Ultrathin and Flat Layer Black Phosphorus Fabricated by Reactive Oxygen and Water Rinse Hyuksang Kwon,†,⊥ Sung Won Seo,†,‡,⊥ Tae Gun Kim,†,‡ Eun Seong Lee,† Phung Thi Lanh,†,‡ Sena Yang,† Sunmin Ryu,*,§ and Jeong Won Kim*,†,‡ †
Korea Research Institute of Standards and Science (KRISS), Daejeon 34113, South Korea Korea University of Science and Technology (UST), Daejeon 34113, South Korea § Department of Chemistry & Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 37673, South Korea ‡
S Supporting Information *
ABSTRACT: Ultrathin black phosphorus (BP) is one of the promising twodimensional (2D) materials for future optoelectronic devices. Its chemical instability in ambient conditions and lack of a bottom-up approach for its synthesis necessitate efficient etching methods that generate BP films of designed thickness with stable and high-quality surfaces. Herein, reporting a photochemical etching method, we demonstrate a controlled layer-by-layer thinning of thick BP films down to a few layers or a single layer and confirm their Raman and photoluminescence characteristics. Ozone molecules generated by O2 photolysis oxidize BP, forming P2O5-like oxides. When the resulting phosphorus oxides are removed by water, the surface of BP with preset thickness is highly flat and self-protective by surface oxygen functional groups. This method provides a fabrication strategy of BP and possibly other 2D semiconductors with band gaps tuned by their thickness. KEYWORDS: phosphorene (black phosphorus), ozone, etching, water, protective layer ince the first exfoliation and electrical measurement in 2014,1,2 atomically thin black phosphorus (BP) layers have shown intriguing potential applications such as field effect transistors with high on−off ratio,1−3 optoelectronic devices,4 and photocatalysis.5,6 This is because BP, as a twodimensional (2D) material, possesses a widely tunable band gap with thickness,7 outstanding electron mobility up to 1000 cm2/ (V s),1,8 and a high photoresponsivity of ∼9 × 104 A/W.9 In addition, its electronic and optical anisotropy, originating from its geometry, endow BP with anisotropic mobility and linear dichroism,10,11 which can be useful in future electronic and optical devices.12−14 Following a recent surge of interest in 2D semiconductors such as transition metal dichalcogenides (TMDs) and single elemental “-cenes”,15−18 BP occupies the most interesting platform of intensive experiment and theory. For further progress of BP-based applications, a few obstacles need to be overcome. First, mass production of wide-area and thin BP has not been reported yet. The fundamental reason for this is that single-crystalline BP is synthesized at a high pressure and a high temperature.10 A liquid exfoliation method to produce very thin BP layers has been suggested,11,19−22 but currently BP flake size is limited to only a few micrometers and
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its thickness uniformity is not fully controllable, leaving much room for improvement when compared to the case of TMDs.23 Thus, further effort in material synthesis is required to achieve wide-area and uniform BP. The second issue concerning BP is its chemical vulnerability to ambient conditions, due to spontaneous surface oxidation24,25 involving hydrated superoxide anions formed by ambient light.26 The performance of BP electronic devices is strongly affected by the surrounding environment and degrades rapidly in ambient conditions,27−29 posing a critical issue regarding device stability and reliability. To avoid ambient oxidation, BP devices are covered by polymer, Al2O3,29−31 graphene, or h-BN32 or are chemically functionalized.33 However, despite the enhanced stability, passivation with foreign materials adds additional cost and complexity in fabrication. Compared to naturally grown oxide layers such as SiO2/Si, surface phosphorus oxide was suggested as a passivation layer, but its thickness and surface roughness did not offer precise control.34,35 Received: June 24, 2016 Accepted: September 1, 2016 Published: September 1, 2016 8723
DOI: 10.1021/acsnano.6b04194 ACS Nano 2016, 10, 8723−8731
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Figure 1. Flat and oxidation-resistant BP flakes after water rinse. AFM topography of a BP flake exposed to ambient conditions for varying times: (a) less than 30 min (pristine), (b) 3 days, (c) 7 days, and (d) 20 days. 3D AFM height images of a BP flake: (e) right after exfoliation, (f) after water rinse, and after (g) 6 h and (h) 24 h air exposure. (i) Root-mean-square (RMS) roughness of three BP flakes as a function of time after water rinse. Scale bars in a−d: 400 nm.
RESULTS AND DISCUSSION Protection against Ambient Oxidation by Water Rinse. We confirmed that freshly exfoliated BP flakes undergo spontaneous oxidation in ambient conditions, as reported previously.26,28,39 Figure 1 shows AFM and optical images obtained for BP flakes exfoliated on a 285 nm SiO2/Si substrate followed by exposure to ambient air for varying times. On the surface of the freshly exfoliated BP flake with a thickness of 14.4 nm (Figure 1a), many small bump-like structures with a height of ≤30 nm are observed, which evolved into larger ones after 3 days (Figure 1b). While the bumps continued to grow for 7 days, their overall number density decreased with increasing height (Figure 1c). After 20 days, the BP flakes disappeared completely, and the bumps eventually turned into submicrometer-sized large droplets (Figure 1d). The change in the appearance could also be recognized in the series of optical images (Figure S1). The bumps generated upon air exposure reveal a significant phase difference (15−30°) in the AFM images with respect to the flat BP region, irrespective of air exposure time (Figure S2).29,39 This indicates that the chemical composition of the bumps is different from that of BP, which is in good agreement with previous studies on the ambient oxidation of BP.29,34,39 The phosphorus atoms on the BP surface irreversibly react with photogenerated superoxide anions (O2−) formed in the presence of ambient light and air.26 Because of its hygroscopic nature, the phosphorus oxide (PxOy) on the BP surface absorbs ambient water and dissolves to form bumps or droplets of PA.26,28,39 Several studies have shown that ambient oxidation is detrimental to unprotected BP flakes and devices.29,39 While one may be tempted to exploit the process to controllably reduce the thickness, the average etching rate is impractically low, ∼0.03 nm/h, when estimated from the AFM images in Figure 1a and c. Moreover, it would be necessary to isolate samples from the ambient conditions (O2, water vapor, or light) to stop the etching, which adds inconvenience in handling samples. By exploiting the high solubility of phosphorus oxide, we tested cleaning the surface with water. One fresh sample in Figure 1e revealed many oxide bumps, tens of nanometers high, which were formed during a ∼30 min exposure to the air between the mechanical exfoliation and AFM scanning. After a brief rinsing with DI water followed by a N2 blow-drying, the
Precise but efficient thickness control of BP to form single (1L) and multilayer (nL) phosphorene stands as another technical challenge. Unlike graphene and various TMDs, chemical vapor deposition or other bottom-up approaches for the synthesis of phosphorene have not been reported yet.36 Photooxidation driven by a focused visible laser beam was used to etch 8-nm-thick BP in O2/H2O, but the laser intensity was too high (200 μW/μm2) to efficiently realize large-area fabrication.26 Ar+ and O2 plasma treatments allowed controllable etching by a few layers35,37 or several nanometers.38 However, they often generated a rough surface, and O2 plasma left surface phosphorus oxide PxOy of several tens of nanometers thickness. Although these treatments generated good-quality samples by removing concomitant oxides and impurities, they still needed prompt passivation to block ambient oxidation.38 Moreover, the plasma treatment typically requires technological expense and careful power optimization. Otherwise it easily induces damage on the remaining surface. For BP to be seriously considered for future applications, other approaches need to be explored for precise layer-by-layer etching and subsequent surface protection against ambient oxidation. Here, we demonstrate a highly efficient and very simple etching method using UV-activated oxygen (UVO) species, mainly ozone, generated at atmospheric pressure. The irradiation of UV light (λ < 250 nm) in an O2 atmosphere enables controllable etching of BP films down to a single layer with a sub-nanometer thickness precision by varying the exposure time. The typical etching rate with a conventional UV lamp is 12 nm/h. The etching reaction consists of a cycle of BP oxidation by the UVO species and surface oxide removal by dissolution into phosphoric acids (PA).26 This process can be repeated to reach the target thickness. Rinsing with deionized (DI) water leads to highly flat BP films of preset thickness with a protective oxide layer that shows a greatly enhanced resistance toward ambient oxidation. This simple yet precise method of controllable BP etching can be applied to manipulate other layered materials into a 2D form for various optical and electronic device fabrications. Moreover, such a nonvacuum etching process facilitates an easy way of simultaneous monitoring of film thickness and surface quality. 8724
DOI: 10.1021/acsnano.6b04194 ACS Nano 2016, 10, 8723−8731
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Figure 2. Photochemical etching of BP by UV-activated oxygen (UVO) species. (a−c) Optical images of BP flakes (A and B, 14.6 and 26.3 nm thick, respectively): pristine (a), after 30 min UVO reaction (b), after rinsing with DI water (c). (d−f) AFM height images corresponding to a−c. The height line profiles were obtained along the white dashed lines. Scale bars: 3 μm. (g) Schematic view of the UVO reaction in the optical cell, where a low-pressure Hg lamp was the source of UV light. (h) Raman spectra of flake A obtained before and after the treatments.
oxide bumps were removed and the surface became highly flat (Figure 1f). Interestingly, the rinsed surface was found to be resistant to ambient oxidation. Figure 1g, obtained after 6 h air exposure following the rinse, lacked such large oxide bumps as were formed immediately after the fresh exfoliation. Even after extended exposure for 24 h, the rinsing significantly suppressed the growth of the bumps (Figure 1h). The root-mean-square roughness (Rq) in Figure 1i clearly presents similar trends for multiple samples. Large Rq values for the pristine state resulted from the presence of nanometer-scaled PxOy and/or PA bumps generated by ambient oxidation.27,28 Although the AFM images were obtained within 30 min after mechanical exfoliation, the formation of bumps was unavoidable.26,27 When rinsed, Rq dropped from 3.5 ± 0.6 nm to 1.7 ± 0.1 nm, which remained essentially identical for several-hour exposure to air. Possible origins of the protective effect will be proposed and discussed with more supporting data below. Considering the high susceptibility of BP to ambient oxidation, the rinse-induced protection can be useful in manipulating BP materials. Efficient and Controllable UVO Etching of BP. In Figure 2, we show that a cycle of UVO etching is highly efficient and produces thin and extremely flat BP films. Figure 2a and d show the respective optical and AFM images obtained for the pristine BP flakes before UVO etching. The two BP flakes in Figure 2a, pale blue (A) and yellow (B), were found to have thicknesses of 14.6 and 26.3 nm, respectively (line profile in the AFM image of Figure 2d). Rq of the untreated regions A and B was 5.1 and 5.4 nm, respectively. For the UVO etching, the pristine sample was placed in an optical gas cell filled with an atmospheric flow of high-purity O2 gas. The sample was then exposed to the emission from a low-pressure Hg lamp through a quartz
window (Figure 2g). UV radiation (λ < 250 nm) photolyzes oxygen molecules (O2) and generates reactive oxygen species such as O radicals and O3, which can oxidize BP.40 The optical image in Figure 2b shows that the 30 min UV irradiation induced a noticeable color change in flake A, indicating successful etching. The AFM image in Figure 2e, which corresponds to the sample exposed to reactive oxygen, reveals two large droplets (∼50 and ∼ 170 nm height) wetting the two BP flakes (A and B). The anomalous volume expansion (∼10 times for flake B) is attributed to dissolution of PxOy into PA by absorbing ambient water vapor.39 To remove the products of the etching reaction, oxide bumps, and PA droplets, the sample was gently rinsed with DI water followed by a N2 blow-drying. As shown in the AFM image (Figure 2f), the surface of the UVO-etched BP flakes became highly flat with an Rq of ∼1 nm but without any apparent defects, bumps, or droplets. As shown for the pristine BP, the rinsed surface of the UVO-etched BP showed a significant resistance to the ambient oxidation, thus essentially serving as a protective layer (Figure S3), which will be discussed below. The AFM height profiles indicate that the thickness of the two flakes, A and B, was reduced to 8.1 and 20.1 nm, by 6.5 and 6.2 nm, respectively. The etching rate averaged for multiple samples was 12 ± 2 nm/h, and it can be further optimized by varying the O2 flow rate and light intensity. Given that ambient oxidation etches BP at a rate of ∼0.03 nm/h, the UVO etching is 2 orders of magnitude faster and can be utilized as an efficient etching process. As will be shown later, the etching rate was hardly affected by the initial BP thickness and remained constant during extended etching processes. 8725
DOI: 10.1021/acsnano.6b04194 ACS Nano 2016, 10, 8723−8731
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Figure 3. Chemical identification of UVO-treated BP. P 2p (a) and O 1s (b) core-level X-ray photoelectron spectra of a bulk BP crystal were obtained during a series of treatments: in the pristine state, after 1 h of UVO treatment, and after rinsing with DI water. The topmost spectra of P2O5 powder, as a reference oxide, were obtained without further treatments.
Moreover, the observed uniform etching of ∼6 nm (∼12 layers of phosphorene) across >10-μm-wide flakes strongly suggests that the reaction occurs essentially in a layer-by-layer manner. The Raman spectra in Figure 2h show that the reduction in the thickness lead to an increase in the Raman intensities of BP (A1g, B2g, A2g) and the Si substrate. The latter is due to increased transmission through the etched BP flakes, and the former is attributed to an optical interference effect,41 as will be described in detail below. Surface Chemical Composition of UVO-Treated BP. To determine the chemical composition of the BP flake surface during the course of the treatments shown in Figure 2, we carried out sequential X-ray photoelectron spectroscopy (XPS) measurements for a large piece of bulk BP sample. As an oxide reference, a P2O5 powder sample was compared. P 2p and O 1s core-level spectra of the BP obtained upon sequential treatments are displayed in Figure 3. The pristine sample showed a P 2p doublet of 2p1/2 and 2p3/2 (spin−orbit splitting of ∼0.86 eV) with a binding energy of 130.1 eV for 2p3/2 and a broad peak at 134.6 eV. The former corresponds to unreacted BP and the latter to residual phosphorus oxides.6,19,42 In spite of the very short duration of ambient exposure (
