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Abstract
The control of defect states is becoming a powerful approach to tune two-dimensional materials. Black phosphorus (BP) is a layered material that offers opportunities in infrared optoelectronics. Its band gap depends strongly on the number of layers and covers wavelengths from 720 to 4000 nm from monolayer to bulk, but only in discrete steps and suffering from poor photostability. Here, we demonstrate tunable and stable infrared emission from defect states in few-layer BP. First, we demonstrate a continuous blue shift of the main photoluminescence peak under laser exposure in air due to the creation of crystal defects during photo-oxidation. The tunable emission spectrum continuously bridges the discrete near-infrared energies of few-layer BP for a decreasing number of layers. Second, using plasma-enhanced encapsulation, we report the creation and protection of defects with peak emission energy between bilayer and trilayer BP. The emission is photostable and has an efficiency comparable to that of pristine layers while retaining the strong polarization anisotropy characteristic of BP. Our results put forward defect engineering in few-layer BP as a flexible strategy for stable and widely tunable infrared sources and detectors in integrated spectrometers and hyperspectral sensors.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The creation of defect states in two-dimensional (2D) materials is emerging as an effective technique to modify material properties to cover different applications [1–3]. Defects can be introduced during growth or by an external agent such as ion or electron irradiation [4,5], UV-ozone exposure [6], chemical treatment for doping [7], and various types of plasma treatments [8–10]. In the context of 2D semiconductors, the introduction of defects in transition metal dichalcogenides by plasma treatment can enhance visible photoluminescence (PL) in some cases [11–13]. A few 2D materials emit light in the infrared telecommunications bands, including black phosphorus (BP) [14], MoTe2 [15], and TiS3 [16]. In particular, BP has attracted interest for photonics and optoelectronics [17,18] due to its high in-plane anisotropy and its direct, step-tunable band gap determined by the number of layers [19]. BP offers a flexible band gap energy from 1.76 eV to 0.3 eV as thickness increases from monolayer to bulk [14], thus covering the near- and mid-infrared electromagnetic spectrum [14,20–22]. These advantages, together with strong absorption and emission, make few-layer BP a promising 2D material for telecommunications applications such as optical signal processing [23] and optical saturable absorbers [24]. In the case of BP, plasma treatments can reduce the thickness [25–27], encapsulate layers to protect against degradation [28–30], and create different types of defects in the crystal structure [29,31]. Oxygen defects can be introduced into a BP crystal by subjecting it to an environment with reactive oxygen atoms like a plasma [29,32]. Oxygen plasma makes it possible to fabricate BP monolayers starting from thicker films, including a protective oxide layer on the surface and resulting in two PL peaks in the visible region related to bridge-type oxygen defects [29].
Despite the efforts to protect BP from oxidation [33,34], the controlled creation of defects in few-layer BP with stable emission requires attention due to the wide range of potential applications. As the available emission wavelengths from BP follow discrete jumps in band gap energy, methods to obtain intermediate energy values would prove useful. Interestingly, defect engineering opens a path towards tunability, potentially enabling spatial patterning of optoelectronic properties in complex devices such as integrated spectrometers. Here, we demonstrate stable and tunable infrared light emission from few-layer BP. Using laser-induced defect creation, we track the PL spectrum over time and observe a continuous blue shift of its main peak. During photo-oxidation in air, the creation of defect states results in a varying emission spectrum. Using plasma-enhanced encapsulation, we show that it is possible to create and protect such defect states. Encapsulated, defect-tuned few-layer BP shows high linear anisotropy in polarization-resolved measurements, suggesting that its quasi-one-dimensional properties are preserved in the defect creation process.
2. Defect creation using laser exposure
To investigate the creation and emission of defect states in few-layer BP, we employ PL spectroscopy at room temperature using an optical microscope coupled to an Andor InGaAs iDus detector and spectrometer [16]. We use a continuous-wave laser at 532 nm as excitation source, with excitation polarization controlled with a Berek compensator (Newport) and the detection polarization analyzed with a wire-grid polarizer (Thorlabs). We focus the laser with a 50× NIR microscope objective (Mitutoyo) on the samples and collect the emitted light with the same objective in an epifluorescence configuration using a long-pass dichroic mirror and a long-pass filter (Thorlabs DMLP605R and FELH0750). We use mechanical exfoliation from a bulk BP crystal (Smart Elements GmbH) to deposit few-layer flakes on a Si substrate with a 270-nm thermal oxide layer under ambient conditions.
To follow the evolution of the emission, we record sequential PL spectra under laser exposure in steps of 10 s over 1400 s (Fig. 1). The initial PL spectrum after 10 s of exposure shows a PL peak centered at 0.85 eV, matching the reported value for partially oxidized four-layer BP [14]. The spectrum, however, changes over time with several peaks emerging. We deconvolute the spectral contributions of such peaks using a multi-peak Gaussian fitting. Three peaks contribute to the spectra during a wide range of times. Two peaks appear after 120 s and 730 s, (blue and green peaks in Fig. 1, respectively), and arise from four-layer and bilayer BP [14]. We attribute another intermediate peak (red) to defects, with energy not matching trilayer BP (pink). To analyze these three peaks, we plot their peak energy, integrated PL, and spectral full width at half maximum after increasing time of laser exposure (Fig. 2). Unlike the four-layer and bilayer energy peaks (blue and green), which are almost constant during a wide range of times, the defect peak (red) shows an apparent blue shift until a specific time (730 s in Fig. 2(a)). We attribute this blue shift to the creation of defects in the crystal during photo-oxidation [35]. The laser exposure can introduce defects in BP, which are preferred centers to bond with oxygen atoms [36]. Consequently, oxygen incorporation into the BP layers leads to an increase of the BP band gap through the formation of a higher band gap oxide [37,38].
Fig. 1. Appearance of defect emission in few-layer black phosphorus during laser-induced photo-oxidation. Evolution of photoluminescence spectra of few-layer BP in air at different times under laser exposure (λ = 532 nm with power density 0.23 kW/cm2). The blue and green peaks match the photoluminescence spectra of pristine four- and bilayer BP, respectively. The red peak shows a broad and spectrally shifting contribution attributed to defect formation.
Fig. 2. Evolution of spectral contributions to emission in few-layer black phosphorus during laser-induced photo-oxidation. (a) Change of emission peak energies under laser exposure. (b) Spectrally integrated photoluminescence of the peaks retrieved from fitting as a function of laser exposure time. (c) Spectral full width at half maximum of the peaks. For the defect contribution (red), all quantities show a gradual increase of defect density due to photo-oxidation until saturation occurs after 730 s. In comparison, the bilayer (green) and four-layer (blue) contributions show stable peak energy and width.
The emission from defects created during photo-oxidation is expected to increase with defect density. On the other hand, degradation should eventually lead also to a decrease in overall PL. Indeed, we observe such an increase and decrease in integrated PL intensity for the red peak (Fig. 2(b)). Additionally, the red peak broadens upon laser exposure (Fig. 2(c)), confirming that it arises from defects created during photo-oxidation [39]. Therefore, our results show that it is possible to create defect states with tunable emission with intermediate energy values by continuous laser exposure of few-layer BP. Such emission is, however, not stable.
3. Defect creation and stabilization using plasma-enhanced encapsulation
Next, we demonstrate that it is possible to create photostable defect states in few-layer BP with robust infrared emission. We use plasma-enhanced chemical vapor deposition (PECVD) to both induce defect states in the BP crystal and to encapsulate it for protection against further oxidation. We deposit a 15-nm SiO2 film on another BP sample using PECVD with an Oxford Plasmalab system 100. We use a working pressure of 1 Torr and a mixture of SiH4:N2:N2O as precursor gases under an applied power of 20 W and a deposition time of 20 s. The BP sample lies on a substrate electrode at 300 °C, which is below the 400 °C reported temperature for decomposition of exfoliated few-layer BP [28]. The existence of reactive species during plasma-enhanced encapsulation introduces defects in the BP crystal.
After SiO2 encapsulation, the BP sample displays bright emission centered around 1055 nm (1.17 eV, red curve in Fig. 3(a)), which is an intermediate value between bilayer (green) and trilayer BP (pink) [14]. Such intermediate emission from SiO2-encapsulated BP is similar to the emission from unstable defects created during laser exposure in unencapsulated BP (Figs. 1 and 2). To examine the photostability of SiO2-encapsulated BP, we excite the sample using a laser power density of 0.23 kW/cm2 and record the emission spectra over time. The PL of bilayer BP without any encapsulation vanishes within 100 s (Fig. 3(b)), demonstrating the degradation of BP as a result of photo-oxidation. On the other hand, the SiO2-encapsulated BP sample shows excellent photostability for longer times, and we do not observe any spectral shift or broadening of the PL spectra. To confirm the origin of the emission in plasma-enhanced encapsulated BP, we measure the spectrally integrated PL as a function of excitation power density (Fig. 3(c)). We fit it with a power-law model with slope α, defined as the ratio of the logarithm of the spectrally integrated PL and the logarithm of the excitation power density. The emission increases sub-linearly with excitation power with α = 0.64, in agreement with photoluminescence from defects. Overall, the emission photostability and power dependence prove that plasma encapsulation is an effective method to tune and protect the optical properties of BP through defect engineering.
Fig. 3. Tunable photoluminescence and photostability of plasma-enhanced encapsulated few-layer black phosphorus. (a) Photoluminescence spectrum of SiO2-encapsulated black phosphorus compared to the peak energies of pristine few-layer flakes. (b) Photostability of the same SiO2-encapsulated black phosphorus sample (red line) using spectrally integrated photoluminescence versus continuous laser exposure time in ambient air. Bilayer black phosphorus without any protection in air (green line and spectra in the inset) degrades quickly in comparison. (c) Excitation power dependence of spectrally integrated photoluminescence. The fitting to a power law shows the sub-linear dependence characteristic of defects.
Finally, we examine the in-plane linear anisotropy of SiO2-encapsulated BP, which is a prominent characteristic of the optical response of BP [40]. We use polarization-resolved PL spectroscopy. We name the polarization direction with maximum PL as X and the perpendicular direction with minimum intensity as Y (Fig. 4(a)). Our measurements show that BP displays linear polarization anisotropy after plasma-enhanced encapsulation. Furthermore, by plotting and fitting the integrated PL as a function of polarization detection angle for SiO2/BP sample to a cosine-squared dependence (Fig. 4(b)), we find that the optical transition is effectively forbidden along the Y direction, consistent with the zigzag direction in the BP crystal [41].
Fig. 4. Linear polarization anisotropy of plasma-enhanced encapsulated black phosphorus. (a) Polarization-resolved photoluminescence spectra measured at room temperature for SiO2-encapsulated few-layer black phosphorus, revealing strong in-plane linear polarization anisotropy in emission. X (Y) denotes the crystal direction with maximum (minimum) emission corresponding to the armchair and zigzag directions, respectively. Inset: bright-field reflection microscopy image of encapsulated sample showing crystal axes and atomic force microscopy image, including a line section (green) of the area of interest. (b) Spectrally integrated photoluminescence as a function of linear polarization angle in detection for the same sample. The solid gray line is a fitted curve to a cosine-squared dependence, which confirms that the optical transition is forbidden along the Y-direction, as known for the zigzag direction in unencapsulated black phosphorus.
4. Conclusion
In summary, we have demonstrated tunable and stable infrared emission from defect-engineered few-layer BP. By recording its PL spectrum under continuous laser exposure, we report a blue shift of the main PL peak, which we attribute to the creation of crystal defects due to photo-oxidation. This blue shift needs to be taken into account for accurate determination of the number of BP layers and the corresponding band gap using photoluminescence. Remarkably, using plasma-enhanced encapsulation, it is possible to obtain photostable infrared emission in few-layer BP with efficiency comparable to that of pristine BP. The peak emission energies lie at intermediate values between different numbers of layers. The preservation of the strong optical anisotropy after encapsulation demonstrates that the main optoelectronic properties of few-layer BP are retained while adding spectral tunability and photostability as highly desirable improvements. The ability to tailor and stabilize the response of few-layer BP could lead to broadband sources and detectors spanning several octaves in the near- and mid-infrared without energy gaps despite consisting of a single material. Monolithic patterning of black phosphorus could, therefore, lead to integrated infrared spectrometers and hyperspectral sensors.
Funding
Netherlands Organization for Scientific Research (NWO) (Gravitation grant “Research Centre for Ingregrated Nanophotonics” 024.002.033); Iran National Science Foundation (PN: 96002336); Ministry of Science Research and Technology.
Acknowledgments
We thank J. Gomez Rivas for access to the InGaAs CCD camera and R. H. Godiksen for discussions and technical assistance. A. Khatibi is grateful for the financial support from Iran National Science Foundation (INSF) and the Iran Ministry of Science, Research, and Technology.
Disclosures
The authors declare no conflicts of interest.
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