Patent Application
20240405153
The present invention generally relates to material science, particularly to electroluminescence from megasonically solution-processed nanosheet inks, fabricating methods, and applications of the same.
The background description provided herein is to present the context of the invention generally. The subject matter discussed in the background of the invention section should not be assumed to be prior art merely due to its mention in the background of the invention section. Similarly, a problem mentioned in the background of the invention section or associated with the subject matter of the background of the invention section should not be assumed to have been previously recognized in the prior art. The subject matter in the background of the invention section merely represents different approaches, which in and of themselves may also be inventions. Work of the presently named inventors, to the extent it is described in the background of the invention section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the invention.
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 semiconductors have been widely studied for electroluminescent devices due to their diverse and tunable optoelectronic and photonic properties. In particular, transition metal dichalcogenides (TMDs) have been of interest due to their layer-dependent bandstructure, direct bandgap at the monolayer limit, strong light-matter interactions, tightly bound excitons, trions, and multi-excitons that can be controlled by strain and doping, solution-processibility for printed optoelectronics, high carrier mobility, and mechanical flexibility. Furthermore, the ability to stack van der Waals materials without the need for lattice matching has enabled a variety of light-emitting device architectures including lateral p-n homojunctions with a split-gate geometry, vertical p-n heterojuctions, and quantum well structures. Light emission through bipolar carrier injection in metal-insulator-semiconductor-metal (MISM) capacitor structures has also been pursued for 2D material light-emitting devices for multiple reasons. First, the light-emitting capacitor structure is simpler to fabricate than p-n heterojunctions or quantum well structures. Second, it avoids stringent materials selection for contact metals since Schottky barriers and work function potential differences can be overcome at moderately high turn-on voltages. Third, the metal-semiconductor-insulator-metal (MSIM) architecture is less sensitive to morphological nonuniformity in the semiconductor layer, which has been exploited in other solution-processed electroluminescent materials such as organic semiconductors.
An outstanding challenge in 2D material optoelectronics is the scalable, solution-based fabrication of large-area electroluminescent devices. For example, the majority of studies on electroluminescent TMDs have utilized mechanically exfoliated nanosheets that cannot be mass-produced. While wafer-scale electroluminescent TMD monolayers can be synthesized by chemical vapor deposition (CVD), these CVD-grown TMD materials suffer from two principal limitations. First, individual monolayers must be delicately transferred from the growth substrate to the device substrate, limiting the manufacturing scalability of this approach. Second, the net fluence of CVD-grown TMD electroluminescent devices is limited by either the atomic-scale thickness of monolayer films or by the low quantum efficiency of thicker TMD films that have indirect bandgaps. In contrast, liquid phase exfoliation has emerged as a low-cost, scalable approach to obtaining TMD nanosheets. However, most studies of solution-processed TMD crystals have resulted in indirect-bandgap few-layer nanosheets with low carrier mobilities in percolating films, which limit their performance in optoelectronic devices such as photodetectors and phototransistors. Consequently, previous attempts to study the EL of solution-processed TMDs have focused on MoS2 quantum dots or composite materials such as MoS2—MoO3 and MoS2—PEDOT:PSS, all of which have optoelectronic properties that are fundamentally different from monolayer MoS2 nanosheets.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.
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.
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.
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 MoS2, WSe2, TaS2, ReS2, SnS2 and/or MoTe2; trichalcogenides including NbSe3, GaInS3, Bi2Se3, and/or In2Se3; 2D semiconducting oxides including MnO3 and/or V2O5; 2D semiconducting metal chalcophosphates including SnP2Se6, Sn2P2Se6, NiPS3, ZnPS3, FePS3; 2D metal halides including CrI3, NiI2, RuCl3, VI3; and/or semiconducting MXenes including Mn2CO2, Ti2C, Sc2CF2, and/or Cr2CF2.
In one embodiment, the first solvent contains high boiling point organic solvent such as 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.
In one embodiment, wherein said discarding the supernatant comprises removing excess PVP and precipitating poorly exfoliated semiconductor.
In one embodiment, each of the second nanosheets has a residual polymer (e.g., PVP) coating on its nanosheet surface.
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).
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.
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.
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.
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.
In another aspect, this invention relates to a nanomaterial ink that is formed by the above disclosed methods.
In one embodiment, the second nanosheets in the MS semiconductor ink are significantly thinner than the first nanosheets in the AP semiconductor ink.
In one embodiment, the second nanosheets are averagely of monolayer nanosheets, and the first nanosheets are averagely of multilayer nanosheets.
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.
In one embodiment, each of the second nanosheets has a residual polymer (e.g., PVP) coating on its nanosheet surface.
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.
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.
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.
In one embodiment, the second nanosheets maintain high crystallinity after megasonication.
In one embodiment, the distance between the in-plane (E12g) and out-of-plane (A1g) peaks of Raman spectra of the second nanosheets is about 19.5 cm−1, and wherein the distance between the E12g and A1g peaks of Raman spectra of the first nanosheets to 22.5 cm−1.
In one embodiment, the photoluminescence (PL) intensity of the first nanosheets is significantly lower than that of the second nanosheets.
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.
In one embodiment, the spectral shape of the PL of the second nanosheets strongly resembles that of a mechanically exfoliated MoS2 monolayer.
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.
In a further aspect, the invention relates to a device comprising at least one element formed of the nanomaterial ink as disclosed above.
In one embodiment, the at least one element is formed by dropcasting of the nanomaterial ink on the substrate.
In one embodiment, the device further comprises electrodes coupled with the at least one element.
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.
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.
In one embodiment, the device is a vertical metal-semiconductor-insulator-metal (MSIM) device.
In one embodiment, the device comprises a gate electrode formed of a transparent conductive material on the glass substrate; a dielectric film formed of Al2O3 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.
In one embodiment, the transparent conductive material comprises transparent conducting oxides including fluorine doped tin oxide (FTO) and indium tin oxide (ITO).
In one embodiment, the metal material comprises gold, silver, chromium, indium, nickel, aluminum, platinum, palladium, bismuth, and/or titanium.
In one embodiment, the semiconductor film has a thickness in a range of about 1-100 nm.
In one embodiment, the photoluminescence (PL) intensity of the semiconductor film is peaked at 1.89 eV.
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.
In one embodiment, the direct-bandgap character of individual monolayer nanosheets is retained in the composite, semiconductor film independently of film thickness.
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.
In one embodiment, the EL intensity of the MSIM device increases with the thickness of the semiconductor film.
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 Vg to the gate electrode while the source electrode is grounded.
In one embodiment, the square wave is centered about Vg=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.
In one embodiment, while applying the square wave with Vg=±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.
In one embodiment, the EL intensity of the MSIM device is modulatable by the square wave voltage parameters.
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.
In one embodiment, the EL intensity of the MSIM device increases as a function of Vg above the turn-on voltage.
In one embodiment, the EL spectra of the semiconductor film red shift slightly with increasing frequency and voltage amplitude.
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.
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.
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.
In one embodiment, the vertical MSIM device is usable as pixels in miniaturized light sources including micro light emitting diodes (micro-LEDs).
These and other aspects of the present invention will become apparent from the following description of the preferred embodiment taken in conjunction with the following drawings, although variations and modifications therein may be affected without departing from the spirit and scope of the novel concepts of the invention.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 semiconductors—particularly, 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.
Several techniques have been attempted to obtain monolayer-rich molybdenum disulfide (MoS2) inks. Lithium intercalation of MoS2 powders or crystals is a decades-old approach to obtain a high yield of MoS2 monolayers. However, this approach induces a phase change from semiconducting 2H-MoS2 to conductive 1T-MoS2, 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-MoS2. More recently, standard electrochemical exfoliation has been reported to yield MoS2 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 μm). 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.
Large-area electroluminescence from TMD films has been attempted using CVD-gr own monolayer films of MoS2 and WS2 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 MoS2 for the vertical capacitor device enables planar and uniform light-emission in a proof-of-concept 100 μm×300 μm area using a single continuous source electrode. There are no fundamental limits to scaling this process to larger areas.
Previous attempts to study the EL of solution-processed TMDs have focused on MoS2 quantum dots or composite materials such as MoS2—MoO3 and MoS2—PEDOT:PSS, all of which have optoelectronic properties that are inferior to monolayer MoS2 nanosheets.
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 cm2 V−1 s−1, which is comparable to that of mechanically exfoliated MoS2. While most previous studies have utilized as-prepared electrochemically exfoliated MoS2 inks in isopropanol (IPA), we have recently shown that secondary exfoliation with megasonication (i.e., bath sonication at megahertz frequencies) further thins electrochemically exfoliated MoS2 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 MoS2 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 MoS2 nanosheets with micron-scale lateral sizes, megasonically exfoliated and printed MoS2 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.
In this disclosure, we overcome these limitations and demonstrate EL from MoS2 nanosheet films by developing and employing a monolayer rich MoS2 ink produced by electrochemical intercalation and megasonic exfoliation. Megasonicated MoS2 films demonstrate characteristic monolayer MoS2 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 MoS2 monolayer surfaces that impedes strong electronic coupling between adjacent nanosheets. Additionally, these results indicate that megasonically exfoliated MoS2 monolayers retain their direct-bandgap character in thick electrically percolating thin films even following multi-step solution processing. Overall, this work establishes megasonicated MoS2 inks as an additive manufacturing platform for flexible, patterned, and miniaturized light sources that can likely be expanded to other TMD semiconductors.
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 MoS2 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 MoS2 nanosheets may be utilized in a number of light-emitting technologies.
Without intent to limit the scope of the invention, exemplary embodiments of the invention are given below.
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.
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.
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.
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 MoS2, WSe2, TaS2, ReS2, SnS2 and/or MoTe2; trichalcogenides including NbSe3, GaInS3, Bi2Se3, and/or In2Se3; 2D semiconducting oxides including MnO3 and/or V2O5; 2D semiconducting metal chalcophosphates including SnP2Se6, Sn2P2Se6, NiPS3, ZnPS3, FePS3; 2D metal halides including CrI3, NiI2, RuCl3, VI3; and/or semiconducting MXenes including Mn2CO2, Ti2C, Sc2CF2, and/or Cr2CF2.
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).
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.
In one embodiment, wherein said discarding the supernatant comprises removing excess PVP and precipitating poorly exfoliated semiconductor.
In one embodiment, each of the second nanosheets has a residual PVP coating on its nanosheet surface.
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).
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.
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.
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.
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.
In another aspect, this invention relates to a nanomaterial ink that is formed by the above disclosed methods.
In one embodiment, the second nanosheets in the MS semiconductor ink are significantly thinner than the first nanosheets in the AP semiconductor ink.
In one embodiment, the second nanosheets are averagely of monolayer nanosheets, and the first nanosheets are averagely of multilayer nanosheets.
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.
In one embodiment, each of the second nanosheets has a residual polymer (e.g., PVP) coating on its nanosheet surface.
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.
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.
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.
In one embodiment, the second nanosheets maintain high crystallinity after megasonication.
In one embodiment, the distance between the in-plane (E12g) and out-of-plane (A1g) peaks of Raman spectra of the second nanosheets is about 19.5 cm−1, and wherein the distance between the E12g and A1g peaks of Raman spectra of the first nanosheets to 22.5 cm−1.
In one embodiment, the photoluminescence (PL) intensity of the first nanosheets is significantly lower than that of the second nanosheets.
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.
In one embodiment, the spectral shape of the PL of the second nanosheets strongly resembles that of a mechanically exfoliated MoS2 monolayer.
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.
In a further aspect, the invention relates to a device comprising at least one element formed of the nanomaterial ink as disclosed above.
In one embodiment, the at least one element is formed by dropcasting of the nanomaterial ink on the substrate.
In one embodiment, the device further comprises electrodes coupled with at least one element.
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.
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.
In one embodiment, the device is a vertical metal-semiconductor-insulator-metal (MSIM) device.
In one embodiment, the device comprises a gate electrode formed of a transparent conductive material on the glass substrate; a dielectric film formed of Al2O3 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.
In one embodiment, the transparent conductive material comprises transparent conducting oxides including fluorine doped tin oxide (FTO) and indium tin oxide (ITO).
In one embodiment, the metal material comprises gold, silver, chromium, indium, nickel, aluminum, platinum, palladium, bismuth, and/or titanium.
In one embodiment, the semiconductor film has a thickness in a range of about 1-100 nm.
In one embodiment, the photoluminescence (PL) intensity of the semiconductor film is peaked at 1.89 eV.
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.
In one embodiment, the direct-bandgap character of individual monolayer nanosheets is retained in the composite, semiconductor film independently of film thickness.
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.
In one embodiment, the EL intensity of the MSIM device increases with the thickness of the semiconductor film.
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 Vg to the gate electrode while the source electrode is grounded.
In one embodiment, the square wave is centered about Vg=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.
In one embodiment, while applying the square wave with Vg=±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.
In one embodiment, the EL intensity of the MSIM device is modulatable by the square wave voltage parameters.
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.
In one embodiment, the EL intensity of the MSIM device increases as a function of Vg above the turn-on voltage.
In one embodiment, the EL spectra of the semiconductor film red shift slightly with increasing frequency and voltage amplitude.
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.
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.
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.
In one embodiment, the vertical MSIM device is usable as pixels in miniaturized light sources including micro light emitting diodes (micro-LEDs).
Among other things, the invention provides at least the following advantages.
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 (tave≈0.7 nm for MoS2) 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.
A high yield of MoS2 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 MoS2 also yield a strong photoluminescence (PL) spectrum that matches isolated MoS2 monolayers.
The solution-processed, electroluminescent monolayer MoS2 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 MoS2 films (approaching 100 nm in thickness) that retain the direct bandgap of monolayer MoS2 that is essential for high quantum yield EL.
Fabricating vertical light-emitting capacitors with electroluminescent megasonicated MoS2 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.
The PL and EL intensities of megasonicated MoS2 vertical light-emitting capacitors (LECs) scale with increasing MoS2 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 MoS2 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 MoS2 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 MoS2 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 MoS2 film.
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.
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.
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.
In this example, we overcome these limitations and demonstrate electroluminescence from molybdenum disulfide (MoS2) nanosheet films by employing a monolayer rich MoS2 ink produced by electrochemical intercalation and megasonic exfoliation. Characteristic monolayer MoS2 photoluminescence and electroluminescence spectral peaks at 1.88-1.90 eV are observed in megasonicated MoS2 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 MoS2 monolayers retain their direct bandgap character in electrically percolating thin films even following multi-step solution processing. Overall, this work establishes megasonicated MoS2 inks as an additive manufacturing platform for flexible, patterned, and miniaturized light sources that can likely be expanded to other TMD semiconductors.
Specifically, we employ megasonication to achieve electroluminescence (EL) from large-area, solution-processed MoS2 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 MoS2 nanosheets that exhibit a strong photoluminescence (PL) spectral peak at 1.90 eV. Moreover, even 70-nm-thick dropcasted films produced from the megasonicated MoS2 inks display strong PL peaked at 1.89 eV, confirming that the monolayer MoS2 optoelectronic response is preserved in large-area percolating films. By integrating large-area percolating films of megasonicated MoS2 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 MoS2 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.
A synthetic MoS2 crystal (2D Semiconductors) was divided into narrow slivers (approx. 0.5 mm wide×2 cm long×1 mm thick) using a razor blade. An electrochemical cell was constructed with a sliver of MoS2 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 MoS2 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 MoS2 nanosheet ink.
To further increase the monolayer nanosheet concentration, the MoS2 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 MoS2 ink during megasonication. For volumes >50 mL, the ink was poured into a quartz linear tank (panel (a) of
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 Al2O3 was grown on part of the ITO chip via atomic layer deposition (Cambridge Nanotech Savannah 5100 ALD). Next, the MoS2 ink was dropcasted onto the region of the chip covered in Al2O3(
Photoluminescence (PL) spectra were obtained using a Horiba XploRa PLUS confocal microscope with a 473 nm laser and 600 mm−1 grating. All point spectra were acquired at 20× magnification (NA=0.4) at 1% laser power (92 μW) with 0.5 s acquisition times and averaged over 10 accumulations.
Electroluminescence (EL) of the MoS2 films was measured at 20× magnification (NA=0.4) using a Horiba XploRa PLUS confocal microscope coupled with a spectrometer via a 600 mm−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).
The thickness of MoS2 nanosheets was studied with atomic force microscopy (AFM). Dilute aliquots of MoS2 in IPA were prepared and dropcasted onto 300 nm SiO2 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 spectra were obtained at 100× magnification (NA=0.9) using a Horiba XploRa PLUS confocal microscope with a 473 nm laser and 2400 mm−1 grating. Spectra were acquired at 10% laser power (1.13 mW) with 10 s acquisition time and averaged over 6 accumulations.
Morphological characterization of the MoS2 film was performed with top-down and cross-sectional scanning electron microscopy (Hitachi SU8030 SEM). Samples were prepared by dropcasting MoS2 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.
The principles of acoustic exfoliation of 2D materials have been derived from studies of acoustic technology in other fields. Since the 20th 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.
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 frequencies—ultrasound is defined as acoustic frequencies >18 kHz, while megasonic frequencies are >350 kHz—the dominant microscale and nanoscale processes in these two regimes are markedly different.
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.
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.
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 μL) 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 MoS2 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.
Following primary exfoliation, the AP-MoS2 ink is subjected to a secondary megasonic exfoliation step. Previously, we disclosed secondary exfoliation of MoS2 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/in2 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/in2 (PCT Systems Inc., San Jose, CA 95131). This megasonicator is typically used for cleaning semiconductor wafers, but it was adapted for megasonication of MoS2 inks by installing a quartz liner tank with a minimum fill volume of about 60 mL inside the water bath (panel (d) of
After megasonication, the MS-MoS2 ink is a brighter green color than the AP-MoS2 ink (panel (a) of
A variety of spectroscopic techniques were subsequently applied to further characterize the physical properties of the MS-MoS2 ink and nanosheets. For example, optical absorbance spectroscopy reveals that the A and B exciton peaks for MS-MoS2 are blue-shifted and narrower compared to those of AP-MoS2 (panel (f) of
Raman and photoluminescence (PL) spectroscopy were also performed on isolated AP-MoS2 and MS-MoS2 nanosheets that were dropcasted onto SiO2/Si substrates without additional processing (a schematic is provided in the inset of panel (h) of
Having confirmed the superlative optoelectronic properties of MS-MoS2 nanosheets, the MS-MoS2 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 MoS2 film upon application of an alternating current (AC) bias to the gate contact (panel (a) of
Top-down and cross-sectional scanning electron microscopy (SEM) imaging of a 100-nm-thick dropcasted MS-MoS2 film confirmed a percolating and dense nanosheet network (panels (a)-(c) of
The optical properties of the MoS2 films in the MSIM geometry were characterized using photoluminescence spectroscopy by focusing the 473 nm excitation laser through the glass/ITO substrate at the MoS2-alumina interface (measurement schematic is provided in the inset of panel (d) of
For electroluminescence measurements, light emission from the MSIM devices were detected through the ITO/glass substrate (see measurement schematic in panel (a) of
The electroluminescence of MS-MoS2 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-MoS2 film thickness (panel (a) of
The MS-MoS2 film EL spectra red shift slightly with increasing frequency and voltage amplitude (panel (a) of
Finally, a high-sensitivity CMOS camera was utilized for spatial mapping of the light emission from the vertical MSIM devices. The MS-MoS2 film shows uniform EL over the entire 100 μm×300 μm active device region between the Au source and ITO gate electrodes, with the integrated EL intensity increasing with frequency (panel (d) of
In summary, we have developed a megasonically exfoliated, monolayer-rich MoS2 nanosheet ink that can be used to fabricate electroluminescent MoS2 nanosheet films. Even at thicknesses approaching 100 nm, these MS-MoS2 films possess PL properties comparable to mechanically exfoliated or CVD-grown MoS2 monolayers, revealing that the MS-MoS2 films preserve the direct bandgap properties of their constituent monolayer nanosheets. These MS-MoS2 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-MoS2 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.
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.
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.
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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Polarity Tunable Trionic Electroluminescence in Monolayer WSe2. Nano Lett. 2019, 19, 7470-7475. https://doi.org/10.1021/acs.nanolett.9b03215.
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This application claims priority to and the benefit of U.S. Provisional Application No. 63/533,175, filed Aug. 17, 2023, which is incorporated herein in its entirety by reference. This application is also a continuation in part application of PCT Patent Application No. PCT/US2023/022664, filed May 18, 2023, which itself claims priority to and the benefit of U.S. Provisional Application No. 63/349,179, filed Jun. 6, 2022, which are incorporated herein in their entireties by reference.
This invention was made with government support under grant number 70NANB19H005 awarded by the National Institute of Standards and Technology and grant number DMR1720139 awarded by the National Science Foundation. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63533175 | Aug 2023 | US | |
| 63349179 | Jun 2022 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/US2023/022664 | May 2023 | WO |
| Child | 18802169 | US |
