Assembling of Sulfur Quantum Dots in Fission of Sublimed Sulfur

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Assembling of Sulphur Quantum Dots in Fission of Sublimed Sulphur Lihua Shen, Hongni Wang, Shengnan Liu, Zhuangwei Bai, Sichun Zhang, Xinrong Zhang, and Chengxiao Zhang J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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Journal of the American Chemical Society

Assembling of Sulphur Quantum Dots in Fission of Sublimed Sulphur Lihua Shen,*,†, ‡ Hongni Wang,† Shengnan Liu,† Zhuangwei Bai,† Sichun Zhang,‡ Xinrong Zhang,*,‡ and Chengxiao Zhang*,§ †

College of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, Xi’an, 710054, China;

‡

Beijing Key Laboratory for Microanalytical Methods, Instrumentation, Department of Chemistry, Tsinghua University, Beijing 100084, China;

§

Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education), School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an, 710062, China ABSTRACT: A novel kind of quantum dots, sulphur quantum dots(S dots) is synthesized by simply treating sublimated sulphur powders with alkali using polyethylene glycol-400 as passivation agents. The synthesized S dots exhibits excellent aqueous dispersibility, eminent photostability and temperature dependent photoluminescence (PL). An “Assemble-fission” mechanism is proposed for the S dots formation in which “assembling” and “fission” are involved and contest each other. The ultimate morphologies of the S dots are dependent on the balance of the two forces. Guided by the “assemble-fission” mechanism, weakening the assembling effect is beneficial for obtaining monodisperse S dots, which can be achieved by pretreating of sulphur powder with nitric acid. PL wavelength of the S dots has been successfully tuned between green and blue light (from 550 to 440 nm) by simply controlling reaction time. A satisfactory quantum yield of 3.8% is obtained. Significant electrochemiluminescence of the S dots is observed in an annihilation reaction. Chemiluminescence from the S dots has been observed by direct oxidation. Taking advantage of unique and inherent antimicrobial activity of the sulphur particles, it is believed that this new emerging luminescent nanomaterial is highly promising in the development of new types of optoelectronic devices and tracer for live cells, in vivo imaging and diagnostics.

and remarkable inherent biological activities. 16-17 Sulphur nanomaterials are overwhelmingly superior to carbon nanomaterials and often used as antimicrobial or antifungal agents. 16-17 However, by so far, only Li’s group reported the synthesis and the PL property of sulphur quantum dots(S dots). 15 They converted CdS quantum dots into S dots by reaction with HNO3. This kind of S dots has a low PL quantum yield of 0.549% and merely emits blue light. More importantly, no research to date has been reported that S dots can generate ECL or chemiluminescence (CL) emission. In the theoretical research field of quantum dot synthesis, synthetic approaches are generally classified into two categories, i.e., “bottom-up” and “top-down”. The crystal growth mechanism, kinetics, and microstructure development play a fundamental role in tailoring the materials with controllable sizes and morphologies. The “bottom-up” approaches are mainly based on the classical crystal growth kinetics-Ostwold ripening (OR) theory in which larger particles grow at the expense of smaller particles.18 Another

INTRODUCTION Luminescent quantum dots have generated much excitement for a wide variety of promising applications in optoelectronic devices, biological labeling and biomedicine because of their unique size-dependent electronic, magnetic, optical and electrochemical properties.1, 2 Moreover, some semiconductor quantum dots can be used as electrochemiluminescence (ECL) reagents in bioanalytical applications.3 However, the potential toxicity and environmental hazards,2 associated with heavy metal containing quantum dots (e.g., CdSe,4 CdTe,5 ZnS6), limit their applications. Therefore, recent efforts are mainly devoted to searching for heavy-metal-free quantum dots, especially pure elemental quantum dots, such as silicon, 7-9 carbon, 1, 10-12 phosphorus 13-14 and sulphur. 15 These pure elemental quantum dots, with their low toxicity, excellent solubility and stable photoluminescence (PL) are considered to be the next generation of alternative nanomaterials. Among the reported pure elemental quantum dots, S nanomaterials find a prominent place due to their unique chemical properties

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significant mechanism named “oriented attachment” (OA) has been widely reported, and revealed remarkable effects on directing and mediating the self-assembly of nanocrystals.19 The “top-down” approaches involve breaking down larger bulk structure by methods such as laser and electrochemical oxidation. However, there is little known theory that accounts for mechanism of the process. Herein, a new and simple method for the synthesis of water-dispersed S dots is developed by simply treating sublimated sulphur powders with alkali using polyethylene glycol-400 (PEG-400) as passivation agent. These S dots are expected to have high PL quantum yield and tuning emission light behaviors. The size and shape of S dots at different duration are observed in detail by transmission electron microscopy (TEM) assisted with characterization by PL and ultraviolet-visible absorption (UV) spectroscopy. A theory of “assemble-fission” for the formation of the S dots is proposed, which can provide an example or ideas to account for the “topdown” approaches. For the first time, significant ECL emission has been observed by applying potential cycled scanning or potential pulse step. It is also found that S dots can generate CL emission by direct oxidation.

broadened and the λem varied with λex. For 72 h sample, no significant broadening of PL spectra was observed compared to a 54 h sample except that the PL intensity enhanced remarkably. When the reaction ran up to 100 h, the λem was fixed near 460 nm as λex changed from 300 to 360 nm. Then λem started shifting towards longer wavelength (465 to 524 nm) as λex varied from 380 to 460 nm. A similar behavior was observed for 125 h sample where λem was also found excitation-independent at around 440 nm when the λex was from 300 nm to 360 nm (see Table 1). And excitationdependent PL was also exhibited when the λex varied from 380 nm to 460 nm.

RESULTS AND DISCUSSION Synthesis of the S dots Sublimed sulphur powder (1.4 g), ultra-pure water (50 mL), PEG-400 (3 mL) and sodium hydroxide (4 g) were added into a 100 mL round-bottom flask. This mixture was stirred at 70oC for a fixed time (30 h~125 h). The bulk sulphur powder gradually dissolved and the color of the mixture changed from colorless to transparent yellow, to orange, and even to red as the heating time went on. This phenomenon reflects the formation of sodium polysulfide and this process is considered as dissolution of bulk sulphur powder. Extension of heating time results in lighter color of the mixture. The product obtained was used as S dots samples and the nanomaterial in the product was referred to as S dots. Here, PEG-400 acts as passivation agent. When PEG is not used during the synthesis, the resulting non-passivated particles express a poor luminescence with irregular spectra and weak intensity (Figure. S1). Thus the use of PEG-400 during the synthesis plays a passivation role which is essential to improve the PL activity of S dots.

Figure 1. PL spectra of S dots sample at different duration and photographs (inset) of S dots in daylight( left) and irradiated by UV light at 365nm (right)

It can be inferred: firstly, the PL intensity enhances gradually with increasing duration. Secondly, all the samples exhibited excitation-dependent PL feature, which is attributed to the inhomogeneous size distribution of quantum size effect.1, 9Thirdly, besides the excitation-dependent PL property, samples of 100 h and 125 h also exhibited excitation-independent PL feature. Lastly, there existed a blue shift in λem at the same λex for different duration samples. For example, the λem gradually shifted to shorter wavelength (529 to 488 nm) when the λex was fixed at 420 nm for 30 to 125 h samples (see Table 1). Since quantum dots have sizedependent emission, 2 this blue shift in λem between different duration samples might suggest that the size of the S dots particles gradually became smaller with the increase in duration. The brightness of the PL reflected the high emission quantum yields. For the 125 h sample, the quantum yield of blue light at 440 nm was determined to be 3.8 % .20, 21 For the 30 h sample, the quantum yield of green light at 540 nm was 0.19 % (Figure S2). Similar to carbon and silicon quantum dots, the surface-passivated S dots exhibited a tunable emission property. This excitation-dependent PL feature demonstrates that its PL originated from the quantum confinement effect of recombination of electron-hole pairs within the bandgap transition. 1, 12, 22

Photoluminescence and UV absorption spectra Figure 1 demonstrates the PL diversities of the S dots at different time intervals. It is highly worth noting that any sample with heating time less than 30 h is incapable of emitting PL. After 30 h, the sample can emit green light under UV light at 365 nm which gradually shift to bright blue (the inset of Figure 1). The transparency of the solution demonstrates that the S dots are homogenous due to excellent water-dispersibility. In terms of the S dots sample which was heated for 30 h (referred as 30 h sample), the emission wavelengths (λem) shifts toward longer wavelength (from 529 to 561 nm) when the excitation wavelength (λex) varied from 420 nm to 500 nm (Table 1), which exhibited an excitationdependent PL property. For 54 h sample, the λem appeared at 507 nm when λex = 420 nm. Moreover, PL spectra are slightly

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Table 1 Emission wavelengths of S dots samples for different durations at different excitation wavelengths 500 480 460 440 420 400 380 360 340 320 300

λex(nm)

30h

λem(nm)

561

550

547

532

529

54h

544

521

507

498

491

474

72h

537

524

499

493

479

467

100h

524

509

495

485

465

461

452

459

451

125h

528

502

488

486

475

457

437

436

444

absorption band emerged at 337 nm (Figure S3) when the duration was up to 72 h. Although the origin of the absorption band at 337 nm is unknown, it is certain that 72 h is a turning point in the synthesis. From 72 to 125 h, these three absorption bands tend to have a blue shift gradually, which is ascribed to a quantum size confinement of the dots (the size diameter less than 10 nm). This means that the size of S dots can be controlled by the duration, and the sizes of S dot particles gradually became smaller with the prolongation of duration from 72 to 125 h. However, there were also two exceptions of red shifts (222 to 224 nm, 370 to 376 nm) from 30 to 54 h, which are largely due to the assembling of S dots.

Figure 2 shows the UV absorption spectra for 30 h samples. It can be seen that there are three absorption bands centered at 222 nm (strong band I), 303 nm (weak band II) and 370 nm (relatively weaker band III). Generally, heteroatoms (S, O) contain nonbonding electrons and the transition of n→ σ* can occur in the range of 150-250 nm, 15 thus, band I at 222 nm might arise from n→σ* transition. Appearance of band II at 303 nm and band III at 370 nm might be an evidence for the existence of S22- and S82- species, respectively. It is because S22- shows absorption peaks at 280 nm in DMF solution while S82- shows absorption peak at 370 nm in DMF and 355 nm in DMSO. 23 It is likely that these species adsorbed on the surface of the S dots. 2

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A) 222nm

1 303nm

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B) 303 nm

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Abs

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1 370 nm

0.5

0

0 200 250 300 350 Wavelength(nm)

260 300 340 380 420 Wavelength(nm)

Figure 3. TEM images of different S dots samples

Figure 2. UV absorption spectra of 30 h S dots sample diluted 160000 times (A) and diluted 400 times (B) with water

TEM observation

Table 2 Absorption band position of S dots samples for different durations time band I band II band III New band 30 h

222 nm

303 nm

370 nm

-

54 h

224 nm

300 nm

376 nm

-

72 h

224 nm

300 nm

376 nm

337 nm

100 h

223 nm

-

374 nm

329 nm

125 h

213 nm

-

366 nm

323 nm

The variation in size and shape of the S dots for different duration was observed by TEM images (Figure 3). The enlarged TEM images are illustrated in Figure S4. The particles in 30 h sample are well dispersed and little assembling of particles can be observed. For 54 h sample, the dots (approximate 3.5 to 9.3 nm with ambiguous boundary) are likely to aggregate and form large assembling particles (~21 to 79 nm). For 72 h sample, the assembling particles have uniform and smaller sizes distribution (20 to 42 nm). So it can be deduced that the assembling particles got best monodispersed in 72 h sample. These monodispersed assembling particles is most likely to give rise to the appearance of new absorption band at 337 nm, which could not be understood during UV spectra characterization. For 100 h sample, there is again a large variation or non-uniformity in the size of assembling particles (26 to 64 nm). These assembling particles undergo further fission to give rise to small dots of size about 3.6 nm which are more clearly

The absorption bands of the S dots for different samples were recorded (Table 2). It can be seen that the absorption peaks of the S dots varied with the duration. Band II disappeared when the heating duration was up to 100 h. A new

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outlined. With increasing the duration from 100 to 125 h, fission of assembling particles was enhanced which resulted in further smaller size of dots (~2.9 nm) with more defined and clear boundary.

assemble together due to high surface energy. Even adsorption layer of PEG on the surface cannot stop these nanoparticles from assembling or aggregation. Instead, terminal hydroxyl group of PEG physically cross-linked between S dots, which enhanced the assembling. 25 Although the assembling and fission coexist and compete with each other during the whole formation process, the assembling effect played a leading role during 54 h to 72 h. The excellent mono-dispersibility (TEM) and an appearance of a new absorption band at 337 nm (UV) all appeared at 72 h, which suggest that there is a dynamic equilibrium or balance between assembling and fission effect at 72 h. Lastly, after 72 h, assembling effect weakened and fission effect played a leading role which results in more defined and clear view of the S dots in TEM images. According to the proposed “assembling-fission” mechanism, the assembling phenomenon will reduce when the assembling effect is weakened and the obtained S dots will be in a monodisperse state rather than in assembling state. Based on this hypothesis, monodispersed S dots (referred as Sm sample) have been successfully synthesized by treating sulphur powder with nitric acid before the reaction. The detailed procedure is shown in the experimental part of the supporting information.

Formation mechanism It can be seen that the assembling phenomenon is much enhanced after 30 h. The size of the assembling particles varies with the increase in duration. The assembling particles change from non-uniform to monodisperse, and again to nonuniform. The boundary of the S dots gradually becomes clearer, and the size of the S dots gradually becomes smaller with the increase in heating duration. This is in accordance with the results of PL and UV spectra. Based on the TEM results, we propose an “assemble-fission” mechanism for the formation of the S dots, as shown in Figure 4A. According to this mechanism, bulk sulphur powder does not undergo direct fission into S dots; instead the whole formation includes three steps: dissolution, assembling, fission. During formation of these S dots, assembling and fission compete with each other and obtain a dynamic equilibrium after definite time. Moreover, the resulting morphology of S dots depends on the balance and contest of assembling and fission effects.

S+6HNO3→H2SO4+6NO2+2H2O (3) During this process, the pretreatment with nitric acid mostly corrodes the surface state of the sulphur particles (Eq. 3) and brings the group of SO42- to the surface. The electrostatic repulsive force of the SO42- group on the surface of sulphur particles helps to reduce the assembling effect during the subsequent fission process.

Figure 4. Schematic illustration of the basic steps (A) and the detailed process (B) of S dots formation

Figure 5.TEM images, size distribution and PL of 116 h Sm dots sample

3S+6NaOH =2Na2S +Na2SO3+2H2O (1) (x-1)S +Na2S=Na2Sx X =2~5 (2) The detailed formation process is shown in Figure 4B, whereby the first step represents the dissolution. For a pH greater than 7, the bulk sulphur powder is likely to be dissolved, 24 as shown in the reaction (1) and (2). The reaction (1), as the driving force, results in the splitting of bulk sulphur powders into small particles. The reaction (2) is the process in which bulk sulphur powder reacts with sodium sulfide (Na2S) to form sodium polysulfide. Reaction (1) and (2) played a leading role within first 30 h. Within these 30 h, bulk sulphur powder was dissolved and converted to inhomogeneous sodium polysulfide particles. Meanwhile, the PEG-400 physically adsorbed on the surface of the sulphur powder and tried to prevent the assembling. Thus a spherical shape of sulphur particles was observed during this period. In the second step, sodium polysulfide nanoparticles are inclined to

From Figure 5, it can be seen that the Sm dots has excellent monodispersibility. A single Sm dots does not aggregate with other Sm dots with narrow diameter distributions (4.1+0.88 nm) for 116 h Sm dots sample. This Sm dots sample was transparent yellowish under daylight and emitted a blue-green light under UV light at 365 nm (the inset of Figure 5).The PL of the Sm dots has also been investigated. An excitation-independent PL λem of the Sm dots was fixed near 518 nm when the λex was in a wide range from 320 to 440 nm. Moreover, the size of Sm dots can also be easily controlled by controlling the heating duration. For 153 h Sm dots sample, the size decreased to 2.8±0.52 nm (Figure S5). The PL properties of these two kinds of the Sm dots samples are related with their sizes. When the size decreased from 4.1 to 2.8 nm, the PL λem would shift to a shorter wavelength under the same λex of 400 nm. That phenomenon is the consequence of the well-known quantum confinement effect.

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The high resolution TEM images (Figure S4) recorded from individual S dots clearly show that the spacing between the two adjacent lattice fringes is 2.16 Å, which is different from (206) planes of orthorhombic S8 phase.25 The XRD pattern of the S dots (Figure S6) also revealed different diffraction peaks from orthorhombic S8 phase according to JCPDS file No.83-2285.25 This is probably due to the fact that synthesized S dots are somewhat amorphous in nature. The Raman spectrum of the S dots (Figure 6b) perfectly coincided with that of sublimed sulphur powder (Figure 6a). As a consequence, it sufficiently proved that synthesized product is sulphur nanomaterial. The chemical composition and structure of the S dots were investigated (Figure 6c and d). The XPS spectrum mainly contained C, O and S elements. The highresolution XPS spectrum contained five different peaks. Two peaks at 163.25 and 164.2 eV were assigned to the atomic sulphur. The binding energies at 167.45 eV, 168.3 eV and 169.2 eV were attributed to the sulphur of SO2 − (2p 2/3), SO 22− (2p 1/2) or SO32− (2p2/3) and SO32− (2p1/2), respectively.15 Thus the S dots were mainly composed of atomic sulphur and abundant sulfonyl/sulfonate groups on the surface, which was similar to S dots reported previously.

200 350 500

a) Raman

Shift(cm-1 )

50

200 350 500

b) Raman

Shift(cm-1)

0

c)

200

570 420

163.25

600

Binding Energy(eV)

130 80 30 -20 -2.2 -1.1 0 1.1 2.2 Potential(V)

164.2

168.3 167.4

169.2

120

d)

180

a)

270 400

ECLintensity(a.u)

ECL intensity(a.u)

S + 4H2O= SO42- + 8H + + 6e (5)

160 162 164 166 168 170 Binding Energy(eV)

600

1

400

-2 200

c)

Figure 6. Raman spectra and XPS spectra of sublimed sulphur powder (a) and S dots (b, c, d)

4

-5

0

Potential(V)

Intensity

Intensity

Intensity

S(2p)

50

720

O(1s)

electrode (saturated KCl). The photomultiplier tube was operated at -1000 V. The electrolytic cell contained 3.0 mL of 125 h S dots sample. When the potential was cycled between +2.2 V and 2.2 V at a scan rate of 50 mV/s, significant ECL emissions were observed at both negative and positive potential regions (Figure 7a). Moreover, an increasing tendency in ECL intensity was observed at high scan rate (Figure S8-1). Meanwhile, Figure 7b reveals a relatively stable ECL response of the S dots, making it a potential candidate for ECL sensing applications.28,29 According to a previously reported literature about sulphur’s electrochemistry,24,30 it was observed that the cathodic peak appears at -0.8 V, which is near the onset ECL potential at -1.0 V in the cathodic scan direction. Thus, the ECL signal at negative potential was related to the transformation of S dots from S0 to S2- (Eq. 4). At high potentials of +1.25V (vs SHE), sulphur is oxidized to sulphate (Eq.5).29 nS0+2e-=Sn2(4)

ECLintensity(a.u)

C(1s)

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300 200 100 0

b) CL intensity(a.u)

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3 6 9 12 Time(s)

50 100 150 200 Time(s)

9000 6000 3000

-8 0

0

d)

0 0

10 20 Time(s)

30

Figure 7. a) ECL response obtained from 125 h S dots sample during the potential was cycled between -2.2 V and 2.2 V at a scan rate of 50 mV/s at Pt disk electrode. b) ECL response of the S dots sample obtained during a continuous potential scan between -2.2 V and +2.2 V at a scan rate of 0.5 V/s. c) ECL transient (solid line) of the S dots sample by pulsing potential between +2.2 V and -2.2 V (dotted line). d) CL kinetic profile obtained when the oxidizing reagent of potassium ferricyanide was added into the S dots sample. All potentials were applied vs Ag/AgCl(sat. KCl).

Furthermore, in order to study any interaction between PEG and S atom, FTIR spectra of the S dots samples were recorded (Figure S7). All the characteristic peaks of PEG in S dots sample are observed: 3363 cm-1, 2887 cm-1, 1297 cm-1, 1248 cm-1, 946 cm-1 (Figure S7a). 26, 27 Peaks at 1452 and 1346 cm-1 in PEG overlap to give somewhat broader band at 1390 cm-1 in the S dots sample. The broad signal at 1097 cm-1 in PEG splits up into two relatively sharper peaks at 999 and 1115 cm-1 in the S dots samples. These signals are caused by stretching vibration of C–O–H or C-O-C in both PEG and the S dots samples. No other additional new peak appeared in the S dots samples. This indicates that there is no chemical interaction between PEG and S dots. These results verified the effect of PEG in the discussion of the formation mechanism. Presence of this physical interaction was evidenced from the fact that there was no aggregation within 30 h. It was due to the fact that PEG was physically adsorbed on the surface of S dots and this adsorbed layer helped to prevent assembling. 25 After its

Evidently, the ECL behavior of S dots is similar to that of other semiconductor nanoparticles, such as CdSe, 31 CdTe, 32 Si33 and carbon nanocrystals.3, 28 ECL transients of the S dots were measured by applying potential pulsing between +2.2 and -2.2 V (Figure 7c). No ECL signal was observed at the first positive or negative potential step (Figure S8-2). However, obvious ECL signals have been detected at the subsequent potential steps. Apparently, both oxidized (R·+) and reduced (R·-) species are necessary for the annihilation ECL reaction. At the first positive (or negative) potential step, only one kind of species (R·+ or R·-) is generated, therefore, no ECL can be observed via the annihilation in this case. However, at the subsequent potential steps, both R·+and R·- can be produced, thus the annihilation of R·+ and R·- happened, and eventually generate the ECL emission. In addition, the observation of subsequent cathodic and anodic ECL signals implies that both R·- and R·+ radicals are stable enough to

aggregation (54 h), the layer of PEG on the surface of S dots cannot prevent S dots from assembling, so PEG may physically cross-linked between S dots.25

Electrochemiluminescence The ECL measurement was conducted with a three-electrode system, a platinum disk working electrode (φ = 4.0 mm), a platinum wire counter electrode, and Ag/AgCl reference

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Journal of the American Chemical Society transfer charge upon colliding and produce the excited stated S dots, R*, in solution.

simple reaction of S powder with alkali by using PEG-400 as surface passivation agent. The newly produced S dots exhibited excellent dispersibility, high stability over two months and temperature dependent PL property. Emission color of S dots can easily be tuned between green and blue (emission wavelength between 550 to 440 nm) by controlling the heating time. A theory of “assemble-fission” has been proposed. According to this mechanism, two forces i.e., assembling and fission are involved in formation of S dots after dissolution of bulk sulphur powder. The final state and morphology of S dots depend on the balance and contest of these two forces. Inspired by this mechanism, we have successfully synthesized a monodispersed Sm dots sample with an average size of 4.1 nm for 116 h duration. Moreover, for the first time, potential ECL activities for these S dots has been explored by cyclic or pulse voltammetry. CL of the S dots has also been observed by directly oxidation. The S dots are envisioned to have promising potential applications in the development of new types of biosensors, display devices and agriculture fungicides assays on the basis of the PL, ECL and CL activity, good water-solubility, high stability and environmental friendliness. More importantly, with the inherent extraordinary antimicrobial activity, the S dots will bring us a new time of visualization of antimicrobial bioanalysis.

Chemiluminescence The CL behaviors of the S dots with various oxidants were also investigated with a static injection system (Table S1). K3Fe(CN)6 can directly oxidize S dots, generating strong CL emission (Figure 7d). The dynamic CL intensity-time profiles indicate that the CL reaction is quite quick and the CL intensity reaches a maximum after ~1.2 s after the reaction was initiated. Other oxidants such as KIO4, K2S2O8, and H2O2 were also used for the oxidation of S dots, but these produce weak CL emissions. The CL behavior of the S dots was similar to that of CdTe nanocrystal 34 and carbon nitride quantum dots. 35 They all can be oxidized directly generating CL emission.

PL.intensity(a.u)

1st

72nd

370 380 390 400

800 600 400 200 0

400

c)

17th

600 400 200

500

500

200 150 100

200

ASSOCIATED CONTENT

140 80

Supporting Information

270 300 330 T(K)

50 0

d)

600

Wavelength(nm)

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600

Wavelength(nm)

400

b)

PL

600

PL intensity(a.u)

500

Wavelength(nm)

1000

800

0 400

a)

PL intensity(a.u)

1000

1000 800 600 400 200 0

PL intensity(a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

400 550 Wavelength(nm)

The Supporting Information is available free of charge on the ACS Publications website.

333K 313K 303K 293K 273K

Experimental procedures and analytical data (PDF)

Figure 8. PL spectra of 54 h S dots sample at 1st (a), 17th (b) and 72ed(c) days respectively and temperature dependent PL of S dots sample at an excitation wavelength of 365 nm.

AUTHOR INFORMATION Corresponding Authors *[email protected];

Stability and Temperature dependence Figure 8 shows the PL stability of 54 h S dots sample. The λem remained at about 500 nm with a slightly blue shift after 71 days and only the PL intensity increased slightly. The reaction medium for prepared S dots was alkaline because the S dots aggregated under acidic conditions. However, after the pH was adjusted to neutral (adding dilute acid slowly) and then dialyzed in water, PL activity of the S dots remained stable for at least 5 days (Figure S9). The S dots revealed satisfactory stability that was attributed to PEG-400, which kept S dots to be in a colloidal state. The PL intensity of the S dots sample was also found to be temperature dependent (Figure 8d). When the temperature is increased from 273 to 333 K, the PL intensity decreased linearly. There is no obvious blue shift in the emission maximum over this narrow temperature range. This temperature dependence property from S dots is similar to that of carbon dots and other semiconductor nanoparticles.36-39

*[email protected]; *[email protected] ORCID Lihua Shen: 0000-0001-8882-8985 Sichun Zhang: 0000-0001-8927-2376 Author Contributions All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported financially by the National Natural Science Foundation of China (No. 21505102, 21621003, 21475082, 21390410). Thanks to Mr. Zhang Hu from Xi'an University of Technology for provision of TEM analyses, and thanks to Dr. Azhar Abbas,

CONCLUSIONS In summary, here we have reported a facile method to synthesize S dots (with a quantum yield up to 3.8 %) via a

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Assembling of Sulfur Quantum Dots in Fission of Sublimed Sulfur

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