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1Department of Material Science and NanoEngineering, Rice University, Houston,TX 77005, USA. 2School of Materials Science and Engineering, Hefei University ofTechnology, Key Laboratory of Advanced Functional Materials and Devices of AnhuiProvince, Hefei 230009, P. R. China. 3College of Polymer Science and Engineering,Sichuan University, State Key Laboratory of Polymer Materials Engineering, Chengdu610065, P. R. China. 4School of Chemistry and Chemical Engineering, Institute forEnergy Research, Jiangsu University, Zhenjiang 212013, P. R. China. 5Saudi BasicIndustries Corporation, Sugar Land, TX 77478, USA.*These authors contributed equally to this work.†Corresponding author. Email: wu2jj@ucmail.uc.edu (J.W.); ycwu@hfut.edu.cn (Y.Wu);ajayan@rice.edu (P.M.A.)

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Cryo-mediated exfoliation and fracturing of layeredmaterials into 2D quantum dotsYan Wang,1,2* Yang Liu,1,3* Jianfang Zhang,2* Jingjie Wu,1† Hui Xu,1,4 Xiewen Wen,1

Xiang Zhang,1 Chandra Sekhar Tiwary,1 Wei Yang,3 Robert Vajtai,1 Yong Zhang,2 Nitin Chopra,5

Ihab Nizar Odeh,5 Yucheng Wu,2† Pulickel M. Ajayan1†

Atomically thin quantum dots from layered materials promise new science and applications, but their scalablesynthesis and separation have been challenging. We demonstrate a universal approach for the preparation ofquantum dots from a series of materials, such as graphite, MoS2, WS2, h-BN, TiS2, NbS2, Bi2Se3, MoTe2, Sb2Te3,etc., using a cryo-mediated liquid-phase exfoliation and fracturing process. The method relies on liquid nitrogenpretreatment of bulk layered materials before exfoliation and breakdown into atomically thin two-dimensionalquantum dots of few-nanometer lateral dimensions, exhibiting size-confined optical properties. This process isefficient for a variety of common solvents with a wide range of surface tension parameters and eliminates theuse of surfactants, resulting in pristine quantum dots without surfactant covering or chemical modification.

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INTRODUCTIONQuantum dots (QDs) with diameters in the range of 2 to 10 nm exhibitunique electronic and optical properties due to confinement of chargecarriers in all spatial dimensions (1–5). Because of the tunability of opto-electronic properties and band structures, QDs are of interest in manyapplications, such as photovoltaics (6), bioimaging (7, 8), catalysis (9),light-emitting devices (10), photodetectors (11), etc. Two-dimensional(2D) materials, including typical transition metal dichalcogenides,graphene, and hexagonal boron nitride (h-BN), intrinsically exhibit ex-ceptional physiochemical properties when the vertical dimension is re-duced into the thickness of a monolayer or few layers (12–19). Thereduction of lateral size into nanometer scale of a 2D monolayer orfew-layer nanosheets to form 2D QDs can result in synergistic effects,such as a combination of the distinctive properties associated with both2D nanosheets and QDs. Before exploring promising physiochemicalproperties of 2D QDs from a range of available compositions, it is es-sential to develop effective, scalable, and generic processes to producethese structures. Some recent reports showed top-down cutting meth-odologies for preparing graphene and MoS2 QDs via hydrothermal,electrochemical, microwave, nanolithography, or ultrasonic shearingprocesses but these processes involved multiple steps and requiredharsh conditions (concentrated acids, strong oxidizing agents, energyintensive grinding, or high temperature, etc.) (20–23). Here, we report asimple, universal, and scalable synthetic approach based on cryo-mediatedliquid-phase exfoliation (LPE) to prepare 2D QDs directly from bulk-layered material powders in a series of solvents within a short time.

In our approach, we introduce a cryo-mediated pretreatment step inwhich the bulk-layered material powders are soaked in liquid nitrogenfor a period of time. Then, the bulk powders are dispersed in a solvent,such as isopropanol (IPA)/H2O mixture (for example, IPA/H2O = 1:1

by volume), followed by LPE with the assistance of bath ultrasonication(24, 25). First, to the best of our knowledge, the cryogenic pretreatmentby soaking the raw powders in liquid nitrogen was adopted for the firsttime to prepare 2DQDs. The cryo-pretreatment facilitates the efficiencyof peeling off few-layer (comprisingmostly monolayer) sheets from thebulk powders in the following bath ultrasonication-assisted LPE whilesimultaneouslymediates the fracture of these few-layer sheets into high-quality pristineQDswithin short time. This is because the interlayer vander Waals force between layers is weakened, and some small cracks inintralayer are formed after cryo-pretreatment process. Second, the cryo-pretreatment process only needs to adopt the pristine raw powders asthe precursors without introducing any other impurity elements, andthus the pristine QDs can be directly achieved without any surfac-tant and other chemicals contamination (20, 26). Third, the cryo-pretreatment process exhibits less dependency on solvent properties,which means various common low–boiling point solvents with a widerange of surface tensions and even pure water can be selected for LPE oflayered materials into QDs, showing great improvements comparedwith the existing LPE technique for graphene and 2D nanosheets intypical high–boiling point organic solvents, such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), and dimethylsulfoxide (24, 27, 28). Fourth, the as-developed cryo-mediated LPEprocess can be applied to a wide range of layered materials, and aseries of 2DQDs can be fabricated with pristine state, demonstratinga universal approach for the production of 2D QDs. Last, but not theleast important, the quality of the as-prepared pristine QDs can becontrolled to have predominant mono- and bilayer thickness and auniform lateral size of 2 to 3 nm.

RESULTS AND DISCUSSIONFigure 1A illustrates the procedure as exemplified by preparing 2DMoS2 QDs. In the early ultrasonication (delamination) stage, the bulkpowders are easily exfoliated into few-layer or even monolayer nano-sheets in a relatively high yield [~8 weight % (wt %)]. These nanosheetshave lateral sizes in a wide range of 100 to 5000 nm, as shown in the rep-resentative case of MoS2 in Fig. 1B. The origin of facilitation of LPE ef-ficiency may lie in crack formation in the layered powder and weakenedinterlayer van derWaals force between 2D layers. Our process involvestwo steps: An initial soaking of room temperature bulk layered powder

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Fig. 1. Procedure of cryo-exfoliation for preparing pristine 2D QDs (represented by MoS2). (A) Schematic illustration of the steps for preparing 2D QDs. Theoutlined step in a red rectangle is the cryo-pretreatment. (B to D) AFM results showing the evolution of nanosheets to QDs, (B) ultrathin MoS2 nanosheets at thedelamination stage, (C) small MoS2 nanosheets with tiny holes (the red arrows point to the holes.), and (D) pure MoS2 2D QDs after vacuum filtration.

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Fig. 2. Morphological and crystalline structure of the 2D QDs. (A to C) AFM images and their statistic thickness distribution (insets). (A) GQDs, (B) MoS2 QDs, and (C)WS2 QDs. (D to F) Low-resolution TEM images of (D) GQDs, (E) MoS2 QDs, and (F) WS2 QDs, respectively (insets showing the lateral size distribution). (G to I) High-resolution TEM images of (G) GQDs, (H) MoS2 QDs, and (I) WS2 QDs, respectively.

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in liquid nitrogen (quench step), and then dispersion of the soaked rawpowder into room temperature solvent following by sonication (recov-ery step). This procedure results in amarked temperature change of 2Dpowder in a cycle, room temperature/liquid nitrogen temperature/room temperature. The fast thermal relaxation in the quenching stepwould form small cracks in the 2D powder, which is a similar feature tothe effect of the quenching treatment for the preparation of graphenenanosheets (29). These cracks, serving as capillaries, allow permeationof solvent, which facilitates the LPE by bath sonication (30). More-over, the thermal expansion in the recovery step helps weaken the in-terlayer van der Waals force of 2D materials. With the extension ofultrasonication treatment time, nanosheets are further disintegratedinto QDs by ultrasonic wave–induced lattice breakup as evidencedby the emergence of lots of nanoscaled holes on the basal planes ofnanosheets (Fig. 1C). The liquid nitrogen pretreatment of the rawpowders will affect the mechanical properties, especially the intralayermolecular bond embrittlement induced by the ultralow temperaturetreatment (31), which subsequently promotes the fracturing of few-/monolayer nanosheets into QDs. The QDs are finally separated fromrelatively larger nanosheets by centrifugation and vacuum filtrationresulting in a QD yield of ~1 wt % (Fig. 1D). The whole process isfree of concentrated acid and surfactant.Without the cryo-pretreament,only nanosheets (thickness, ~1.6 nm; and lateral size, ~100 nm) can beobtained by identical LPE conditions (fig. S1).

The thicknesses of the 2D QDs are in the range of 0.5 to 1.2 nm,corresponding to one to two layers, as analyzed by atomic forcemicroscopy (AFM) (Fig. 2B, showing an example of MoS2 QDs). The

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transmission electronmicroscopy (TEM) image reveals thatMoS2 QDsare nearly circular in shape, with a relatively narrow diameter distri-bution ranging from 0.78 to 2.4 nm (Fig. 2E). The high-resolution TEM(HRTEM) image shows the lattice fringes of MoS2 QDs with a spacingof ~0.27 nm corresponding to the (100) faces ofMoS2 crystals (Fig. 2H),suggesting the high crystallinity of theMoS2 QDs. This cryo-exfoliationstrategy toward fabricating 2D QDs can be universally applied to other2Dmaterials, including graphite, h-BN,TiS2,NbS2,WS2, Bi2Se3,MoSe2,MoTe2, and Sb2Te3. The quality of these QDs in terms of thickness, di-ameter, and crystallinity is similar to that of MoS2 QDs. For examples,an average thickness of 0.6 and 1.0 nm is observed for graphene QDs(GQDs) and WS2 QDs, respectively (Fig. 2, A and C), whereas the di-ameter ranges from 0.77 to 3.20 nm (mean value of 1.71 ± 0.32 nm) forGQDs (Fig. 2D) and 2.18 to 5.99 nm for WS2 QDs (mean value of3.73 ± 0.56 nm) (Fig. 2F). Both GQDs and WS2 QDs are crystallinewith exposed (100) facets, respectively (Fig. 2, G and I). The otherQDs made from bulk powders of h-BN, TiS2, NbS2, Bi2Se3, MoTe2,and Sb2Te3 are also composed of mono- or bilayers and exhibit meanlateral size of 2 to 3 nm (Fig. 3, A to F).

The crystalline structure of the QDs, taking graphene, MoS2, andWS2 QDs as examples, was studied by x-ray diffraction (XRD). Com-pared to the coexistence of a variety of XRD peaks for interlayer andintralayer crystal faces in the bulk powders, only one peak for the inter-planar face (002) appears in the XRD patterns of all QDs samplesprepared by drop casting, indicating a random restacking of QDs along<001> direction (fig. S2). In addition, the (002) peaks for QDs samplesbecome broader due to the reduced size in both thickness and lateral size

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Fig. 3. Cryo-exfoliation applied to fabricate 2D QDs from different bulk-layered material powders in various solvents. AFM images of QDs for (A) h-BN, (B) TiS2,(C) NbS2, (D) Bi2Se3, (E) MoTe2, and (F) Sb2Te3. (G) Statistic average thickness of MoS2 QDs as a function of cryo-pretreatment duration (the blue curve, with fixed 1-hourultrasonication time) and sonication time (the red curve, with fixed 1-hour cryo-pretreatment time). The average thickness of MoS2 QDs as-exfoliated in differentsolvents with varied polar/dispersive ratio (H) and surface tension (I).

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(32, 33). The x-ray photoelectron spectroscopy (XPS) results reveal thattheseQDs exhibit a pristine chemical state with no obvious oxidation orfunctionalization (figs. S3 and S4), which is distinctly different andunique compared to QDs prepared by vigorous chemical treatments(34).

Two key process parameters, the cryo-pretreatment duration andthe subsequent ultrasonication time, potentially determine the qualityof as-prepared 2D QDs. When the sonication time is fixed to 1 hour,the resulting QDs gradually become thinner with the increase of cryo-pretreatment time and finally become steady when the pretreatmentlasting for 1 hour or longer (Fig. 3G, blue curve). With the fixed cryo-pretreatment time of 1 hour, the thickness gradually decreases withthe increase of ultrasonication time until 2 hours (Fig. 3G, red curve).A combined cryo-pretreatment and sonication for 1 hour each couldresult in 2D QDs with a thickness <1 nm corresponding to mono-/bilayer of 2D materials from normal AFM measurement. To furtherimprove the yield and concentration of QDs, we can cycle the cryo-pretreatment and the LPE steps, during which the residual mono- orfew-layer nanosheets could be further disintegrated into QDs (figs. S5and S6).

From previous work on the controlled LPE of 2Dmaterials, match-ing of surface tension parameters of solvents to that of 2D materialsplays a critical role in producing nanosheets from bulk powders (25, 35).We sonicated the cryo-treated MoS2 powders in various solvents witha wide range of surface tensions and surface tension component polar/dispersive ratios to explore the solvent effect on the production of QDs.The results show that high-quality MoS2 QDs could be produced in allof these solvents (Fig. 3, H and I; table S1; and fig. S7), demonstratingthat the cryo-pretreatment could effectively reduce the dependence of

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exfoliation efficiency on solvent. The obtained results also apply to otherlayered materials such as graphite (fig. S8). This is of great significancefor the future development of 2DQDs in a variety of solvent systems forpractical applications.

The optical properties of the QDs were explored by ultraviolet–visible (UV-vis) absorption and photoluminescence (PL) spectroscopy.Under irradiation with UV light at 365 nm, the GQDs solution emitsblue light (inset in Fig. 4A). Both MoS2 QDs and WS2 QDs solutionexibit blue fluorescence as well but with higher brightness (fig. S9).All QDs solutions exhibit an excitation-dependent PL behavior (Fig.4A and fig. S9). The PL peak redshifts upon increasing the excitationwavelength, similar to that of previously reported 2D materials–basedQDs (36–38). Visible (3.05 eV) pump–midinfrared (0.25 eV) probetransient absorption experiments were performed to compare the ultra-fast dynamics of monolayer graphene and GQDs. Figure 4B shows thefree photoexcited carrier recombination lifetime ofmonolayer grapheneand GQDs. Monolayer graphene exhibits a biexponential decay con-taining a fast process of 0.18 ps and a slow process of 2.0 ps, whereasthe recombination rate for GQDs slows down by several times to 2.7 ps.The fluorescence quantum yield (QY) of these 2D QDs is typically be-low 10% (Fig. 4C). The 2DQDs (for example,MoS2QDs) can uniformlydisperse in an epoxymatrix to form a photoluminescent composite (insetin Fig. 4C). The 2D QDs present larger optical bandgaps between 3.0and 6.0 eV, as compared to their bulk and monolayer counterparts, re-sulting from the quantum confinement effect (Fig. 4D and fig. S10) (39).

The cryo-mediated LPE serves as a powerful and universal approachto produce 2D QDs directly from bulk-layered material powders. Withthe introduction of cryo-pretreatment, the efficiency of LPE is enhancedand exhibits less dependency on solvent properties. These exfoliated

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Fig. 4. Optical properties of the pristine 2D QDs. (A) PL spectra excited at different wavelengths of GQDs. Insets show digital images of the corresponding QDs under dayand UV light. a.u., arbitrary units. (B) Pump-probe transient absorption spectra of GQDs and monolayer graphene. (C) The QY and (D) collective optical bandgap for selected 2DQDs. The inset in (C) shows epoxy filled with MoS2 QDs exhibiting blue fluorescence under UV irradiation.

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mono-/few-layer sheets are more amenable to fracturing into QDs com-pared to nanosheets from LPE procedure alone. This methodology appliesto a variety of layeredmaterials processed in a broad range of common sol-vents without harsh chemical modifications and with no use of surfactantsor additives, which enables the synthesis of pristine QDs.

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MATERIALS AND METHODSCryo-mediated exfoliation and fracturing process for QDsCommercially available h-BN (average, 45 mm; 99.5%) and Bi2Se3(99.999%)were purchased fromAlfa Aesar. Graphite (<45 mm, 99.99%),MoS2 (average, 2 mm; 99%), andWS2 (average, 2 mm; 99%); and MoSe2(~325 mesh, 99.9%), TiS2 (~325 mesh, 99.9%), and Sb2Te3 (~325 mesh,99.96%) were purchased from Sigma-Aldrich. All materials were used assupplied. All solvents were purchased from Sigma-Aldrich and used assupplied without purification.

MoTe2 powder was synthesized using a tube furnace. A porcelainboat containing mixed Mo (99.9%; Sigma-Aldrich) and Te (99.999%;Sigma-Aldrich) powder was placed in the center of the furnace. Themole ratio of Mo/Te was about 1:2. Ar and H2 gases were used as thecarrier gas andwere kept flowing during the whole process. The furnacewas heated to 600°C andwas kept at the reaction temperature for 20min.Then, the furnace was cooled down naturally. The phase of MoTe2 pow-der was further confirmed by XRD (fig. S11).

The cryo-mediated exfoliation and fracturingprocessmainly includ-ed the following two steps: First was the cryo-pretreatment, performedby soaking the raw 2D materials bulk powder in liquid nitrogen for aperiod of time (10 min to several hours), then the bulk powders wereimmediately dispersed into the solvent IPA/H2O (the volume ratio ofIPA/H2O is 1:1), and finally liquid-exfoliated with the assistance ofbath ultrasonication. The initial concentration of the dispersions was3 mg ml−1 for all 2D materials. After the cryo-mediated exfoliationand fracturing treatment, the resulting dispersions were centrifugedat 6000 rpm (rounds perminute) for 30min to remove nonexfoliatedmaterials. To further separate the QDs from the mixed dispersions(containing QDs and the exfoliated nanosheets), the supernatants werecollected by pipette followed by vacuum filtration through ultrafinemembranes with pore size of 25 nm (VSWP02500, Millipore). Forthe AFM characterization, the as-obtained QDs solution was dilutedand then directly dropped onto a new mica substrate, which wouldbe first dried naturally at room temperature and then further treatedin a vacuum oven at 80°C for 4 hours to completely eliminate the influ-ence of residual solvents. The as-filtered QDs samples were droppedonto ultrathin carbon type A (3 to 4 nm) with removable formvar grid(400 mesh) and then vacuum-dried for TEM and HRTEM test. TheXRD and XPS samples were prepared by dropping the QDs samplesonto glass substrates and drying in a vacuum oven. The yield of nano-sheet and QDs were determined by inductively coupled plasma massspectrometry (ICP-MS) measurement.

Solvents with different surface tensions and polar/dispersiveratio parameters usually have crucial influence on the LPE for the prep-aration ofmono- or few-layer nanosheets of 2Dmaterials. It is desirablefor the LPE process to optimize the experimental results by matchingthe surface tension of solvents to that of 2D materials (24, 25, 35). Toexplore and determine the effect of solvent types on the production pro-cess andmorphology of QDs, some other typical organic and inorganicsolvents (including NMP, DMF, acetone, and ethanol) with varied sur-face tension and polar/dispersive ratio parameters were chosen to dis-perse the cryo-pretreated MoS2 powders for the following sonication

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treatment, and the corresponding produced QDs were characterizedby AFM (fig. S7 and table S1).

We also conducted a comparative study on the effect of the liquidnitrogen pretreatment. Without cryo-pretreatment, we can only obtainnanosheets of 2Dmaterials with lateral sizes ranging from tens of nano-meters to a few hundred nanometers (fig. S1) after identical LPE (forexample, for 4 hours in IPA/H2O = 1:1 mixture). In our normal cryo-mediated exfoliation and fracturing process, we immediately dispersedthe cryo-pretreated powders into solvents followed by bath sonication.There was no time gap between the cryo-pretreatment and sonicationprocesses. However, we had also preliminarily studied the effect of gapon themorphology of the as-prepared sample.With a 1-hour gap, it waspossible to obtain the 2DQDs but with lower yield. There existed someincompletely fractured nanosheets with lateral size >10 nm (fig. S12).

We tried to collect the WS2 QDs by vacuum filtration with a mem-brane pore size of 25 nm.As the ICP-MS test show, the concentration ofWS2QDswill be greatly improvedwith the extension of ultrasonic time(fig. S13) under the same cryo-pretreatment duration (1 hour), indicat-ing the increase of yield. After 4 hours of sonication treatment (for ex-ample, WS2-1h-4h), the concentration of WS2 QDs could achieve 28.7parts per million (28.7 mg liter−1). The yield of the QDs could becalculated up to 1 wt % of the raw powders. In actuality, the obtainedQDs concentration was underestimated because many QDs settled onthe remaining nanosheets and were filtered out. If the as-prepared QDscould be directly used together with the as-exfoliated nanosheets in adispersion (for example, a photocatalytic water splitting application),the superiority of our high-concentrationQDs in low–boiling point sol-vents would be much more remarkable.

Characterization and calculation of QDsIn all cases, the ultrasonication was performed using a Branson 5510model bath sonicator with a frequency of 40 kHz. The resulting disper-sions were centrifuged by the Thermo Fisher Scientific Sorvall LegendX1 centrifuge at 6000 rpm for 30min to remove nonexfoliatedmaterials.The topographic height/thickness of the as-obtained QDs of 2D ma-terials was characterized by an AFM (Bruker NanoScope VMultiMode8.0) on mica substrates. TEM images were recorded on a JEM-2100F atan accelerating voltage of 200 kV. XRD patterns of the samples wereperformed on D/Max 2500 V with Cu Ka radiation (l = 1.54056 Å).The chemical valences of the sample elements were measured by XPSusing an ESCALAB 250 with a monochromatic Al Ka x-ray beam(1486.60 eV). All the binding energies of the elements were calibratedto the C 1s binding energy of 284.8 eV. PL spectroscopy experimentswere conducted using a HORIBA Jobin Yvon NanoLog spectrofluo-rometer with varied excitation wavelengths in the range of 300 to400 nm. A Shimadzu UV-2450 UV-vis spectrophotometer wasadopted for the UV-vis absorption spectra characterizations.

We calculated the thickness of the as-obtained QDs mainly fromthe AFM images, as shown in Fig. 2 and fig. S14. As we can see fromthe fig. S14A, when we draw a horizontal line in the NanoScope Anal-ysis software (shown in fig. S14A, blue line), we can obtain the thick-ness distribution data, from which we can see that there are somenegative values (fig. S14B). We guess that this kind of negative valueshould be generated by the instrument itself or the mica substrate. Then,we characterized the pure mica substrate by AFM (fig. S14C). When wedraw a horizontal line on the mica substrate, we can find some periodicthickness fluctuations ranging from several tens to 300 pm (fig. S14 D).We suppose that this kind of fluctuation would affect the thicknessdistribution statistics of the as-obtained QDs.

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The optical bandgaps of various 2D QDs were extracted accordingto the Tauc plots (40)

ða ⋅hvÞn ¼ Cðhv � EgÞ

where a is the diffuse absorption coefficient at wavelength l, h is thePlanck’s constant, v is the light frequency, n is 2 or 0.5 for direct andindirect transitions, respectively,C is constant, andEg is optical bandgapof materials.

The QYs of 2D QDs were measured at an excitation wavelength of370 nm using [Ru(bpy)3]Cl2 (QY = 0.04) as a reference. The QY wascalculated by (41)

F ¼ FR � IntIntR

AR

An2

n2R

whereF is theQYof samplewith respect to referencematerial, Int is thearea under the emission peak at a certain wavelength,A is absorbance atthe excitation wavelength, and n is the refractive index of the solvent.

For the preparation ofQDs/epoxy composite, a liquid epoxy resin sys-tem 2000 based on diglycidyl ether of bisphenol A and a curing agent2020 based on ethylenediamine were supplied by the Fibre Glast Devel-opments Corporation and used as received. TheQDs solution was addedto the epoxy resin andmagnetically stirred at 80°C under vacuum for48 hours. The final mixture was cooled down to room temperature andthe curing agent was added (Phr 23). The viscous mixed composite wasplaced in siliconemolds, cured at ambient temperature, and further post-cured at 100°C for 2 hours. Then, the as-preparedQDs/epoxy compositewas placed under day light and UV light for the optical characterization.

SUPPLEMENTARY MATERIALSSupplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/12/e1701500/DC1fig. S1. The AFM images of the as-exfoliated MoS2 nanosheets with only sonication-assistedLPE and without cryo-pretreatment.fig. S2. XRD patterns of the as-obtained QDs and raw powder of layered materials.fig. S3. XPS spectra of the as-obtained QDs.fig. S4. XPS spectra of the corresponding raw bulk powders.fig. S5. UV-Vis absorbance spectra of MoS2 samples with a series of cycles of cryo-pretreatmentand liquid exfoliation treatment.fig. S6. Digital image of MoS2 supernatants without vacuum filtration with series cycles ofcryo-pretreatment and liquid exfoliation.fig. S7. AFM images of MoS2 QDs produced in different solvents.fig. S8. The thickness statistics of the as-exfoliated GQDs with different cryo-pretreatment durationand solvents.fig. S9. Photoluminescence spectra excited at different wavelengths for MoS2 QDs and WS2 QDs.fig. S10. Tauc plots used to determine the optical bandgaps of various 2D QDs derived fromUV-vis spectra.fig. S11. XRD pattern of the as-synthesized MoTe2 powders.fig. S12. The AFM image of the MoS2 sample obtained in a process with 1-hour gap between thecryo-pretreatment and the ultrasonication.fig. S13. ICP-MS results of the WS2 QDs concentration with different sonication time at a fixedcryo-pretreatment duration of 1 hour.fig. S14. Thickness statistics of the as-exfoliated MoS2 QDs and the background thicknesssignal of the new mica substrate.table S1. Surface tension and polar/dispersive ratio of the solvents adopted for the cryo-exfoliationof MoS2.

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Acknowledgments: We appreciate A.A. Martí (Department of Chemistry, Rice University) andA.Aliyan for the UV-vis and PL test and meaningful discussion. Funding: This research isfinancially supported by SABIC (Saudi Basic Industries Corporation) and National NaturalScience Foundation of China (51772072). Y. Wang also would like to acknowledge thefinancial support received from the China Scholarship Council during his visit to RiceUniversity. Author contributions: Y. Wang and J.W. conceived the idea and designed theexperiments. Y. Wu and P.M.A. supervised the project. Y. Wang and Y.L. performed thesynthesis experiment and the physical characterization in collaboration with J.Z. and J.W.Y. Wang and J.W. wrote the manuscript. Y. Wang is responsible for the Figs. 1, 2 (A to C), 3; andfigs. S1, S5 to S8, and S12 to 14. J.Z. claims responsibility for the Fig. 2 (D to I) and figs. S2to S4. Y.L. and X.W. were responsible for the Fig. 4 and figs. S9 and S10. X.Z. takes responsibilityfor fig. S11. All authors discussed the results, analyzed the data, and commented on themanuscript. Competing interests: Y. Wang, P.M.A., Y.L., J.W., H.X., N.C., and I.N.O. are authorson a patent application related to this work filed by Rice University and SABIC (no. 62/380,902,filed 29 August 2016). All other authors declare that they have no competing interests.Data and materials availability: All data needed to evaluate the conclusions in the paperare present in the paper and/or the Supplementary Materials. Additional data related to thispaper may be requested from the authors.

Submitted 8 May 2017Accepted 13 November 2017Published 15 December 201710.1126/sciadv.1701500

Citation: Y. Wang, Y. Liu, J. Zhang, J. Wu, H. Xu, X. Wen, X. Zhang, C. S. Tiwary, W. Yang,R. Vajtai, Y. Zhang, N. Chopra, I. N. Odeh, Y. Wu, P. M. Ajayan, Cryo-mediated exfoliationand fracturing of layered materials into 2D quantum dots. Sci. Adv. 3, e1701500 (2017).

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Cryo-mediated exfoliation and fracturing of layered materials into 2D quantum dots

Robert Vajtai, Yong Zhang, Nitin Chopra, Ihab Nizar Odeh, Yucheng Wu and Pulickel M. AjayanYan Wang, Yang Liu, Jianfang Zhang, Jingjie Wu, Hui Xu, Xiewen Wen, Xiang Zhang, Chandra Sekhar Tiwary, Wei Yang,

DOI: 10.1126/sciadv.1701500 (12), e1701500.3Sci Adv 

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