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Bright Mid-Infrared Photoluminescence from Thin-Film Black Phosphorus Chen Chen, Feng Chen, Xiaolong Chen, Bingchen Deng, Brendan Eng, Daehwan Jung, Qiushi Guo, Shaofan Yuan, Kenji Watanabe, Takashi Taniguchi, Minjoo Larry Lee, and Fengnian Xia Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b04041 • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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Bright Mid-Infrared Photoluminescence from Thin-Film Black Phosphorus Chen Chen1†, Feng Chen1, 5†, Xiaolong Chen1, 6, Bingchen Deng1, Brendan Eng2, Daehwan Jung3, Qiushi Guo1, Shaofan Yuan1, Kenji Watanabe4, Takashi Taniguchi4, Minjoo L. Lee2 and Fengnian Xia1* 1Department 2Department

of Electrical Engineering, Yale University, New Haven, Connecticut 06511, USA

of Electrical and Computer Engineering, University of Illinois Urbana–Champaign, Champaign, Illinois 61801, USA

3Institute

for Energy Efficiency, University of California Santa Barbara 93106, USA

4National

Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan

*To whom correspondence should be addressed: [email protected]

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Abstract Recently rediscovered layered black phosphorus (BP) provides rich opportunities for the investigations of device physics and applications. The bandgap of BP is widely tunable by its layer number and a vertical electric field, covering a wide electromagnetic spectral range from visible to mid-infrared. Despite much progress in BP optoelectronics, the fundamental photoluminescence (PL) properties of thin-film BP in mid-infrared have rarely been investigated. Here, we report bright PL emission from thin-film BP (with thickness of 4.5- to 46-nm) from 80 to 300 K. The PL measurements indicate a bandgap of 0.3080.003 eV in 46-nm thick BP at 80 K and it increases monotonically to 0.3340.003 eV at 300 K. Such an anomalous blueshift agrees with the previous theoretical and photoconductivity spectroscopy results. However, the observed blueshift of 26 meV from 80 to 300 K is about 60% of the previously reported value. Most importantly, we show that the PL emission intensity from thin-film BP is only a few times weaker than that of an indium arsenide (InAs) multiple quantum well (MQW) structure grown by molecular beam epitaxy. Finally, we report the thickness-dependent PL spectra in thin-film BP in mid-infrared regime. Our work reveals the mid-infrared light emission properties of thin-film BP, suggesting its promising future in mid-infrared tunable light emitting and lasing applications.

Key Words Black phosphorus; photoluminescence; tunability; bandgap; mid-infrared; anomalous blueshift


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Main Text Recently rediscovered black phosphorus (BP)1-4 has attracted wide attention from device and physics communities due to its high carrier mobility5-11, large in-plane anisotropy12-15, and the widely tunable band structure16-20. The bandgap of BP strongly depends on its layer number, ranging from around 2 eV in monolayer14, 21 to 0.3 eV in bulk form22. This widely tunable bandgap provides an extraordinary platform for infrared optoelectronics23-26. Recently, an external out-ofplane electric field has been leveraged to realize the bandgap tuning in thin-film BP27-29, extending the mid-infrared application of BP into longer wavelength region up to around 8 m30. In contrast, the light emitting properties of BP have not been investigated extensively. For example, only photoluminescence (PL) spectra of 1-5 layers have been explored21, 31, which is limited to visible and near-infrared region. Interestingly, the mid-infrared light emission properties of thin-film BP with thickness greater than 6 layers have rarely been reported, probably due to the challenges in PL measurements in mid-infrared wavelength range, which significantly limits its potential for mid-infrared light emitting applications. Here, we report thickness and temperature dependent mid-infrared PL in thin-film BP. Bright PL at 2485 cm-1 was observed in ~46-nm thick BP at 80 K, indicating a bandgap of 0.308 eV. In this work, we do not consider excitonic effect, because the BP explored here is rather thick and always close to the bulk limit. The PL emission from thin-film BP is quite strong, which is only seven times less bright than that of a control indium arsenide (InAs) multiple quantum well (MQW) sample at 80K. With increasing temperature, the PL spectrum shows a blue shift and reaches 2686 cm-1 at 300 K, corresponding to a bandgap of 0.334 eV. Although such anomalous temperature dependence of bandgap in BP has previously been predicted theoretically32 and observed in photoconductivity spectroscopy33, the observed blue shift in our PL experiment is only about 62% of the previously reported value33. We obtain a temperature coefficient of 1.53×10-4 eV/K at temperature range from 80 to 120 K and 1.04×10-4 eV/K from 160 to 300 K. We further investigated the PL spectra of thinner BP flakes with thickness down to ~ 6-nm. At 80 K, the bandgap increases from 0.308 eV for ~46-nm BP, to 0.338 and 0.441 eV for 9- and 4.5-nm BP, respectively. Our observations provide the first demonstration of mid-infrared light emission in the wavelength range of 2.8 to 4 μm in two-dimensional layered materials, which opens up the


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opportunities for the realization of the mid-infrared light-emitting diodes and lasers using layered materials. BP flakes for optical characterizations were directly deposited onto the 285-nm SiO2/Si substrate in an argon-filled glove box using the standard mechanical exfoliation method. Then, BP flakes were covered with hexagonal boron nitride (hBN) flakes using the polymer-free dry transfer method34 to prevent BP from oxidation. Fig. 1a shows the optical image of a typical hBN-covered BP flake. Its thickness is about 46-nm, determined by the atomic force microscopy (AFM) measurements. The PL measurement setup is shown in Fig. 1b. The BP sample was placed in a temperature-controlled cryostat and the excitation photon energy is 1.94 eV. The emitted light was collected by a mid-infrared microscope objective and its spectrum was analyzed by the Fourier transform infrared (FTIR) spectrometer. The detailed information on the measurement setup and procedures are presented in the Method section.

Figure 1. (a) Optical image of the hexagonal boron nitride (hBN) covered 46-nm thick black phosphorus (BP) flake on a 285-nm SiO2/Si substrate. (b) Schematic illustration of the midinfrared photoluminescence (PL) measurement setup.

Fig. 2a shows the PL spectrum of the thin-film BP flake (46-nm) measured at 80K with excitation power of 10 µW/µm2. The PL spectrum exhibits a peak position at 0.308 eV, corresponding to the bandgap of the bulk BP since the excitonic effect is negligible in thin-film BP thicker than 8 layers. 4 ACS Paragon Plus Environment

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The insignificant excitonic effect can be attribute to the reduced screening effect and relatively large dielectric constant in multilayer BP22. In fact, in bulk BP, the exciton binding energy is only around 10 meV35. The bandgap of the thin-film BP extracted from PL spectrum also agrees with the previous theoretical calculations22 and the infrared absorption measurements19. To best our knowledge, the optical bandgap of BP single crystal was previously determined by absorption measurements35, while its mid-infrared PL emission has not been reported. To ensure the PL spectroscopy is in the linear response regime, we measured the PL spectrum of BP under different excitation power densities (See Supporting Fig. S1). As shown in the inset of Fig. 2a, the PL peak intensity shows a linear relationship with the power intensity ranging from 8 to 20 µW/µm2. Since it is difficult to accurately quantify the mid-infrared loss spectrum of various optical components in the collection pathway of the home-made PL system and the Fourier transform infrared spectrometer, we do not report the absolute quantum efficiency directly in this work. Instead, we evaluated the relative PL emission intensity of thin-film BP by comparing its PL with an indium arsenide (InAs) multiple quantum well (MQW) sample grown by molecular beam epitaxy with peak emission at 2.45 𝜇m. Growth details about the InAs MQW structure used in this experiment are presented in the Method and Supporting Information S2. Figure 2b plots the PL spectra of both BP and InAs MQW measured at the same excitation power density of 10 µW/µm2 and 80 K. The absolute PL intensity of black phosphorus is only seven times weaker than that of the InAs MQW, demonstrating that the PL emission from the thin-film BP is quite bright. Here we want to emphasize that our measurement results do not directly indicate that the quantum efficiency of PL in BP is seven times smaller than that in the InAs MQW. The absorbed powers of the pumping light (640 nm) by BP and InAs can be different and the collection and detection efficiency at 2.45 and 4 m can also be different. However, this comparison shows that the PL signal from BP is quite bright such that the BP PL spectrum can be plotted together with the InAs MQW sample; where similarly grown MQW samples can serve as the active region for electrically pulsed, roomtemperature lasers36. Therefore, the natural next step will be exploring the cavity controlled light emission and possible lasing based on BP, since the emission efficiency depends on the competition between the radiative and non-radiative processes and the radiative decay time can be controlled by the local electromagnetic environment37.


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Figure 2. (a) Mid-infrared PL spectrum of the 46-nm thick BP at 80 K. The full-width-halfmaximum of the spectrum is 30 meV. Inset: Power dependence of the BP PL peak intensity. (b) The PL emission spectra of thin-film BP and the InAs quantum well (The PL intensity of BP is multiplied by a factor of 7). Both spectra were measured under the same measurement condition (80 K and excitation power density of 10 µW/µm2).

We further explored the anisotropy of the PL emission in thin-film BP. The crystal orientation of BP flake was first identified by the polarization-resolved Raman measurements38, 39 as reported previously (See Method). Supporting Fig. S3a exhibits the Raman spectrum of the BP. The intensity ratio of Ag2 and Ag1 mode reaches the maximum when the excitation laser polarization was aligned along the armchair (x) direction of BP flakes40, as shown in the inset of the Supporting Fig. S3a. We investigated the emission anisotropy of 46-nm thick BP, where PL emission spectrum along armchair (x) and zigzag (y) directions were plotted in Supporting Fig. S3b, respectively. The PL emission was almost perfectly polarized along the armchair (x) direction under the excitation photon energy of 1.94 eV (640 nm). The angular resolved PL intensity of BP was well fitted with the function of 𝑎𝑐𝑜𝑠2𝜃 + 𝑏 (See the inset of Supporting Fig. S3b), where  is the angle between emission polarization direction and the armchair-direction of BP, and a, b are fitting parameters. This linear polarized PL emission from BP is because the band-edge optical transition along the zigzag-direction was forbidden according to the symmetry analysis19, 21. In fact, the similar PL 6 ACS Paragon Plus Environment

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emission anisotropy has been previously observed in its monolayer to trilayer counterparts14, 21. The observation reported here also agrees with previously reported linearly polarized, band-edge infrared absorption in thin-film BP19, since emission and absorption are two reciprocal processes.

Figure 3. a) Temperature dependence of the PL spectrum of the 46-nm thick BP. Clear blueshift is seen at higher temperature, opposite to that in traditional semiconductors. b) Temperature dependence of the BP PL peak intensity. c) Temperature-dependent bandgap of BP extracted from the PL peak position.

The temperature dependence of the PL spectrum was then measured to investigate the temperature dependence of the thin-film black phosphorus (46-nm) bandgap. Fig. 3a shows the PL spectra at different temperatures ranging from 80 to 300 K at a step of 20 K. The PL intensity decreases as 7 ACS Paragon Plus Environment

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temperature increases, as plotted in Fig. 3b. The decreasing PL intensity with increasing temperature probably arises from the thermal activation of the nonradiative recombination center at higher temperature41, 42. At higher temperature, the probability of nonradiative recombination of the electron-hole pairs increases, which suppresses the PL emission. Interestingly, the PL peak position of black phosphorus exhibits an anomalous blueshift with increasing temperature. This phenomenon reveals that the bandgap of thin-film black phosphorus increases monotonically with temperature, which is opposite to conventional semiconductors43-45. Previously, such an anomalous temperature dependence of BP bandgap was explored both theoretically32 and experimentally based on photoconductivity spectroscopy33. Figure 3c shows the BP bandgap at different temperatures extracted from the PL spectra, which increases from 0.308 eV at 80 K to 0.334 eV at 300 K. The anomalous temperature-dependent PL position of BP can be attributed to the renormalization of bandgap from both electron-phonon coupling and lattice thermal expansion, which has been theoretically predicted32 and also experimentally observed in monolayer BP41. As shown in Fig. 3c, the anomalous temperature-dependent trend can be divided into two temperature regions: 80-120 K and 160-300 K, giving different temperature coefficient (dEg/dT)33. Under low temperature (< 120 K), electron-phonon coupling and lattice thermal expansion play comparable role in the renormalization of the bandgap of BP. In contrast, at high temperature (> 160 K), the contributions from electron-phonon interaction become weaker while the lattice thermal expansion dominates. Our observation is also consistent with the recent studies20, 46, 47 on mechanical straininduced bandgap renormalization in thin-film BP, since lattice thermal expansion can induce a similar strain effect. The linear fitting was performed to extract the temperature coefficient (dEg/dT) of the optical bandgap in respective temperature ranges, which gives 1.53×10-4 eV/K and 1.04×10-4 eV/K within 80-120 K and 160-300 K, respectively. Interestingly, our measured temperature coefficient in both temperature ranges is about 40% smaller than previous theoretical prediction32 and measurement results based on photoconductivity spectroscopy33. As a result, the overall bandgap increases from 80 to 300 K measured in our experiment of 26 meV is also about 40% smaller than that reported in Ref. 31. Such a difference could be due to the different methods used for the estimation of the BP bandgap. We also observe a side peak at around 283.5 meV in Fig. 3a, whose position is almost independent of the temperature. This side peak may be attributed to a defect level. Since the bandgap renormalization is due to the anharmonic properties of phonons and defects experience different local lattice vibrations, it is not surprising that the defect level 8 ACS Paragon Plus Environment

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exhibits a different temperature behavior from that of the bandgap. On the other hand, it is also possible that the side peak is caused by the absorption of residue carbon dioxide in the detection optical pathway. Regardless of its origin, this side peak has minimal impact on our conclusion due to its low amplitude. The main PL properties are hardly affected by this side peak. The anomalous temperature-dependent bandgap of BP can also be described based on a twooscillator model48, 49, which contains two isolated Einstein oscillators governed by two discrete phonon energies. The two-oscillator model has been leveraged to fit the temperature dependence of the bandgap of semiconductors including monolayer black phosphorus41,

50, 51,

where the

bandgap Eg can be expressed as41, 48, 51

(

𝐸𝑔 = 𝐸0 + 𝐸1

2 ℏ𝜔1

) (

+ 1 + 𝐸2

2 ℏ𝜔2

𝑒 𝑘𝐵𝑇 ― 1

𝑒 𝑘𝐵𝑇 ― 1

)

+1

(1)

Here, E0 is bare bandgap representing the low-temperature bandgap without considering the zeropoint motion energy, ℏ𝜔1 and ℏ𝜔2 denotes two oscillator energies, and E1 and E2 govern the renormalization energy strength of two oscillators. Here we choose ℏ𝜔1 = 17.2 meV and ℏ𝜔2 = 52.8 meV, extracted from the phonon densities of monolayer BP52. Although the phonon densities may vary slightly with thickness, the changes are expected to be small such that the fitting is hardly affected15, 52. As shown in Fig. 3c, the fitting results based on the two-oscillator model (dashed line) agree with the experiments well. The fitting results yield the fitting parameters E0=339 meV, E1=21 meV and E2=-55 meV, respectively. The bandgap at low temperature (~308 meV at 80 K) is smaller than the bare bandgap (339 meV), indicating that the electron-phonon interactions can significantly renormalize the bandgap of the thin-film BP. We further investigated the thickness-dependent bandgap through PL measurements on thin-film BP with thicknesses varying from 4.5- to 46-nm at 80 K. The optical image and detailed AFM analysis of these flakes were provided in Supporting Fig. S4. Fig. 4a exhibits the normalized PL spectra of thin-film BP at various thickness. With increasing layer numbers, the PL peak position shows a redshift, revealing the decreasing bandgap of BP. The layer-dependent bandgap of thinfilm BP was summarized in Fig. 4b, which decreases monotonously from 0.441 eV at 4.5-nm (~8 layers), to 0.338 eV at 10-nm (~19 layers) and eventually to 0.308 eV in 46-nm thick BP (~91 layers). We added an error bar of ±1 layer as shown in Fig. 4b to take the possible errors in atomic 9 ACS Paragon Plus Environment

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force microscopy measurements into account. As reported previously, the layer-dependent bandgap of BP can be well described by a one-dimensional tight-binding model, taking only the nearest-neighbor interlayer interaction into consideration19, 21. For an N-layer BP, both conduction band and valence band will split into N subbands at the Γ point of the Brillouin zone due to the interlayer interactions. Then the energy gap between conduction band and valence band with the same subband index n (n = 1, 2, 3,…, N) 𝐸𝑛𝑁 can be expressed as19, 21 𝑛𝜋 𝐸𝑛𝑁 = 𝐸𝑔 ― 2(𝛾𝑐 ― 𝛾𝑣)cos ( ) 𝑁+1 Here, 𝐸𝑔 represents the bandgap of monolayer BP, and 𝛾𝑐 and 𝛾𝑣denote the nearest neighbor interlayer interactions of the conduction band and the valence band, respectively. The bandgap is determined by the energy difference between the conduction band minimum and the valence band maximum in subband n = 1. Using 𝐸𝑔 = 2.12 eV and 𝛾𝑐 ― 𝛾𝑣 = 0.905 eV, we fit the experimental data based on Eq. (1), as shown in Fig. 4b. The fitted curve agrees with the measured thickness dependent bandgap very well.

Figure 4. a) Thickness dependence of the PL spectra of BP. b) Thickness dependence of BP bandgap in mid-infrared. An error bar of ±1 layer is added to the layer number axis.


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In conclusion, we report a comprehensive study on mid-infrared light emission properties of thinfilm BP. We observe a bright and robust PL emission from 46-nm thin-film BP, revealing the bandgap of 0.308 and 0.334 eV at 80 and 300 K, respectively. Remarkably, such bright emission intensity is only about seven times less intense than that of an InAs MQW structure, which is used as the active region of a room-temperature laser. The bandgap shows an anomalous blueshift with increasing temperature, with a temperature coefficient of 1.53×10-4 eV/K and 1.04×10-4 eV/K within 80-120 K and 160-300 K, respectively. Such an anomalous blueshift agrees with previous photoconductivity spectroscopy experiments, but the measured temperature coefficient is about 40% less than the previously reported values. We further complete the thickness-dependent bandgap diagram of BP through studying the mid-infrared PL emissions in thin-film BP with thicknesses varying from 4.5- to 46-nm. Our study reports the basic mid-infrared light emission properties of BP, unravelling its thickness- and temperature-dependent tunability in the mid-infrared region. Combining with its high mobility and electrical-field/strain tunability, our observations reveal the promising future of BP in mid-infrared light emitting and lasing applications.

Method Optical Measurement. The Raman scattering spectroscopy was performed in Horiba LabRAM HR Evolution Raman Microscope with an excitation photon energy of 2.33 eV. We performed the polarization-resolved Raman measurement by rotating the sample with respect to the excitation laser polarization. The mid-infrared PL spectroscopy was performed in a system as shown in Fig. 1b. The sample was placed in a low-temperature stage (model HFS600E-PB4 from Linkam Scientific Instruments). A chopped 640 nm solid-state laser was used as an excitation source, which was focused on the sample by optical lenses and a prism attached to 15× mid-infrared objective lens in a Hyperion 2000 microscope. The radius of the laser spot focused onto the sample was optimized to be around 20 µm, which is smaller than the sample size. The PL signal was collected by this 15× objective, and was further analyzed using a Brucker Fourier transform infrared spectrometer (FTIR). An external lock-in scheme similar to that reported in previous works 53, 54 was used to suppress the noise arising from the background thermal emission as shown in Fig. 1b. The Lock-in amplifier (Model SR380) was in reference to the mechanical chopper at a chopping frequency of 10 kHz. Each spectrum is smoothed by averaging 512 measurements in 11 ACS Paragon Plus Environment

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FTIR to lower the noise level. The resolution of the directly measured spectra is 8 cm-1, and all the spectra presented in this work were obtained by averaging eight adjacent points to minimize the background noise. The polarization-resolved PL measurements of emission anisotropy were performed by rotating an infrared polarizer in the detection path. Growth of InAs MQW. InAs MQW samples were grown on semi-insulating (001) InP in a Veeco Mod Gen-II molecular beam epitaxy chamber. The substrate was first desorbed at a thermocouple temperature of 500 °C to remove the surface oxide under a P2 beam equivalent pressure of 1.1E-5 Torr. The thermocouple temperature was lowered to 450 °C, and an InP buffer and InAsxP1-x stepgraded buffers were grown at 1 𝜇m/hour. InAsxP1-x compositions were changed by increasing the As2 flux while keeping the P2 flux constant based off previous calibrations. A 9-step graded buffer was grown from InP to InAs0.45P0.55 with 150-nm steps and 5% As increments and capped with 600-nm InAs0.5P0.5. During the last 50-nm of the InAs0.5P0.5 growth, the In cell temperature was lowered to reduce the growth rate to 0.5 µm/hour. A strain-balanced active region of compressively strained InAs QWs and tensile strained In0.54Ga0.46As barriers were then grown and capped with a 50-nm InAs0.5P0.5 cap. The schematic structure of InAs is shown in Figure. S2a. Cross-sectional transmission electron microscopy (X-TEM) showed a coherently strained MQW active region and also revealed clear QW/barrier interfaces. Reflection high-energy electron diffraction (RHEED) patterns were observed during growth showed streaky 2×4 patterns for the InAsxP1-x graded buffer and the InAs QWs along with a streaky 1×3 pattern for the tensile strained In0.54Ga0.46As, as shown in Figure S2c, both indicating good crystal quality55-57.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website PL spectra of BP at different excitation powers; Growth condition of the InAs MQW PL emission anisotropy of the 46-nm BP flake; 12 ACS Paragon Plus Environment

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Optical micrographs and AFM results of BP of different thicknesses; Author Information Corresponding Author *E-mail: [email protected] Author Contribution †C.C

and F.C contribute equally to this work. The manuscript was written through contributions

of all authors. All authors have given approval to the final version of the manuscript. Present Address 5(F.C.) 6(X.C.)

Department of Physics, Fudan University, Shanghai 200433, China Department of Electrical and Electronic Engineering, Southern University of Science and

Technology, Shenzhen 518055, China

Notes The authors declare no competing financial interest.

Acknowledgement We acknowledge the financial support from the National Science Foundation EFRI-2DARE program (1542815). M. L. L. acknowledges financial support from NSF Award No. 1713068. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan and the CREST (JPMJCR15F3), JST. We thank Professor Li Yang of Washington University in St Louis for valuable discussions and providing helpful comments to our manuscript. We thank Zishan Wu of Yale University for assistance on Raman measurements.

Reference 13 ACS Paragon Plus Environment

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Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Content


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Page 17 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Nano Letters

Figure 1

a

b

ACS Paragon Plus Environment

Nano Letters

Figure 2 a 16 12

8

4

0.6

80 K 0.8

0.8 0.6 0.4

0.4

87 6

5

4

3

2

1.0

Intensity (a.u.)

0.8

Intensity (a.u.)

1.0

Wavelength (m)

b

Wavelength (m) 1.0

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

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8 10 12 14 16 18 20 2

Pump Density (W/m )

0.2 0.0

80 K Black Phosphorus InAs Quantum Well 7

0.6 0.4 0.2 0.0

100

150

200

250

300

350

400

200

300

400

500

Energy (meV)

Energy (meV)


600

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Nano Letters

Figure 3

Wavelength (m)

a

5.0 4.5

4.0

3.5

b 3.0

Intensity (a.u.)

1.0

80K

0.8 100K 120K

0.6

1.0 Experimental data Fitting Line

0.8 0.6 0.4 0.2 0.0 50

140K

100

c

180K

0.4

200K 220K 240K 260K 280K 300K

0.2

150

200

250

300

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160K

Bandgap (meV)

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

336 330 324

Experimental data Two-oscillator model d𝐸𝑔 = 0.153 𝑚𝑒𝑉/𝐾 𝑑𝑇

318

d𝐸𝑔 = 0.104 𝑚𝑒𝑉/𝐾 𝑑𝑇

312

0.0 306

240

280

320

360

Energy (meV)


100

150

200

250

Temperature (K)

300

Nano Letters

Figure 4 Wavelength (m)

a 1.0

4.8 4.4

4

3.6

3.2

b

2.8

450

4.5 nm 6 nm 7 nm 10 nm

17 nm 30 nm 46 nm

Experiment Fitting Curve

420

Bandgap (meV)

0.8

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

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0.6

0.4

0.2

390

360

330

0.0

300 250

300

350

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Energy (meV) ACS Paragon Plus Environment

0

20

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80

100

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Nano Letters

TOC

Lock in

Filter

Spectrometer Mirror

Wavelength (m) 2

4

8 6

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Polarizer

0.9 Black Phosphorus 7

InAs Quantum Well

Chopper

0.6

15× Lens

0.3 80 K 0.0

150

Laser BP Sample

600 450 300 Energy (meV)

Stage


Bright Mid-Infrared Photoluminescence from Thin ... - ACS Publications

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