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Bi-stable electronic states of cobalt phthalocyanine molecules on two-dimensional vanadium diselenide
Applied Materials Today 18 (2020) 100535
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Bi-stable electronic states of cobalt phthalocyanine molecules on two-dimensional vanadium diselenide Lei Zhang a,b,1 , Tong Yang a,1 , Wen Zhang a , Dongchen Qi c,d , Xiaoyue He e , Kaijian Xing d , Ping Kwan Johnny Wong b,∗ , Yuan Ping Feng a,∗ , Andrew Thye Shen Wee a,b,∗∗ a
Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore c School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, Queensland 4001, Australia d Department of Chemistry and Physics, La Trobe Institute for Molecular Science, La Trobe University, Melbourne, VIC, 3086, Australia e Songshan Lake Materials Laboratory, Dongguan 523808, China b
a r t i c l e
i n f o
Article history: Received 25 September 2019 Accepted 12 December 2019 Keywords: Cobalt phthalocyanine Vanadium diselenide Bi-stable states Scanning tunneling microscopy Two-dimensional transition-metal dichalcogenides
a b s t r a c t Hybrid organic/2D interfaces combine the wide spectrum of 2D material properties with the major advantages of organic materials, such as low cost, mechanical flexibility, and chemical tunability. Here, we report the electronic properties of cobalt phthalocyanine (CoPc) molecules adsorbed on molecular beam epitaxy-grown monolayer vanadium diselenide (VSe2 ). Using scanning tunneling microscopy/spectroscopy, we provide evidence of highly ordered molecular assembly on monolayer VSe2 , with two distinctive bright and regular molecular contrasts, which are not observed on graphite. These contrasts also lead to a distinct difference in the electronic state of the molecule’s central Co atom, for which density functional theory calculations indicate the regular state as the ground state. A correlation between these two molecular states and the charged states of individual molecules is postulated, as demonstrated by the possibility of switching the bright state to the regular state using a negative tip voltage pulse. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Benefiting from their multitude of intrinsic and unique properties, two dimensional (2D) transition-metal dichalcogenides (TMDs) have received enormous attention in the recent decade for both fundamental studies and emerging applications [1]. Many opportunities exist for engineering and manipulating these properties, for example, by stacking similarly layered materials as van der Waals (vdW) heterostructures, or interfacing them with other non-layered materials. The functionalization of 2D TMDs with organic molecules is a particular example of the latter approach, via which the 2D properties can be optimized by coupling with organic materials that are low cost, mechanically flexible and
∗ Corresponding authors. ∗∗ Corresponding author at: Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore; Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore E-mail addresses: [email protected] (P.K.J. Wong), [email protected] (Y.P. Feng), [email protected] (A.T.S. Wee). 1 L.Z. and T.Y. contributed equally. https://doi.org/10.1016/j.apmt.2019.100535 2352-9407/© 2019 Elsevier Ltd. All rights reserved.
chemically tunable. The hybrid organic/2D interface may also host unprecedented interfacial phenomena, inaccessible in the individual constituents [2–10]. Here, our work focuses on the electronic properties of cobalt phthalocyanine (CoPc) molecules deposited on molecular beam epitaxy (MBE)-grown monolayer VSe2 on highly oriented pyrolytic graphite (HOPG). The rationale for this particular organic/2D combination is largely motivated by a number of attributes: (1) 2D vanadium dichalcogenides, VX2 (X= S, Se, Te), are an exciting class of TMDs with a great variety of exotic properties. These include the ultra-high electrical conductivity (up to 106 Sm−1 ) in VSe2 [11], enhanced charge-density waves (CDW) in monolayer VSe2 [12–14], VS2 [15] and VTe2 [16], catalytic activities of VSe2 for hydrogen evolution reaction (HER) [17–21], and magnetism in VS2 [15,22,23], VTe2 [15] and VSe2 [12,24–29]. These properties in 2D VX2 are mainly derived from the 3d1 odd electronic configuration of V4+ alongside the strong electronic coupling between neighboring V4+ −V4+ pairs [30,31]; (2) CoPc is a typical spin-1/2 transition-metal phthalocyanine molecule, with its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) being largely dictated by the central Co 3d orbital electrons of the molecule [32]. It is thus fundamentally interesting
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to explore how these electrons would interact with those in the V 3d bands of VX2 . Our study, using scanning tunneling microscopy/spectroscopy (STM/STS), reveals that CoPc molecules form a highly ordered selfassembly on 2D VSe2 and exhibit two distinctive bright and regular molecular contrasts, in contrast to CoPc adsorbed on HOPG. These molecular contrasts are accompanied with a distinct difference in the electronic state of the central Co atom, which DFT calculations suggest the regular state as the ground state. We have also demonstrated the ability to switch the bright state to the regular by applying a negative voltage pulse through an STM tip. This might suggest a correlation between the observed molecular states and the charged states of the individual molecules. 2. Experimental methods and computational details 2.1. Sample preparation and characterization Monolayer VSe2 was grown on HOPG substrate by MBE in an ultrahigh vacuum (UHV) chamber with a base pressure of ∼1 × 10−9 mbar. To avoid possible contamination, the substrate was cleaved in the vacuum chamber and then annealed at 720 K for 1 h. Se (Sigma, 99.999 %) was evaporated from a Knudsen cell at 470 K and V (ESPI Metals, 99.999 %) from an electron-beam evaporator. During growth, the substrate was kept at 650 K, a temperature higher than the sublimation temperature of Se, and the whole growth process was maintained under Se-rich conditions to ensure sufficient Se atoms to react with V. After growth, the sample was cooled to 320 K and capped with Se. For CoPc deposition and STM/STS characterization, the monolayer VSe2 was transferred to the STM preparation chamber. After annealing at 520 K for 1 h, the Se capping layer was completely desorbed, leaving a clean VSe2 surface for the stepwise CoPc deposition. CoPc (Fluka, >97 %) was thoroughly degassed at its evaporation temperature (573 K) in a Knudsen cell for at least 3 h, then idled at 500 K during the entire experiment. For deposition, the molecules were first heated to 573 K and stabilized for 1 h to obtain a stable evaporation rate, then deposited onto monolayer VSe2 at room temperature at a deposition rate of ∼0.17 layer/min. The thickness of the CoPc film was determined by a quartz crystal microbalance (QCM) thickness monitor, with its reading calibrated against STM line profiles. 2.2. DFT calculations Calculations based on spin-polarized DFT were carried out using the Vienna ab initio simulation package (VASP) [33,34]. The projector augmented-wave (PAW) method and the generalized gradient approximation within the Perdew, Burke and Ernzerhof parametrization were adopted for the core-valence electrons interaction and the exchange-correlation functional, respectively [35,36]. The electronic wave functions were expanded in a plane wave basis with a cutoff kinetic energy of 500 eV. In accordance with the experimentally observed molecular arrangement of CoPc on √ 1 T-VSe √ 2 (see Fig. 3 f), a monolayer VSe2 supercell including 21 × 21 unit cells (15.672 Å × 15.672 Å) was constructed and the in-plane orientations of the adsorbed CoPc molecules were adjusted so as to be similar to the experimental observation. To diminish the spurious interlayer interaction, a vacuum layer of around 20 Å was inserted in the direction perpendicular to the basal plane. A -centered k-grid of 2 × 2 × 1 was chosen to sample the first Brillouin zone for structural optimizations, and 4 × 4 × 1 for energy calculations. The electronic and ionic convergence criteria were set to 1.0 × 10−4 eV and −0.01 eV/Å, respectively. To take into account the strong on-site Coulomb interaction, the effective Hubbard U values of 1.0 and 3.0 eV, which were taken from previous
literatures, were added onto the V 3d and Co 3d electrons using the simplified rotationally invariant approach proposed by Dudarev et al. [32,37,38]. The long-range vdW interaction was also included via the DFT-D3 correction method [39]. The STM images of the freestanding and adsorbed CoPc molecules were simulated using the Tersoff method, where the tip height was kept at about 2 Å [40]. 3. Results and discussion 3.1. MBE-grown monolayer VSe2 The MBE-grown monolayer VSe2 on HOPG was characterized by STM/STS at 77 K. Fig. 1a shows that VSe2 forms ordered and smooth triangular islands with a height of ∼8 Å, indicating its high crystallinity and monolayer thickness. The atomic resolution √ STM image of VSe2 is shown in Fig. 1b, in which a well ordered 3a × √ 7a superlattice caused by CDW-related lattice distortions driven by the Se−Se dimerization at low temperature can be distinguished [12]. STS measured on VSe2 (Fig. 1c) shows a small band gap of ∼50 meV in local density of states (LDOS) near the Fermi level, due to CDW order, consistent with previous theoretical [30,38,41] and experimental results [12,13,24,26]. For comparison, the STS of the substrate is also shown in Fig. 1c. Its peakless feature verifies that the substrate has no effect to the observed structures in the STS of VSe2 . At this point, our STM characterization demonstrates the good crystalline quality of the MBE-grown VSe2 for CoPc adsorption to be discussed next. 3.2. Submonolayer CoPc on VSe2 We first studied the growth behavior of submonolayer CoPc on VSe2 . As shown in Fig. 2a, CoPc molecules preferentially adsorb on HOPG, forming a closely packed structure, while only a small portion of the molecules are seen to loosely adsorb on VSe2 , indicating a weaker interaction between CoPc and VSe2 than with HOPG. Fig. 2b−d are three successive STM scans of the same area, illustrating the dynamic changes of several molecular domains (confined within blue, green and red squares) on VSe2 . These domains change in shape or even disappear under the perturbation of the STM tip, thus revealing the mobility of the weakly interacting CoPc on VSe2 . Another obvious feature in Fig. 2 is the distinctive contrasts at the central core of the molecules, with some being brighter than the others, as marked by yellow arrows in Fig. 2b−d. A closer inspection further reveals those brighter molecules to preferentially appear at the molecular domain boundaries or when isolated. These distinctive molecular contrasts are the same as those observed for a CoPc monolayer (as discussed next) due to the resemblance between the bias-dependent STM images of the submonolayer (Figure S1) and those of the monolayer (Fig. 6). 3.3. Monolayer CoPc on VSe2 3.3.1. Adsorption structures For an as-deposited monolayer, CoPc molecules are loosely assembled on VSe2 , forming short-ranged molecular chains or domains (Fig. 3a and its inset). However, on HOPG, the molecules are closely packed. Upon annealing at 420 K for an hour, an ordered molecular arrangement is achieved on VSe2 , as shown in Fig. 3b. Fig. 3c and d are zoom-in STM images of CoPc molecules adsorbed on HOPG and VSe2 , respectively, showing different adsorption structures. Clearly, the molecular contrast on HOPG is singular and homogeneous, while there are two differing contrasts on VSe2 , consistent with the submonolayer case (Fig. 2). The minority of molecules display a flower-like shape, with a brighter central core (referred to as bright molecule hereafter), and the rest having a
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Fig. 1. STM/STS characterization of MBE-grown monolayer VSe2 on HOPG. (a): STM image of VSe2 showing its high crystallinity. The√ height √ profile along the red dashed line shows a thickness of ∼8 Å, indicating a monolayer thickness (Vtip = −0.5 V, I = 39 pA). (b): Atomic resolution STM image of VSe2 . A 3 × 7 superlattice is marked by blue dotted lines (Vtip = 0.2 V, I = 200 pA). (c): Averaged STS spectra of monolayer VSe2 (red line) and HOPG (black line), respectively (Vtip = −0.5 V, I = 100 pA). Monolayer VSe2 shows a CDW-induced band gap of ∼50 meV around the Fermi level.
Fig. 2. STM images of submonolayer CoPc on VSe2 . (a): A large scale STM image showing the preferential adsorption of CoPc molecules on HOPG (Vtip = −2 V, I = 16 pA, 150 × 120 nm2 ). (b), (c) and (d): Three successive STM scans of the same area on VSe2 showing the mobile nature of the weakly adsorbed CoPc on VSe2 (Vtip = −1 V, I = 22 pA, 30 × 30 nm2 ). Accordingly, small molecular domains encased by blue, green and red squares either change in shape or even disappear for different scans. Some of those bright molecules are marked by yellow arrows.
cross-like shape with a lower core brightness (referred to as regular molecule). The height profile along the dashed red line in Fig. 3d shows that, at the given imaging parameters (Vtip = −2.0 V, I = 19 pA), the bright molecules are ∼0.3 Å higher than the regular ones. Fig. 3e and f depicts the respective CoPc adsorption structure models on HOPG and VSe2 , constructed according to the molecularresolution STM images. On HOPG, CoPc molecules form a square lattice with lattice vectors b1 = b2 = ∼1.49 nm and = ∼90◦ (Fig. 3e), while on VSe2 , the molecules are arranged in a quasi-rhombus lattice with lattice vectors a1 = ∼1.63 nm, a2 = ∼1.58 nm and = ∼76.5◦ (green quadrangle in Fig. 3f). For comparison, an undistorted rhombus lattice (yellow) is displayed as well in Fig. 3f. 3.3.2. DFT calculations According to DFT calculations, we find that the Se-top-site adsorption configuration (Co atop the Se atom of VSe2 ) is the most energetically stable, with a charge transfer of 0.104 electron per CoPc molecule to VSe2 (see Figure S2 and Table S1). The polarized density of states (PDOS) and simulated STM images of the HOMO and LUMO of CoPc, based on such adsorption configuration, are illustrated in Fig. 4. It is shown in Fig. 4a that the LUMO consists of contributions from C, N and Co, indicating its delocalization on both the arm and center, consistent with the simulated STM image in Fig. 4c. On the other hand, the LUMO + 1 is mainly contributed by Co in a relatively wider energy landscape. Fig. 4a also shows that the HOMO is mainly contributed by C, meaning that
the HOMO orbital is localized on the arm, as indicated in the simulated STM image (Fig. 4b) with the arm being bright and the center dark. The HOMO + 1 is mainly contributed by Co with slight contributions from C and N. In order to clearly articulate each energy level in the following discussion, the molecular energy levels are separately labeled with regard to the arm and center. For example, HOMO + 1center indicates the HOMO + 1 energy level of the center. As such, the energy level diagram extracted from the DFT calculations is HOMOcenter < HOMOarm < LUMOcenter = LUMOarm < LUMO + 1center < LUMO + 1arm , as summarized in Fig. 5. Note that the energy level separation values are only indicative. 3.3.3. Electronic structure of the regular molecules The STS and bias-dependent STM images of the bright and regular molecules are summarized in Fig. 6. At negative tip bias (LUMO region), LDOS are seen to emerge from ∼ −0.34 V for all spectra, as indicated by the purple dashed line (Fig. 6a). For regular molecules, the positions of LUMOarm and LUMOcenter are comparable (∼ −0.50 V), but the LUMO + 1arm (∼ −1.57 V) is at higher negative tip bias than LUMO + 1center (∼ −1.46 V) by ∼0.11 V. The HOMOarm of the regular molecules (∼1.53 V) is, on the other hand, closer to the Fermi level by ∼0.23 V compared to the HOMOcenter (∼1.76 V). We can thus draw the energy level diagram of the regular molecules from HOMOcenter < HOMOarm < LUMOcenter = LUMOarm < LUMO + 1center < LUMO + 1arm , the relation of which is in good agreement with that calculated by DFT. The STM images obtained
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Fig. 3. STM images of monolayer CoPc on VSe2 . (a): As-deposited monolayer CoPc on VSe2 (150 × 150 nm2 , Vtip = −1.1 V, I = 20 pA). The inset shows a zoom-in image of the molecules on VSe2 (15 × 15 nm2 , Vtip = −1.2 V, I = 16 pA). (b): Monolayer CoPc on VSe2 after annealing at 420 K for an hour (200 × 200 nm2 , Vtip = −1.0 V, I = 34 pA). (c) and (d): Zoom-in STM images showing the molecular arrangements of CoPc on HOPG and VSe2 , respectively (20 × 20 nm2 , Vtip = −1.8 V, I = 19 pA for (c); 20 × 20 nm2 , Vtip = −2.0 V, I = 19 pA for (d)). The height profile along the red dashed line in (d) indicates that the centers of the bright molecules are ∼0.3 Å higher than those of the regular molecules at the given scanning parameters. Two bright molecules on the profile line are marked by red and green triangles. (e) and (f): Adsorption structure models of CoPc on HOPG and VSe2 with experimental lattice constants and angles displayed, respectively. Green quadrangles in (e) and (f) are primitive cells. Yellow lines in (f) show an undistorted rhombus.
Fig. 4. (a) The PDOS of CoPc on monolayer VSe2 based on the Se-top-site adsorption. The LUMO is contributed by C, N and Co, while the HOMO mainly by C, indicating the delocalization of LUMO on both the arm and center and the localization of HOMO on the arm. (b) and (c): The corresponding simulated STM images of the HOMO and LUMO, respectively.
at −0.5 V (Fig. 6d) and −1.6 V (Fig. 6b) show the delocalization of the orbital of the regular molecules on both the arm and center, also in agreement with the STS results and the simulated STM image of
the LUMO (Fig. 4c), in which both the center and arm are bright. On the other hand, localization of molecular orbital is observed on the arm at 1.5 V and on the center at 1.8 V (Fig. 6e and g), which are
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Fig. 5. The energy level diagrams of the regular and bright molecules extracted from Fig. 6a as well as that extracted from DFT calculations (Fig. 4a). Note that the diagrams just reflect relative alignments, rather than the absolute separations between different energy levels.
Fig. 6. (a): STS measured on the centers and arms of the regular and bright molecules on monolayer VSe2 . STM settings before opening feedback loop: −1.0 V, 30 pA for bright centers and −1.2 V, 15 pA for the rest. The STS intensities of the regular centers and bright centers were multiplied by 3 for a better comparison. (b)−(g): Bias-dependent STM images of CoPc on monolayer VSe2 scanned on the same area with both regular and bright molecules. Scan size is 10 × 10 nm2 and setpoint current is 20 pA for all images.
consistent with both our experimental STS and the simulated STM image of the HOMO (Fig. 4b). The general agreement here suggests the ground state of the observed regular molecules. 3.3.4. Electronic structure of the bright molecules For the bright molecules, the LUMOarm and LUMO + 1arm remain unchanged with respect to those of the regular molecules. However, their HOMOarm shift away from the Fermi level to ∼1.80 V, as revealed in Fig. 6a. Both the LUMO + 1center and HOMOcenter of
the bright molecules shift towards the Fermi level to ∼ −1.00 V and ∼1.63 V respectively, leading to the HOMOarm being below the HOMOcenter (i.e. HOMOarm < HOMOcenter ). The dramatic shift of the LUMO + 1center and the change from HOMOarm > HOMOcenter for the regular molecules to HOMOarm < HOMOcenter for the bright molecules are indications that the central Co atom is responsible for the emergence of the bright molecules. Apparent broadening of the HOMOcenter of the bright molecules is another key feature different from that of the regular molecules. The STS features and the
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Fig. 7. Tip bias pulses of −2.0 (a−b), −2.2 (b−c), −2.4 (c−d), 2.0 (e−f), 2.2 (f−g) and 2.5 V (g−h) were applied to the centers of the bright and regular molecules marked by yellow and green circles, respectively. All pulse times were 0.5 s. (i) STS recorded in the bias range from −2.0 to −2.8 V where the state switching occurred. (Setpoints: Vtip = −1.1 V, I = 20 pA). A sudden jump can be seen in the STS when the bright molecules were switched to the regular molecules. For comparison, no such jump was observed for the regular molecules.
bias-dependent STM images correspond well. At −0.5 V (Fig. 6d), the orbital of the bright molecules delocalizes on both the center and arm due to LUMOarm = LUMOcenter . The orbital mainly localizes on the center at −1.0 V (Fig. 6c), but on the arm at −1.6 V (Fig. 6b) due to LUMO + 1center < LUMO + 1arm . The orbital mainly localizes on the center at 1.6 V (Fig. 6f) and on both the center and arm at 1.8 V (Fig. 6g) due to the aforementioned broadening of the HOMOcenter . The comparative inequalities are again summarized in Fig. 5. The energy level diagram of the bright molecules is then HOMOarm < HOMOcenter < LUMOcenter = LUMOarm < LUMO + 1center < LUMO + 1arm , which is not fully consistent with our DFT calculations which predicts that HOMOcenter < HOMOarm .
bias pulse larger than 2.6 V, the molecule was likely to be pulled out from the substrate surface, meaning that either the bias needed for the switching from the regular to the bright state is too large or the state switching itself is unidirectional. Based on the tipbias induced state switching, we postulate that the bright CoPc molecules might be in a more positively charged metastable state than the regular molecules. As such, a negative tip bias above the threshold can force the bright molecules to overcome the energy barrier toward the regular ground state. On the other hand, a larger energy barrier could exist for the reverse switching as it is barely achieved.
3.4. Tip-bias induced state switching
4. Conclusion
Difference in molecular contrasts are often of structural and/or electronic origins. The former effects are, for example, the protrusion or depression of central metal atom [42], and the interfacial atom substitution between the molecule and substrate [43], etc., while electronic effects can arise from adsorption [44–46], delicate energy level alignment between molecule and substrate [47], conformational change without charge transfer [48], or Moire-induced charged states [8,49,50]. For the present case of closely packed CoPc monolayer film on VSe2 , the random distribution of the bright molecules excludes the possibility of the Moire-induced effect. We also show and discuss in Supporting Information that further DFT calculations and control experiments preclude the other possibilities mentioned above. The bi-stable electronic states of CoPc on monolayer VSe2 could be attributed to the different charged states of CoPc since switching from the bright to the regular state can be achieved by applying a tip bias pulse to the core Co atom. Fig. 7 illustrates such switching, in which tip bias pulses were applied to those bright molecules marked by yellow circles. At −2.0 V, no switching occurred (Fig. 7a−b), while increasing to −2.2 V led to the switching of one bright molecule to the regular state (Fig. 7b−c). The tip bias of −2.4 V is the threshold, at which all of the bright molecules were switched (Fig. 7c−d). Our observation was further confirmed by the STS recorded in the bias range from −2.0 to −2.8 V, within which a sudden jump at ∼ −2.35 V was observed only for the bright molecules (Fig. 7i). It is also evident that such switching is irreversible. When positive tip bias pulses were applied to the regular molecules (marked by green circles in Fig. 7e−h), the switching from regular to bright, was not observed. In most cases with a tip
In summary, the adsorption and electronic structure of CoPc on MBE-grown monolayer VSe2 /HOPG were studied using STM/STS and DFT calculations. Two distinct molecular contrasts, i.e. the bright and regular, were observed on VSe2 , in contrast to those adsorbed on HOPG where only a single molecular contrast was observed. STM/STS results reveal a major difference in the electronic state of the central Co atom between these two states on VSe2 . The switching from the bright state to the regular state can be achieved by applying a negative tip bias pulse to the molecule center, but the reverse switching can be barely achieved. Based on DFT calculations, we ascribe the regular state to the ground state and speculate that the bright molecules are in a more positively charged metastable state. The origin of the bi-stable electronic states of CoPc on VSe2 remains inconclusive, and addressing this might provide a future pathway to manipulate the electronic states of molecules by a switching bias. This could find important applications, for instance, in single-molecule devices for memory storage.
Author contributions L.Z. and T.Y. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Declaration of Competing Interest The authors declare no competing interests.
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Acknowledgments L.Z. acknowledges financial support from the RS (PhD)CA2DM/Graphene Cte IS Scholarship. This work is financially supported by the Singapore Ministry of Education Tier 2 grant (MOE2016-T2-2-110), the National Research Foundation Medium Sized Centre Programme (R-723-000-001-112) and the A*STAR 2D PHAROS Grant (R-144-000-359-305). This work was partially performed on the soft X-ray spectroscopy beamline at the Australian Synchrotron, Victoria, Australia. We also acknowledge the technical services and beamtimes provided by the National Synchrotron Radiation Research Center, a national user facility supported by the Ministry of Science and Technology of Taiwan, ROC. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apmt.2019. 100535.
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Applied Materials Today journal homepage: www.elsevier.com/locate/apmt
Bi-stable electronic states of cobalt phthalocyanine molecules on two-dimensional vanadium diselenide Lei Zhang a,b,1 , Tong Yang a,1 , Wen Zhang a , Dongchen Qi c,d , Xiaoyue He e , Kaijian Xing d , Ping Kwan Johnny Wong b,∗ , Yuan Ping Feng a,∗ , Andrew Thye Shen Wee a,b,∗∗ a
Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore c School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, Queensland 4001, Australia d Department of Chemistry and Physics, La Trobe Institute for Molecular Science, La Trobe University, Melbourne, VIC, 3086, Australia e Songshan Lake Materials Laboratory, Dongguan 523808, China b
a r t i c l e
i n f o
Article history: Received 25 September 2019 Accepted 12 December 2019 Keywords: Cobalt phthalocyanine Vanadium diselenide Bi-stable states Scanning tunneling microscopy Two-dimensional transition-metal dichalcogenides
a b s t r a c t Hybrid organic/2D interfaces combine the wide spectrum of 2D material properties with the major advantages of organic materials, such as low cost, mechanical flexibility, and chemical tunability. Here, we report the electronic properties of cobalt phthalocyanine (CoPc) molecules adsorbed on molecular beam epitaxy-grown monolayer vanadium diselenide (VSe2 ). Using scanning tunneling microscopy/spectroscopy, we provide evidence of highly ordered molecular assembly on monolayer VSe2 , with two distinctive bright and regular molecular contrasts, which are not observed on graphite. These contrasts also lead to a distinct difference in the electronic state of the molecule’s central Co atom, for which density functional theory calculations indicate the regular state as the ground state. A correlation between these two molecular states and the charged states of individual molecules is postulated, as demonstrated by the possibility of switching the bright state to the regular state using a negative tip voltage pulse. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Benefiting from their multitude of intrinsic and unique properties, two dimensional (2D) transition-metal dichalcogenides (TMDs) have received enormous attention in the recent decade for both fundamental studies and emerging applications [1]. Many opportunities exist for engineering and manipulating these properties, for example, by stacking similarly layered materials as van der Waals (vdW) heterostructures, or interfacing them with other non-layered materials. The functionalization of 2D TMDs with organic molecules is a particular example of the latter approach, via which the 2D properties can be optimized by coupling with organic materials that are low cost, mechanically flexible and
∗ Corresponding authors. ∗∗ Corresponding author at: Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore; Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore E-mail addresses: [email protected] (P.K.J. Wong), [email protected] (Y.P. Feng), [email protected] (A.T.S. Wee). 1 L.Z. and T.Y. contributed equally. https://doi.org/10.1016/j.apmt.2019.100535 2352-9407/© 2019 Elsevier Ltd. All rights reserved.
chemically tunable. The hybrid organic/2D interface may also host unprecedented interfacial phenomena, inaccessible in the individual constituents [2–10]. Here, our work focuses on the electronic properties of cobalt phthalocyanine (CoPc) molecules deposited on molecular beam epitaxy (MBE)-grown monolayer VSe2 on highly oriented pyrolytic graphite (HOPG). The rationale for this particular organic/2D combination is largely motivated by a number of attributes: (1) 2D vanadium dichalcogenides, VX2 (X= S, Se, Te), are an exciting class of TMDs with a great variety of exotic properties. These include the ultra-high electrical conductivity (up to 106 Sm−1 ) in VSe2 [11], enhanced charge-density waves (CDW) in monolayer VSe2 [12–14], VS2 [15] and VTe2 [16], catalytic activities of VSe2 for hydrogen evolution reaction (HER) [17–21], and magnetism in VS2 [15,22,23], VTe2 [15] and VSe2 [12,24–29]. These properties in 2D VX2 are mainly derived from the 3d1 odd electronic configuration of V4+ alongside the strong electronic coupling between neighboring V4+ −V4+ pairs [30,31]; (2) CoPc is a typical spin-1/2 transition-metal phthalocyanine molecule, with its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) being largely dictated by the central Co 3d orbital electrons of the molecule [32]. It is thus fundamentally interesting
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to explore how these electrons would interact with those in the V 3d bands of VX2 . Our study, using scanning tunneling microscopy/spectroscopy (STM/STS), reveals that CoPc molecules form a highly ordered selfassembly on 2D VSe2 and exhibit two distinctive bright and regular molecular contrasts, in contrast to CoPc adsorbed on HOPG. These molecular contrasts are accompanied with a distinct difference in the electronic state of the central Co atom, which DFT calculations suggest the regular state as the ground state. We have also demonstrated the ability to switch the bright state to the regular by applying a negative voltage pulse through an STM tip. This might suggest a correlation between the observed molecular states and the charged states of the individual molecules. 2. Experimental methods and computational details 2.1. Sample preparation and characterization Monolayer VSe2 was grown on HOPG substrate by MBE in an ultrahigh vacuum (UHV) chamber with a base pressure of ∼1 × 10−9 mbar. To avoid possible contamination, the substrate was cleaved in the vacuum chamber and then annealed at 720 K for 1 h. Se (Sigma, 99.999 %) was evaporated from a Knudsen cell at 470 K and V (ESPI Metals, 99.999 %) from an electron-beam evaporator. During growth, the substrate was kept at 650 K, a temperature higher than the sublimation temperature of Se, and the whole growth process was maintained under Se-rich conditions to ensure sufficient Se atoms to react with V. After growth, the sample was cooled to 320 K and capped with Se. For CoPc deposition and STM/STS characterization, the monolayer VSe2 was transferred to the STM preparation chamber. After annealing at 520 K for 1 h, the Se capping layer was completely desorbed, leaving a clean VSe2 surface for the stepwise CoPc deposition. CoPc (Fluka, >97 %) was thoroughly degassed at its evaporation temperature (573 K) in a Knudsen cell for at least 3 h, then idled at 500 K during the entire experiment. For deposition, the molecules were first heated to 573 K and stabilized for 1 h to obtain a stable evaporation rate, then deposited onto monolayer VSe2 at room temperature at a deposition rate of ∼0.17 layer/min. The thickness of the CoPc film was determined by a quartz crystal microbalance (QCM) thickness monitor, with its reading calibrated against STM line profiles. 2.2. DFT calculations Calculations based on spin-polarized DFT were carried out using the Vienna ab initio simulation package (VASP) [33,34]. The projector augmented-wave (PAW) method and the generalized gradient approximation within the Perdew, Burke and Ernzerhof parametrization were adopted for the core-valence electrons interaction and the exchange-correlation functional, respectively [35,36]. The electronic wave functions were expanded in a plane wave basis with a cutoff kinetic energy of 500 eV. In accordance with the experimentally observed molecular arrangement of CoPc on √ 1 T-VSe √ 2 (see Fig. 3 f), a monolayer VSe2 supercell including 21 × 21 unit cells (15.672 Å × 15.672 Å) was constructed and the in-plane orientations of the adsorbed CoPc molecules were adjusted so as to be similar to the experimental observation. To diminish the spurious interlayer interaction, a vacuum layer of around 20 Å was inserted in the direction perpendicular to the basal plane. A -centered k-grid of 2 × 2 × 1 was chosen to sample the first Brillouin zone for structural optimizations, and 4 × 4 × 1 for energy calculations. The electronic and ionic convergence criteria were set to 1.0 × 10−4 eV and −0.01 eV/Å, respectively. To take into account the strong on-site Coulomb interaction, the effective Hubbard U values of 1.0 and 3.0 eV, which were taken from previous
literatures, were added onto the V 3d and Co 3d electrons using the simplified rotationally invariant approach proposed by Dudarev et al. [32,37,38]. The long-range vdW interaction was also included via the DFT-D3 correction method [39]. The STM images of the freestanding and adsorbed CoPc molecules were simulated using the Tersoff method, where the tip height was kept at about 2 Å [40]. 3. Results and discussion 3.1. MBE-grown monolayer VSe2 The MBE-grown monolayer VSe2 on HOPG was characterized by STM/STS at 77 K. Fig. 1a shows that VSe2 forms ordered and smooth triangular islands with a height of ∼8 Å, indicating its high crystallinity and monolayer thickness. The atomic resolution √ STM image of VSe2 is shown in Fig. 1b, in which a well ordered 3a × √ 7a superlattice caused by CDW-related lattice distortions driven by the Se−Se dimerization at low temperature can be distinguished [12]. STS measured on VSe2 (Fig. 1c) shows a small band gap of ∼50 meV in local density of states (LDOS) near the Fermi level, due to CDW order, consistent with previous theoretical [30,38,41] and experimental results [12,13,24,26]. For comparison, the STS of the substrate is also shown in Fig. 1c. Its peakless feature verifies that the substrate has no effect to the observed structures in the STS of VSe2 . At this point, our STM characterization demonstrates the good crystalline quality of the MBE-grown VSe2 for CoPc adsorption to be discussed next. 3.2. Submonolayer CoPc on VSe2 We first studied the growth behavior of submonolayer CoPc on VSe2 . As shown in Fig. 2a, CoPc molecules preferentially adsorb on HOPG, forming a closely packed structure, while only a small portion of the molecules are seen to loosely adsorb on VSe2 , indicating a weaker interaction between CoPc and VSe2 than with HOPG. Fig. 2b−d are three successive STM scans of the same area, illustrating the dynamic changes of several molecular domains (confined within blue, green and red squares) on VSe2 . These domains change in shape or even disappear under the perturbation of the STM tip, thus revealing the mobility of the weakly interacting CoPc on VSe2 . Another obvious feature in Fig. 2 is the distinctive contrasts at the central core of the molecules, with some being brighter than the others, as marked by yellow arrows in Fig. 2b−d. A closer inspection further reveals those brighter molecules to preferentially appear at the molecular domain boundaries or when isolated. These distinctive molecular contrasts are the same as those observed for a CoPc monolayer (as discussed next) due to the resemblance between the bias-dependent STM images of the submonolayer (Figure S1) and those of the monolayer (Fig. 6). 3.3. Monolayer CoPc on VSe2 3.3.1. Adsorption structures For an as-deposited monolayer, CoPc molecules are loosely assembled on VSe2 , forming short-ranged molecular chains or domains (Fig. 3a and its inset). However, on HOPG, the molecules are closely packed. Upon annealing at 420 K for an hour, an ordered molecular arrangement is achieved on VSe2 , as shown in Fig. 3b. Fig. 3c and d are zoom-in STM images of CoPc molecules adsorbed on HOPG and VSe2 , respectively, showing different adsorption structures. Clearly, the molecular contrast on HOPG is singular and homogeneous, while there are two differing contrasts on VSe2 , consistent with the submonolayer case (Fig. 2). The minority of molecules display a flower-like shape, with a brighter central core (referred to as bright molecule hereafter), and the rest having a
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Fig. 1. STM/STS characterization of MBE-grown monolayer VSe2 on HOPG. (a): STM image of VSe2 showing its high crystallinity. The√ height √ profile along the red dashed line shows a thickness of ∼8 Å, indicating a monolayer thickness (Vtip = −0.5 V, I = 39 pA). (b): Atomic resolution STM image of VSe2 . A 3 × 7 superlattice is marked by blue dotted lines (Vtip = 0.2 V, I = 200 pA). (c): Averaged STS spectra of monolayer VSe2 (red line) and HOPG (black line), respectively (Vtip = −0.5 V, I = 100 pA). Monolayer VSe2 shows a CDW-induced band gap of ∼50 meV around the Fermi level.
Fig. 2. STM images of submonolayer CoPc on VSe2 . (a): A large scale STM image showing the preferential adsorption of CoPc molecules on HOPG (Vtip = −2 V, I = 16 pA, 150 × 120 nm2 ). (b), (c) and (d): Three successive STM scans of the same area on VSe2 showing the mobile nature of the weakly adsorbed CoPc on VSe2 (Vtip = −1 V, I = 22 pA, 30 × 30 nm2 ). Accordingly, small molecular domains encased by blue, green and red squares either change in shape or even disappear for different scans. Some of those bright molecules are marked by yellow arrows.
cross-like shape with a lower core brightness (referred to as regular molecule). The height profile along the dashed red line in Fig. 3d shows that, at the given imaging parameters (Vtip = −2.0 V, I = 19 pA), the bright molecules are ∼0.3 Å higher than the regular ones. Fig. 3e and f depicts the respective CoPc adsorption structure models on HOPG and VSe2 , constructed according to the molecularresolution STM images. On HOPG, CoPc molecules form a square lattice with lattice vectors b1 = b2 = ∼1.49 nm and = ∼90◦ (Fig. 3e), while on VSe2 , the molecules are arranged in a quasi-rhombus lattice with lattice vectors a1 = ∼1.63 nm, a2 = ∼1.58 nm and = ∼76.5◦ (green quadrangle in Fig. 3f). For comparison, an undistorted rhombus lattice (yellow) is displayed as well in Fig. 3f. 3.3.2. DFT calculations According to DFT calculations, we find that the Se-top-site adsorption configuration (Co atop the Se atom of VSe2 ) is the most energetically stable, with a charge transfer of 0.104 electron per CoPc molecule to VSe2 (see Figure S2 and Table S1). The polarized density of states (PDOS) and simulated STM images of the HOMO and LUMO of CoPc, based on such adsorption configuration, are illustrated in Fig. 4. It is shown in Fig. 4a that the LUMO consists of contributions from C, N and Co, indicating its delocalization on both the arm and center, consistent with the simulated STM image in Fig. 4c. On the other hand, the LUMO + 1 is mainly contributed by Co in a relatively wider energy landscape. Fig. 4a also shows that the HOMO is mainly contributed by C, meaning that
the HOMO orbital is localized on the arm, as indicated in the simulated STM image (Fig. 4b) with the arm being bright and the center dark. The HOMO + 1 is mainly contributed by Co with slight contributions from C and N. In order to clearly articulate each energy level in the following discussion, the molecular energy levels are separately labeled with regard to the arm and center. For example, HOMO + 1center indicates the HOMO + 1 energy level of the center. As such, the energy level diagram extracted from the DFT calculations is HOMOcenter < HOMOarm < LUMOcenter = LUMOarm < LUMO + 1center < LUMO + 1arm , as summarized in Fig. 5. Note that the energy level separation values are only indicative. 3.3.3. Electronic structure of the regular molecules The STS and bias-dependent STM images of the bright and regular molecules are summarized in Fig. 6. At negative tip bias (LUMO region), LDOS are seen to emerge from ∼ −0.34 V for all spectra, as indicated by the purple dashed line (Fig. 6a). For regular molecules, the positions of LUMOarm and LUMOcenter are comparable (∼ −0.50 V), but the LUMO + 1arm (∼ −1.57 V) is at higher negative tip bias than LUMO + 1center (∼ −1.46 V) by ∼0.11 V. The HOMOarm of the regular molecules (∼1.53 V) is, on the other hand, closer to the Fermi level by ∼0.23 V compared to the HOMOcenter (∼1.76 V). We can thus draw the energy level diagram of the regular molecules from HOMOcenter < HOMOarm < LUMOcenter = LUMOarm < LUMO + 1center < LUMO + 1arm , the relation of which is in good agreement with that calculated by DFT. The STM images obtained
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Fig. 3. STM images of monolayer CoPc on VSe2 . (a): As-deposited monolayer CoPc on VSe2 (150 × 150 nm2 , Vtip = −1.1 V, I = 20 pA). The inset shows a zoom-in image of the molecules on VSe2 (15 × 15 nm2 , Vtip = −1.2 V, I = 16 pA). (b): Monolayer CoPc on VSe2 after annealing at 420 K for an hour (200 × 200 nm2 , Vtip = −1.0 V, I = 34 pA). (c) and (d): Zoom-in STM images showing the molecular arrangements of CoPc on HOPG and VSe2 , respectively (20 × 20 nm2 , Vtip = −1.8 V, I = 19 pA for (c); 20 × 20 nm2 , Vtip = −2.0 V, I = 19 pA for (d)). The height profile along the red dashed line in (d) indicates that the centers of the bright molecules are ∼0.3 Å higher than those of the regular molecules at the given scanning parameters. Two bright molecules on the profile line are marked by red and green triangles. (e) and (f): Adsorption structure models of CoPc on HOPG and VSe2 with experimental lattice constants and angles displayed, respectively. Green quadrangles in (e) and (f) are primitive cells. Yellow lines in (f) show an undistorted rhombus.
Fig. 4. (a) The PDOS of CoPc on monolayer VSe2 based on the Se-top-site adsorption. The LUMO is contributed by C, N and Co, while the HOMO mainly by C, indicating the delocalization of LUMO on both the arm and center and the localization of HOMO on the arm. (b) and (c): The corresponding simulated STM images of the HOMO and LUMO, respectively.
at −0.5 V (Fig. 6d) and −1.6 V (Fig. 6b) show the delocalization of the orbital of the regular molecules on both the arm and center, also in agreement with the STS results and the simulated STM image of
the LUMO (Fig. 4c), in which both the center and arm are bright. On the other hand, localization of molecular orbital is observed on the arm at 1.5 V and on the center at 1.8 V (Fig. 6e and g), which are
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Fig. 5. The energy level diagrams of the regular and bright molecules extracted from Fig. 6a as well as that extracted from DFT calculations (Fig. 4a). Note that the diagrams just reflect relative alignments, rather than the absolute separations between different energy levels.
Fig. 6. (a): STS measured on the centers and arms of the regular and bright molecules on monolayer VSe2 . STM settings before opening feedback loop: −1.0 V, 30 pA for bright centers and −1.2 V, 15 pA for the rest. The STS intensities of the regular centers and bright centers were multiplied by 3 for a better comparison. (b)−(g): Bias-dependent STM images of CoPc on monolayer VSe2 scanned on the same area with both regular and bright molecules. Scan size is 10 × 10 nm2 and setpoint current is 20 pA for all images.
consistent with both our experimental STS and the simulated STM image of the HOMO (Fig. 4b). The general agreement here suggests the ground state of the observed regular molecules. 3.3.4. Electronic structure of the bright molecules For the bright molecules, the LUMOarm and LUMO + 1arm remain unchanged with respect to those of the regular molecules. However, their HOMOarm shift away from the Fermi level to ∼1.80 V, as revealed in Fig. 6a. Both the LUMO + 1center and HOMOcenter of
the bright molecules shift towards the Fermi level to ∼ −1.00 V and ∼1.63 V respectively, leading to the HOMOarm being below the HOMOcenter (i.e. HOMOarm < HOMOcenter ). The dramatic shift of the LUMO + 1center and the change from HOMOarm > HOMOcenter for the regular molecules to HOMOarm < HOMOcenter for the bright molecules are indications that the central Co atom is responsible for the emergence of the bright molecules. Apparent broadening of the HOMOcenter of the bright molecules is another key feature different from that of the regular molecules. The STS features and the
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Fig. 7. Tip bias pulses of −2.0 (a−b), −2.2 (b−c), −2.4 (c−d), 2.0 (e−f), 2.2 (f−g) and 2.5 V (g−h) were applied to the centers of the bright and regular molecules marked by yellow and green circles, respectively. All pulse times were 0.5 s. (i) STS recorded in the bias range from −2.0 to −2.8 V where the state switching occurred. (Setpoints: Vtip = −1.1 V, I = 20 pA). A sudden jump can be seen in the STS when the bright molecules were switched to the regular molecules. For comparison, no such jump was observed for the regular molecules.
bias-dependent STM images correspond well. At −0.5 V (Fig. 6d), the orbital of the bright molecules delocalizes on both the center and arm due to LUMOarm = LUMOcenter . The orbital mainly localizes on the center at −1.0 V (Fig. 6c), but on the arm at −1.6 V (Fig. 6b) due to LUMO + 1center < LUMO + 1arm . The orbital mainly localizes on the center at 1.6 V (Fig. 6f) and on both the center and arm at 1.8 V (Fig. 6g) due to the aforementioned broadening of the HOMOcenter . The comparative inequalities are again summarized in Fig. 5. The energy level diagram of the bright molecules is then HOMOarm < HOMOcenter < LUMOcenter = LUMOarm < LUMO + 1center < LUMO + 1arm , which is not fully consistent with our DFT calculations which predicts that HOMOcenter < HOMOarm .
bias pulse larger than 2.6 V, the molecule was likely to be pulled out from the substrate surface, meaning that either the bias needed for the switching from the regular to the bright state is too large or the state switching itself is unidirectional. Based on the tipbias induced state switching, we postulate that the bright CoPc molecules might be in a more positively charged metastable state than the regular molecules. As such, a negative tip bias above the threshold can force the bright molecules to overcome the energy barrier toward the regular ground state. On the other hand, a larger energy barrier could exist for the reverse switching as it is barely achieved.
3.4. Tip-bias induced state switching
4. Conclusion
Difference in molecular contrasts are often of structural and/or electronic origins. The former effects are, for example, the protrusion or depression of central metal atom [42], and the interfacial atom substitution between the molecule and substrate [43], etc., while electronic effects can arise from adsorption [44–46], delicate energy level alignment between molecule and substrate [47], conformational change without charge transfer [48], or Moire-induced charged states [8,49,50]. For the present case of closely packed CoPc monolayer film on VSe2 , the random distribution of the bright molecules excludes the possibility of the Moire-induced effect. We also show and discuss in Supporting Information that further DFT calculations and control experiments preclude the other possibilities mentioned above. The bi-stable electronic states of CoPc on monolayer VSe2 could be attributed to the different charged states of CoPc since switching from the bright to the regular state can be achieved by applying a tip bias pulse to the core Co atom. Fig. 7 illustrates such switching, in which tip bias pulses were applied to those bright molecules marked by yellow circles. At −2.0 V, no switching occurred (Fig. 7a−b), while increasing to −2.2 V led to the switching of one bright molecule to the regular state (Fig. 7b−c). The tip bias of −2.4 V is the threshold, at which all of the bright molecules were switched (Fig. 7c−d). Our observation was further confirmed by the STS recorded in the bias range from −2.0 to −2.8 V, within which a sudden jump at ∼ −2.35 V was observed only for the bright molecules (Fig. 7i). It is also evident that such switching is irreversible. When positive tip bias pulses were applied to the regular molecules (marked by green circles in Fig. 7e−h), the switching from regular to bright, was not observed. In most cases with a tip
In summary, the adsorption and electronic structure of CoPc on MBE-grown monolayer VSe2 /HOPG were studied using STM/STS and DFT calculations. Two distinct molecular contrasts, i.e. the bright and regular, were observed on VSe2 , in contrast to those adsorbed on HOPG where only a single molecular contrast was observed. STM/STS results reveal a major difference in the electronic state of the central Co atom between these two states on VSe2 . The switching from the bright state to the regular state can be achieved by applying a negative tip bias pulse to the molecule center, but the reverse switching can be barely achieved. Based on DFT calculations, we ascribe the regular state to the ground state and speculate that the bright molecules are in a more positively charged metastable state. The origin of the bi-stable electronic states of CoPc on VSe2 remains inconclusive, and addressing this might provide a future pathway to manipulate the electronic states of molecules by a switching bias. This could find important applications, for instance, in single-molecule devices for memory storage.
Author contributions L.Z. and T.Y. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Declaration of Competing Interest The authors declare no competing interests.
L. Zhang, T. Yang, W. Zhang et al. / Applied Materials Today 18 (2020) 100535
Acknowledgments L.Z. acknowledges financial support from the RS (PhD)CA2DM/Graphene Cte IS Scholarship. This work is financially supported by the Singapore Ministry of Education Tier 2 grant (MOE2016-T2-2-110), the National Research Foundation Medium Sized Centre Programme (R-723-000-001-112) and the A*STAR 2D PHAROS Grant (R-144-000-359-305). This work was partially performed on the soft X-ray spectroscopy beamline at the Australian Synchrotron, Victoria, Australia. We also acknowledge the technical services and beamtimes provided by the National Synchrotron Radiation Research Center, a national user facility supported by the Ministry of Science and Technology of Taiwan, ROC. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apmt.2019. 100535.
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