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Actively variable-spectrum optoelectronics with black phosphorus

Abstract

Room-temperature optoelectronic devices that operate at short-wavelength and mid-wavelength infrared ranges (one to eight micrometres) can be used for numerous applications1,2,3,4,5. To achieve the range of operating wavelengths needed for a given application, a combination of materials with different bandgaps (for example, superlattices or heterostructures)6,7 or variations in the composition of semiconductor alloys during growth8,9 are used. However, these materials are complex to fabricate, and the operating range is fixed after fabrication. Although wide-range, active and reversible tunability of the operating wavelengths in optoelectronic devices after fabrication is a highly desirable feature, no such platform has been yet developed. Here we demonstrate high-performance room-temperature infrared optoelectronics with actively variable spectra by presenting black phosphorus as an ideal candidate. Enabled by the highly strain-sensitive nature of its bandgap, which varies from 0.22 to 0.53 electronvolts, we show a continuous and reversible tuning of the operating wavelengths in light-emitting diodes and photodetectors composed of black phosphorus. Furthermore, we leverage this platform to demonstrate multiplexed nondispersive infrared gas sensing, whereby multiple gases (for example, carbon dioxide, methane and water vapour) are detected using a single light source. With its active spectral tunability while also retaining high performance, our work bridges a technological gap, presenting a potential way of meeting different requirements for emission and detection spectra in optoelectronic applications.

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Fig. 1: Strain-tunable bandgap in black phosphorus.
Fig. 2: Strain-induced modification of the bandgap in bP.
Fig. 3: Strain-tunable MWIR LEDs based on a bP–MoS2 heterostructure.
Fig. 4: NDIR gas sensing using strain-tunable LEDs.

Data availability

All data generated or analysed during this study are included in this published article. Source data are provided with this paper.

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Acknowledgements

We thank H. M. Fahad for help with setting up the gas-sensing instrument. Luminescence studies were funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under contract DE-AC02-05-CH11231 (EMAT program KC1201). Photodetector fabrication and testing were funded by the Defense Advanced Research Projects Agency under contract HR0011-16-1-0004. K.B.C. acknowledges funding from the Australian Research Council (grant numbers DP180104141 and FT140100577). H.K. acknowledges support from Samsung Scholarship.

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Authors

Contributions

H.K. and A.J. conceived the idea for the project and designed the experiments. H.K., S.Z.U. and D.-H.L. performed optical measurements. H.K., M.Y. and T.K. fabricated devices. S.Z.U., N.S.A., S.B. and K.B.C. performed optical simulations. N.G. helped with gas-sensing experiments. Y.R. and C.P.G. helped with Raman measurements. H.K. wrote the manuscript. All authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to Ali Javey.

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The authors declare no competing interests.

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Peer review information Nature thanks Kah Wee Ang, Andres Castellanos-Gomez and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Strain applied in bP.

a, Photographic image of the two-point bending apparatus used here. An electrical linear actuator that can push/pull one point of the two-point bending apparatus applies a continuous and precise amount of uniaxial tensile strain to bP. b, Schematic of the two-point bending apparatus. Strain is calculated as ε = tsinθ/a, where ε is the amount of strain; t is the thickness of the substrate; a is the length of the substrate; and θ represents the angle of bending, which is equal to a/(2R) where R is the radius of the curvature60. Note that the circular arc approximation is not satisfied when θ is large at strains of 20% or more61. c, Raman spectra of the bP measured in Fig. 1 and Fig. 2. d, Schematic showing the atomic vibrations that correspond to Raman modes of \({{\rm{A}}}_{{\rm{g}}}^{1}\) (out-of-plane), \({{\rm{B}}}_{{\rm{2g}}}\) (in-plane; zigzag), and \({{\rm{A}}}_{{\rm{g}}}^{2}\) (in-plane; armchair). e, Optical image of the strained bP flake on the PETG substrate.

Extended Data Fig. 2 Detailed optical characterization of strained bP.

a, Photoluminescence (PL) peak wavelength as a function of transfer temperature. We characterized the photoluminescence peak wavelength as a function of biaxial compressive strain before and after the application of tensile strain (1.21%, zigzag). Each measurement was performed for five bP samples with thickness 20–22 nm. Samples were compressively strained by different amounts via different transfer temperatures (Ttr = 20 °C, 50 °C, 70 °C, 90 °C and 95 °C). The photoluminescence peak shift resulting from tensile strain increased with increasing biaxial compressive strain (as determined by transfer temperature). This is understood to be the result of the following. At high biaxial strain, the larger friction-induced resistance prevents the sliding of the 2D materials55. At lower transfer temperatures—that is, with reduced biaxial compressive strain—the bP is thus more likely to slip during bending of the substrate, such that the intended uniaxial tensile strain is not efficiently delivered to the bP. This could be because, at lower values of biaxial strain (that is, lower transfer temperatures), the friction-induced resistance that would prevent sliding of the bP is reduced. b, Laser power dependence of strain effect. We characterized the laser-induced heating effect on the bandgap shift by measuring photoluminescence peak wavelengths as a function of laser power. As the excitation spot size was similar or slightly smaller than the bP size, this helped to prevent the thermal expansion of the surrounding PETG by laser excitation (this thermal expansion could have elicited unexpected strain or slippage of bP from the PETG62). Regardless of the strain in bP, an excitation power higher than 1,500 W cm−2 always resulted in blueshift of the photoluminescence, attributed to thermal heating by the laser63. Although the photoluminescence peak position recovered after cooling to room temperature without excitation, when the laser intensity was even higher (higher than around 20 kW cm−2), there was visible damage to bP, which did not return to its original photoluminescence peak position. Therefore, we kept the laser incident power for photoluminescence measurements below 600 W cm−2, such that the photoluminescence peak of the exfoliated sample remained constant. This laser incident power is much less than that of the least powerful laser pump (roughly 20 kW cm−2), a value that is known to have a laser thermal effect in bP and MoS2 transferred on polyimide or PDMS62,63. c, Bandgap shift under different directions of strain with respect to the crystal orientation of bP. As the direction of tensile strain changed with respect to the crystal orientation of bP, there was no apparent difference in the strain-induced bandgap shift. This observation is consistent with previous results from a similar bending experiment performed on six-layer bP atop a polyethylene terephthalate (PET) substrate22. AC, armchair; ZZ, zigzag. d, Absorption at excitation wavelength for bP under zero strain, compressive strain and 1.21% of tensile strain. We found that strain had no notable effects on the absorption of light by bP at the photoluminescence excitation wavelength. Even though the bP bandgap was being modulated by strain, because our excitation wavelength was far from the absorption edge, the enhancement in photoluminescence cannot be attributed to increased absorption. e, Reversibility and repeatability of bandgap tuning in bP using compressive strain (0.6%) and tensile strain (1.2%). The photoluminescence peak from 20 nm bP shifts and recovers throughout ten cycles of bending and relaxation. At much higher strain, the PETG is subject to plastic deformation, exhibiting no return to its original state.

Extended Data Fig. 3 Detailed characterization of variable-spectrum bP LED.

a, Dependence on current density of the electroluminescence peak wavelength, showing the reliability of strain-tunable emission at different injection levels. To prevent degradation at high temperatures and to minimize the effect of localized hot spots on device performance, we used a polyimide film with high thermal conductivity, coupled with a Peltier module, to facilitate heat dissipation and to keep a constant temperature during operation. We also maintained a forward current density of less than 20% of the lowest injection level where thermal failure started to take place. When the current density was high, the devices failed sooner, and visible degradation was observed in the channel region. We therefore kept the current levels within the range shown (around 4 A cm−2 to 90 A cm−2) and the device showed stable operations over roughly 8 h (see Extended Data Fig. 5d, e). b, Distribution of angular intensity of the strain-tunable bP LED, calculated with finite-difference time domain (FDTD) simulations (FDTD Solutions, Lumerical). Precise computations await further study on changes in the refractive index of bP with strain (compressive and tensile), but here we simply calculated the angular distribution of the bP LED at two different peak wavelengths, using the published refractive indices of bP without strain. c, Left, schematic of the device architecture. Right, table showing published28,47,64,65,66 complex refractive indices of the polyimide substrate, bP and MoS2; these values were used for simulations. We found the angular distributions at two different wavelengths to be close enough that we could assume there was no discrepancy between the power collections at these two emission wavelengths using an objective lens with a fixed collection angle. d, IV curves of a strain-tunable bP–MoS2 LED measured at 0.20% of compressive strain and 1.06% of tensile strain.

Extended Data Fig. 4 Temperature-dependent performance of strain-tunable bP LED.

a, Electroluminescence spectra for the bP LED on a polyimide substrate, operating at a constant current density of 20 A cm−2 and at different temperatures, under compressive strain (0.2%) or tensile strain (1.0%). b, c, Peak wavelength (b) and peak intensity (c) of the electroluminescence from the bP LED under different strains and at different temperatures. Note that, to achieve heat dissipation and a uniform temperature control during device operation, dry N2 gas was consistently purged, and a mechanically flexible heat sink was installed, connected to the cold finger of the cryostat.

Extended Data Fig. 5 Tuning emission wavelengths to detect different gases.

a, Normalized electroluminescence spectra of bP LED with 0.2% compressive strain, 0.3% tensile strain and 1.0% tensile strain, for detecting CO2, CH4 and H2O, respectively. b, c, Sensor response from the device under 0.2% compressive strain in the presence of CH4 gas (b) and under 0.3% tensile strain in the presence of CO2 gas (c). Our approach showed minimal CH4 detection at a concentration of 2.5% under 0.2% compressive strain, and at 0.3% tensile strain, it could no longer detect CO2 gas. d, e, Stability of the gas-sensing setup for the bP LED measured under 0.2% compressive strain (d) and 0.3% tensile strain (e). Both measurements were performed at a current density of 20 A cm−2 with fmod = 1 kHz. Over 8 h of measurement, the device exhibited a maximum drift of 0.90% and 1.22% for 0.2% compressive strain and 0.3% tensile strain, respectively.

Extended Data Fig. 6 Strain-tunable photoconductors based on bP.

a, Schematic of a strain-tunable bP photoconductor. b, Schematic of the device architecture, showing the generation of a photocurrent at a bias voltage. c, Optical micrograph of the device. D, drain; S, source. d, Strain-dependent spectral photoresponse. A/W, amps/watts. e, Polarization-dependent responsivity at 4.0 μm and 2.0 μm for the device under 0.4% compressive strain and 1.0% tensile strain, respectively. f, IV curves for the strain-tunable bP photoconductor measured in the dark and under illumination by a 1,000 K black body. g, Rise and fall times (between photoresponses of 10% and 90%) under 0.4% compressive and 1.0% tensile strain, using a 1,650-nm laser at roughly 10 mW cm−2. All measurements were conducted at a bias voltage of 100 mV from a device comprising bP of thickness 22 nm.

Extended Data Fig. 7 Detailed characterization of a variable-spectrum bP photoconductor.

a, Spectral noise density under 0.4% compressive and 1.0% tensile strain. The dashed line indicates the 1/f curve at low frequency. b, Normalized photoresponse of the strain-tunable bP photoconductor measured as a function of modulation frequency. The device was measured at a Vd = 100 mV and excited by a 1,650-nm laser, showing a 3-dB frequency of 10 kHz. c, Specific detectivity (D*) as a function of wavelength at room temperature, for the bP device with 0.4% compressive and 1.0% tensile strain and for various commercially available photodetectors.

Extended Data Fig. 8 Setups for measuring photoluminescence and electroluminescence.

a, The IRPL setup. b, The infrared electroluminescence (IREL) setup. For both measurements, the emission from bP was collected by a reflective objective and sent to the external port of the FT-IR spectrometer, with fmod = 5 kHz, τLock-in = 300 μs and optical velocity = 0.0633 cm s−1. Ref., reference; Vac. chamber, vacuum chamber. The total interferogram from the HgCdTe (MCT) detector and the modulated interferogram from the voltage preamplifier were used to separate the photoluminescence/electroluminescence signal from the thermal background.

Extended Data Table 1 Strain-induced bandgap shift of semiconductors in the SWIR/MWIR range
Extended Data Table 2 Parameters used to calculate the theoretical quantum yield from the ABC model

Supplementary information

Supplementary Video 1

Dynamic emission spectrum tuning of bP-LED. Real-time measurement of emission spectrum from actively variable spectrum bP-LED. When tensile strain is applied by bending, the emission has a peak around 2.7 μm. When tensile strain is released and there is only compressive strain, the emission peak is shifted to 4.1 μm. The time period between the bending and release is 21 s. The spectrum is modulated without drift over 500 times of bending cycles and 6 hours of operation time.

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Kim, H., Uddin, S.Z., Lien, DH. et al. Actively variable-spectrum optoelectronics with black phosphorus. Nature 596, 232–237 (2021). https://doi.org/10.1038/s41586-021-03701-1

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