a wavelength of 633 nm; (k) UV-Vis absorption spectrum of BiQDs in NMP. Inset: Photos of the BiQDs suspensions (left) and Tyndall effect of the BiQDs suspensions (right).


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Figure 2. Self-driven photo-response behavior of BiQDs evaluated using PEC measurements. The typical on/off switching behavior of the current density of BiQDs was triggered by irradiation of various wavelengths of light (350, 365, 380, and 400 nm) with increasing light intensity (namely, levels I, II, III, IV, and VI). The electrolyte used was KOH aqueous solution with various concentrations. (a) and (b): 0.1 M KOH; (c and d): 0.5 M KOH; (e and f): 1.0 M KOH. Note that (b), (d), and (f) are the typical current density–time (I–t) curves under irradiation of light with an incident intensity of level IV. (g) and (h): curves of photocurrent density (Iph) and photo-responsivity (Rph) as a function of wavelength; (f) curves of Iph and Rph as a function of light intensity (Pλ). The data for Iph and Rph were calculated from Figure 2b, 2d, and 2f, respectively.

The optical absorption of BiQDs in Figure 1k demonstrates that the BiQDs show a broad absorption spanning from 200 nm to 600 nm (i.e., from ultraviolet to visible). In view of this finding, several wavelengths of lights (350, 365, 380, 400, 475, and 520 nm) were employed to


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illuminate a BiQDs-based photodetector to evaluate its self-driven photo-response performance (i.e., at a bias potential of 0 V). Linear sweep voltammogram (LSV) curves show that the BiQDs-based photo-detector possesses a positive response with respect to applied external light (see Figure S1 in the Supporting Information). The influences of light intensity and concentration of electrolyte KOH aqueous solution on the photoresponse behavior of the BiQDs-based photodetector were systematically investigated in KOH electrolyte, as shown in Figure 2. Another electrolyte, Na2SO4, was also employed, and its insufficient practicability is demonstrated in Figure S2 and S3 in the Supporting Information. Results show that the lower photocurrent signal in Na2SO4 electrolyte can be explained with the much larger interfacial resistance of electrode (i.e. BiQDs-coated ITO glass) with Na2SO4 electrolyte as compared to that in KOH electrolyte under the same condition. As a control, naked ITO glass without the BiQDs sample was also investigated. The naked ITO glass exhibited negligible photo-response behavior in 0.1 KOH, as shown in Figure 2a, regardless of light intensity when exposed to the illumination of a mixed light (i.e. simulated light) with wavelengths from 300 nm to 800 nm. In sharp contrast, BiQDs showed clear ON/OFF switching behavior under irradiation of light at 350 nm, 365 nm, 380 nm, and 400 nm. In addition, the current signals gradually increased with increasing light intensity (i.e., from level I, II, III, IV, to level VI with explicit values of various lights in Table S1 in the Supporting Information) in these cases. Moreover, higher KOH concentrations of 0.5 M and 1.0 M led to a large increase in the current signals, as shown in Figure 2c and 2e, respectively. Although the naked ITO glass also possessed a self-driven photoresponse behavior that can also be strengthened by elevating the concentrations of KOH, its considerably low photocurrent value compared with that of BiQDs-based sample suggests that the dominant self-driven photoresponse behavior of the BiQDs-based photodetector stemmed


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from BiQDs. It is believed that BiQDs can generate a large amount of photo-induced electronhole pairs when exposed upon the illumination of lights due to the strong BiQDs-light interactions. The efficient separation of those electron-hole pairs can give rise to photocurrents, especially under an external electric filed. In the present study, it is interesting to find that the BiQDs-based photodetector can generate strong photocurrents without any bias potential, demonstrating an excellent self-driven photoresponse performance for BiQDs, which has rare been reported in the Bi-film based photodetector in the previous study.

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In spite of their broad

absorption (from 200 nm to 600 nm), shown in the above optical results, BiQDs exhibited little photoresponse toward light at 475 nm and 520 nm. This can be attributed to the separation of the photogenerated electron-hole pairs that can form a photocurrent, which is also determined by an external bias potential (this will be discussed later). To quantitatively evaluate the self-driven performance of the BiQDs-based photodetector under various conditions, photocurrent density (Iph), photoresponsivity (Rph), rise time, and decay time were calculated using the following equations 57: Iph=Ilight-Idark

(1)

Rph=Iph/(Pλ·S) (2) where Ilight and Idark indicate the drain current density with and without light, respectively; Pλ and S denote the light power intensity and effective area of the BiQDs sample on ITO glass, respectively. The calculated Iph and Rph values as a function of wavelength of light are shown in Figure 2g and 2h, obtained from Figure 2b, 2d, and 2f under a fixed Pλ (i.e., level IV). As shown in Figure 2g, the Iph values first increased and then gradually decreased with a red-shift of light wavelength, regardless of the concentration of KOH. The highest Iph value in different concentrations of KOH was observed at 365 nm. Moreover, with a fixed irradiation of light, the


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Iph value can be significantly improved by increasing the concentration of KOH. For instance, the Iph value at 365 nm in 0.1 M KOH was calculated to be 4.75×10-3 µA/cm2, which can be increased to 4.80×10-2 µA/cm2 (0.5 M KOH) and 5.51×10-2 µA/cm2 (1.0 M KOH), respectively. In terms of PEC measurement for BiQDs, the factors associated with the electrochemical properties, such as electrolytes and their concentrations, can also play vital roles in the photoresponse behaviors of BiQDs. As a result, higher concentrations of NaOH can further reduce the interfacial resistance of BiQDs with electrolyte, thus resulting larger photocurrent (see Figure S4 in Supporting Information). However, the Rph, according to Equation 2, is associated with both Iph and Pλ when the effective active area is fixed. Therefore, the highest Rph value of 19.3 µA/W was observed at 350 nm in 1.0 M KOH in Figure 2h. Additionally, with a red-shift of light wavelength, the Rph values with various KOH concentrations always showed a decreasing trend from UV to visible regions. As seen in Figure 2i, the Iph value was found to have a positive correlation with Pλ, while Rph has a negative one, taking the case of 350 nm in 0.1 M KOH as an example. Larger light intensity of lights provided BiQDs sample with more amount of photos (thus more energies), generating much more amounts of electron-hole pairs and thus larger photocurrent.


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Figure 3 Photoresponse behavior of BiQDs under the illuminations of light (350, 365, 380, 400, 475, 520, 550, 650, and 700 nm) at bias potentials of 0.3 V and 0.6 V in 0.1 M KOH. (a) and (b): 0.3 V; (c) and (d): 0.6 V; (e): Iph values as a function of wavelength; (f): Rph values as a function of wavelength.

To shed much more light on the photoresponse behavior of the BiQDs, the influence of a bias potential on the Iph and Rph in 0.1 M KOH was also investigated, as shown in Figure 3. In contrast to the self-driven performance at a bias potential of 0 V in Figure 2, the BiQDs-based photodetector displayed stronger ON/OFF switching signals both at 0.3 V (Figure 3a and 3b) and


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0.6 V (Figure 3c and 3d) in 0.1 M KOH. Much higher concentrations of KOH were also used in Figure S5 in the Supporting Information. For instance, as seen in Figure 3e, under the irradiation of 365 nm, the Iph values can reach 6.33×10-2 µA/cm2 (at 0.3 V) and 1.02 ×100 µA/cm2 (at 0.6 V), approximately 13 and 215 times higher than that at 0 V, respectively. This phenomenon can be explained by the fact that higher external electric field can improve the separation efficiency of electron-hole pairs triggered by the illumination of light, leading to an increase in the photocurrent. In addition, higher bias potential can also result in a significant increase in the Rph value, as shown in Figure 3f. For instance, at a bias potential of 0.6 V, the Rph values can reach 294.9 µA/W at 350 nm and 285.7 µA/W at 365 nm, which are 169 and 215 times higher than those at 0 V under the same conditions, respectively. It should be noted that the photocurrent signals of BiQDs irradiated by light at 475 nm, 520 nm, and 550 nm can be obviously enhanced by applying bias potentials of 0.3 V and 0.6 V, compared with those at 0 V, which suggests that the photoresponse behavior of BiQDs is determined not only by its band gap (Eg) or the energy of light but also by the experimental conditions. The above photoresponse results are in strong agreement with the optical absorption results in Figure 1k. Additionally, a parameter of specific detectivity (D*, Jones or cm⋅Hz1/2⋅W-1) was also calculated according to the following equation 57 (assuming that the dark current is the major source of short noise): D*= Rph⋅S1/2/(2q⋅Idark)1/2 (3) From Equation 3, several typical D* values for the BiQDs-based photodetector were calculated to be 1.79×108 Jones (0.3 V, 2.17 mW/cm2) and 8.68×108 Jones (0.6 V, 2.17 mW/cm2) at 350 nm and 1.35×108 Jones (0.3 V, 3.57 mW/cm2) and 9.09×108 Jones (0.6 V, 3.57 mW/cm2) at 365 nm. Furthermore, the response time (tres) and recovery time (trec) of BiQDs photodetector are assigned to the time interval for the rise and decay from 10 % to 90 % and from 90 % to 10 % of


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its peak value (Figure S6 in the Supporting Information), respectively. Results show that the BiQDs photodetector exhibited quite fast tres as well as trec in the present study. For instances, the values of tres and trec were calculated to be 0.1 s and 0.2 s (350 nm, 1.0 M KOH) and 0.2 s and 0.2 s (365 nm, 1.0 M KOH) for the self-driven photoresponse cases. Moreover, these values were not significantly influenced by applying bias potentials or altering the concentration of KOH electrolyte (Figure S6 in the Supporting Information), indicating an intrinsic photoresponse behaviors of the BiQDs-based photodetector. It should be noted that the above values of tres as well as trec are much lower than those of BPNF-based photodetector, respectively,

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that BiQDs photodetector have an appealing potential used in the PEC-type detector fields.


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Figure 4 Photoresponse stability of the BiQD-based photo-detector under the illumination of light at 365 nm before and after 1 month. (a) and (b): self-driven behavior in 0.5 M KOH without bias potential; (c) and (d): photoresponse behavior in 0.1 M KOH under a bias potential of 0.6 V. The stability of the photoresponse of BiQDs-based photodetectors is of great importance for their long-term application. The cyclic and temporal stability of the BiQDs-based photodetector before and after 1 month was evaluated under the illumination of light at 365 nm, shown in Figure 4. For the fresh BiQDs-based photodetector in 0.5 M KOH toward its self-driven photoresponse, as seen in Figure 4a, the Iph value initially decreased and then reached an equilibrium state. The reduced Iph value phenomenon may have a close correlation with the wetting degree of the BiQDs-coated ITO glass in electrolytes. Much more immersion time for


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the sample in electrolyte may shorten the time required to reach an equilibrium state. Once the Iph value reached the equilibrium state, as shown in Figure 4b, the BiQDs-based photodetector exhibited excellent photoresponse performance stability. An approximate reduction of 56.7% of Iph was found when the same sample was measured again after 1 month in 0.5 M KOH. This obvious reduction may be attributed to the peeling off of BiQDs from the ITO glass and the possible oxidation of Bi in 0.5 M KOH over time. In spite of this decrease in Iph relative to that of the fresh sample, BiQDs also exhibited a good stability in the equilibrium state, as shown in Figure 4b. Similarly, at a bias potential of 0.6 V, as seen in Figure 4c and 4d, the BiQDs also displayed a good stability before and after 1 month. Only a 48.6% reduction of the Iph value was observed in 0.1 M KOH. In spite of the obvious reduction of Iph values obtained under illumination at 365 nm with or without bias potential, the BiQDs-based photodetector still displayed clear as well as appropriate ON/OFF switching behavior, indicating the good cyclic and temporal stability of its photoresponse.

Conclusion Uniform-sized BiQDs were produced by a facile LPE method. They exhibited an average lateral dimension of 4.9 ± 1.0 nm and an average thickness of 2.6 ± 0.7 nm. The optical absorption of the BiQDs not only demonstrated a uniform size distribution but also revealed a broad absorption range spanning from 200 nm to 600 nm, the latter of which is a prerequisite for the potential application of photodetectors. Under the illumination of light at 350 nm, 365 nm, 380 nm, and 400 nm, the BiQDs-based photodetector had an appropriate capacity for self-driven photoresponse, with the largest Iph of 55.1 nA/cm2 at 365 nm and the largest Rph of 19.3 µA/W at 350 nm in 1.0 M KOH. More importantly, with a bias potential of 0.6 V and under the same conditions, the corresponding Iph and Rph significantly increased to 1020 nA/cm2 and 294.9


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µA/W, respectively. This phenomenon can be ascribed to the more effective separation of the photogenerated electron-hole pairs under an external filed. Additionally, other light at 475 nm, 520 nm, and 550 nm, which were absent in the cases without a bias potential, can trigger the photoresponse behavior of the BiQDs-based photodetector with bias potentials of 0.3 V and 0.6 V, which is strongly supported by the optical characteristics. Moreover, the good temporal stability of the photoresponse performance of the BiQDs-based photodetector is demonstrated in 0.1 M KOH by a decrease of only 50% of the Iph value after 1 month without any protection. This contribution may pave the way to realizing uniform-sized BiQDs, and the resultant BiQDs show promise for potential application in photo-detectors in the UV-visible band and other photoelectric fields. ASSOCIATED CONTENT Supporting Information. Linear sweep voltammogram (LSV) curves, typical photoresponse behavior of BiQDs-based photodetectors under various experimental conditions, electrochemical impedance spectrum (EIS) profiles and power intensity (Pλ) of incident light with various wavelengths. (PDF) AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (H. Z.) Author Contributions ‡

C. Xing and W. Huang contributed equally.

Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENT The financial support from by the Science and Technology Development Fund (No. 007/2017/A1), Macao SAR, China, National Natural Science Fund (Grant Nos. 61435010, 61575089, and 81701819), Science and Technology Innovation Commission of Shenzhen (KQTD2015032416270385 and JCYJ20150625103619275), China Postdoctoral Science Foundation (Grant Nos. 2017M612712 and 2017M612730) and Natural Science Foundation of Guangdong Province (Grant Nos. 2017A030310495) is grateful acknowledged. Graduate school at Shenzhen, Tsinghua University materials and devices testing centre is also gratefully acknowledged. And We also thank Joshua Green, MS, from Liwen Bianji, Edanz Editing China (www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript.

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For Table of Contents Use Only TOC

Ultra-Small Bismuth Quantum Dots: Facile LiquidPhase Exfoliation, Characterization, and Application in High-Performance UV-Vis Photo-detector

Chenyang Xing1,‡ Weichun Huang1,‡ Zhongjian Xie,1 Jinlai Zhao2, Dingtao Ma2, Taojian Fan1, Weiyuan Liang1, Yanqi Ge1, Biqin Dong3, Jianqing Li2 and Han Zhang1 * 1

Shenzhen Engineering Laboratory of Phosphorene and Optoelectronics, International

Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of Ministry of Education, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China. 2

Faculty of Information Technology, Macau University of Science and Technology, Macao

3

School of Civil Engineering, Guangdong Province Key Laboratory of Durability for Marine

Civil Engineering, Shenzhen University, Shenzhen 518060, China


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ACS Photonics

Uniformly sized 2D Bi quantum dots (BiQDs) based photodetectors have possessed excellent self-driven photoresponse performance ranging from UV to visible light in association with longterm stability of the ON/OFF switching behavior by virtue of a novel photoelectrochemical (PEC) measurement.


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Ultra-Small Bismuth Quantum Dots: Facile Liquid- Phase Exfoliation

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