MEGASONICALLY SOLUTION-PROCESSED NANOSHEET INKS, FABRICATING METHODS, AND APPLICATIONS OF THE SAME

Abstract

One aspect of this invention relates to a method of forming a nanomaterial ink comprising providing an as-prepared (AP) semiconductor ink containing first nanosheets of at least one semiconductor; and megasonically exfoliating the AP semiconductor ink to form a megasonicated semiconductor ink containing second nanosheets of the at least one semiconductor.

Claims

1. A method of forming a nanomaterial ink, comprising: providing an as-prepared (AP) semiconductor ink containing first nanosheets of at least one semiconductor; and megasonically exfoliating the AP semiconductor ink to form a megasonicated (MS) semiconductor ink containing second nanosheets of the at least one semiconductor.

2. The method of claim 1, wherein said providing the AP semiconductor ink comprises: electrochemically intercalating crystals of the at least one semiconductor to obtain intercalated crystals of the at least one semiconductor; exfoliating the intercalated crystals of the at least one semiconductor in a first solvent using bath sonication to obtain a suspension of exfoliated nanosheets of the at least one semiconductor; and performing solvent transfer via centrifugation to disperse the exfoliated nanosheets of the at least one semiconductor in a second solvent while discarding supernatant, thereby forming the AP semiconductor ink containing the first nanosheets of the at least one semiconductor.

3. The method of claim 2, wherein the at least one semiconductor comprises: elemental semiconductors including phosphorene, germanene, tellurene, selenene, and/or stanene; monochalcogenides including GeS, InSe, GaTe, PbTe, SnS, and/or SnSe; dichalcogenides including MoS.sub.2, WSe.sub.2, TaS.sub.2, ReS.sub.2, and/or MoTe.sub.2; trichalcogenides including NbSe.sub.3, GaInS.sub.3, Bi.sub.2Se.sub.3, and/or In.sub.2Se.sub.3; 2D semiconducting oxides including MnO.sub.3 and/or V.sub.2O.sub.5; 2D semiconducting metal chalcophosphates including SnP.sub.2Se.sub.6, Sn.sub.2P.sub.2Se.sub.6, NiPS.sub.3, ZnPS.sub.3, FePS.sub.3; 2D metal halides including CrI.sub.3, NiI.sub.2, RuCl.sub.3, VI.sub.3; and/or semiconducting MXenes including Mn.sub.2CO.sub.2, Ti.sub.2C, Sc.sub.2CF.sub.2, and/or Cr.sub.2CF.sub.2.

4. The method of claim 2, wherein the first solvent contains polyvinylpyrrolidone (PVP) dissolved in dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), propylene carbonate (PC), N-cyclohexyl-2-pyrrolidone (CHP), dimethylcarbonate (DMC), and/or dimethyl sulfoxide (DMSO). In one embodiment, the first solvent contains a stabilizing agent comprising small molecule stabilizers including bile salts (e.g., sodium cholate), linear chain surfactants (e.g., sodium dodecyl sulfate), and/or pyrene-derivative salts (e.g., 1-pyrenesulfonic acid); and/or polymers including polyvinylpyrrolidone (PVP), Triton X-100, poly(ethylene oxide) (PEO), poly(propylene oxide) (PPO), and/or poloxamers.

5. The method of claim 4, wherein said discarding the supernatant comprises removing excess PVP and precipitating poorly exfoliated semiconductor.

6. The method of claim 5, wherein each of the second nanosheets has a residual PVP coating on its nanosheet surface.

7. The method of claim 2, wherein the second solvent contains water, methanol, ethanol, isopropanol (IPA), butanol, acetone, acetonitrile, triethanolamine, terpineol, Cyrene, dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), propylene carbonate (PC), N-cyclohexyl-2-pyrrolidone (CHP), dimethylcarbonate (DMC), and/or dimethyl sulfoxide (DMSO).

8. The method of claim 1, wherein said megasonically exfoliating the AP semiconductor ink is performed in a megasonicator having an acoustic medium containing a water bath, wherein the AP semiconductor ink is poured into a quartz liner tank installed inside the water bath.

9. The method of claim 8, wherein the quartz liner tank has a slant-bottom quartz liner fabricated such wall thickness and slope of the bottom of the quartz liner tank enable maximum transfer of acoustic energy from the water bath to the AP semiconductor ink in the quartz liner tank.

10. The method of claim 1, wherein said megasonically exfoliating the AP semiconductor ink is performed in a megasonicator having an acoustic medium containing a water bath, wherein the AP semiconductor ink is injected into a sealed plastic pouch placed inside the water bath.

11. The method of claim 10, wherein the sealed plastic pouch is solvent resistant and thin enough to ensure maximum transfer of acoustic energy from the water bath to the AP semiconductor ink in the sealed plastic pouch.

12. A nanomaterial ink, being formed by the method of claim 1.

13. The nanomaterial ink of claim 12, wherein the second nanosheets in the MS semiconductor ink are significantly thinner than the first nanosheets in the AP semiconductor ink.

14. The nanomaterial ink of claim 13, wherein the second nanosheets are averagely of monolayer nanosheets, and the first nanosheets are averagely of multilayer nanosheets.

15. The nanomaterial ink of claim 13, wherein the second nanosheets have a log-normal mean thickness of about 0.75 nm, and the first nanosheets have a log-normal mean thickness of about 2.3 nm.

16. The nanomaterial ink of claim 12, wherein each of the second nanosheets has a residual polymer coating on its nanosheet surface.

17. The nanomaterial ink of claim 12, wherein the optical absorbance spectrum of the second nanosheets has A and B exciton peaks that are blue-shifted and narrower compared to those of the first nanosheets.

18. The nanomaterial ink of claim 17, wherein the A exciton peak shifts from about 675 nm of the first nanosheets to about 650 nm of the second nanosheets, wherein the A exciton peak of about 650 nm corresponds to that of monolayer nanosheets.

19. The nanomaterial ink of claim 12, wherein the monolayer fraction in the MS semiconductor ink is about 68% of the total number of flakes, as calculated from optical absorption spectroscopy analysis, and the monolayer fraction in the AP semiconductor ink is about 2% of the total number of flakes.

20. The nanomaterial ink of claim 12, wherein the second nanosheets maintain high crystallinity after megasonication.

21. The nanomaterial ink of claim 12, wherein the distance between the in-plane (E.sup.1.sub.2g) and out-of-plane (A.sub.1g) peaks of Raman spectra of the second nanosheets is about 19.5 cm.sup.1, and wherein the distance between the E.sup.1.sub.2g and A.sub.1g peaks of Raman spectra of the first nanosheets to 22.5 cm.sup.1.

22. The nanomaterial ink of claim 12, wherein the photoluminescence (PL) intensity of the first nanosheets is significantly lower than that of the second nanosheets.

23. The nanomaterial ink of claim 12, wherein the A exciton PL peak of the first nanosheets is centered at about 1.8 eV, which is indicative of a trion-dominated resonance that is corresponding to the PL of multilayer nanosheets.

24. The nanomaterial ink of claim 12, wherein the spectral shape of the PL of the second nanosheets strongly resembles that of a mechanically exfoliated MoS.sub.2 monolayer.

25. The nanomaterial ink of claim 12, wherein the spectral peak of the PL of the second nanosheets is positioned at 1.90 eV, which reflects the A exciton direct bandgap transition at the K point of the Brillouin zone.

26. A device, comprising: at least one element formed of the nanomaterial ink according to claim 12 on a substrate.

27. The device of claim 26, wherein the at least one element is formed by dropcasting of the nanomaterial ink on the substrate.

28. The device of claim 26, further comprising electrodes coupled with the at least one element.

29. The device of claim 26, wherein the device is an electronic device including a transistor, a memristor, a diode, a power converter, a sensor, a battery, a resistor, integrated circuit elements, or combinations of them.

30. The device of claim 26, wherein the device is an optoelectronic device including a photodetector, a photosensor, a photodiode, a solar cell, a phototransistor, a light-emitting diode, a laser diode, integrated optical circuit (IOC) elements, a photoresistor, a charge-coupled imaging device, or combinations of them.

31. The device of claim 24, wherein the device is a vertical metal-semiconductor-insulator-metal (MSIM) device.

32. The device of claim 31, wherein the device comprises: a gate electrode formed of a transparent conductive material on the glass substrate; a dielectric film formed of Al.sub.2O.sub.3 on the gate electrode by atomic layer deposition (ALD); a semiconductor film formed of the second nanosheets on the dielectric film by dropcasting the nanomaterial ink; and a source electrode formed of a metal material on top of the semiconductor film.

33. The device of claim 32, wherein the transparent conductive material comprises transparent conducting oxides including fluorine doped tin oxide (FTO) and indium tin oxide (ITO).

34. The device of claim 32, wherein the metal material comprises gold, silver, chromium, indium, nickel, aluminum, platinum, palladium, bismuth, and/or titanium.

35. The device of claim 32, wherein the semiconductor film has a thickness in a range of about 1-100 nm.

36. The device of claim 32, wherein the photoluminescence (PL) intensity of the semiconductor film is peaked at 1.89 eV.

37. The device of claim 36, wherein the PL intensity of the semiconductor film increases with increasing film thickness while the PL peak remains at about 1.89 eV.

38. The device of claim 32, wherein the direct-bandgap character of individual monolayer nanosheets is retained in the composite, semiconductor film independently of film thickness.

39. The device of claim 32, wherein the MSIM device is configured to achieve electroluminescence (EL) through oscillatory bipolar carrier injection from the source electrode into the semiconductor film upon application of an alternating current (AC) bias to the gate electrode.

40. The device of claim 39, wherein the EL intensity of the MSIM device increases with the thickness of the semiconductor film.

41. The device of claim 39, wherein in operation, a waveform generator connected to a high bandwidth (1 MHz) voltage amplifier is used to apply a bipolar square wave signal V.sub.g to the gate electrode while the source electrode is grounded.

42. The device of claim 41, wherein the square wave is centered about V.sub.g=0 V with amplitudes in a range of about 1 to about 50 V and frequencies (f) in a range of about 1 to about 600 kHz.

43. The device of claim 42, wherein while applying the square wave with V.sub.g=20 V and f=100 kHz, the semiconductor EL is peaked at about 1.88 eV, indicating that the EL is emitted from a similar excitonic state as the semiconductor PL.

44. The device of claim 41, wherein the EL intensity of the MSIM device is modulatable by the square wave voltage parameters.

45. The device of claim 44, wherein the EL intensity of the MSIM device increases linearly with frequency due to an increased number of voltage transitions per unit time.

46. The device of claim 44, wherein the EL intensity of the MSIM device increases as a function of V.sub.g above the turn-on voltage.

47. The device of claim 44, wherein the EL spectra of the semiconductor film red shift slightly with increasing frequency and voltage amplitude.

48. The device of claim 44, wherein measuring EL with decreasing frequency from 400 kHz to 50 kHz results in a blue shifting of the EL peaks, indicating that any charge trapping or heating effects are reversible.

49. The device of claim 44, wherein the semiconductor film shows uniform EL over an entire active device region between the source electrode and the gate electrodes, with the EL intensity increasing with frequency.

50. The device of claim 44, wherein the emission area in the MSIM device is directly determined by the patterned electrodes due to the vertical device architecture and sufficiently conductive semiconductor film with monolayer properties.

51. The device of claim 39, wherein the vertical MSIM device is usable as pixels in miniaturized light sources including micro light emitting diodes (micro-LEDs).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0062] The accompanying drawings illustrate one or more embodiments of the invention and together with the written description, serve to explain the principles of the invention. Wherever possible, the same reference numbers are used throughout the drawings to refer to the same or like elements of an embodiment.

[0063] FIG. 1 shows a schematic representation of megasonicated MoS.sub.2 ink preparation according to embodiments of the invention. Panel (a): Electrochemical intercalation of an MoS.sub.2 crystal with tetraheptylammonium cations (THA.sup.+) in acetonitrile electrolyte. Br.sub.2 is evolved at the Pt foil counter electrode. Panel (b): Bath ultrasonication of intercalated MoS.sub.2 in dimethylformamide (DMF) and polyvinylpyrrolidone (PVP) produces a brown-colored suspension of exfoliated MoS.sub.2 nanosheets. Panel (c): Multiple rounds of centrifugation are used to perform solvent transfer from DMF to isopropanol (IPA) in addition to removing excess PVP and precipitating poorly exfoliated MoS.sub.2. The result of this primary exfoliation step is the as-prepared MoS.sub.2 ink. Panel (d): Megasonication induces secondary exfoliation of the MoS.sub.2 nanosheets down to the monolayer limit with minimal nanosheet lateral size reduction. Panel (e): The final megasonicated MoS.sub.2 ink is bright green and contains a high concentration of MoS.sub.2 monolayers.

[0064] FIG. 2 shows comparative characterization of as-prepared (AP) and megasonicated (MS) MoS.sub.2 nanosheets according to embodiments of the invention. Panel (a): The MoS.sub.2 ink changes color from brown to green after megasonication. Panel (b): Representative atomic force microscopy (AFM) images of AP-MoS.sub.2 (top) and MS-MoS.sub.2 (bottom). AFM line profiles extracted from 200 nanosheets of each MoS.sub.2 sample were used to produce comparative histograms of (c) nanosheet length and (d) nanosheet thickness. The average lengths and thicknesses of each sample are indicated on the respective plots. Panel (e): Line profiles of representative AP-MoS.sub.2 and MS-MoS.sub.2 nanosheets. Panel (f): Optical absorbance spectra of AP-MoS.sub.2 and MS-MoS.sub.2 inks. The A and B exciton peaks are indicated. Panel (g): Raman spectra of representative isolated AP-MoS.sub.2 and MS-MoS.sub.2 nanosheets. The in-plane (E.sup.1.sub.2g) and out-of-plane (A.sub.1g) vibration modes are indicated. Panel (h): Photoluminescence spectra of representative isolated AP-MoS.sub.2 and MS-MoS.sub.2 nanosheets.

[0065] FIG. 3 shows characterization of megasonicated MoS.sub.2 nanosheet films according to embodiments of the invention. Panel (a): Schematic of the metal-semiconductor-insulator-metal (MSIM) electroluminescent device. Panel (b): Top-down and (c) cross-sectional scanning electron microscopy images of a about 100-nm-thick dropcasted MS-MoS.sub.2 nanosheet percolating film, respectively. The nanosheet film is partially hanging over the edge of the substrate in each of the SEM images. Panel (d): Photoluminescence spectra of the as-prepared MoS.sub.2 and megasonicated MoS.sub.2 nanosheet films. The inset provides a schematic of the photoluminescence measurement in the MSIM device geometry. Panel (e): Photoluminescence spectra of MS-MoS.sub.2 nanosheet films with different thicknesses.

[0066] FIG. 4 shows generating electroluminescence in megasonicated MoS.sub.2 nanosheet films according to embodiments of the invention. Panel (a): Schematic of the MSIM device during operation. Electroluminescence is generated close to the Au contact and emitted through the transparent ITO/glass substrate. Panel (b): Schematic of the AC square wave voltage applied to the ITO gate electrode. Panel (c): Band diagrams for the points labeled 1 and 2 in panel (b). In particular, label 1 is for the transition from positive to negative V.sub.g, whereas label 2 is for the transition from negative to positive V.sub.g. Panel (d): Electroluminescence spectra of as-prepared and megasonicated MoS.sub.2 films that are both about 70 nm thick.

[0067] FIG. 5 shows characterization of electroluminescence in megasonicated MoS.sub.2 films according to embodiments of the invention. Panel (a): Thickness dependence of electroluminescence (EL) in MS-MoS.sub.2 films. Panel (b): EL as a function of the square wave frequency with fixed V.sub.g=20 V. Panel (c): EL as a function of the square wave voltage amplitude with fixed f=100 kHz. Panel (d): Spatial map of EL in an MS-MoS.sub.2 MSIM device, with EL intensity increasing as a function of the square wave frequency.

[0068] FIG. 6 shows megasonication details according to embodiments of the invention. Panel (a): Photograph of the quartz liner tank used for large-volume sonication. The tank is filled with 80 mL of as-prepared MoS.sub.2ink. Panel (b): Photograph of heat-sealed plastic pouches used for small volume megasonication experiments. Panel (c): Optical absorbance spectroscopy of various primary and secondary exfoliation experiments for electrochemically exfoliated MoS.sub.2 with the bath sonicator and megasonicator. Intercalated MoS.sub.2 in PVP/DMF cannot be thinned to the monolayer by a single step (primary exfoliation, curves 1 and 2) in either the bath ultrasonicator or megasonicator. Rather, the primary exfoliation of MoS.sub.2 in PVP/DMF in the ultrasonicator must be followed by solvent transfer to IPA (as prepared optical absorbance spectra in panel (f) of FIG. 2). Even then, additional processing in the ultrasonicator for up to 18 h does not induce significant secondary exfoliation of the MoS.sub.2 (curve 3). Ultimately, only megasonication of electrochemically exfoliated MoS.sub.2 in IPA allows for secondary exfoliation of nanosheets (curve 4, identical to megasonicated optical absorbance spectra in panel (f) of FIG. 2).

[0069] FIG. 7 shows additional atomic force microscopy according to embodiments of the invention. Panel (a): Atomic force microscopy image of megasonicated MoS.sub.2 flakes. The sample was annealed in a tube furnace in Ar at 220 C. to remove PVP and other solvents from the nanosheet surface. Panel (b): Line profiles corresponding to the nanosheets marked in (a). The average thickness of all nanosheets is 0.7 nm.

[0070] FIG. 8 shows estimation of nanosheet thicknesses from optical absorbance spectra according to embodiments of the invention. Panel (a): Optical absorbance spectra for as-prepared (AP) and megasonicated (MS) MoS.sub.2 inks (from panel (a) of FIG. 3) are plotted as a function of photon energy. The A and B exciton peaks are indicated. Panel (b): The second derivative of the optical absorbance spectra of AP-MoS.sub.2 around the A exciton peak. Lorentzian fits to this curve reveal four populations of nanosheet thicknesses: monolayer (1L), bilayer (2L), trilayer (3L), and multilayer (4L+). Panel (c): The second derivative of the optical absorbance spectra of MS-MoS.sub.2 around the A exciton peak. Lorentzian fits to this curve reveal two populations of nanosheet thicknesses: monolayer (1L) and bilayer (2L). Panel (d): Percentage of nanosheets in AP-MoS.sub.2 and MS-MoS.sub.2 dispersions at various thicknesses, estimated from the fit areas shown in panels (b) and (c).

[0071] FIG. 9 shows megasonicated MoS.sub.2 film formation via dropcasting according to embodiments of the invention. Optical images of various dropcasted MS-MoS.sub.2 films. A severe coffee ring effect is observed when dropcasting inks that only have isopropanol (IPA) as a solvent. Adding 20% H.sub.2O to the ink enables more uniform films from dropcasting. Multiple layers (1-3L shown here) of MoS.sub.2 ink can be dropcasted to adjust the film thickness.

[0072] FIG. 10 shows thermal annealing of megasonicated MoS.sub.2 nanosheet films according to embodiments of the invention. Panel (a): Photoluminescence, and Panel (b): normalized photoluminescence of megasonicated MoS.sub.2 films as casted and after annealing in an Ar tube furnace at 400 C. for 30 min.

[0073] FIG. 11 shows photoluminescence and electroluminescence of as-prepared MoS.sub.2 films according to embodiments of the invention. Panels (a)-(b): Raw and normalized PL spectra, respectively, of a about 70-nm-thick AP-MoS.sub.2 film acquired at different laser power filters. The PL counts increase and the peak red shifts from 1.75 eV to 1.70 eV with increasing power. Panel (c): EL spectra of AP-MoS.sub.2 at various frequencies of the gate voltage signal with fixed gate voltage amplitude of V.sub.g=20 V. The EL counts are much lower than MS-MoS.sub.2 films of similar thickness.

[0074] FIG. 12 shows fitting megasonicated MoS.sub.2 photoluminescence and electroluminescence spectra according to embodiments of the invention. Panel (a): PL and Panel (b): EL spectra of a 70-nm-thick MS-MoS.sub.2 film were fit with two Lorentzians to extract the neutral exciton (X) and negatively charged trion (X.sup.) emission contributions. The trion intensity is slightly larger in the EL spectra.

[0075] FIG. 13 shows frequency, voltage, and thickness dependence trends in the electroluminescence of megasonicated MoS.sub.2 films according to embodiments of the invention. Panel (a): Integrated EL intensity as a function of the frequency of the gate voltage signal at fixed gate voltage amplitude V.sub.g=20. The intensity increases linearly with frequency but begins to roll off at f>100 kHz. Panel (b): Integrated EL intensity in counts/cycle as a function of frequency. The cycle-normalized EL intensity decreases with frequency due to distortion of the V.sub.g square waveform. Panel (c): Integrated EL intensity and (d) integrated EL intensity in counts per cycle as a function of gate voltage amplitude V.sub.g for fixed f=100 kHz. The intensity increases linearly with V.sub.g for both cases.

[0076] FIG. 14 shows quantifying redshifts in megasonicated MoS.sub.2 electroluminescence according to embodiments of the invention. Panel (a): The peak energy of EL as a function of the frequency of the applied gate voltage signal. The peak location red shifts linearly with increasing frequency, ranging from 1.885 eV at 5 kHz to 1.865 eV at 400 kHz. The dotted line serves as a guide to the eye. Panel (b): The EL spectra for the 42-nm-thick MS-MoS.sub.2 film were fit to Lorentzian curves representing the trion and exciton contributions to EL (as in panel (b) of FIG. 11). The peak positions of the trion and exciton fits are plotted as a function of frequency on the left y-axis. The ratio of the exciton intensity to trion intensity versus frequency is plotted on the right y-axis. The contribution of the neutral exciton decreases with increasing frequency. Panel (c): EL spectra of a 70-nm-thick MS-MoS.sub.2 film were acquired while decreasing the frequency of the gate voltage signal for successive measurements. The EL peaks blue shift with decreasing frequency, indicating that the trion emission is a consequence of temporary Joule heating or charge trapping in the MoS.sub.2 film.

DETAILED DESCRIPTION OF THE INVENTION

[0077] The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. However, this invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this specification will be thorough and complete and fully convey the invention's scope to those skilled in the art. Like reference numerals refer to like elements throughout.

[0078] The terms used in this specification generally have their ordinary meanings in the art, within the context of the invention, and in the specific context where each term is used. Certain terms used to describe the invention are discussed below, or elsewhere in the specification, to provide additional guidance to the practitioner regarding the description. For convenience, certain terms may be highlighted, for example using italics and/or quotation marks. The use of highlighting has no influence on the scope and meaning of a term; the scope and meaning of a term are the same, in the same context, whether or not it is highlighted. It will be appreciated that same thing can be said in more than one way. Consequently, alternative language and synonyms may be used for any one or more of the terms discussed herein, nor is any special significance to be placed upon whether or not a term is elaborated or discussed herein. Synonyms for certain terms are provided. A recital of one or more synonyms does not exclude the use of other synonyms. The use of examples anywhere in this specification including examples of any terms discussed herein is illustrative only, and in no way limits the scope and meaning of the invention or of any exemplified term. Likewise, the invention is not limited to various embodiments given in this specification.

[0079] It will be understood that, as used in the description herein and throughout the claims that follow, the meaning of a, an, and the includes plural reference unless the context clearly dictates otherwise. Also, it will be understood that when an element is referred to as being on another element, it can be directly on the other element or intervening elements may be present therebetween. In contrast, when an element is referred to as being directly on another element, there are no intervening elements present. As used herein, the term and/or includes any and all combinations of one or more of the associated listed items.

[0080] It will be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, or section without departing from the invention's teachings.

[0081] Furthermore, relative terms, such as lower or bottom and upper or top, may be used herein to describe one element's relationship to another element as illustrated in the figures. It will be understood that relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the figures. For example, if the device in one of the figures. is turned over, elements described as being on the lower side of other elements would then be oriented on upper sides of the other elements. The exemplary term lower, can, therefore, encompasses both an orientation of lower and upper, depending on the particular orientation of the figure. Similarly, if the device in one of the figures is turned over, elements described as below or beneath other elements would then be oriented above the other elements. Therefore, the exemplary terms below or beneath can encompass both an orientation of above and below.

[0082] It will be further understood that the terms comprises and/or comprising, or includes and/or including or has and/or having, or carry and/or carrying, or contain and/or containing, or involve and/or involving, and the like are to be open-ended, i.e., to mean including but not limited to. When used in this specification, they specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof.

[0083] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and this specification, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0084] As used in this specification, around, about, approximately or substantially shall generally mean within 20 percent, preferably within 10 percent, and more preferably within 5 percent of a given value or range. Numerical quantities given herein are approximate, meaning that the term around, about, approximately or substantially can be inferred if not expressly stated.

[0085] As used in this specification, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical OR. As used herein, the term and/or includes any and all combinations of one or more of the associated listed items.

[0086] The description below is merely illustrative in nature and is in no way intended to limit the invention, its application, or uses. The broad teachings of the invention can be implemented in a variety of forms. Therefore, while this invention includes particular examples, the true scope of the invention should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. It should be understood that one or more steps within a method may be executed in a different order (or concurrently) without altering the principles of the invention.

[0087] Electroluminescence (EL), which is the conversion of an electrical current into light, is central to diverse technologies such as lighting, displays, data communication, and sensing. Over the last decade, two-dimensional (2D) monolayer semiconductorsparticularly, transition metal dichalcogenides (TMDs)have been widely studied for EL devices due to their diverse and tunable optoelectronic and photonic properties. However, challenges in isolating optoelectronically active TMD monolayers using scalable liquid phase exfoliation have precluded the realization of electroluminescence in large-area, solution-processed TMD films.

[0088] Several techniques have been attempted to obtain monolayer-rich molybdenum disulfide (MoS.sub.2) inks. Lithium intercalation of MoS.sub.2 powders or crystals is a decades-old approach to obtain a high yield of MoS.sub.2 monolayers. However, this approach induces a phase change from semiconducting 2H-MoS.sub.2 to conductive 1T-MoS.sub.2, resulting in entirely different structural, optical, and electronic properties that are not suitable for light-emitting devices. Techniques to reverse the phase change do not fully recover semiconducting 2H-MoS.sub.2. More recently, standard electrochemical exfoliation has been reported to yield MoS.sub.2 inks containing monolayers; however, the photoluminescence of these inks does not show the direct bandgap of monolayer MoS2, which undermines the claim of monolayer-rich inks. More broadly, acoustic-based exfoliation techniques such as horn-tip and bath sonication can be used to produce 2D material monolayers. However, the monolayer yield is quite low and the nanosheet length is significantly smaller (about 100 nm) than that of electrochemically exfoliated materials (0.5-2 m). Monolayer enrichment through techniques such as liquid cascade centrifugation and density gradient ultracentrifugation is time-consuming and ultimately discards over 90% of the exfoliated material.

[0089] Large-area electroluminescence from TMD films has been attempted using CVD-gr own monolayer films of MoS.sub.2 and WS.sub.2 with a vertical capacitor device geometry. However, the electroluminescence in these devices is spatially localized near the contact edges. Large-area EL (millimeter-scale) is obtained by creating a dense array of source electrodes that limits the overall emitting area. Furthermore, the monolayer thickness in these devices also limits the net light output. In our invention, megasonically exfoliated MoS.sub.2 for the vertical capacitor device enables planar and uniform light-emission in a proof-of-concept 100 m300 m area using a single continuous source electrode. There are no fundamental limits to scaling this process to larger areas.

[0090] Previous attempts to study the EL of solution-processed TMDs have focused on MoS.sub.2 quantum dots or composite materials such as MoS.sub.2MoO.sub.3 and MoS.sub.2PEDOT:PSS, all of which have optoelectronic properties that are inferior to monolayer MoS.sub.2 nanosheets.

[0091] Among the previously investigated TMD solution processing methods, electrochemical exfoliation has yielded the best device performance in percolating nanosheet films for thin-film transistors, photodetectors, and memristors. Electrochemically exfoliated TMD inks are notable for their superior nanosheet size uniformity compared to inks obtained through alternative liquid phase exfoliation methods such as horn sonication or shear mixing. The high degree of nanosheet size uniformity results in solution-processed films with exceptional percolating charge transport, yielding carrier mobilities on the order of 10 cm.sup.2 V.sup.1 s.sup.1, which is comparable to that of mechanically exfoliated MoS.sub.2. While most previous studies have utilized as-prepared electrochemically exfoliated MoS.sub.2 inks in isopropanol (IPA), we have recently shown that secondary exfoliation with megasonication (i.e., bath sonication at megahertz frequencies) further thins electrochemically exfoliated MoS.sub.2 nanosheets down to the monolayer limit, which is disclosed in PCT Patent Application No. PCT/US2023/022664, which is incorporated herein in their entireties by reference. Importantly, megasonication enriches the monolayer concentration of the MoS.sub.2 ink by converting thicker nanosheets to monolayers, unlike post-exfoliation centrifugal separation processes that discard thicker material, thus providing significantly higher monolayer yields and scalability. The mechanistic differences between ultrasonic and megasonic exfoliation of 2D materials are discussed in detail in the section of METHODS AND MATERIALS. Briefly, while traditional liquid phase exfoliation processes based on violent inertial cavitation at kilohertz sonication frequencies cause nanosheet fragmentation in addition to exfoliation, the gentler and more stable cavitation produced with megahertz sonication frequencies results in nanosheet delamination with minimal fracture. Due to the high concentration of monolayer MoS.sub.2 nanosheets with micron-scale lateral sizes, megasonically exfoliated and printed MoS.sub.2 films have yielded the highest reported responsivities among all-printed, visible-light photodetectors, suggesting that megasonication could also enable other solution-processed TMD optoelectronic devices.

[0092] In this disclosure, we overcome these limitations and demonstrate EL from MoS.sub.2 nanosheet films by developing and employing a monolayer rich MoS.sub.2 ink produced by electrochemical intercalation and megasonic exfoliation. Megasonicated MoS.sub.2 films demonstrate characteristic monolayer MoS.sub.2 photoluminescence and electroluminescence spectral peaks, with emission intensity increasing with film thickness over the range of 10-70 n m. Furthermore, employing a vertical light-emitting capacitor geometry enables uniform electroluminescence in large-area devices. These novel properties are attributed to a polymer coating on the MoS.sub.2 monolayer surfaces that impedes strong electronic coupling between adjacent nanosheets. Additionally, these results indicate that megasonically exfoliated MoS.sub.2 monolayers retain their direct-bandgap character in thick electrically percolating thin films even following multi-step solution processing. Overall, this work establishes megasonicated MoS.sub.2 inks as an additive manufacturing platform for flexible, patterned, and miniaturized light sources that can likely be expanded to other TMD semiconductors.

[0093] Inexpensive, high-quality 2D materials must be available to drive commercial applications. The invention of a scalable megasonication process to obtain a high yield of MoS.sub.2 monolayers via solution processing achieves this goal. Simultaneously, compelling applications must be developed to utilize these inks. Towards this end, scalably manufactured films of megasonicated MoS.sub.2 nanosheets may be utilized in a number of light-emitting technologies.

[0094] Without intent to limit the scope of the invention, exemplary embodiments of the invention are given below.

[0095] In one aspect, this invention relates to a method of forming a nanomaterial ink. The method comprises providing an as-prepared (AP) semiconductor ink containing first nanosheets of at least one semiconductor; and megasonically exfoliating the AP semiconductor ink to form a megasonicated (MS) semiconductor ink containing second nanosheets of the at least one semiconductor.

[0096] It should be noted that the term first nanosheets, used in the disclosure, refers to nanosheets of the at least one semiconductor in the AP semiconductor ink, which are produced by the primary exfoliation of intercalated crystals of the at least one semiconductor using bath sonication, and the term second nanosheets, used in the disclosure, refers to nanosheets of the at least one semiconductor in the MS semiconductor ink, which are produced by the secondary exfoliation of the AP semiconductor ink using megasonication.

[0097] In one embodiment, said providing the AP semiconductor ink comprises electrochemically intercalating crystals of the at least one semiconductor to obtain intercalated crystals of the at least one semiconductor; exfoliating the intercalated crystals of the at least one semiconductor in a first solvent using bath sonication to obtain a suspension of exfoliated nanosheets of the at least one semiconductor; and performing solvent transfer via centrifugation to disperse the exfoliated nanosheets of the at least one semiconductor in a second solvent while discarding supernatant, thereby forming the AP semiconductor ink containing the first nanosheets of the at least one semiconductor.

[0098] In one embodiment, the at least one layered semiconductor comprises elemental semiconductors including phosphorene, germanene, tellurene, selenene, and/or stanene; monochalcogenides including GeS, InSe, GaTe, PbTe, SnS, and/or SnSe; dichalcogenides including MoS.sub.2, WSe.sub.2, TaS.sub.2, ReS.sub.2, SnS.sub.2 and/or MoTe.sub.2; trichalcogenides including NbSe.sub.3, GaInS.sub.3, Bi.sub.2Se.sub.3, and/or In.sub.2Se.sub.3; 2D semiconducting oxides including MnO.sub.3 and/or V.sub.2O.sub.5; 2D semiconducting metal chalcophosphates including SnP.sub.2Se.sub.6, Sn.sub.2P.sub.2Se.sub.6, NiPS.sub.3, ZnPS.sub.3, FePS.sub.3; 2D metal halides including CrI.sub.3, NiI.sub.2, RuCl.sub.3, VI.sub.3; and/or semiconducting MXenes including Mn.sub.2CO.sub.2, Ti.sub.2C, Sc.sub.2CF.sub.2, and/or Cr.sub.2CF.sub.2.

[0099] In one embodiment, the first solvent contains dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), propylene carbonate (PC), N-cyclohexyl-2-pyrrolidone (CHP), dimethylcarbonate (DMC), and/or dimethyl sulfoxide (DMSO).

[0100] In one embodiment, the first solvent contains a stabilizing agent comprising small molecule stabilizers including bile salts (e.g., sodium cholate), linear chain surfactants (e.g., sodium dodecyl sulfate), and/or pyrene-derivative salts (e.g., 1-pyrenesulfonic acid); and/or polymers including polyvinylpyrrolidone (PVP), Triton X-100, poly(ethylene oxide) (PEO), poly(propylene oxide) (PPO), and/or poloxamers.

[0101] In one embodiment, wherein said discarding the supernatant comprises removing excess PVP and precipitating poorly exfoliated semiconductor.

[0102] In one embodiment, each of the second nanosheets has a residual PVP coating on its nanosheet surface.

[0103] In one embodiment, the second solvent contains water, methanol, ethanol, isopropanol (IPA), butanol, acetone, acetonitrile, triethanolamine, terpineol, Cyrene, dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), propylene carbonate (PC), N-cyclohexyl-2-pyrrolidone (CHP), dimethylcarbonate (DMC), and/or dimethyl sulfoxide (DMSO).

[0104] In one embodiment, said megasonically exfoliating the AP semiconductor ink is performed in a megasonicator having an acoustic medium containing a water bath, wherein the AP semiconductor ink is poured into a quartz liner tank installed inside the water bath.

[0105] In one embodiment, the quartz liner tank has a slant-bottom quartz liner fabricated such wall thickness and slope of the bottom of the quartz liner tank enable maximum transfer of acoustic energy from the water bath to the AP semiconductor ink in the quartz liner tank.

[0106] In one embodiment, said megasonically exfoliating the AP semiconductor ink is performed in a megasonicator having an acoustic medium containing a water bath, wherein the AP semiconductor ink is injected into a sealed plastic pouch placed inside the water bath.

[0107] In one embodiment, the sealed plastic pouch is solvent resistant and thin enough to ensure maximum transfer of acoustic energy from the water bath to the AP semiconductor ink in the sealed plastic pouch.

[0108] In another aspect, this invention relates to a nanomaterial ink that is formed by the above disclosed methods.

[0109] In one embodiment, the second nanosheets in the MS semiconductor ink are significantly thinner than the first nanosheets in the AP semiconductor ink.

[0110] In one embodiment, the second nanosheets are averagely of monolayer nanosheets, and the first nanosheets are averagely of multilayer nanosheets.

[0111] In one embodiment, the second nanosheets have a log-normal mean thickness of about 0.75 nm, and the first nanosheets have a log-normal mean thickness of about 2.3 nm.

[0112] In one embodiment, each of the second nanosheets has a residual polymer (e.g., PVP) coating on its nanosheet surface.

[0113] In one embodiment, the optical absorbance spectrum of the second nanosheets has A and B exciton peaks that are blue-shifted and narrower compared to those of the first nanosheets. It should be noted that the term A exciton PL peak, used in the disclosure, refers to a photoluminescence (PL) emission feature that corresponds to a ground state exciton in monolayer transition metal dichalcogenides (TMDs), and the term B exciton PL peak, used in the disclosure, refers to another PL emission feature that corresponds to higher spin-orbit split state exciton in monolayer TMDs.

[0114] In one embodiment, the A exciton peak shifts from about 675 nm of the first nanosheets to about 650 nm of the second nanosheets, wherein the A exciton peak of about 650 nm corresponds to that of monolayer nanosheets.

[0115] In one embodiment, the monolayer fraction in the MS semiconductor ink is about 68% of the total number of flakes, as calculated from optical absorption spectroscopy analysis, and the monolayer fraction in the AP semiconductor ink is about 2% of the total number of flakes.

[0116] In one embodiment, the second nanosheets maintain high crystallinity after megasonication.

[0117] In one embodiment, the distance between the in-plane (E.sup.1.sub.2g) and out-of-plane (A.sub.1g) peaks of Raman spectra of the second nanosheets is about 19.5 cm.sup.1, and wherein the distance between the E.sup.1.sub.2g and A.sub.1g peaks of Raman spectra of the first nanosheets to 22.5 cm.sup.1.

[0118] In one embodiment, the photoluminescence (PL) intensity of the first nanosheets is significantly lower than that of the second nanosheets.

[0119] In one embodiment, the A exciton PL peak of the first nanosheets is centered at about 1.8 eV, which is indicative of a trion-dominated resonance that is corresponding to the PL of multilayer nanosheets.

[0120] In one embodiment, the spectral shape of the PL of the second nanosheets strongly resembles that of a mechanically exfoliated MoS.sub.2 monolayer.

[0121] In one embodiment, the spectral peak of the PL of the second nanosheets is positioned at 1.90 eV, which reflects the A exciton direct bandgap transition at the K point of the Brillouin zone.

[0122] In a further aspect, the invention relates to a device comprising at least one element formed of the nanomaterial ink as disclosed above.

[0123] In one embodiment, the at least one element is formed by dropcasting of the nanomaterial ink on the substrate.

[0124] In one embodiment, the device further comprises electrodes coupled with at least one element.

[0125] In one embodiment, the device is an electronic device including a transistor, a memristor, a diode, a power converter, a sensor, a battery, a resistor, integrated circuit elements, or combinations of them.

[0126] In one embodiment, the device is an optoelectronic device including a photodetector, a photosensor, a photodiode, a solar cell, a phototransistor, a light-emitting diode, a laser diode, integrated optical circuit (IOC) elements, a photoresistor, a charge-coupled imaging device, or combinations of them.

[0127] In one embodiment, the device is a vertical metal-semiconductor-insulator-metal (MSIM) device.

[0128] In one embodiment, the device comprises a gate electrode formed of a transparent conductive material on the glass substrate; a dielectric film formed of Al.sub.2O.sub.3 on the gate electrode by atomic layer deposition (ALD); a semiconductor film formed of the second nanosheets on the dielectric film by dropcasting the nanomaterial ink; and a source electrode formed of a metal material on top of the semiconductor film.

[0129] In one embodiment, the transparent conductive material comprises transparent conducting oxides including fluorine doped tin oxide (FTO) and indium tin oxide (ITO).

[0130] In one embodiment, the metal material comprises gold, silver, chromium, indium, nickel, aluminum, platinum, palladium, bismuth, and/or titanium.

[0131] In one embodiment, the semiconductor film has a thickness in a range of about 1-100 nm.

[0132] In one embodiment, the photoluminescence (PL) intensity of the semiconductor film is peaked at 1.89 eV.

[0133] In one embodiment, the PL intensity of the semiconductor film increases with increasing film thickness while the PL peak remains at about 1.89 eV.

[0134] In one embodiment, the direct-bandgap character of individual monolayer nanosheets is retained in the composite, semiconductor film independently of film thickness.

[0135] In one embodiment, the MSIM device is configured to achieve electroluminescence (EL) through oscillatory bipolar carrier injection from the source electrode into the semiconductor film upon application of an alternating current (AC) bias to the gate electrode.

[0136] In one embodiment, the EL intensity of the MSIM device increases with the thickness of the semiconductor film.

[0137] In one embodiment, in operation, a waveform generator connected to a high bandwidth (1 MHz) voltage amplifier is used to apply a bipolar square wave signal V.sub.g to the gate electrode while the source electrode is grounded.

[0138] In one embodiment, the square wave is centered about V.sub.g=0 V with amplitudes in a range of about 1 to about 50 V and frequencies (f) in a range of about 1 to about 600 kHz.

[0139] In one embodiment, while applying the square wave with V.sub.g=20 V and f=100 kHz, the semiconductor EL is peaked at about 1.88 eV, indicating that the EL is emitted from a similar excitonic state as the semiconductor PL.

[0140] In one embodiment, the EL intensity of the MSIM device is modulatable by the square wave voltage parameters.

[0141] In one embodiment, the EL intensity of the MSIM device increases linearly with frequency due to an increased number of voltage transitions per unit time.

[0142] In one embodiment, the EL intensity of the MSIM device increases as a function of V.sub.g above the turn-on voltage.

[0143] In one embodiment, the EL spectra of the semiconductor film red shift slightly with increasing frequency and voltage amplitude.

[0144] In one embodiment, measuring EL with decreasing frequency from 400 kHz to 50 kHz results in a blue shifting of the EL peaks, indicating that any charge trapping or heating effects are reversible.

[0145] In one embodiment, the semiconductor film shows uniform EL over an entire active device region between the source electrode and the gate electrodes, with the EL intensity increasing with frequency.

[0146] In one embodiment, the emission area in the MSIM device is directly determined by the patterned electrodes due to the vertical device architecture and sufficiently conductive semiconductor film with monolayer properties.

[0147] In one embodiment, the vertical MSIM device is usable as pixels in miniaturized light sources including micro light emitting diodes (micro-LEDs).

[0148] Among other things, the invention provides at least the following advantages.

[0149] A scalable method has been achieved for megasonication of electrochemically exfoliated van der Waals crystals. The process yields nanosheets with a tight distribution of thickness (t.sub.ave0.7 nm for MoS.sub.2) with a large fraction (about 70%) of monolayers. This megasonication method utilizes a commercially available megasonic cleaner for a high net yield. This secondary exfoliation technique has advantages over primary exfoliation techniques such as tip sonication, bath ultrasonication, and shear mixing that produce a large distribution of nanosheet sizes in dispersions. In this manner, megasonically exfoliation conserves the mass of exfoliated material in solution and increases the yield of monolayers compared to centrifugal processing techniques such as liquid cascade centrifugation and density gradient ultracentrifugation.

[0150] A high yield of MoS.sub.2 monolayers from the megasonication process allows the realization of technology that relies exclusively on the high quantum yield of optical transitions across the direct electronic bandgap such as electroluminescent (EL) or light-emitting devices. Composite films from megasonicated MoS.sub.2 also yield a strong photoluminescence (PL) spectrum that matches isolated MoS.sub.2 monolayers.

[0151] The solution-processed, electroluminescent monolayer MoS.sub.2 inks are compatible with scalable manufacturing techniques, such as ink printing, to fabricate large-area light-emitting devices. Conventionally, electroluminescence in 2D semiconductors has been demonstrated with mechanically exfoliated nanosheets for proof-of-concept devices; however, these devices are not scalable due to the labor-intensive nanosheet synthesis approach. Chemical vapor deposition (CVD) has also been utilized to obtain large-area monolayer films of 2 D nanosheets for EL, but this method requires several cleanroom-based film transfer and patterning steps that limit the quality and scalability of EL technology. Moreover, CVD monolayers are only 1-nm-thick which limits the net optical output of EL. Here, megasonication allows thicker percolating MoS.sub.2 films (approaching 100 nm in thickness) that retain the direct bandgap of monolayer MoS.sub.2 that is essential for high quantum yield EL.

[0152] Fabricating vertical light-emitting capacitors with electroluminescent megasonicated MoS.sub.2 inks uniquely enables large-area, planar EL devices. Other attempts to utilize this capacitor architecture with large-area CVD-grown TMD films have resulted in spatially localized electroluminescence at electrode edges, requiring a high electrode density to mimic planar electroluminescence where electrodes disrupt the spatial homogeneity of the light output.

[0153] The PL and EL intensities of megasonicated MoS.sub.2 vertical light-emitting capacitors (LECs) scale with increasing MoS.sub.2 nanosheet film thickness, which is a notable advance for practical technologies. As an example, organic EL molecules used in conventional solution-processed light-emitting diodes (LEDs) must be assembled in uniform, thick films to maximize light emission. Percolating films of 2D materials such as MoS.sub.2 have similar charge transport characteristics as organic molecules, but scalability in the vertical direction is the key to realizing practical light-emitting devices. In this work, there is sufficient charge carrier mobility in the vertical direction in megasonicated MoS.sub.2 films to allow band bending to occur at the metal contact. The band bending is reversed at a high rate by the application of an alternating current (AC) voltage at the other contact in a two-probe vertical LEC. Furthermore, the polymer coating on the surface of MoS.sub.2 monolayers, which is a consequence of the initial electrochemical exfoliation process, impedes undesirable electronic coupling between nanosheets and maintains monolayer characteristics in bulk films. Consequently, light emission (both PL and EL) is obtained from the depth of the nanosheet film determined by the depletion region and extinction coefficient of the percolating MoS.sub.2 film.

[0154] The invention may have widespread applications in electroluminescent devices, light-emitting capacitors, photodetectors, flexible photodetectors, two-dimensional transistors, thin-film transistors, neuromorphic computing devices, printed electronics, micro-pixel flat panel displays, nanomaterials synthesis, printed optoelectronics, wearable sensors, and the like.

[0155] These and other aspects of the invention are further described below. Without intent to limit the scope of the invention, exemplary instruments, apparatus, methods, and their related results according to the embodiments of the invention are given below. Note that titles or subtitles may be used in the examples for convenience of a reader, which in no way should limit the scope of the invention. Moreover, certain theories are proposed and disclosed herein; however, in no way they, whether they are right or wrong, should limit the scope of the invention so long as the invention is practiced according to the invention without regard for any particular theory or scheme of action.

Example

Electroluminescence from Megasonically Solution-Processed MoS.SUB.2 .Nanosheet Films

[0156] Due to their unique optoelectronic properties, monolayer two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted significant attention for electroluminescent devices. However, challenges in isolating optoelectronically active TMD monolayers using scalable liquid phase exfoliation has precluded electroluminescence in large-area, solution-processed TMD films.

[0157] In this example, we overcome these limitations and demonstrate electroluminescence from molybdenum disulfide (MoS.sub.2) nanosheet films by employing a monolayer rich MoS.sub.2 ink produced by electrochemical intercalation and megasonic exfoliation. Characteristic monolayer MoS.sub.2 photoluminescence and electroluminescence spectral peaks at 1.88-1.90 eV are observed in megasonicated MoS.sub.2 films with the emission intensity increasing with film thickness over the range of 10-70 nm. Furthermore, employing a vertical light-emitting capacitor architecture enables uniform electroluminescence in large-area devices. These results indicate that megasonically exfoliated MoS.sub.2 monolayers retain their direct bandgap character in electrically percolating thin films even following multi-step solution processing. Overall, this work establishes megasonicated MoS.sub.2 inks as an additive manufacturing platform for flexible, patterned, and miniaturized light sources that can likely be expanded to other TMD semiconductors.

[0158] Specifically, we employ megasonication to achieve electroluminescence (EL) from large-area, solution-processed MoS.sub.2 nanosheet films. Unlike previous work that achieved megasonication using a low-volume atomizer chamber, this study utilizes a large-volume megasonication bath that increases processing throughput and results in a high fraction of monolayer MoS.sub.2 nanosheets that exhibit a strong photoluminescence (PL) spectral peak at 1.90 eV. Moreover, even 70-nm-thick dropcasted films produced from the megasonicated MoS.sub.2 inks display strong PL peaked at 1.89 eV, confirming that the monolayer MoS.sub.2 optoelectronic response is preserved in large-area percolating films. By integrating large-area percolating films of megasonicated MoS.sub.2 with a parallel-plate metal-semiconductor-insulator-metal (MSIM) capacitor geometry, an alternating current (AC) biasing scheme can generate EL with tunable intensity controlled by the film thickness in addition to the AC voltage amplitude and frequency. The EL spectra from these large-area devices possess peaks at 1.88 eV that closely match the PL spectra, thus confirming that monolayer MoS.sub.2 properties are preserved in thin films in the MSIM architecture. Overall, this work establishes a scalable, solution-based pathway for producing large-area EL devices based on monolayer TMDs.

Methods and Materials

Ink Preparation:

[0159] A synthetic MoS.sub.2 crystal (2D Semiconductors) was divided into narrow slivers (approx. 0.5 mm wide2 cm long1 mm thick) using a razor blade. An electrochemical cell was constructed with a sliver of MoS.sub.2 as the working electrode, platinum foil as the counter electrode, and an electrolyte of 5 mg/mL tetraheptylammonium bromide (99%, Acros Organics/Thermo Scientific Chemicals) in acetonitrile (99.8%, Sigma-Aldrich). A fixed potential of 10 V was applied across the cell for 1-2 hours to drive intercalation. Next, the expanded crystal sliver was dried in air and chopped with a razor blade into a chunky powder. The aggregate powder of several crystal slivers was bath sonicated in 40 mL of 0.2 M polyvinylpyrrolidone (PVP) (powder, average Mw about 29,000, Sigma-Aldrich) in dimethylformamide (DMF) (anhydrous, 99.8%, Sigma-Aldrich) for 90 min. The exfoliated slurry was washed with isopropanol (IPA; ACS Grade, Fisher Chemical) three times by pelletizing the MoS.sub.2 by ultracentrifugation, discarding the supernatant, and redispersing the pellet in fresh IPA. Finally, the dispersion was centrifuged at 7500 rpm for 5 min (Avanti J26XPI with a JS-7.5 rotor) to obtain the as-prepared MoS.sub.2 nanosheet ink.

Megasonication:

[0160] To further increase the monolayer nanosheet concentration, the MoS.sub.2 ink was subjected to megasonication. A megasonic cleaner and generator (PCT Systems Megasonic Hyperclean) operating at about 0.95 MHz were utilized for secondary exfoliation experiments. The acoustic medium was water, and the water bath temperature was regulated with an ultrasonic bath cooling coil (Fisher brand) attached to a chiller operating at 5-20 C. The megasonicator was operated at full power with a mux time of 10 ms. Two techniques were utilized to contain the MoS.sub.2 ink during megasonication. For volumes >50 mL, the ink was poured into a quartz linear tank (panel (a) of FIG. 6). The slant-bottom quartz liner was fabricated such the wall thickness and slope of the bottom enable the maximum transfer of acoustic energy. For volumes <10 mL, the ink was injected into a heat-sealed plastic pouch (panel (b) of FIG. 6). These pouches were solvent resistant and thin enough to ensure maximum transfer of acoustic energy between the water bath and the ink. Depending on the volume and concentration of the ink, maximum megasonic exfoliation was observed after 2-72 hours of megasonication. The degree of monolayer conversion in the ink was tracked via optical absorbance spectroscopy (Agilent Cary 5000 Spectrophotometer), as shown in panel (c) of FIG. 6.

Device Fabrication:

[0161] Glass chips coated with a thin layer of indium tin oxide (ITO) were obtained from Thin Film Devices, Inc. (Anaheim, California). The ITO chips were cleaned by successive bath sonication in acetone, isopropanol, and deionized water. A 60-nm-thick film of Al.sub.2O.sub.3 was grown on part of the ITO chip via atomic layer deposition (Cambridge Nanotech Savannah 5100 ALD). Next, the MoS.sub.2 ink was dropcasted onto the region of the chip covered in Al.sub.2O.sub.3(FIG. 9). Gold electrodes were fabricated by physical vapor deposition through a shadow mask to contact the ITO and the MoS.sub.2 films, forming a metal-semiconductor-insulator-metal (MSIM) stack.

Photoluminescence Spectroscopy:

[0162] Photoluminescence (PL) spectra were obtained using a Horiba XploRa PLUS confocal microscope with a 473 nm laser and 600 mm.sup.1 grating. All point spectra were acquired at 20 magnification (NA=0.4) at 1% laser power (92 W) with 0.5 s acquisition times and averaged over 10 accumulations.

Electroluminescence Spectroscopy:

[0163] Electroluminescence (EL) of the MoS.sub.2 films was measured at 20 magnification (NA=0.4) using a Horiba XploRa PLUS confocal microscope coupled with a spectrometer via a 600 mm.sup.1 grating. A waveform generator (Agilent 33500B Series Trueform Waveform Generator) was used to output a square bipolar wave with programmed peak-to-peak amplitude (Vpp) of 50 mV to 1.5 V. A voltage amplifier (Thorlabs HVA200 High Voltage Amplifier) was used to amplify this signal 40-fold such that the absolute applied potential ranged from 1 V to 30 V. The frequency of the waveform was adjusted between 1 kHz and 500 kHz. An oscilloscope (Tektronix TBS 2000 Series Digital Oscilloscope) was used to verify the applied potential and identify leaky and short circuits. EL spectra shown in this work were obtained with an acquisition time of 20 s and averaged over 5 accumulations. Spatial visualization of EL was obtained at 5 magnification using a CMOS camera (Teledyne Photometrics PRIME BSI Express Scientific CMOS).

Atomic Force Microscopy:

[0164] The thickness of MoS.sub.2 nanosheets was studied with atomic force microscopy (AFM). Dilute aliquots of MoS.sub.2 in IPA were prepared and dropcasted onto 300 nm SiO.sub.2 wafer chips. To remove excess PVP on the surface of nanosheets, the chips were annealed under Ar flow in a tube furnace (Thermo Scientific Lindberg Blue M) at 220 C. for 1 h. Isolated nanosheets were characterized by atomic force microscopy (Oxford Instruments Asylum Research Cypher AFM) in tapping mode using Si cantilevers with a resonance frequency of about 320 kHz. AFM images were post-processed in Gwyddion, and line profiles across isolated nanosheets were obtained. A MATLAB program was used to extract the nanosheet thicknesses and lengths from the line profile data.

Raman Spectroscopy:

[0165] Raman spectra were obtained at 100 magnification (NA=0.9) using a Horiba XploRa PLUS confocal microscope with a 473 nm laser and 2400 mm.sup.1 grating. Spectra were acquired at 10% laser power (1.13 mW) with 10 s acquisition time and averaged over 6 accumulations.

Scanning Electron Microscopy:

[0166] Morphological characterization of the MoS.sub.2 film was performed with top-down and cross-sectional scanning electron microscopy (Hitachi SU8030 SEM). Samples were prepared by dropcasting MoS.sub.2 ink on glass slides and cleaving through the film by scoring the bottom of the glass slide. The samples were sputter coated with a about 9 nm thick film from an Au/Pd target (Denton Desk IV Sputter Coater) to avoid charging artifacts.

Acoustic Exfoliation of 2D Materials:

[0167] The principles of acoustic exfoliation of 2D materials have been derived from studies of acoustic technology in other fields. Since the 20.sup.th century, ultrasound technology has been utilized in fields as diverse as naval navigation, medicine, food science, and industrial solids processing. Meanwhile, megahertz-frequency acoustic technology, also called megasonics, emerged in the late 1970s for use in semiconductor cleaning applications. In the decades since, megasonic cleaning has been adopted as a gentler, more effective means of removing nanoparticles from the surfaces of wafers covered with delicate nanoscale structures.

[0168] The basic process of ultrasonication and megasonication in 2D material liquid phase exfoliation (LPE) is as follows. Acoustic waves are produced by piezoelectric transducers and propagate through an acoustic medium. In bath sonication, the acoustic medium is primarily water and secondarily the exfoliation solvent that is contained in a vessel; in tip sonication, the medium is simply the exfoliation solvent. As the acoustic wave travels, the pressure in the medium changes as a function of time and space. Bubbles grow in regions of negative pressure and shrink or collapse in regions of high pressure; this process is called cavitation. On average, smaller bubbles are produced at higher frequencies. Furthermore, acoustic streaming, or fluid flow induced by acoustic waves, occurs in the medium. While the nominal difference between ultrasonics and megasonics is the range of excitation frequenciesultrasound is defined as acoustic frequencies >18 kHz, while megasonic frequencies are >350 kHzthe dominant microscale and nanoscale processes in these two regimes are markedly different.

[0169] In ultrasonication, the bubbles that form are primarily composed of vapor that evolves when the acoustic liquid is ruptured under negative pressure. During high pressure cycles, a fraction of these bubbles collapses violently and rapidly, releasing large amounts of energy, in a process known as inertial cavitation. This shockwave of energy facilitates 2D material exfoliation through mechanisms such as delamination, fragmentation, compressive stress, and shearing. Indeed, experiments with inertial cavitation generated by a tip sonotrode in the presence of a graphite crystal have shown the production of tensile stresses of 3-7 MPa, which is sufficient to overcome interlayer van der Waals forces. Acoustic streaming in ultrasonication primarily involves microjetting, in which bubble implosions generate a high-velocity jet of solvent that can plastically deform the sample surface and create pits. While some researchers have hypothesized that microjetting plays a vital role in 2D material exfoliation, recent in situ visualization experiments have not observed significant microjetting during exfoliation; rather, shockwaves induced by inertial cavitation were observed to be the primary exfoliation mechanism.

[0170] In megasonication, cavitation bubbles are primarily composed of gas. Under the influence of the acoustic field, these bubbles may coalesce, dissolve, or float out of the acoustic medium. Stable cavitation is achieved when a bubble has a minimum radius and can grow through a rectified diffusion process. Similarly to the case of ultrasonication, while acoustic streaming was long theorized to play a crucial role in megasonic cleaning, in situ visualization of this process reveals that stable oscillating bubbles are responsible for detaching nanoparticles from the surface of a substrate. Moreover, in situ visualization of stable cavitation in the presence of a graphite flake shows that pressure oscillations induce delamination from the edge of the flake. Therefore, maximizing stable cavitation through strategies such as optimizing acoustic pulse parameters, adding surfactants to the acoustic medium, or inducing liquid atomization is critical for applications in megasonic cleaning, and analogously in megasonic exfoliation. It is also valuable to note that the acoustic boundary layer decreases with increasing frequencies, causing higher viscous stresses at a substrate surface (e.g., the nanosheet surfaces). Furthermore, long range acoustic streaming, called Eckart streaming, can separate delaminated nanosheets, preventing reaggregation.

[0171] The majority of acoustic-based exfoliation research has focused on bath ultrasonication and horn tip sonication, where sonication parameters such as power, time, and temperature have been explored. Meanwhile, interest in megasonic exfoliation techniques has developed over the last five years. Several studies have demonstrated megasonic exfoliation-on-chip technology that applies about 10-20 MHz acoustic waves to a small volume (10 s L) of 2D material powder dispersed in water, inducing exfoliation through acoustic streaming or liquid atomization mechanisms. Telkhozhayeva et al. reported that larger, thinner 2D nanosheets can be exfoliated by increasing the acoustic frequency, although the maximum frequency studied was only 80 kHz. Kuo et al. reported similar results by using megasonic atomization at 1.65 MHz to exfoliate large-area MoS.sub.2 nanosheets down to the monolayer limit. These promising examples motivate more exploratory work for optimizing megasonic exfoliation conditions as well as finding the best applications for megasonically exfoliated 2D materials.

Results and Discussions

[0172] FIG. 1 provides a schematic representation of the megasonicated MoS.sub.2 ink preparation. The ink production process begins with electrochemical intercalation (panel (a) of FIG. 1), for which an electrochemical cell was constructed with an MoS.sub.2 crystal as the working electrode and platinum foil as the counter electrode. The electrolyte includes tetraheptylammonium bromide (THAB) salt dissolved in acetonitrile. A bias of 10 V is applied across the cell to drive THA.sup.+ cations to intercalate between the MoS.sub.2 crystal layers. The intercalated MoS.sub.2 crystals are then exfoliated by traditional bath sonication at 40 kHz (panel (b) of FIG. 1) in 0.2 M polyvinylpyrrolidone (PVP) dissolved in dimethylformamide (DMF). Although intercalating multiple MoS.sub.2 crystals can be time-consuming, this process can be scaled up by intercalating large pellets of pressed MoS.sub.2 powder. Next, solvent transfer via centrifugation (panel (c) of FIG. 1) is utilized to disperse the MoS.sub.2 nanosheets in IPA while removing excess PVP and crashing out poorly exfoliated MoS.sub.2. The resulting MoS.sub.2 ink following this primary exfoliation step is brown in color and is labeled as-prepared MoS.sub.2 (AP-MoS.sub.2) throughout this study.

[0173] Following primary exfoliation, the AP-MoS.sub.2 ink is subjected to a secondary megasonic exfoliation step. Previously, we disclosed secondary exfoliation of MoS.sub.2 through megasonication within the atomization chamber of an Optomec AJ200 aerosol jet printer (AJP), in PCT Patent Application No. PCT/US2023/022664, which is incorporated herein in their entireties by reference. The AJP has a piezoelectric transducer that outputs a power of about 30 W/in.sup.2 at 1.65 MHz to aerosolize approximately 1.5 mL volume of ink. To overcome this small volume limitation and decouple megasonication from the AJP, we instead employed a PCT Systems 0.95 MHz bath megasonicator with a power output of 36 W/in.sup.2 (PCT Systems Inc., San Jose, CA 95131). This megasonicator is typically used for cleaning semiconductor wafers, but it was adapted for megasonication of MoS.sub.2 inks by installing a quartz liner tank with a minimum fill volume of about 60 mL inside the water bath (panel (d) of FIG. 1 and panel (a) of FIG. 6). The quartz liner was fabricated by PCT Systems to allow maximum coupling of acoustic energy from the water bath to the ink in the liner tank. Alternatively, for megasonication of smaller volumes, the AP-MoS.sub.2 ink was injected into a heat-sealed plastic pouch that was thin enough to transmit the acoustic energy through the membrane (panel (b) of FIG. 6). Utilizing the heat-sealed plastic pouch, we investigated whether megasonication could replace ultrasonication as the primary exfoliation step; however, as characterized through optical absorbance spectroscopy (data not shown), this processing change resulted in a lower concentration of exfoliated multilayer MoS.sub.2 compared to primary ultrasonication, and no monolayer MoS.sub.2 exciton absorbance peak was observed. Ultimately, the multi-step procedure as depicted in FIG. 1 uniquely enables the scalable preparation of large volumes of megasonicated MoS.sub.2 (MS-MoS.sub.2) ink (panel (e) of FIG. 1).

[0174] After megasonication, the MS-MoS.sub.2 ink is a brighter green color than the AP-MoS.sub.2 ink (panel (a) of FIG. 2), providing a visual indication of megasonic exfoliation. Quantitative verification of the efficacy of the 0.95 MHz bath megasonicator for inducing megasonic exfoliation was obtained through atomic force microscopy (AFM). To prepare samples for AFM analysis, an aliquot of each batch of ink was diluted with IPA, dropcasted on 300-nm-thick SiO.sub.2/Si substrates, and then annealed in a tube furnace at 220 C. for 1 hour to remove residual polymer and solvent from the nanosheet surfaces. AFM topographical imaging (panel (b) of FIG. 2 and FIG. 7) reveals that the MS-MoS.sub.2 nanosheets are significantly thinner than the AP-MoS.sub.2 nanosheets. Extraction of nanosheet lateral sizes from AFM line scans (N=200 for each sample) yielded nanosheet length distributions for the AP-MoS.sub.2 and MS-MoS.sub.2 samples with similar log-normal mean lengths of 0.75 m and 0.74 m, respectively (panel (c) of FIG. 2). On the other hand, the nanosheet thickness distributions show that the MS-MoS.sub.2 nanosheets are significantly thinner than the AP-MoS.sub.2nanosheets with respective log-normal mean thicknesses of 0.75 nm and 2.3 nm (panels (d)-(e) of FIG. 2). Since monolayer MoS.sub.2 typically appears to be approximately 0.7 nm thick in AFM measurements, the AFM results here correspond to average megasonication-induced nanosheet thinning from about 3 layers to monolayer, confirming that our scaled-up megasonication process produces a large population of monolayer MoS.sub.2 nanosheets.

[0175] A variety of spectroscopic techniques were subsequently applied to further characterize the physical properties of the MS-MoS.sub.2 ink and nanosheets. For example, optical absorbance spectroscopy reveals that the A and B exciton peaks for MS-MoS.sub.2 are blue-shifted and narrower compared to those of AP-MoS.sub.2 (panel (f) of FIG. 2). In particular, the A exciton peak shifts from about 675 nm to about 650 nm, which is consistent with previous reports of monolayer MoS.sub.2. The optical absorbance spectra also allow the fraction of monolayer MoS.sub.2 nanosheets to be estimated using a protocol devised by Backes et al. Specifically, the A exciton absorbance contributions are extracted from various thickness populations by fitting the second derivative of the optical absorbance spectra as a function of photon energy with Lorentzian curves (panels (a)-(c) of FIG. 8). Accordingly, we determined that the monolayer fraction significantly increases from 2% in the AP-MoS.sub.2 ink to 68% in the MS-MoS.sub.2 ink (panel (d) of FIG. 8).

[0176] Raman and photoluminescence (PL) spectroscopy were also performed on isolated AP-MoS.sub.2 and MS-MoS.sub.2 nanosheets that were dropcasted onto SiO.sub.2/Si substrates without additional processing (a schematic is provided in the inset of panel (h) of FIG. 2). Representative Raman spectra of both samples (panel (g) of FIG. 2) show the characteristic in-plane (E.sup.1.sub.2g) and out-of-plane (A.sub.1g) vibrational modes, confirming that the MoS.sub.2 nanosheets maintain high crystallinity after megasonication. Furthermore, the distance between the E.sup.1.sub.2g and A.sub.1g peaks decreases from 22.5 cm.sup.1 for AP-MoS.sub.2 to 19.5 cm.sup.1 for MS-MoS.sub.2, which is consistent with megasonication decreasing the nanosheet thickness down to the monolayer limit. The PL spectra in panel (h) of FIG. 2 further reflect the differences between multilayer AP-MoS.sub.2 nanosheets and monolayer MS-MoS.sub.2 nanosheets. First, the PL intensity of AP-MoS.sub.2 is significantly lower than that of MS-MoS.sub.2 because the PL quantum yield (QY) significantly decreases with increasing MoS.sub.2 nanosheet thickness. Second, the AP-MoS.sub.2 A exciton PL peak is centered at about 1.8 eV, which is indicative of a trion-dominated resonance that is commonly observed in the PL of multilayer MoS.sub.2 nanosheets at room temperature. Third, the spectral shape of the MS-MoS.sub.2 PL strongly resembles that of a mechanically exfoliated MoS.sub.2monolayer. The MS-MoS.sub.2 PL spectral peak position at 1.90 eV further reflects the A exciton direct bandgap transition at the K point of the Brillouin zone. In addition, a trion resonance peaked at about 1.85 eV contributes to the MS-MoS.sub.2 PL, as has been previously reported for monolayer MoS.sub.2. Interestingly, the B exciton peak is not observed in either the AP-MoS.sub.2 or MS-MoS.sub.2 PL spectra. This result can be attributed to the high structural quality of the MoS.sub.2 nanosheets in both samples, as a low B-to-A exciton ratio indicates low defect concentration. Notably, the structural and optical properties of MS-MoS.sub.2 monolayers agree well with those of highest quality mechanically exfoliated MoS.sub.2 monolayers utilized in electroluminescent devices and field-effect transistors, which possess a Raman interpeak distance of about 20 cm.sup.1 as well as direct bandgap PL spectra.

[0177] Having confirmed the superlative optoelectronic properties of MS-MoS.sub.2 nanosheets, the MS-MoS.sub.2 inks were used to fabricate vertical MSIM devices, which were designed to achieve EL through oscillatory bipolar carrier injection from the metal source contact into the semiconducting MoS.sub.2 film upon application of an alternating current (AC) bias to the gate contact (panel (a) of FIG. 3). Indium tin oxide (ITO) coated on a glass substrate was utilized as the gate electrode since its optical transparency enables detection of light emission. Atomic layer deposition (ALD) was then used to grow a 60-nm-thick Al.sub.2O.sub.3 gate dielectric film on the ITO. Dropcasting was selected as an efficient method of fabricating large-area MoS.sub.2 films without losing excess ink, as is unavoidable in other solution-based deposition techniques such as spin coating. The MS-MoS.sub.2 ink was diluted to approximately 0.33 mg/mL in IPA as verified by optical absorbance spectroscopy (based on an extinction coefficient of 1987 L g.sup.1 m.sup.1 at 525 nm), after which 20% H.sub.2O was added to the ink to enable dropcasting of uniform percolating MS-MoS.sub.2 films at 60 C. with negligible coffee ring effects (panel (a) of FIG. 9). Films of various thicknesses were realized by dropcasting multiple layers of MS-MoS.sub.2 ink (panel (a) of FIG. 9). Finally, source contacts were deposited by evaporating 100-nm-thick Au on top of the MS-MoS.sub.2 film through a stainless-steel shadow mask to complete the MSIM device geometry.

[0178] Top-down and cross-sectional scanning electron microscopy (SEM) imaging of a 100-nm-thick dropcasted MS-MoS.sub.2 film confirmed a percolating and dense nanosheet network (panels (a)-(c) of FIG. 3). Four-point-probe electrical measurements revealed a film conductivity of 0.06 S/m0.04 S/m, which is notable since the film was not subjected to any additional annealing steps. While this conductivity is sufficient for achieving EL in the vertical MSIM devices as will be discussed below, it is likely that even higher film conductivities can be achieved by controlled annealing treatments that have been shown to enhance electrical conductivity in inkjet-printed films derived from electrochemically exfoliated MoS.sub.2.

[0179] The optical properties of the MoS.sub.2 films in the MSIM geometry were characterized using photoluminescence spectroscopy by focusing the 473 nm excitation laser through the glass/ITO substrate at the MoS.sub.2-alumina interface (measurement schematic is provided in the inset of panel (d) of FIG. 3). AP-MoS.sub.2 and MS-MoS.sub.2 films (thickness about 70 nm) exhibit similar PL spectra as the respective isolated nanosheets (panel (d) of FIG. 3 and panel (h) of FIG. 2). Specifically, the AP-MoS.sub.2 film shows weak PL that is peaked at roughly 1.75 eV due to a low fraction of monolayers, whereas the monolayer-rich MS-MoS.sub.2 film shows bright PL that is peaked at 1.89 eV. Furthermore, the PL intensity of the MS-MoS.sub.2 films increases with increasing film thickness while the PL peak remains at about 1.89 eV (panel (e) of FIG. 3). This trend contrasts what was reported for thin films of lithium-exfoliated MoS.sub.2 monolayers, in which PL quenching and PL red shifting was observed as a function of increasing film thickness up to about 10 nm. These PL data indicate that individual MoS.sub.2 monolayer nanosheets retain their direct-bandgap character in a composite film independently of film thickness, likely due to the residual PVP coating on the nanosheet surfaces that restricts strong electronic interactions between the nanosheets. To test this hypothesis, a MS-MoS.sub.2 EL device was annealed in an Ar tube furnace at 400 C. for 30 min to completely pyrolyze the PVP coating. Subsequent characterization revealed that the PL intensity of the annealed MS-MoS.sub.2 nanosheet film was substantially degraded (panel (a) of FIG. 10). Furthermore, a weak PL peak was observed for the annealed MS-MoS.sub.2 at around 1.80 eV (panel (b) of FIG. 10), resembling the PL spectral shape of an AP-MoS.sub.2 nanosheet. Thus, the PVP coating plays a critical role in maintaining the monolayer direct-bandgap optical properties of MS-MoS.sub.2 in solution-deposited films.

[0180] For electroluminescence measurements, light emission from the MSIM devices were detected through the ITO/glass substrate (see measurement schematic in panel (a) of FIG. 4). A waveform generator connected to a high bandwidth (1 MHz) voltage amplifier was used to apply a bipolar square wave signal V.sub.g to the ITO electrode (gate) while the bottom Au electrode (source) was grounded (panel (b) of FIG. 4). During device operation, holes are injected into the MoS.sub.2 film at negative biases, whereas electrons are injected at positive biases. When the voltage polarity is reversed, injected carriers from the Au source contact radiatively recombine with a fraction of oppositely charged carriers that remain in the semiconducting layer (panel (c) of FIG. 4). The square wave was centered about V.sub.g=0 V with amplitudes in the range 5 to 25 V and frequencies (f) in the range of 1 to 400 kHz. Voltage amplitudes and frequencies beyond this range led to rapid device degradation including shorting due to dielectric breakdown of the alumina gate dielectric. Panel (d) of FIG. 4 compares the EL responses of about 70-nm-thick AP-MoS.sub.2 and MS-MoS.sub.2 films while applying a square wave with V.sub.g=20 V and f=100 kHz. The AP-MoS.sub.2 film EL is peaked at about 1.75 eV with an intensity significantly lower than the MS-MoS.sub.2 film EL, which is attributed to the lower QY of the thicker AP-MoS.sub.2 nanosheets. In contrast, the MS-MoS.sub.2 EL is peaked at about 1.88 eV, which indicates that EL is emitted from a similar excitonic state as the MS-MoS.sub.2 PL. These MS-MoS.sub.2 EL results underscore that the electroluminescent properties of megasonicated MoS.sub.2 closely resemble mechanically exfoliated and CVD-grown MoS.sub.2 monolayers.

[0181] The electroluminescence of MS-MoS.sub.2 MSIM devices was subsequently studied as a function of film thickness and square wave voltage parameters. As was observed with PL, the EL intensity increases with MS-MoS.sub.2 film thickness (panel (a) of FIG. 5). In thin films, the EL intensity is expected to depend on both the optical absorption length and spatial extent of band bending from the Au source contact, which is a function of the MoS.sub.2 film thickness, effective dielectric constant, morphology, and carrier mobility. For MS-MoS.sub.2 thin films, the net effect is increasing EL with increasing film thickness up to several tens of nanometers in thickness, which enables significantly higher EL intensity than achievable with mechanically exfoliated or CVD-grown MoS.sub.2 monolayers. In addition to film thickness, the EL intensity can be modulated by the square wave voltage parameters. Working with a about 70-nm-thick MS-MoS.sub.2 nanosheet film, EL spectra were acquired as function of square wave frequency at a fixed amplitude V.sub.g=20 V (panel (b) of FIG. 5) and as a function of square wave amplitude at a fixed frequency f=100 kHz (panel (c) of FIG. 5). These measurements reveal that the EL intensity can be tuned over three orders of magnitude. As expected, the integrated EL intensity increases linearly with frequency due to an increased number of voltage transitions per unit time (panel (a) of FIG. 13). However, the applied waveform becomes distorted as the frequency approaches the voltage amplifier bandwidth (1 MHz), resulting in EL intensity roll-off due to the maximum V.sub.g being up to 20% lower than the programmed V.sub.g (panel (b) of FIG. 13) in addition to the possibility that the period of the square wave approaches the time required for effective carrier injection and transport into the depth of the film. The integrated EL intensity also increases as a function of V.sub.g above the turn-on voltage (panels (c)-(d) of FIG. 13). An exponential V.sub.g dependence is observed in the sub-threshold regime, which then transitions into a linear dependence as the Fermi level begins to enter the band extrema. This dependence of the EL intensity on the square wave voltage amplitude is consistent with previous MSIM device reports.

[0182] The MS-MoS.sub.2 film EL spectra red shift slightly with increasing frequency and voltage amplitude (panel (a) of FIG. 14). This red shift may be attributed to trapped electrons in the film or Joule heating in the device. Nanosheet films are known to be susceptible to Joule heating at high biases due to energy concentration at atomically sharp nanosheet edges, with the resulting elevated temperature affecting charge transport in the film. Moreover, the displacement current in the metal electrode lines could further contribute to Joule heating in these devices. It is important to note that measuring EL with decreasing frequency from 400 kHz to 50 kHz results in a blue shifting of the EL peaks (panel (c) of FIG. 14), indicating that any charge trapping or heating effects are reversible.

[0183] Finally, a high-sensitivity CMOS camera was utilized for spatial mapping of the light emission from the vertical MSIM devices. The MS-MoS.sub.2 film shows uniform EL over the entire 100 m300 m active device region between the Au source and ITO gate electrodes, with the integrated EL intensity increasing with frequency (panel (d) of FIG. 5). Notably, previous reports of MSIM devices based on 2D materials have only demonstrated spatially localized EL within a Debye length of the source contacts. In those previous studies, the spatial uniformity was enhanced by fabricating strategically-spaced windows in the source contact or by utilizing a porous carbon nanotube network to overcome low mobility in the emitter film. In contrast, the emission area in our MSIM devices is directly determined by the patterned electrodes due to the vertical device architecture and sufficiently conductive MS-MoS.sub.2 films with monolayer properties. The negligible EL emission beyond the edges of the source contact can be attributed to highly localized band bending directly above the electrode. The ability to spatially localize EL simply via electrode patterning suggests that these MS-MoS.sub.2 vertical MSIM devices can be used as pixels in miniaturized light sources such as micro light emitting diodes (micro-LEDs).

CONCLUSIONS

[0184] In summary, we have developed a megasonically exfoliated, monolayer-rich MoS.sub.2 nanosheet ink that can be used to fabricate electroluminescent MoS.sub.2 nanosheet films. Even at thicknesses approaching 100 nm, these MS-MoS.sub.2 films possess PL properties comparable to mechanically exfoliated or CVD-grown MoS.sub.2 monolayers, revealing that the MS-MoS.sub.2 films preserve the direct bandgap properties of their constituent monolayer nanosheets. These MS-MoS.sub.2 films have also been integrated into vertical MSIM devices, where the resulting EL can be tuned by orders of magnitude by varying the frequency and amplitude of the driving square wave voltage. In addition, the vertical MSIM device architecture provides a means for patterning the EL emission in a manner suitable for applications in display technologies. Since the EL emission is normal to patterned contacts, this MSIM device architecture is amenable to large-area additive manufacturing by sequentially printing the MS-MoS.sub.2 ink, dielectric layer, and electrodes. Overall, this work establishes a scalable, solution-based platform for incorporating optoelectronically active monolayer TMD nanosheets into printed electronics, sensors, and related applications.

[0185] The foregoing description of the exemplary embodiments of the invention has been presented only for the purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the above teaching.

[0186] The embodiments were chosen and described to explain the principles of the invention and their practical application to enable others skilled in the art to utilize the invention and various embodiments and with various modifications as are suited to the particular use contemplated. Alternative embodiments will become apparent to those skilled in the art to which the invention pertains without departing from its spirit and scope. Accordingly, the scope of the invention is defined by the appended claims rather than the foregoing description and the exemplary embodiments described therein.

[0187] Some references, which may include patents, patent applications, and various publications, are cited and discussed in the description of this invention. The citation and/or discussion of such references is provided merely to clarify the description of the invention and is not an admission that any such reference is prior art to the invention described herein. All references cited and discussed in this specification are incorporated herein by reference in their entireties and to the same extent as if each reference was individually incorporated by reference.

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