WO2020093063A1

WO2020093063A1 - Method of creating scalable broadband and tunable light emitter at the nanoscale using layered black phosphorus

Info

Publication number: WO2020093063A1
Application number: PCT/US2019/059715
Authority: WO
Inventors: Mohammed R. AMER; Sarah A. ALODAN
Original assignee: University of California Berkeley; University of California San Diego UCSD
Current assignee: University of California Berkeley; University of California San Diego UCSD
Priority date: 2018-11-02
Filing date: 2019-11-04
Publication date: 2020-05-07
Anticipated expiration: 2021-05-02

Abstract

A method for making a room temperature stable and nanoscale broadband tunable light emitter. The method is scalable and suitable for industry applications. The fabricated device exhibits tunable light emission in a wavelength range between 590nm and 720nm (corresponding to a 130 nm bandwidth). This tunable broadband light emitter can be used for optoelectronic applications including fiber optics technologies, optical communication technologies, nanoscale light emitting diodes, and nanolasers.

Description

METHOD OF CREATING SCALABLE BROADBAND AND TUNABLE LIGHT EMITTER AT THE NANOSCALE USING LAYERED BLACK PHOSPHORUS

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. Section 119(e) of co- pending and commonly-assigned U.S. Provisional Patent Application Serial No 62/754,972, filed on November 2, 2018, and entitled“METHOD OF CREATING SCALABLE BROADBAND AND TUNABLE LIGHT EMITTER AT THE NANOSCALE USING LAYERED BLACK PHOSPHORUS” which application is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention.

A composition of matter useful as a tunable light emitter and a method of making the same.

2. Description of the Related Art.

(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers in brackets, e.g., [x] A list of these different publications ordered according to these reference numbers can be found below in the section entitled“References.” Each of these publications is incorporated by reference herein.)

New emerging two dimensional materials such as layered black phosphorus have attracted researchers due to their extraordinary mechanical, electrical, and optical properties at the nanoscale [1-5] Unlike graphene, phosphorene exhibits a layer dependent band gap ranging between 0.3eV to 1 7eV for bulk to monolayer, respectively [6] This feature is desirable for light emission since the band gap is only thickness dependent. Moreover, phosphorene has shown exceptional properties at the nanoscale including high carrier mobility [7,8], layer dependent photoemission [6,9], and anisotropic thermoelectric behavior [10], which makes it desirable for a wide variety of applications. In fact, electronic and optoelectronic devices with remarkable performance have been demonstrated using black phosphorus [11,12] However, phosphorene still suffers from major obstacles including long term stability in ambient [13-16] conditions, which can be temporarily overcome by layer passivation or layer functionalization [17,18] Accordingly, those skilled in the art continue with research and development efforts to increase tunability and stability of wideband light emitters. The present disclosure satisfies this need.

SUMMARY OF THE INVENTION

Fabricating a wideband tunable light emission source has been a desired feature in newly explored two dimensional (2D) materials and devices. However, band gap tunability by layer dependence can only cover specific emission

wavelengths, which corresponds to specific band gap values of the 2D material. A method to produce a tunable light source with high spectral resolution is yet to be realized.

The present disclosure describes a composition of matter capable of being tuned to emit electromagnetic radiation over a wide range of wavelengths. The composition of matter can be embodied in many ways including, but not limited to, the following examples.

1. A composition of matter, comprising:

a layered structure comprising phosphorus oxide (PxOi-x), wherein:

the phosphorus oxide emits electromagnetic radiation having an emission spectrum including a peak emission wavelength tunable between 590 nanometers and 720 nanometers, and

the peak emission wavelength is the wavelength at a peak intensity of the emission spectrum.

2. The composition of matter of example 1, wherein the layered structure comprises heat treated and oxidized black phosphorus, the layered structure including a first layer on a second layer, the first layer comprising the phosphorus oxide and the second layer comprising black phosphorus.

3. The composition of example 1 or example 2, wherein the emission wavelength is tunable with 10 nm or less wavelength resolution.

4. The composition of example 1 or example 2, wherein the emission wavelength is tunable with 5 nm or less wavelength resolution.

5. The composition of matter of any of the preceding examples, wherein the layered structure comprises black phosphorus oxide annealed at a temperature in a range of 200-370°C for one or more time periods each having a duration of less than 1 minute.

6. The composition of matter of any of the preceding examples, wherein the layered structure comprises black phosphorus oxide annealed at a temperature in a range of 200-370°C for between 20 and 40 seconds.

7. The composition of matter of any of the preceding examples, wherein the layered structure comprises a nanosheet (e.g., 2D nanosheet) having a thickness Tl between 50 nm and 200 nm, e.g., 50 nm < T < 200 nm, 70 nm < T < 120 nm, or 70 nm < T < 160 nm. In one or more examples, the first layer l404a and the second layer l404b each independently have thicknesses T2 and T3 in a range 5 nm < T < 200 nm, 70 nm < T < 120 nm, or 70 nm < T < 160 nm.

8. The composition of matter of example 7, wherein the nanosheet is on a substrate comprising silicon dioxide on silicon.

9. The composition of matter of any of the preceding examples, wherein the emission is photoemission and the phosphorus oxide is a phosphorescent material.

10. A device comprising the composition of matter of example 9, wherein the phosphorescent material is coupled to a light emitting device or an optical fiber optically pumping the phosphorescent material with pump electromagnetic radiation (e.g., comprising green electromagnetic radiation or radiation having a frequency corresponding to a photon energy larger than a bandgap of the phosphorus oxide). 11. The device of example 10, wherein the light emitting device comprises a light emitting diode or a laser.

12. The composition of matter of any of the preceding examples, wherein the emission spectrum has a full width at half maximum of less than 120 nm.

13. The composition of matter of any of the preceding examples, wherein the layered structure comprises the black phosphorus oxide having the formula PxOi-x, wherein 0 < x < 1.

14. The composition of matter of any of the preceding examples, wherein the black phosphorus oxide is passivated and stored in a dark evacuated environment.

15. The composition of matter of any of the preceding examples, wherein: the layered structure comprises heat treated and oxidized black phosphorus including the phosphorus oxide, and

the phosphorus oxide emits the electromagnetic radiation comprising photoemission comprising the peak emission wavelength corresponding to a red wavelength or in a range of 625 nm to 720 nm,

so that a surface of the layered structure appears red to the naked eye, when the layered structure/phosphorus oxide is optically pumped.

16. The device or the composition of matter of any of the preceding examples, wherein:

the layered structure comprises heat treated and oxidized black phosphorus including the phosphorus oxide, and

the phosphorus oxide emits the electromagnetic radiation including

photoemission having a peak wavelength corresponding an orange wavelength, or in a range of 590 - 625 nm,

so that a surface of the layered structure appears orange to the naked eye when the phosphorus oxide/layered structure is optically pumped.

17. The device or the composition of matter of any of the preceding examples, wherein: the layered structure comprises heat treated and oxidized black phosphorus including the phosphorus oxide, and

the phosphorus oxide emits the electromagnetic radiation including photoemission having a peak wavelength corresponding a yellow wavelength, so that a surface of the layered structure/phosphorus oxide appears yellow to the naked eye when the phosphorus oxide is optically pumped.

18. The device or the composition of matter of any of the preceding examples, wherein the phosphorus oxide has a bandgap tunable in a range of 1.7 electron volt to 2.1 electron volts.

19. The device or the composition of matter of any of the preceding examples, wherein the electromagnetic radiation is emitted over a surface of the layered structure having an area of at least 1 centimeter by 1 centimeter.

20. The device or the composition of matter of any of the preceding examples, wherein the layered structure has a thickness, a thickness of the phosphorus oxide comprising PxOi-x, wherein 0 < x < 1, an oxygen content, and/or a value of x, such that the peak emission wavelength (l) is in the range 590 nanometers < l < 720 nanometers and/or such that the peak emission or the layered structure is

characterized by any of the features in examples 1-19.

21. A method of making a composition of matter, comprising:

heating a layer comprising black phosphorus in an environment so as to form the black phosphorus oxide having an emission spectrum including a peak emission wavelength tunable between 590 nanometers and 720 nanometers, wherein the peak emission wavelength is the wavelength at a peak intensity of the emission spectrum.

22. The method of example 21, wherein the heating comprises:

heating the layer for j heating intervals, wherein:

j is an integer such that l £j £ n,

n is an integer, the (j+l)^th heating interval has a duration of 2-8 seconds longer than the j^th heating interval so as to reduce the peak emission wavelength by 10 nm or less or by 5 nm or less.

23. The method of example 22, wherein for j=l, the heating interval is 20 seconds or less so as to reduce the peak emission wavelength by 10 nm or less.

24. The method of example 23, wherein for j=l, the heating interval is 20 seconds or less so as to reduce the peak emission wavelength by 5 nm or less.

25. The method of examples 22 or 23, wherein n is between 5 and 10.

26. The method of any of the examples 21-25, further comprising:

selecting the peak emission wavelength;

heating the layer for one or more periods of time so as to form the black phosphorus oxide having the selected peak emission wavelength.

27. The method of any of the examples 21-26, further comprising performing a cool down to room temperature in less than 20 seconds after each heating interval.

28. The method of any of the examples 21-27, wherein the layer has a thickness in a range of 70 - 200 nanometers.

29. The method of any of the examples 21-28, further comprising measuring the photoemission wavelength after one or more of the heating intervals so as to determine if the phosphorus oxide having the selected peak emission wavelength has been obtained.

30. A device or composition of matter fabricated according to the method of any of the examples 21-29.

31. The device or composition of matter of any of the examples 1-20 fabricated according to the method of any of the examples 21-29.

In one example, the composition of matter comprises an optically pumped nanoscale wideband tunable light source fabricated using rapidly heat-treated black phosphorus nanosheets (heat treated under ambient conditions). The light emitter comprising the heat treated black phosphorus exhibits tunable light emission between 590 nm to 720 nm and the light emission can be tuned with a spectral resolution of 5 nm. Moreover, this light emission is anisotropic and can be stable for at least 11 days when passivated with a Poly(methyl methacrylate) capping layer and preserved in dark vacuum environment.

The present disclosure further describes the Raman properties of these nanosheets and demonstrates the modulation in the Raman intensity, which is aligned with thickness decrease due to layer oxidation. Thus, the tunable emission

corresponds to black phosphorus oxide formation on the surface of a black phosphorus flake and the oxide is correlated with the measured photoemission wavelength. The findings disclosed herein shed light on a new method to fabricate tunable light sources that is optically pumped at the nanoscale using 2D materials for future optoelectronic applications.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers represent corresponding parts throughout:

Figure 1. Flow chart showing the technique followed to create tunable light emitter using black phosphorus nanosheets (flakes) deposited on Si02 /Si substrate.

Figure 2A Schematic diagram showing the process of rapid heat treatment of exfoliated black phosphorus after exfoliation with no light emission, after cycle 1 treatment and emitting at wavelength (col), and after cycle 2 treatment and emitting at wavelength (co2).

Figures 2B-2L. Characterization of the light emitter fabricated according to Figure 1 and Figure 2 A, showing photoluminescence (PL) intensity (arbitrary units, a.u.) versus (vs.) wavelength (l) at different treatment cycles with treatment times of (Figure 2B) 68 seconds (s), (Figure 2C) 109 s, (Figure 2D) 152 s, (Figure 2E) 180 s, (Figure 2F) 210 s, and (Figure 2G) 240 s for a l20nm exfoliated black phosphorus nanosheet. PL emission wavelength (Figure 2H), PL intensity (Figure 21), and PL linewidth (Full Width at Half Maximum (FWHM), Figure 2J) after fitting each peak in Figures 2B-2G. Figure 2K shows photoluminescence (PL) spectra of another rapidly heat-treated black phosphorus flake at 90° angle difference. Figure 2L shows intensities at different angles showing anisotropic behavior.

Figure 3 A. Photoluminescence measurements of treated black phosphorus (sample as shown in Figure 2A) for different treatment times in seconds (s).

Figure 3B. Different samples showing tunable wideband light emission for different treatment times in seconds (s).

Figure 4A-4E. Spatial photoemission maps of different black phosphorus nanosheets showing the maximum light emission occurring at a wavelength of 700nm (Figure 4A), 670nm (Figure 4B), 640nm (Figure 4C), 6l3nm (Figure 4D), and 595nm (Figure 4E). Each black phosphorus nanosheet is deposited on a different substrate and is treated for different treatment time intervals.

Figures 5A-5C. Optical image (left) and photoluminescence measurement (right) of treated black phosphorus for 3 different samples and using a 5 l4nm laser line (wavelength 514 nm) as the optical pump pumping the PL.

Figures 6A-6E. Photoluminescence measurements of treated black phosphorus showing emission at a wavelength of 711 nanometers (nm), 706 nm, 704 nm, 700 nm, and 694 nm, respectively. Figure 6F shows a normalized photoluminescence spectra measured at different treatment times showing wideband tunable emission.

Figures 7A-7D. Raman spectra of cyclic treated rapidly heat treated black phosphorus. The measurements were taken at the same spot with the same nanosheet orientation. The amplitude of each of the peaks A^xg, B_2g, and A² _g vary depending on treatment times, each of the peak amplitudes A^xg, B₂ , and A² decreasing with increasing treatment time in Figure 7A and increasing with increasing treatment time in Figure 7B. In Figure 7C, each of the peak amplitudes A^xg, B₂ , and A² for the 510 second (s or sec) treatment time are between the respective peak amplitude for the 410 s treatment time (highest) and the peak for the 440 s treatment time (lowest).

Figures 7E-7G show curve fits for all Raman modes showing the intensity (Figure 7E), the Raman shift (Figure 7F), and linewidth (Figure 7G), respectively. The intensity modulation observed in Figure 7E is caused by Raman scattering interference with the incident laser beam, as discussed below.

Figure 8. Photoluminescence of degraded layered black phosphorus after 0 days (black line), 16 days (red line), and 31 days (blue line). Broad emission peaks are observed at 587 nm and 705 nm. The intensity of these emission peaks increases with increasing degradation time.

Figure 9A shows Raman characteristics and Figure 9B shows

photoluminescence measurements of PMMA passivated black phosphorus preserved in a dark and vacuum environment. The black line represents the characteristics after exfoliation and under PMMA passivation, while the red line represents measurements obtained after 11 days have lapsed.

Figure 10. Energy band diagram schematics showing the increase in black phosphorus oxide band gap E_gap with increasing treatment time due to increased oxygen concentration. The defect states (E_D) start to increase also with increasing treatment time. Arrows indicate all possible emissions detected. Also shown are the conduction band Ec and valence band Ev.

Figure 11 A. Raman intensity ratio (L^ /Zs) for different nanosheet thicknesses. The nanosheet thickness is obtained from Atomic Force Microscopy measurements. The red line is obtained from the linear fit.

Figure 11B. Atomic Force Microscope (AFM) images showing the decrease in thickness with increasing treatment time. The thickness for each line scan in the images is plotted in Figure 11C for green line scan and in Figure 11D for red line scan.

Figure 12A-12C. Photoluminescence emission from thicknesses d (Figure 12A) d= 72nm thick (I_Ai /I_st=6A), (Figure 12B) d= 25nm (I_Ai /I_si= \ .7), and (Figure

12C) d> 200nm nanosheets showing no tunability when the our technique is applied. The thick nanosheet shows a broad photoluminescence emission with increasing treatment time which is attributed to multiple emission peaks.

Figure 13. Flowchart illustrating example process steps. Figure 14. Cross sectional schematic of an exemplary device including a composition of matter.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.

Technical Description

First example: example fabrication and structure

Figure 1 shows a method flow chart followed to create a tunable emission between 590nm - 720nm and Figure 2A shows the cyclic thermal treatment process of a deposited black phosphorus nanosheet. In one or more examples, the substrate is rapidly heated to 370°C for few seconds at each cycle.

Referring now to Figure 1 and Figure 2A, the method comprises the following steps.

Block 100 represents exfoliating black phosphorus from bulk, e.g., using scotch tape, so as to form one or more black phosphorus nanosheets, as illustrated in Figure 2 A.

Block 102 represents identifying one or more black phosphorus flakes or nanosheets, e.g., using microscope.

Block 104 represents measuring the black phosphorus flakes and nanosheets.

In one or more examples, the measuring comprises measuring the photoluminescence of the phosphorus flakes or nanosheets using optical pumping and/or obtaining Raman spectra of the black phosphorus flakes or nanosheets and/or using atomic force microscopy (AFM) to identify the thickness of the black phosphorus flakes or nanosheets between l20nm± 30nm and I_Ai /I_Si between 8-13.

Block 106 represents heating or annealing the substrate comprising the black phosphorus flakes or nanosheets, e.g., using a rapid heat treatment, as illustrated by Cycle 1. In one or more examples, the step includes placing the substrate on a hot plate after reaching a temperature between 350°C to 370°C and for a time period between 5 to 10 seconds, so as to form a layered structure comprising phosphorus oxide on the substrate, as illustrated in Figure 2B.

Block 108 represents measuring the layered structure. In one or more examples, the measuring includes measuring confocal photoluminescence spectra (by optically pumping the layered structure) and monitoring the emergence of the peak at a wavelength between 580nm- 750nm.

If the confocal photoluminescence shows a downshift in the wavelength of the photoluminescence, then block 106 is repeated (as illustrated in cycle 2) until there is no downshift.

Block 110 represents the end result (stop) if there is no further downshift in the wavelength of the photoluminescence.

Figure 2A illustrates an example wherein the end result comprises a composition of matter 200 comprising a layer 202 including layered black phosphorus oxide PxOy (wherein P is phosphorus, O is oxygen, x is P fraction, y = l-x is O fraction, and 0 < x < 1) on the surface 204 of black phosphorus (BP) nanosheets 206, wherein the BP nanosheets 206 are on a substrate 208 comprising a silicon dioxide layer (S1O2) on a silicon (Si). The layer 202 comprising heat treated black phosphorus oxide emits electromagnetic radiation 210 comprising a wideband tunable

photoemission that exceeds a wavelength range of 100 nm range. This light emission can be precisely controlled with a high spectral resolution.

The next section further describes the origin of this emission and demonstrates that the formed black phosphorus oxide and suboxides exhibit a remarkable, surprising, and unexpected optoelectronic behavior. Second example: example results for the composition of matter fabricated according to the first example illustrated in Figure 1 and Figures 2A-2C

1. Example Wideband tunable photoemission

Phosphorus oxide on a nanosheet was fabricated according to the first example described above (see methods section in the third example below for a more detailed discussion of the fabrication processes).

Figures 2B-2G show the nanosheet 206 has a tunable broadband

photoemission starting at 720nm. This photoemission peak downshifts and widens at each different thermal treatment cycle, reaching 590 nm. Moreover, the peak starts with low intensity, and then starts to increase. In Figure 3 A and 3B, the measured photoemission peak after several treatment times is illustrated in one plot along with additional sample exhibiting the same light emission pattern, respectively.

In Figures 2H-2J, the wavelength, emission peak intensity, and the emission linewidth are plotted after different cyclic thermal treatment times, respectively. Figure 2H illustrates the emission peak starts to change in profile at 200 seconds treatment time, exhibiting 2 additional peaks while the main peak downshifts to 600nm and the other rising peak does not show any significant shift between 651 nm and 661 nm.

The emission intensity after different treatment times is shown in Figure 21. The main peak intensity increases 11.5 fold while the rising peak shows almost unchanged intensity. Figure 2J shows the linewidth of the main and rising

photoemission peaks. Both peaks show broad linewidth, which is attributed to emission of black phosphorus suboxides and other induced defects, as further explained below. Figure 2K shows 2 different spectra measured at different angles for a treated black phosphorus nanosheet. Figure 2L illustrates anisotropic photoemission based on the measured photoluminescence; thus black phosphorus oxide exhibits anisotropy, similar to layered black phosphorus. In Figures 4A through 4E, spatial photoluminescence maps are obtained on different thermally treated black phosphorus nanosheets, showing emission at 700 nm, 670 nm, 640 nm, 613 nm, and 595 nm, respectively. Uniform emission along the surface 212 (surface of PxO_y, see also Figure 2A) of the nanosheets is observed. There are few spots on the nanosheets that have lower intensity than other areas, as illustrated in Figure 4B and Figure 4E. These low intensity spots are artificial

(manmade) and are caused by longer laser exposure during single spot measurements. Additionally, the photoemission is confirmed using a different laser line (5l4nm). In Figure 5, the measured photoemission shows a broad peak, analogous to light emission observed in Figures 2F and 2G.

The spectral resolution of a rapidly heated black phosphorus nanosheet can be tuned within 5nm peak difference. In Figures 6A to 6E, tunable light emission is achieved, with emission peaks occurring at 711 nm, 706 nm, 704 nm, 700 nm, and 694 nm, respectively. The full photoemission spectra with increasing treatment time is plotted in Figure 6F. These results demonstrate that rapidly heating the black phosphorus nanosheets produces light emission with spectral resolution as high as 5nm.

Raman spectra obtained from treated black phosphorus shows a modulated intensity with increasing treatment time. In Figures 7A-7D, the intensity vs. treatment time shows a non-monotonic profile. This behavior has been attributed to Raman light interference with the incident laser beam, which is caused by a change in the nanosheet thickness [14] This decrease in black phosphorus nanosheet thickness results from the thermal treatment oxidizing the top layers at each treatment cycle.

Surprisingly and unexpectedly, tunable photoemission has been observed only for nanosheets with thickness between 70 nm and 120 nm. The reason behind this thickness range is still not clear. However, one possible reason is the mechanism of oxygen atoms bonding to phosphorene atoms to form the desired PxOy, where the surface to thickness ratio of the black phosphorus nanosheet is large enough for the black phosphorus oxide to form on the surface. This mechanism enables the variation of oxygen concentration and hence, the level of oxidation.

It is important to distinguish between photoemission due to degraded black phosphorus and photoemission due to rapid-heat treated black phosphorus nanosheets. Photoluminescence measurements of degraded black phosphorus nanosheet are shown in Figure 8. After 16 days, the nanosheet starts to exhibit two emission peaks centered at 587 nm and 705 nm. The intensity of these emission peaks dramatically increases with increasing the level of oxidation. Both peaks become very broad and

indistinguishable. Unlike cyclic thermally treated black phosphorus nanosheets, light emission from degraded black phosphorus does not exhibit any tunability.

Nevertheless, cyclic thermal treatment enables control of the level of oxidation, which gives control over the oxygen concentration in black phosphorus oxide, as

demonstrated below.

2. Example Air stability

Passivating the nanosheet with Polymethyl methacrylate (PMMA) and storing the nanosheet in a dark vacuum environment preserves the nanosheet for a prolonged period of time [14] Moreover, black phosphorus oxide can act as a protective layer for the underlying black phosphorus layers [23, 24] Accordingly, the thermally treated nanosheet 206 was passivated with PMMA and was kept in a dark vacuum environment. Raman characteristics and photoluminescence measurements of the thermally treated black phosphorus nanosheet were measured immediately after PMMA passivation and prior to preservation in dark and vacuum environment (Figure 9A and Figure 9B). After 11 days, the measured Raman modes A^lg ,B² g, and A²g of the capped black phosphorus do not show any changes, suggesting preserved nanosheet properties. Figures 9A and 9B show the obtained Raman and

Photoluminescence measurements at 0 days (immediately after PMMA passivation) and after preservation in dark and vacuum environment for 11 days, respectively. 3. Further results

Figure 11 A shows Raman intensity ratio (IAV/S) for different nanosheet thicknesses. The nanosheet thickness is obtained from Atomic Force Microscopy measurements. The red line is obtained from the linear fit. Figure 11B shows Atomic Force Microscope (AFM) images showing the decrease in thickness with increasing treatment time. The thickness for each line scan in the images is plotted in Figure 11C for green line scan and in Figure 11D for red line scan.

Figure 12A-12C show photoluminescence emission from thicknesses d (Figure 12A) d= 72nm thick (I_Ai/I_st=6A), (Figure 12B) d= 25nm (I_Ai /I_si= \ .7), and (Figure 12C) d> 200nm nanosheets showing no tunability when the our technique is applied. The thick nanosheet shows a broad photoluminescence emission with increasing treatment time which is attributed to multiple emission peaks.

4. Discussion

Phosphorus oxide has two different forms, stoichiometric and non- stoichiometric phosphorus oxide, depending on how oxygen atoms bond to phosphorus atoms [25] For each of these forms, the band gap increases with increasing oxygen concentration. However, the bandgap for these two forms is different. Various theoretical calculations suggest that the bandgap can increase from l.62eV to 8 eV with increasing oxygen concentration [19] However, to date, no experimental work has demonstrated phosphorene oxide with wideband tunability as described herein.

In Figures 2H,F, the measured photoemission exhibits 2 peaks, the main tunable peak (which is attributed to the phosphorus oxide band gap emission) and a secondary peak (which is attributed to suboxide defect induced emission). The tunable band gap arises from the increasing oxygen concentration reacting with the black phosphorus nanosheet when thermally treated. The secondary peak arises from the various induced defects created on the surface of the nanosheet when thermally treated.

Figure 10 is a schematic energy band diagram as a function of increasing treatment times. In this illustration, the bandgap of black phosphorus oxide increases with increasing treatment time, mainly due to increasing oxygen reaction (and concentration) with the exposed phosphorus atoms. The experimental results presented herein demonstrate that high temperature annealing in ambient near 370°C with short annealing time is attributed to the formation of black phosphorus oxide with precise control over the photoemission tunability (within 5nm emission change). The results presented herein show that temperatures higher than 370°C can induce much faster reaction with oxygen and hence, higher oxygen concentration, leading to emission at -600 nm and the absence of tunable emission. Moreover, Figure 10 shows the number of defect states increases with increasing treatment time, leading to a broad photoemission as discussed above. We believe these defects can be caused by vacancies in the material or substitution of phosphorus atoms with external atoms.

Third example: methods used to obtain data presented in the second example a. Fabrication: mechanically exfoliated black phosphorus nanosheets were deposited onto Si/SiCk substrates. An optical microscope was used to identify high quality black phosphorus nanosheets. The thicknesses were estimated using the method adopted in references [18,26] where the ratio of the intensity of A¹ g Raman mode (IA¾ to the intensity of the silicon peak Isi. This ratio I A 'y/Is was obtained after Atomic Force Microscopy (AFM) and Raman measurements. Data were fitted to a linear relation as plotted in Figure 10. Chosen nanosheets were treated by a rapid heating and cooling cycles in ambient environment. The temperature was set at 370°C with different treatment durations, as discussed above. Each heating interval was followed by a cool down period for 10- 20 seconds. b. Photoluminescence and Raman measurements: each sample was measured using a confocal Raman spectroscopy setup (Renishaw) with Charge Coupled Device (CCD) detector and 1800mm grating. A 532 nm laser line was used as an excitation source. The laser power used in the measurements was minimal in order to prevent any damage to the nanosheets. A 100X objective lens was used for all samples to ensure nanosheet emission. Photoemission maps were recorded with a 0.3 pm step size.

Example Advantages and Improvements

Oxidized black phosphorus is formed when oxygen atoms interact with layered black phosphorus. Given the correct environment, different combinations of black phosphorus oxide can be produced with different black phosphorus and oxygen concentrations (PxOy). Theoretical calculations suggest that black phosphorus oxide exhibits a wide band gap range, mainly between 1 eV to 8 eV, which is highly oxygen concentration dependent [19] However, very little experimental work has been done on this new emerging oxide. Lu et al. first showed laser induced photoemission from the oxide on black phosphorus nanosheets and measured an emission close to 600nm [20] Gan et al. showed very small tunable photoluminescence from electrochemically oxidized black phosphorus crystals [21] (their data shows a small tunability range that does not exceed 50nm, and their emission peak exhibits a very broad linewidth). Zhao et al. annealed deposited black phosphorus in vacuum and purged argon gas in order to controllably oxidize black phosphorus. The measured photoluminescence is found to occur at 600nm [22]

Surprisingly and unexpectedly, the present disclosure has found that black phosphorus nanosheets treated using cyclic rapid heat treatment at high temperature and using different treatment times exhibits wideband tunable light emission having (1) an emission peak precisely tunable between 590 nm to 720nm and (2) a broad linewidth. The intensity of this emission increases dramatically with increasing treatment time. The spectral resolution obtained is 5nm (with confidence). By preserving the treated black phosphorus in a vacuum and dark environment after PMMA capping, the treated black phosphorus can be stable for at least 11 days. The wideband tunable light emission is attributed to the formation of black phosphorus with increasing oxygen concentration. Thus, the present disclosure demonstrates a reliable technique to produce a nanoscale light source emitting in the wavelength range of 590nm to 720nm and with high spectral resolution (5nm), unlocking doors for future optoelectronic applications. Commercial applications include fiber optics, optical communication, optoelectronic systems and devices, nano light emitting diodes (LEDs), and nanolasers. The tunability feature of this device over a large spectral range (from 590 nm to 720 nm) makes the device desirable for nanoscale applications, for example.

Process Steps

Figure 13 is a flowchart illustrating a method of making a composition of matter.

Block 1302 represents optionally selecting a peak emission wavelength.

Block 1302 represents heating a black phosphorus oxide so as to form the black phosphorus oxide having an emission spectrum including a peak emission wavelength tunable between 590 nanometers and 720 nanometers, wherein the peak emission wavelength is the wavelength at a peak intensity of the emission spectrum.

In one or more examples, the heating comprises heating the black phosphorus oxide for j heating intervals, wherein j is an integer such that 1 < j < n, n is an integer, the (j+l)^th heating interval has a duration of 2-8 seconds longer than the j*¹¹ heating interval so as to reduce the peak emission wavelength by 10 nm or less or by 5 nm or less.

In one or more examples, for j=l, the heating interval is 20 seconds or less so as to reduce the peak emission wavelength by 10 nm or less or 5 nm or less.

In one or more examples, n is between 5 and 10. Block 1304 represents performing a cool down to room temperature in less than 20 seconds after each heating interval.

Block 1306 represents the end result, a composition of matter.

The composition of matter or method can be embodied in many ways including, but not limited to, the following.

1. A composition of matter 1400, as illustrated in Figure 14, comprising: a layered structure 1402 comprising phosphorus oxide (PxOi-x), wherein: the phosphorus oxide emits electromagnetic radiation 1406 having an emission spectrum including a peak emission wavelength tunable between 590 nanometers and 720 nanometers, and

2. The composition of matter of example 1, wherein the layered structure comprises heat treated and oxidized black phosphorus, the layered structure including a first layer l404a on a second layer l404b, the first layer comprising the phosphorus oxide and the second layer comprising black phosphorus.

7. The composition of matter of any of the preceding examples, wherein the layered structure comprises a nanosheet having a thickness Tl between 50 nm and 200 nm, e.g., 50 nm < Tl < 200 nm, 70 nm < Tl < 120 nm, or 70 nm < Tl < 160 nm. In one or more examples, the first layer l404a and the second layer l404b each independently have thicknesses T2 and T3 in a range 5 nm < T, T2, T3 < 200 nm, 70 nm < T, T2, T3 < 120 nm, or 70 nm < T, T2, T3 < 160 nm. Figure 11A highlights an example range (shaded region 1100) of thicknesses Tl and/or IA'g/f for which emission tunability is expected to happen. Figure 13 illustrates there is no tunability when the nanosheet is not within a required thickness range.

10. A device comprising the composition of matter of example 9, wherein the phosphorescent material is coupled to a light emitting device 1408 or an optical fiber optically pumping the phosphorescent material with pump electromagnetic radiation 1410 (e.g., comprising green electromagnetic radiation or radiation having a frequency corresponding to a photon energy larger than a bandgap of the phosphorus oxide).

11. The device of example 10, wherein the light emitting device comprises a light emitting diode or a laser.

15. The composition of matter of any of the preceding examples, wherein: the layered structure comprises heat treated and oxidized black phosphorus including the phosphorus oxide, and the phosphorus oxide emits the electromagnetic radiation comprising photoemission comprising the peak emission wavelength corresponding to a red wavelength or in a range of 625 nm to 720 nm,

the phosphorus oxide emits the electromagnetic radiation including photoemission having a peak wavelength corresponding an orange wavelength, or in a range of 590 - 625 nm,

17. The device or the composition of matter of any of the preceding examples, wherein:

19. The device or the composition of matter of any of the preceding examples, wherein the electromagnetic radiation is emitted over a surface of the layered structure having an area of at least 1 centimeter by 1 centimeter. 20. The device or the composition of matter of any of the preceding examples, wherein the layered structure has a thickness T, Tl, T2, a thickness of the phosphorus oxide comprising PxOi-x, wherein 0 < x < 1, an oxygen content, and/or a value of x, such that the peak emission wavelength (l) is in the range 590 nanometers

< l < 720 nanometers and/or such that the peak emission or the layered structure is characterized by any of the features in examples 1-19.

21. The device or the composition of matter of any of the preceding examples, wherein the layered structure has a thickness T, Tl, T2, a thickness of the phosphorus oxide comprising PxOi-x, wherein 0 < x < 1, an oxygen content, and/or a value of x, such that the peak emission wavelength (l) is in any 5nm range or 10 nm range in the range of 590 nanometers < l < 720 nm (i.e., the peak emission is precisely tuned (e.g., with 5 nm or 10 nm resolution) in the range of 590 nanometers < l < 720 nm).

21. The device or the composition of matter of any of the preceding examples, wherein the layered structure has a thickness T, Tl, T2, a thickness of the phosphorus oxide comprising PxOi-x, wherein 0 < x < 1, an oxygen content, and/or a value of x, such that the peak emission wavelength (l) is in the range of 590 nm < l < 600 nm, 600 nm < l < 610 nm, 610 nm < l < 620 nm, 620 nm < l < 630 nm, 630 nm < l < 640 nm, 640 nm < l < 650 nm, 650 nm < l < 660 nm, 660 nm < l < 670 nm, 670 nm < l < 680 nm, 680 nm < l < 690 nm, 690 nm < l < 700 nm, 700 nm < l < 710 nm, or 710 nm < l < 720 nm.

22. The device or the composition of matter of any of the preceding examples, wherein the layered structure has a thickness T, Tl, T2, a thickness of the phosphorus oxide comprising PxOi-x, wherein 0 < x < 1, an oxygen content, and/or a value of x, such that the peak emission wavelength (l) is in the range of 590 nm < l < 595 nm, 595 nm < l < 600 nm, 600 nm < l < 605 nm, 605 nm < l < 610 nm, 610 nm < l < 615 nm, 615 nm < l < 620 nm, 620 nm < l < 625 nm, 625 nm < l < 630 nm, 630 nm < l < 635 nm, 635 nm < l < 640 nm, 640 nm < l < 645 nm, 645 nm < l < 650 nm, 655 nm

< l < 660 nm, 660 nm < l < 665 nm, 665 nm < l < 670 nm, 670 nm < l < 675 nm,

675 nm < l < 680 nm, 680 nm < l < 685 nm, 685 nm < l < 690 nm, 690 nm < l < 695 nm, 695 nm < l < 700 nm, 700 nm < l < 705 nm, 705 nm < l < 710 nm, 710 nm < l < 715 nm, or 715 nm < l < 720 nm.

23. A method of making a composition of matter, comprising:

24. The method of example 23, wherein the heating comprises:

heating the layer for j heating intervals, wherein:

j is an integer such that l £j £ n,

n is an integer,

the (j+l)^th heating interval has a duration of 2-8 seconds longer than the j*¹¹ heating interval so as to reduce the peak emission wavelength by 10 nm or less or by 5 nm or less.

25. The method of example 24, wherein for j=l, the heating interval is 20 seconds or less so as to reduce the peak emission wavelength by 10 nm or less.

26. The method of example 25, wherein for j=l, the heating interval is 20 seconds or less so as to reduce the peak emission wavelength by 5 nm or less.

27. The method of examples 25 or 26, wherein n is between 5 and 10.

28. The method of any of the examples 22-26, further comprising:

selecting the peak emission wavelength;

29. The method of any of the examples 23-28, further comprising performing a cool down to room temperature in less than 20 seconds after each heating interval.

30. The method of any of the examples 23-29, wherein the layer has a thickness in a range of 70 - 200 nanometers. 31. The method of any of the examples 23-30, further comprising measuring (e.g., using photoluminescence) the photoemission wavelength after one or more of the heating intervals so as to determine if the phosphorus oxide having the selected peak emission wavelength (e.g., with 5 nm or 10 nm resolution) has been obtained.

32. A device or composition of matter fabricated according to the method of any of the examples 23-31.

33. The device or composition of matter of any of the examples 1-22 fabricated according to the method of any of the examples 23-31.

34. A composition of matter, comprising a layered structure comprising phosphorus oxide; and means for treating the layered structure so that the

phosphorous oxide emits electromagnetic radiation having an emission spectrum including a peak emission wavelength tunable between 590 nanometers and 720 nanometers, and the peak emission wavelength is the wavelength at a peak intensity of the emission spectrum.

References

The following references are incorporated by reference herein.

1 Liu, Y. et al. Approaching the Schottky-Mott limit in van der Waals metal- semiconductor junctions. Nature, 1 (2018).

2 Deng, Y. et al. Black Phosphorus-Monolayer MoS2 van der Waals Heterojunction p- n Diode. Acs Nano 8, 8292-8299 (2014).

3 Alrasheed, A. et al. Surface Properties of Laser-treated Molybdenum Disulfide Nanosheets for Optoelectronic Applications. ACS applied materials & interfaces (2018).

4 Mas-Balleste, R., Gomez-Navarro, C., Gomez-Herrero, J. & Zamora,

F. 2D materials: to graphene and beyond. Nanoscale 3, 20-30 (2011).

5 Kim, Y. D. et al. Bright visible light emission from graphene. Nature nanotechnology (2015).

6 Tran, V., Soklaski, R., Liang, Y. & Yang, L. Layer-controlled band gap and anisotropic excitons in few-layer black phosphorus. Physical Review B 89, 235319 (2014).

7 Li, L. et al. Black phosphorus field-effect transistors. Nature nanotechnology 9, 372- 377 (2014).

8 Qiao, L, Kong, X., Hu, Z.-X., Yang, F. & Ji, W. High-mobility transport anisotropy and linear dichroism in few-layer black phosphorus. Nature communications 5, 4475 (2014).

9 Wang, X. et al. Highly anisotropic and robust excitons in monolayer black phosphorus. Nature nanotechnology 10, 517-521 (2015).

10 Fei, R. et al. Enhanced thermoelectric efficiency via orthogonal electrical and thermal conductances in phosphorene. Nano Letters 14, 6393-6399 (2014).

11 Huang, M. et al. Broadband Black - Phosphorus Photodetectors with High Responsivity. Advanced Materials 28, 3481-3485 (2016).

12 Wang, H. et al. Black phosphorus radio-frequency transistors. Nano letters 14, 6424- 6429 (2014).

13 Island, J. O., Steele, G. A., van der Zant, H. S. & Castellanos-Gomez, A. Environmental instability of few-layer black phosphorus. 2D Materials 2, 011002 (2015).

14 Alsaffar, F. et al. Raman Sensitive Degradation and Etching Dynamics of Exfoliated Black Phosphorus. Scientific Reports 7, 44540 (2017).

15 Favron, A. et al. Photooxidation and quantum confinement effects in exfoliated black phosphorus. Nature materials 14, 826-832 (2015).

16 Wood, J. D. et al. Effective passivation of exfoliated black phosphorus transistors against ambient degradation. Nano letters 14, 6964-6970 (2014).

17 Ryder, C. R. et al. Covalent functionalization and passivation of exfoliated black phosphorus via aryl diazonium chemistry. Nature chemistry (2016). 18 Artel, V. et al. Protective molecular passivation of black phosphorus npj 2D Materials and Applications 1, 6 (2017).

19 Ziletti, A. et al. Phosphorene oxides: Bandgap engineering of phosphorene by oxidation. Physical Review B 91, 085407 (2015).

20 Lu, J. et al. Bandgap engineering of phosphorene by laser oxidation toward functional 2D materials. ACS nano 9, 10411-10421 (2015).

21 Gan, Z. et al. Tunable photoluminescence from sheet-like black phosphorus crystal by electrochemical oxidation. Applied Physics Letters 107,

021901 (2015).

22 Zhao, C. et al. Anisotropic visible photoluminescence from thermally annealed few- layer black phosphorus. Nanotechnology 29, 245202 (2018).

23 Pei, J. et al. Producing air-stable monolayers of phosphorene and their defect engineering. Nature communications 7 (2016).

24 Edmonds, M. et al. Creating a stable oxide at the surface of black phosphorus. ACS applied materials & interfaces 7, 14557-14562 (2015).

25 Wang, G., Pandey, R. & Kama, S. P. Phosphorene oxide: stability and electronic properties of a novel two-dimensional material. Nanoscale 7, 524-531 (2015).

26 Castellanos-Gomez, A. et al. Isolation and characterization of few- layer black phosphorus. 2D Materials 1, 025001 (2014).

Conclusion

This concludes the description of the preferred embodiment of the present invention. The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many

modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.

Claims

WHAT IS CLAIMED IS:

1. A composition of matter, comprising:

a layered structure comprising phosphorus oxide, wherein:

2. The composition of matter of claim 1, wherein the layered structure comprises heat treated and oxidized black phosphorus, the layered structure including a first layer on a second layer, the first layer comprising the phosphorus oxide and the second layer comprising black phosphorus.

3. The composition of claim 1 or claim 2, wherein the emission wavelength is tunable with 10 nm or less wavelength resolution.

4. The composition of claim 1 or claim 2, wherein the emission wavelength is tunable with 5 nm or less wavelength resolution.

5. The composition of matter of any of the preceding claims, wherein the layered structure comprises black phosphorus oxide annealed at a temperature in a range of 200- 370°C for one or more time periods each having a duration of less than 1 minute.

6. The composition of matter of any of the preceding claims, wherein the layered structure comprises black phosphorus oxide annealed at a temperature in a range of 200- 370°C for between 20 and 40 seconds.

7. The composition of matter of any of the preceding claims, wherein the layered structure comprises a nanosheet having a thickness between 50 nm and 200 nm.

8. The composition of matter of claim 7, wherein the nanosheet is on a substrate comprising silicon dioxide on silicon.

9. The composition of matter of claim 1, wherein the layered structure has an oxygen content and/or a thickness such that a peak emission wavelength has a desired value, within 10 nm precision, between 590 nanometers and 720 nanometers.

10. The composition of matter of any of the preceding claims, wherein the emission is photoemission and the phosphorus oxide is a phosphorescent material.

11. A device comprising the composition of matter of claim 10, wherein the phosphorescent material is coupled to a light emitting device or an optical fiber optically pumping the phosphorescent material.

12. The device of claim 11, wherein the light emitting device comprises a light emitting diode or a laser.

13. The composition of matter of any of the preceding claims, wherein the emission spectrum has a full width at half maximum of less than 120 nm.

14. The composition of matter of any of the preceding claims, wherein the layered structure comprises black phosphorus oxide having the formula PxOi-x, wherein 0 < x < 1.

15. The composition of matter of any of the preceding claims, wherein the black phosphorus oxide is passivated and stored in a dark evacuated environment.

16. The composition of matter of any of the preceding claims, wherein:

the layered structure comprises heat treated and oxidized black phosphorus including the phosphorus oxide, and the phosphorus oxide emits the electromagnetic radiation comprising photoemission comprising the peak emission wavelength corresponding to a red wavelength or in a range of 625 nm to 720 nm,

so that a surface of the layered structure appears red to the naked eye, when the layered structure is optically pumped.

17. The device or the composition of matter of any of the preceding claims, wherein:

so that a surface of the layered structure appears orange to the naked eye when the layered structure is optically pumped.

18. The device or the composition of matter of any of the preceding claims, wherein:

the phosphorus oxide emits the electromagnetic radiation including photoemission having a peak wavelength corresponding a yellow wavelength, so that the layered structure appears yellow to the naked eye when the layered structure is optically pumped.

19. The device or the composition of matter of any of the preceding claims, wherein the phosphorus oxide has a bandgap tunable in a range of 1.7 electron volt to 2.1 electron volts.

20. The device or the composition of matter of any of the preceding claims, wherein the electromagnetic radiation is emitted over a surface of the layered structure having an area of at least 1 centimeter by 1 centimeter.

21. The device or the composition of matter of any of the preceding claims, wherein the layered structure has a thickness, a thickness of the phosphorus oxide comprising PxOi-x, wherein 0 < x < 1, an oxygen content, and/or a value of x, such that the peak emission wavelength (l) is in the range 590 nanometers < l < 720 nanometers.

22. A method of making a composition of matter, comprising:

heating a layer comprising black phosphorus in an environment so as to form the black phosphorus oxide having an emission spectrum including a peak emission wavelength tunable between 590 nanometers and 720 nanometers, wherein the peak emission wavelength is the wavelength at a peak intensity of the emission spectrum

23. The method of claim 22, wherein the heating comprises:

heating the layer for j heating intervals, wherein:

j is an integer such that l £j £ n,

n is an integer,

24. The method of claim 23, wherein for j=l, the heating interval is 20 seconds or less so as to reduce the peak emission wavelength by 10 nm or less.

25. The method of claim 24, wherein for j=l, the heating interval is 20 seconds or less so as to reduce the peak emission wavelength by 5 nm or less.

26. The method of claims 24 or 25, wherein n is between 5 and 10.

27. The method of any of the claims 22-26, further comprising:

selecting the peak emission wavelength;

28. The method of any of the claims 22-27, further comprising performing a cool down to room temperature in less than 20 seconds after each heating interval.

29. The method of any of the claims 22-28, wherein the layer has a thickness in a range of 70 - 200 nanometers.

30. The method of any of the claims 22-29, further comprising measuring the photoemission wavelength after one or more of the heating intervals so as to determine if the phosphorus oxide having the selected peak emission wavelength has been obtained.

31. A device or composition of matter fabricated according to the method of any of the claims 22-30.

32. The device or composition of matter of any of the claims 1-20 fabricated according to the method of any of the claims 22-30.