Effective Passivation of Exfoliated Black Phosphorus Transistors against Ambient Degradation

Abstract

Unencapsulated, exfoliated black phosphorus (BP) flakes are found to chemically degrade upon exposure to ambient conditions. Atomic force microscopy, electrostatic force microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy are employed to characterize the structure and chemistry of the degradation process, suggesting that O2 saturated H2O irreversibly reacts with BP to form oxidized phosphorus species. This interpretation is further supported by the observation that BP degradation occurs more rapidly on hydrophobic octadecyltrichlorosilane self-assembled monolayers and on HSi(111), versus hydrophilic SiO2. For unencapsulated BP field-effect transistors, the ambient degradation causes large increases in threshold voltage after 6 hours in ambient, followed by a ~10 3 decrease in FET current on/off ratio and mobility after 48 hours. Atomic layer deposited AlOx overlayers effectively suppress ambient degradation, allowing encapsulated BP FETs to maintain high on/off ratios of ~10 3 and mobilities of ~100 cm 2 V -1 s -1 for over two weeks in ambient. This work shows that the ambient degradation of BP can be managed effectively when the flakes are sufficiently passivated. In turn, our strategy for enhancing BP environmental stability will accelerate efforts to implement BP in electronic and optoelectronic applications. On increased ambient exposure, the bubble density eventually decreases, evolving into wider and taller bubbles. These bubbles occur in BP, regardless of flake thickness (Fig. S2). In Fig. 2, we therefore use X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy to assess whether chemical modifications, such as the formation of additional chemical bonds or a change in oxidation state, occur in BP upon ambient exposure. Fig. 2A shows P 2p core level XPS spectra of as-exfoliated BP flakes on SiO2 for 0 hrs, 13 hrs, 1, day, 2 days, and 3 days, respectively, of ambient exposure. All spectra are calibrated to the binding energy of adventitious carbon (284.8 eV), and electrostatic charging is compensated using an Ar + flood gun (see Supporting Information for details). At 0 hrs of ambient exposure (black spectrum in Fig. 2A), the exfoliated BP exhibits a single spin-orbit split doublet at ~130 eV, consistent with previous XPS measurements on BP bulk crystals. 27, 28 Note that these spectra do not match those for red phosphorus (~129.8 eV), white phosphorus, or amorphous P-H. 27 A broad, s photoelectronSi satellite from the substrate 300 nm SiO2 appears at ~126.5 eV. After 13 hrs of ambient exposure (maroon spectrum), the full-width at half-maximum (FWHM) for the BP increases, characteristic of some loss of long range order. After 1, 2, and 3 days in ambient (green, navy, and gray spectra, respectively), an additional doublet appears at ~134 eV. This feature is best assigned to phosphate species, 9, 29 although many oxidized phosphorus compounds exhibit peaks near ~134-135 eV. 30, 31 The la...

Emerging Trends in Phosphorene Fabrication towards Next Generation Devices

“…The studies of Wood et al36 confirmed that the irreversible reaction of BP with O 2 ‐saturated H 2 O was responsible for the oxidized phosphorus species (PO x ), thereby providing an explanation for the structure and chemistry of degradation process 54, 216, 228. They found small topographic protrusions (bubbles) above the plane of the BP flakes shortly after exfoliation, whereas the bubble density and the height of the bubbles increased after 1 day of ambient exposure.…”

“…The LPE of BP becomes problematic due to the degradation caused by the instability of BP in oxygen or water and stabilization is required 35, 36, 156. The encapsulation of BP was proposed as a possible solution to protect its reaction with environmental species 36, 220.…”

“…The monolayer of BP, known as phosphorene, exhibits physical properties that can be significantly different from those of its bulk counterpart 16. Phosphorene has changed the landscape of many research areas in science and technology, particularly in condensed matter physics, and it has received much attention recently for its use as the base component of novel nanodevices, e.g., transistors, nanomechanical resonators, photovoltaics, photodetectors, batteries and sensors 9, 10, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37…”

Polydopamine‐Modified Black Phosphorous Nanocapsule with Enhanced Stability and Photothermal Performance for Tumor Multimodal Treatments

“…However, it has been revealed that BP is very reactive to oxygen and water,[[qv: 4b,10]] resulting in compositional and physical changes of BP 11. The lack of water‐ and air‐stability under ambient conditions hinders its potential biomedical applications.…”

Exploring the Formation of Black Phosphorus Intercalation Compounds with Alkali Metals

“…The individual layers of BP exhibit a honeycomb structure differing from that of graphene in that it has a marked puckering of the sp 3 P atoms with an AB stacking structure driven by van der Waals interactions (Scheme 1). 3, 4 BP has attracted enormous attention owing to its high p‐type charge carrier mobility up to around 6000 cm 2  V −1  s −1 and its tunable direct band gap ranging from about 0.3 eV for bulk to approximately 2 eV for monolayers 2, 9, 10, 11, 12, 13, 14, 15, 16. It has recently been predicted that the electronic properties of BP can be modulated by strain or electric field 17, 18.…”