Article pubs.acs.org/cm
Two-Dimensional GeSe as an Isostructural and Isoelectronic Analogue of Phosphorene: Sonication-Assisted Synthesis, Chemical Stability, and Optical Properties Yuting Ye,† Qiangbing Guo,† Xiaofeng Liu,*,† Chang Liu,† Junjie Wang,*,‡ Yi Liu,† and Jianrong Qiu*,§ †
School of Materials Science & Engineering, Zhejiang University, Hangzhou 310027, China Materials Research Center for Element Strategy, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan § State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡
S Supporting Information *
ABSTRACT: Monochalcogenides of germanium (or tin) are considered as isoelectronic and isostructural analogues of black phosphorus. Here, we demonstrate the synthesis of atomically thin GeSe by direct sonication-assisted liquid phase exfoliation (LPE) of bulk microcrystalline powders in organic solvents. The thickness of the GeSe sheets is dependent on the exfoliation conditions, and highly crystalline few-layer GeSe sheets of 4−10 layer stacks with lateral sizes over 200 nm were obtained. In ambient atmosphere, the LPE sheets deposited on the substrate demonstrate strong resistance against degradation, while decomposition into elemental Ge and Se nanostructures occurs at a moderate rate for ethanol dispersions. Density functional theory calculation together with optical characterizations confirm the blue-shifted bandgap for the GeSe sheets as a result of strong quantum confinement effect. In addition, we show that the few-layer GeSe sheets with favorable optical bandgap allow for efficient solar light harvesting for photocurrent generation based on a photoelectrochemical cell. Our joint theoretical and experimental results suggest that GeSe sheets of atomic thickness could be a new two-dimensional semiconductor that can be exploited for potential applications in optoelectronics and photonics.
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(for each atom pair) for possible 2D semiconductors.10,11 Indeed, the compounds of group IV−VI elements such as the group IV monochalcogenides (i.e., GeS and GeSe), which have bandgaps of 1.0−2.3 eV, usually adopt a puckered layer structure similar to that of BP, while the difference is that the cations in GeSe (or GeS) project out of the layer (see Figures 1a and b). In addition, each cation in this structure is threecoordinated, leaving a lone electron pair pointing also to the interlayer spacing which, therefore, is sensitive to interlayer coupling. The bulk form of these compounds, including both chalcogenides and oxides, have in fact unraveled interesting properties such as carrier bipolarity,12−14 superconductivity,15 and high piezoelectric and thermoelectric performance.16−20 Recent theoretical calculations for the mono- and few-layer (FL) form of these chalcogenides have shown the strong quantum confinement effect and electric properties as well as the valley polarized excitons due to breaking of inversion symmetry in monolayer.21−24 Especially GeSe, with a bulk
INTRODUCTION Recent decades have witnessed the booming of extensive investigations into diverse two-dimensional (2D) materials of atomic-scale thickness since the discovery of graphene in 2004.1,2 The near-zero electron effective mass associated with the liner energy dispersion endows graphene with extremely high electron mobility, promising for novel electronic devices. Its application is limited only by the gapless nature which has plagued the development of switching electronic devices. In comparison, 2D semiconductors based on transition-metal dichalcogenides (TMDs) with bandgaps of 1.5−2.5 eV are considered better semiconducting 2D scaffolds for future electronics, while they usually characterize smaller mobility associated with the relatively large bandgap and the small dispersion near the band edges.3−5 The 2D form of black phosphorus (BP), also known as phosphorene, with a thickness-dependent bandgap of 0.3−1.5 eV therefore becomes a better alternative 2D semiconductor, demonstrating excellent electronic properties yet compromised stability compared with that of 2D TMDs.6−9 In the meantime, it is quite natural to explore BP’s isoelectronic analogues, i.e., compounds with 10 electrons © 2017 American Chemical Society
Received: July 5, 2017 Revised: September 4, 2017 Published: September 6, 2017 8361
DOI: 10.1021/acs.chemmater.7b02784 Chem. Mater. 2017, 29, 8361−8368
Article
Chemistry of Materials
Figure 1. (a) Crystal structure of orthorhombic GeSe (space group: Pnma). Each unit cell consists of two GeSe monolayers. (b) Top view of GeSe (along a-axis).
bandgap close to that of silicon (1.12 eV), has been predicted to experience the indirect to direct bandgap crossover for monolayer,11,22 similar to that of MoS2.25,26 However, the presence of a direct bandgap at the single layer limit has been excluded in recent predictions,27,28 leaving the electronic structure of these 2D materials under intense debate. On the other hand, the heavier atomic mass (as compared to BP) and the absence of inversion symmetry for monolayer of these 2D compounds also makes them a better platform for exploiting spin−orbital physics.28,29 These peculiar features have recently evoked growing interest in the exploration of their synthesis by either colloidal chemistry route or liquid phase exfoliation30−32 as well as in the physical properties of these 2D IV−VI compounds.10−24,33 However, atomically thin GeSe still has not been accessed experimentally by existing process variable to other 2D materials. We demonstrate here successful liquid phase exfoliation (LPE) of the GeSe with atomic thickness and high crystallinity (Figure S1). These FL-GeSe sheets are redispersible in various solvents, forming stable colloidal dispersion with long-term stability, and importantly, high robustness against degradation. First principle calculations and optical measurement both confirm clear quantum confinement effect, and we show that FL-GeSe could be a highly active 2D medium with remarkable photocurrent generation under visible light irradiation, which might be promising for energy-related and photonic applications.
Figure 2. (a and b) Calculated electronic band structure (left) and PDOS (right) for monolayer (a) and bulk GeSe (b). Indirect bandgaps and the associated transitions are indicated in the left panels.
with a recent report.23,24 From the projected DOS (PDOS) (Figure 2, right panels), the top of the VB and the bottom of the CB also undergo significant modifications in the PDOS of both Ge and Se because GeSe is covalent; therefore, Ge and Se orbitals are highly hybridized. A more detailed analysis reveals that the bottom of the conduction band is mainly contributed from orbitals of both the Se 4p and Ge 4p and importantly the Ge 4s (due to 4s lone pair electrons in bivalent Ge), which may be responsible for the high sensitivity of electronic structure to layer stacking (see Figure S3). Stimulated by the peculiar electronic structure of 2D GeSe and the presence of a van der Waals gap, synthesis of few−layer (FL) GeSe was performed by sonication-assisted liquid phase exfoliation (LPE) of bulk GeSe powders (prepared by solid state reaction, Figure S4) in different solvents (see the Supporting Information for more details). During sonication, the cavity bubbles generated by the intense sound wave will collapse to form high energy jet, which breaks the bulk layered compounds into thin sheets. The presence of solvents with proper surface tension and polarity prevents the restacking of thin sheets and stabilizes the dispersion.41 The obtained GeSe nanosheets are redispersible in different solvents, forming stable colloidal dispersion without noticeable precipitation for over 24 h. Presented in Figure 3 is the GeSe nanosheets exfoliated in the solvent of ethanol under ambient conditions. From the representative transmission electron microscopy (TEM) image, the sheet-like structures with a lateral dimension of 50−200 nm are clearly observed. The orthorhombic crystal structure of these FL-GeSe nanosheets is well-corroborated by the selected area electron diffraction (SAED) pattern as well as the high resolution TEM (HRTEM) image (Figure 3c), where lattice spacings of 0.288 and 0.279 nm are ascribed to the (011) and (111) planes. As indicated by SAED pattern, these sheets are clearly single crystalline (the diffused ring structures might be ascribed to carbon support). The homogeneous elemental distribution of both Ge and Se by a single-sheet elemental mapping together with X-ray photoelectron spectroscopy
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RESULTS AND DISCUSSION Monochalcogenides of Ge or Sn adopt the GeS-type orthorhombic structure (space group of Pnma) which can be best described as a binary compound analogue of black phosphorus.34,35 As for GeSe, it characterizes a monolayer thickness of 0.54 nm with a van der Waals (vdW) gap of 0.26 nm (Figure 1a). To understand the role played by interlayer coupling in optical and relevant properties, we first calculated the electronic structure for monolayer, bilayer, and bulk GeSe (Figures 2a and b and Figure S2). Following previous studies,36,37 the vdW-corrected screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE06)38,39 was used to obtain good descriptions of electronic structure of GeSe (the calculation details can be found in the Supporting Information). Overall, the structural two-dimensionality is responsible for the very small dispersion along F-Q-B as well as for the presence of sharp peaks in density of states (DOS) which can be associated with strong optical absorption favorable for photovoltaic applications.28,29,40 For monolayer GeSe, an indirect bandgap of 1.81 eV is found, which is in close agreement with recent reports.27,28 Similar as other 2D semiconductors (e.g., TMDs),25,26 the bandgap expands drastically, from 0.93 eV for the bulk to 1.81 eV at the monolayer limit, in agreement 8362
DOI: 10.1021/acs.chemmater.7b02784 Chem. Mater. 2017, 29, 8361−8368
Article
Chemistry of Materials
Figure 3. (a) TEM and SAED pattern (inset) taken from a single GeSe sheet, as indicated by the red rectangle. (b) TEM image of a single GeSe nanosheet. (c) HRTEM image of GeSe nanosheet with clear lattice fringes. (d) Scanning TEM image of a single GeSe nanosheet. (e and f) Elemental mapping for the same nanosheet shown in panel d.
Figure 4. Height−mode AFM images and height profiles (inset) of GeSe nanosheets collected at centrifugation speed of (a) 3k, (b) 6k, and (c) 9k GeSe. (d−f) Corresponding statistical analysis (over 300 single sheets) for the thickness of GeSe nanosheets.
oxidation state of Ge and Se are found to be +2 and −2, respectively, according to the high resolution XPS spectra.42,43
(XPS) (Figure S5) further confirms the chemical composition of the GeSe nanosheets (Figures 3d−f). In addition, the 8363
DOI: 10.1021/acs.chemmater.7b02784 Chem. Mater. 2017, 29, 8361−8368
Article
Chemistry of Materials
Figure 5. (a) Raman spectra of FL-GeSe nanosheets of varying thickness collected at different centrifugation speeds and bulk GeSe powders. (b) Atomic displacement of the Raman active modes in GeSe. (c) Raman spectra for FL-GeSe aged in different conditions for 60 days: top, FL-GeSe thin film deposited onto silicon substrate; bottom, FL-GeSe dispersion in ethanol. The inset shows the corresponding TEM images. The weak peaks located in 238 and 303 cm−1 can be ascribed to elemental Se and Ge, respectively.
Figure 6. (a) Diffuse reflection spectrum of FL-GeSe dispersion and GeSe powders. (b) Calculation of the direct and indirect bandgap (by using the Tauc plot) from the diffuse reflection spectrum. (c) Absorption spectra of FL-GeSe dispersion separated with different centrifugation speeds. Inset shows the bandgap change of GeSe as a function of the centrifugation speed. (d) Dependence of optical bandgap on the thickness of GeSe sheet calculated based on a 2D quantum well model and experimental bandgap calculated from absorption spectra.
To examine the influence of solvent on LPE of GeSe, we employed a number of solvents with different polarity and surface tension. With appropriate ultrasonic agitation (i.e., time and intensity) conditions, all of the solvents examined yield GeSe nanosheets with quite different lateral size and thickness. These sheets produced in different solvents are all redispersible, forming stable colloidal dispersion with long-term stability (Figures S7−S9). The nanosheets exfoliated in 1-cyclohexyl-2pyrrolidnone (CHP) show the largest lateral size of up to 400 nm, while the products from isopropyl alcohol (IPA) are dominated by much smaller sheets (Figure S10). The results therefore suggest that solvent characteristics such as surface tension and polarity play an important role in the exfoliation and stabilization of 2D materials.41,46 For GeSe of the D16 2h symmetry, there should be totally 12 active Raman modes (4Ag + 2B1g + 4B2g + 2B3g).47,48 Here, three main vibrational modes (the A3g mode at 80 cm−1, B13g mode at 150 cm−1, A1g mode at 187 cm−1) can be identified in Figure 5. Compared with bulk GeSe, the peak intensity is notably reduced. The weakening of Raman peak intensity and
Similar to mechanical exfoliation, LPE always produces a mixture of sheets with a range of thicknesses unless a size (or thickness) selection process is performed by, e.g., centrifugation.44,45 To produce GeSe nanosheets with a relatively smaller thickness distribution, the dispersions were subjected to cascade centrifugation. As shown in Figure 4, the statistical atomic force microscopy (AFM) analysis indicates that the GeSe nanosheets obtained at the centrifugation speed of 3k rpm (designated 3k-GeSe nanosheets) exhibit an average thickness of around 6 nm (corresponding to 12 layers). The supernatant was then further centrifuged at 6k rpm to obtain the GeSe nanosheets with an average thickness of 5.5 nm. Moreover, the thickness of the GeSe nanosheets obtained at the centrifugation speed of 9k rpm show an averaged thickness of 2 nm (4 layers) with a rather focused thickness distribution over 1.5−8 nm (Figures 4b and d and S6). At a higher centrifugation speed of 12k rpm, there are some small and thin flakes that could not be observed in the AFM image. These results suggest that the thickness of the GeSe nanosheets can be controllably adjusted by the centrifugation speed. 8364
DOI: 10.1021/acs.chemmater.7b02784 Chem. Mater. 2017, 29, 8361−8368
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Chemistry of Materials
Figure 7. (a) Cyclic voltammogram of the GeSe nanosheets recorded in the electrolyte solution of acetonitrile containing 0.1 M tetrabutylammonium perchlorate (TBAP). (b) Positions of conduction band minimum (LUMO) and valence band maximum (HOMO) for FLGeSe. For reference, the positions of hydrogen evolution and oxygen evolution energies are shown. (c and d) Time-dependent photocurrent response of the GeSe nanosheets at negative bias potentials of −0.4 and −0.2 V and positive potentials of 0.4 and 0.2 V with light on/off (chopped manually with a time interval of 20 s).
broadening of the peak are always observed in 2D materials, and it can be ascribed to the small sample thickness and presence of rich defects and surface atoms. Nevertheless, the Raman peaks of FL-GeSe sheets reveal a clear blue shift compared to that bulk GeSe. For GeSe of smaller thicknesses, the degree of blue shift becomes larger. Generally, the blue shift can be ascribed to reduction of dielectric screening effect in 2D materials that normally leads to increase in vibrational frequency.49,50 Therefore, these results can be used to determine the average thickness of GeSe nanosheets in the dispersion. However, due to possible aggregation and size distribution, precise determination of layer thickness by the Raman spectroscopy is not possible here. Because the stability under ambient conditions is of paramount importance for practical applications, we employed Raman spectroscopy to examine the possible degradation of the FL-GeSe thin film (deposited onto silicon substrate) and their colloidal dispersions. Unlike FL-BP (or phosphorene) that is highly prone to be oxidized,51−53 the thin film of FL-GeSe nanosheets stored in ambient conditions are stable for over two months without observable degradation, as confirmed by Raman spectroscopy (Figure 5c). In addition, there are also chemically inert in most of the examined organic solvents. However, for ethanol dispersion stored over two months, we observed in the Raman spectra additional weak peaks which can be ascribed to elemental Se and Ge,54,55 suggesting moderate degradation occurs via GeSe → Ge0 + Se0. Interestingly, examination of the aged ethanol dispersion (two months after synthesis) of FL-GeSe by TEM reveals the formation of needle and particle structures, which were confirmed jointly by Raman spectroscopy and elemental analysis to be Se and Ge (see Figure S11 for details). Therefore, the formation of Se nanorods and Ge dots is apparently associated with a dissolution−growth process that requires the participation of solvents. This result implies a novel synthetic route for Ge quantum dots and Se quantum wires.
We then determined the optical bandgap of GeSe nanosheets and bulk GeSe using optical reflectance spectrum, as shown in Figure 6a. After performing Kubelka−Munk transformations, the direct and indirect bandgaps of FL-GeSe found in the Tauc plot are 1.52 and 1.26 eV (Figure 6b), respectively. For bulk GeSe, we found an indirect bandgap of 1.08 eV, slightly lower than that of the FL-GeSe. These results are in agreement with calculations considering that obtained FL-GeSe sheets are dominated by 4−10 stacks and monolayer GeSe are obviously not dominating. Because the thickness of the FL-GeSe sheets can be controlled by adjusting the centrifugation speed, we measured the absorption spectra of FL-GeSe dispersions obtained at different centrifugation speeds. Obviously, the bandgap of GeSe became larger increased centrifugation speed (Figure 6c). The range of variation from bulk GeSe to FLGeSe is 1.08−1.53 eV. In addition, it has to be noted that density functional theory (DFT) calculation always underestimates the bandgap, and scattering cannot be avoided in optical measurement; these measured bandgaps are therefore reasonable. The slight difference in the bandgaps between DFT calculation and experimental might also be strong optical anisotropy for the single layer29,30 as well as the scattering effect for reflectance measurement. To elaborate on the thickness-dependent optical bandgap, we calculate the bandgap for GeSe sheets based on a twodimensional (2D) quantum well of infinite height:56 opt Eg,2D (d) = Eg − E b +
π 2ℏ2 2d 2μc
(1)
where Eg is the bandgap of the bulk GeSe (1.08 eV), Eb (
