Letter pubs.acs.org/NanoLett
Controlled Synthesis of High-Mobility Atomically Thin Bismuth Oxyselenide Crystals Jinxiong Wu,†,§ Congwei Tan,†,‡,§ Zhenjun Tan,†,‡ Yujing Liu,† Jianbo Yin,† Wenhui Dang,† Mingzhan Wang,† and Hailin Peng*,†,‡ †
Center for Nanochemistry, Beijing National Laboratory for Molecular Sciences (BNLMS), College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China ‡ Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, P. R. China S Supporting Information *
ABSTRACT: Non-neutral layered crystals, another group of two-dimensional (2D) materials that lack a well-defined van der Waals (vdWs) gap, are those that form strong chemical bonds in-plane but display weak out-of-plane electrostatic interactions, exhibiting intriguing properties for the bulk counterpart. However, investigation of the properties of their atomically thin counterpart are very rare presumably due to the absence of efficient ways to achieve large-area high-quality 2D crystals. Here, high-mobility atomically thin Bi2O2Se, a typical non-neutral layered crystal without a standard vdWs gap, was synthesized via a facial chemical vapor deposition (CVD) method, showing excellent controllability for thickness, domain size, nucleation site, and crystal-phase evolution. Atomically thin, large single crystals of Bi2O2Se with lateral size up to ∼200 μm and thickness down to a bilayer were obtained. Moreover, optical and electrical properties of the CVD-grown 2D Bi2O2Se crystals were investigated, displaying a size-tunable band gap upon thinning and an ultrahigh Hall mobility of >20000 cm2 V−1 s−1 at 2 K. Our results on the high-mobility 2D Bi2O2Se semiconductor may activate the synthesis and related fundamental research of other non-neutral 2D materials. KEYWORDS: Two-dimensional materials, bismuth oxyselenide, non-neutral layered semiconductor, high mobility, chemical vapor deposition, controlled synthesis
F
representative of 2D materials without a well-defined vdWs gap. As indicated in Figure 1a, the Bi2O2Se, a traditional thermoelectric material,11,12 exhibits a layered crystal structure (I4/mmm, a = 3.88 Å, c = 12.16 Å, Z = 2) in which the strong covalently bonded [Bi2O2]n2n+ layers were sandwiched by planar [Se]n2n− layers with relatively weak electrostatic interaction13 (Figure 1a). Additionally, Bi2O2Se is a highmobility layered semiconductor,12,14−17 suggesting the potential application in flexible logic transistors and optoelectronics. For those applications to be achieved, the prerequisite challenge lies in the controlled synthesis of large-area high-quality atomically thin layers. Herein, we present the controlled synthesis of large-area high-mobility atomically thin 2D Bi2O2Se crystals via facile chemical vapor deposition (CVD). The thickness, domain size, nucleation site, and crystal-phase evolution of Bi2O2Se nanoplates were well-controlled by tuning the growth conditions.On the basis of the optical and Hall measurements, the CVD-grown 2D Bi2O2Se semiconductor exhibited a sizetunable band gap upon thinning and an ultrahigh hall mobility of >20000 cm2 V−1 s−1 at 2 K. Our results imply the potential
ollowing the observation of the usual physical properties of graphene, a significant amount of research into twodimensional (2D) materials has been pursued because of their unique planar structures, rich electronic band structures, and fascinating optical, electrical, and optoelectrical properties.1,2 Although possessing a big family, current research in 2D materials primarily focuses on several “star materials” such as graphene,3 transition metal dichalcogides (MoS2, WS2),4 black phosphorus,5 and topological insulators (Bi2Se3, Bi2Te3).6,7 Interestingly, an obvious structural similarity among them is the existence of a van der Waals (vdWs) gap in all of their parent counterparts. However, there is another group of 2D materials that lack of a well-defined vdWs gap2 and possess a wide variety of electronic properties including superconductor (KFe2Se2),8 semiconductor (BiCuOSe),9 and insulator (layered double hydroxides, mica),10 which are alternated with chargecompensating cations and anions (rather than neutral layers) and are held together with the weak electrostatic interactions.2 Compared to those materials with a standard vdWs gap, the reports on the above-mentioned charge-carried layered materials are very rare due to the possible absence of efficient ways to achieve large-area high-quality atomically thin crystals. Layered Bi2Se3 has recently been demonstrated to be a reference topological insulator with an insulating bulk gap and gapless Dirac-type surface states. Partial replacement of selenium atoms with lighter oxygen atoms in Bi2Se3 can form the ternary oxyselenide compound Bi2O2Se, which is a © 2017 American Chemical Society
Received: January 24, 2017 Revised: March 27, 2017 Published: April 11, 2017 3021
DOI: 10.1021/acs.nanolett.7b00335 Nano Lett. 2017, 17, 3021−3026
Letter
Nano Letters
Figure 1. (a) Layered crystal structure of Bi2O2Se with alternated tetragonal [Bi2O2]n2n+ layers and [Se]n2n− layers. For clarity, the weak electrostatic interactions between the [Bi2O2]n2n+ and [Se]n2n− layers are not presented. (b) Schematic illustration of a CVD setup to synthesize Bi2O2Se nanoplates on mica with the coevaporation sources of Bi2O3 and Bi2Se3. (c) Typical OM image of as-synthesized 2D Bi2O2Se crystals grown on mica. (d−g) OM images of large-area atomically thin Bi2O2Se crystals with thicknesses of (d) 4, (e) 3, and (f) 2 layers. (g) AFM topography of a monolayer of Bi2O2Se.
Figure 2. (a) Low-magnification TEM image of a thin Bi2O2Se nanoplate with folded edges transferred onto a holey carbon TEM grid. (b) Atomicresolution TEM image projected along the c-axis. Clear atomic arrangements with lattice spacing of 0.19 nm (a dark lattice fringe alternating with a brighter one) along the direction of [100] were observed, which are in accordance with the atomic arrangement of the ab-plane in Bi2O2Se (as shown inset in b). (c) HRTEM images recorded from the folded edge of the Bi2O2Se nanoplate. A well-defined spacing of 0.61 nm matches well with the layer thickness of Bi2O2Se (0.608 nm). (d) The EDX spectra of as-synthesized nanoplates, showing a stoichiometry of Bi2O2Se. The Cu signal comes from the Cu grid. (e, f) XPS characterization of CVD-grown Bi2O2Se thin film in which the clear Bi 4f spectrum (e) and Se 3d (f) core level photoemission spectra were acquired.
were passivated and neutralized during the growth, atomically thin Bi2O2Se crystals would be achieved. Herein, the atomically flat and freshly cleaved fluorophlogopite mica [KMg3(AlSi3O10)F2] was utilized as substrate for CVD growth.19,20 Mica is a typical non-neutral layered material with positively charged K+ layers separated by negatively charged [Mg3(AlSi3O10)F2]− layers,21 as illustrated in Figure S1. Combined with the atomic scale smooth surface of mica, the stronger electrostatic interaction between the epitaxial layer (Bi2O2Se) and mica substrates may facilitate the lateral growth of Bi2O2Se and result in 2D forms due to the relatively lower
applications of atomically thin Bi2O2Se crystals in highperformance logic transistors and optoelectronics. The layered lattice structure with a strong interlayer/ intralayer bonding anisotropy in Bi2O2Se makes it easier to obtain ultrathin 2D crystals on a suitable substrate. However, compared to the van der Waals interaction in neutral layered materials such as Bi2Se3 and MoS2, the weak electrostatic interaction between charge-carried layers in Bi2O2Se is relatively stronger, which would result in a higher tendency of vertical growth on a substrate due to higher surface energy if not rationally designed.18 If high surface energy sites, however, 3022
DOI: 10.1021/acs.nanolett.7b00335 Nano Lett. 2017, 17, 3021−3026
Letter
Nano Letters
Figure 3. Control over the domain size and nucleation site of 2D Bi2O2Se crystals. (a, b) OM images of Bi2O2Se nanoplates grown at (a) 570 and (b) 550 °C for 30 min exhibiting obvious morphological evolution in domain sizes. (c) Statistics for average domain sizes of as-synthesized 2D Bi2O2Se crystals as a function of growth temperature. (d) Schematic of nucleation site control of 2D Bi2O2Se crystals via a method of patterning growth with pretreatment of the mica substrate. (e) OM image of well-ordered 2D Bi2O2Se crystal arrays. (f) Corresponding AFM image of rectangular area marked with red dashed line in (e). The relatively larger thicknesses (∼16.1 and 18.2 nm) achieved in 2D Bi2O2Se crystals may be ascribed to the relatively lower energetic barrier for the nucleation and the use of longer patterning growth time (1 h).
than 5 μm without breaking, which suggests that ultrathin nanoplates are rather flexible. The single-crystalline nature of Bi2O2Se was confirmed by high-resolution TEM (HRTEM). As shown in Figure 2b, welldefined lattice spacing of 0.19 nm (a dark lattice fringe alternating with a brighter one) and 0.28 nm was obtained, which is identical with the theoretical value of lattice distance of 0.19 nm for (200) and 0.27 nm for (110) in Bi2O2Se. Moreover, the HRTEM recorded from the folded edge indicated a layer spacing of 0.61 nm along the [001] stacking orientation (Figure 2c), corresponding to the layer thickness of Bi2O2Se (0.608 nm). Energy-dispersive X-ray spectroscopy (EDX) revealed the chemical composition of the as-synthesized nanoplates in which obvious signal peaks for Bi, Se, and O were observed (Figure 2d). X-ray photoelectron spectroscopy (XPS) further verified the chemical bonding character of as-grown 2D Bi2O2Se crystals. As elucidated in Figure 2e and f, the two symmetric peaks of Bi 4f7/2 and Bi 4f5/2 were centered at 158.7 and 164.0 eV (higher than those in Bi2Se3),27 which can be assigned to the chemical bonding of Bi(III)-Ox28 that exists in Bi2O2Se. On the other hand, the Se 3d spectra, fitted to two Lorenz peaks at 53.3 and 54.2 eV, also located among the reasonable chemical shift range of Se 3d core level in Bi2O2Se. These spectroscopic characterizations indicated as-synthesized 2D Bi2O2Se crystals are of high quality. The ability to tailor the domain-size of 2D crystals for specific applications is essential and challenging for synthetic chemistry. Interestingly, the domain size of Bi2O2Se nanoplates can be controlled by altering the growth temperature. As shown in Figure 3a−c and Figure S4, the average domain size of the as-synthesized Bi2O2Se nanoplates reached a peak value of ∼135 μm as the temperature varied from 540 to 570 °C, whereas it shrank gradually when further elevating the growth temperature. The approximate reason for the above phenomenon is summarized as follows. When the substrate temperature was relatively low (540 °C < T < 570 °C), the absorbing rate of
energy barrier for lateral growth. Notably, according to the phase diagram of the Bi/O/Se ternary system22 (Figure S2), the ternary phase of Bi2O2Se thermodynamically exists on the binary equilibrium line of Bi2O3−Bi2Se3. Therefore, atomically thin Bi2O2Se crystals were synthesized on the mica substrate by utilizing the Bi2Se3 and Bi2O3 as coevaporation sources (Figure 1b). Figure 1c shows a typical optical image (OM) of Bi2O2Se nanoplates grown on mica substrate. Square 2D Bi2O2Se crystals have a domain size of several tens of micrometers. Notably, by precisely tuning growth parameters such as growth temperature, flow rate, and growth time, ultrasmooth large single crystals with domain sizes of ∼200 μm and thicknesses ranging of 2−4 layers were obtained (Figure 1d−f and Figure S3). Atomic force microscopy (AFM) was performed to determine the thickness and step height of few-layer Bi2O2Se. Moreover, Bi2O2Se single layer with a domain size of several tens of micrometers and thickness of ∼0.9 nm has been achieved (Figure 1g). Note that the height of the single layer is slightly higher than the theoretical valve of ∼0.61 nm, presumably due to the structural relaxation as the thickness shrinks or measurement uncertainty of AFM, which was often observed in other 2D materials such as monolayer Bi2Se323 and black phosphorus.24 To evaluate the crystal structure and chemical compositions of as-synthesized 2D Bi2O2Se crystals, we performed characterization by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The as-grown Bi2O2Se nanoplates can be readily transferred onto the holey carbonsupported Cu grid for TEM characterization via a poly(methyl methacrylate) (PMMA)-mediated method.25 Interestingly, artificially folded edges in Bi2O2Se can occasionally be formed (Figure 2a), which can reveal cross-sectional structure in 2D materials using TEM.25,26 We found that thin Bi2O2Se nanoplates can be easily bent to 180° with a radius of less 3023
DOI: 10.1021/acs.nanolett.7b00335 Nano Lett. 2017, 17, 3021−3026
Letter
Nano Letters
Figure 4. Controlled crystal phase evolution from Bi2Se3 to Bi2O2Se during CVD growth with the assistance of O2. (a) Schematic demonstration and related chemical equations for the evolutions of Bi2Se3 and Bi2O2Se. (b−d) Morphological evolution of CVD growth products using Bi2Se3 as the sole evaporation source assisted by O2 with different concentrations of 0, 0.2, and 0.4 sccm. (e) Corresponding Raman spectra of as-synthesized 2D crystals in (b−d) clearly verifying the gradual evolution of crystal phases from Bi2Se3 to Bi2O2Se assisted by O2.
Figure 5. Optical and electrical measurements of atomically thin 2D Bi2O2Se crystals. (a) OM image of the CVD-grown atomically thin Bi2O2Se crystals with different layer numbers (from 1 to 6 L) on mica with the reflectance mode (top) and transmittance mode (bottom). (b) Micro-optical measurements for Bi2O2Se nanoplates with various layer numbers in (a). (c) Extracted optical band gaps as a function of layer number, and deduction of the band gap for bulk Bi2O2Se is shown in the inset in (c). (d) OM image of CVD-grown 2D Bi2O2Se device with a Hall bar configuration. (e) Resistance (Rxx) as a function of temperature. (f) Hall mobility (μHall) and carrier density of 2D Bi2O2Se crystals as a function of temperature.
originating from the lower concentration of precursor absorbed on substrate at higher temperature. In addition to thickness and domain size control during growth, precise control of the positions of 2D crystal arrays is crucial in the subsequent integration of these nanostructures. To this end, the polydimethylsiloxane (PDMS)-mediated printing growth method, using acetone as the ink solvent, was verified as a general approach that can efficiently modify the mica substrates with the PMDS oligomer29 and precisely control the nucleation sites (Figure 3d). By this means, wellordered arrays of 2D Bi2O2Se crystals were obtained (Figure 3e) that had smooth surfaces with thicknesses ranging from
the precursor is relatively high; thereby, the absorbed precursors would accumulate and nucleate on the substrate with more probability, then crystallize into Bi2O2Se nanoplates with smaller domain sizes and larger density of nucleation sites (Figure 3a and b). On the contrary, upon further increasing the temperature (T > 570 °C), the precursor’s absorbing rate diminishes and the absorbing behavior gradually becomes the dominating elemental step of crystal growth, thereby also resulting in smaller domain size of Bi2O2Se nanoplates at the opposite extreme of temperature. In addition, the average thicknesses of 2D Bi2O2Se crystals decrease gradually when the substrate temperature increases (Figure S5), presumably 3024
DOI: 10.1021/acs.nanolett.7b00335 Nano Lett. 2017, 17, 3021−3026
Letter
Nano Letters
determined from Hall coefficient RH measurements (n = 1/ eRH). As shown in Figure 5f, the Hall mobility reaches a value of ∼313 cm2 V−1 s−1 at 300 K, which is favorably comparable to the best CVD-grown transition metal dichalcogenides and increased dramatically to a superior value of ∼20660 cm2 V−1 s−1 upon cooling to 2 K. The behavior of the temperaturedependent mobility (increasing rapidly upon cooling) can be elucidated perfectly by a phonon-dominated charge transport mechanism and implies significantly depressed scattering of charge impurities. Such an ultrahigh mobility validates the high quality of our CVD-grown 2D Bi2O2Se crystals and would facilitate the investigation of low-temperature quantum phenomena such as ballistic transport and the quantum Hall effect. In conclusion, we reported the controlled synthesis of highquality CVD-grown Bi2O2Se semiconducting crystals on mica with ultrahigh electron mobility, which is a typical non-neutral layered material without a standard vdWs gap. The thickness, domain size, nucleation site, and crystal-phase evolution of Bi2O2Se crystals were well-controlled by tuning the growth condition. The convenient CVD growth and ultrahigh mobility feature suggest that Bi2O2Se is a promising 2D material for fundamental investigations and high-performance electronic applications.
16.1 to 18.2 nm (Figure 3f), providing an excellent platform for future integration of optoelectronic devices and logic transistors. As it pertains to elemental component, Bi2O2Se can be derived from its parent compound of layered Bi2Se3 by partially substituting Se with O. Hence, it is quite interesting to determine whether Bi2O2Se can be evolved from Bi2Se3 by controlling the concentration of O2, whose chemical reactions can be represented as follows: Bi2Se3 + 3O2 = Bi2O2Se + 2SeO2 (Figure 4a). If we used the sole evaporation source Bi2Se3 for the CVD growth, only triangular or truncated triangle-like Bi2Se3 nanoplates were obtained without introducing O2 in the system (Figure 4b). However, when a small amount of O2 (0.2 sccm) was added during CVD growth, square-shaped 2D Bi2O2Se crystals emerged among the triangular Bi2Se3 nanoplates (Figure 4c). This phenomenon can presumably be ascribed to the concentration fluctuations of oxygen precursors at the onset of nucleation. Notably, the Bi2Se3 can be completely changed to tetragonal Bi2 O 2 Se with an O 2 concentration of 0.4 sccm (Figure 4d). The phase evolution from Bi2Se3 to Bi2O2Se as a function of O2 concentration was monitored with Raman spectroscopy excited by a 632.8 nm laser, as shown in Figure 4e. A typical Raman signal of Bi2Se3 crystals composed of two peaks located at ∼131 and ∼174 cm−1 is associated with the E2g and A21g vibrational modes, respectively.23 The Raman shift of as-synthesized 2D Bi2O2Se crystals was centered at ∼100 and ∼159 cm−1, displaying identical values with the bulk Bi2O2Se crystal synthesized via a modified Bridgman method. Therefore, the oxygen concentration plays a vital role in the evolution of crystal phases from Bi2Se3 to Bi2O2Se. Large-area atomically thin 2D Bi2O2Se crystals grown on a transparent and insulating mica substrate facilitate the investigation of optical and electrical properties without transfer. Optical absorption spectroscopy, a fundamental feature for a material, helps us to experimentally investigate the band structure evolution and extract the optical band gap of as-synthesized materials. We conducted the optical measurements of CVD-grown 2D Bi2O2Se crystals with varied thicknesses as indicated by the optical contrast of OM image with the reflectance and transmittance modes (Figure 5a and Figure S6). In detail, the absorption spectroscopy was acquired on a homemade microabsorption spectrometer that has a spatial resolution of
